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General Discussion => Advanced Concepts => Topic started by: kfsorensen on 12/30/2005 01:56 pm

Title: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 12/30/2005 01:56 pm
There was previous discussion of the role of nuclear thermal propulsion/bimodal nuclear thermal propulsion in a previous thread.  Is there interest in continuing this discussion in terms of the roles these technologies, as well as nuclear electric propulsion, might play in future exploration architectures?

This is an image of a nuclear-electric powered Mars transfer vehicle that would rotate to provide artificial gravity to the crew.  The mass of the reactor and power conversion system roughly balance the mass of the pressurized module.  The electric thrusters are located in the middle of the truss.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: lmike on 12/30/2005 02:06 pm
Mars bound, LEO constructed nuke powered ships?  Certainly, I would think.  The question of the total mass needed to be lofted in pieces (such as the radiators/truss), and the LEO construction needed appears to dominate the considerations, and also the (non)availability of rare gases if required as the working body (xenon, argon).  It's a question of how we get it all to LEO.  Practically, I think it's a question of either an HLV being available, and/or significantly cheaper medium lift from Earth available.  (NASA vs. commercial, here, it seems)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: FransonUK on 12/30/2005 03:23 pm
Wow, cool image. How long would that take to construct in orbit?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: realtime on 12/30/2005 06:20 pm
Lo-o-ong transit times with nuclear electric.  About 225 days from LEO to LMO if you believe the Mars Institute.  Doesn't mean it won't do better as the technology advances.

Interesting design for artificial gravity.  Like the BNTR, only NEP thrusts continuously.  Too bad all that radiator mass has to be boosted there and back again.  Maybe this would make a decent "cycler"?

http://www.marsinstitute.info/rd/faculty/dportree/rtr/at05.html
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 12/30/2005 06:57 pm
Trip time has more to do with orbital mechanics than propulsion systems.  The trip time is a variable...you plug it into the orbital mechanics and out pops a propulsion requirement.  Then you see if the technology can meet the propulsion requirement.  A NEP system will probably want about 9 months to get to Mars, whereas a high-thrust system can do it in about 6 months.  This assumes that the NEP system essentially starts in a C3=0 orbit and has to transition to a Mars transfer orbit, and then transition again to a Martian heliocentric orbit.  By virtue of the injection burn, the high-thrust case will leave Earth on a C3~15 trajectory and then also encounter Mars at a positive C3, requiring a propulsive maneuver or aerobraking.

Either way, you really need to spend the next 18 months at Mars waiting for the right moment to depart, so what does it really matter if it took you 6 months or 9 months to get there?  With artificial gravity you've had no bone or immune system deterioration.  And I assume you've carried sufficient polyethylene or water to keep the radiation exposure levels down.

Bottom line, it takes 2 1/2 to 3 years to go to Mars in a reasonable fashion.  If you use high-thrust propulsion and aerobraking, your outbound and inbound legs will be about 6 months each.  With this NEP system those legs are about 9 months.  Either way you're gone the same amount of time.  And this system might be able to be reused, unlike the high-thrust systems.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 12/30/2005 07:33 pm
The previous image shows a typical low-thrust trajectory.  This one starts in October 2026 and has nine-month inbound and outbound legs.  The green sections are thrusting arcs, the blue sections are coasting arcs.  The total trajectory takes 1000 days, or 2 3/4 years, returning in July 2029.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: GirlygirlShuttlefan on 12/30/2005 08:01 pm
Silly question, but why do you have to travel from Earth to Mars in a big curve? Wouldn't it be half the distance to go straight, timing it so Mars in at the end of the straight line for when you arrive in the area?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 12/30/2005 08:19 pm
No, asking why we don't travel in a straight line isn't a dumb question, but instead a common one.  Because we are used to traveling around on the Earth in pretty straight lines, and we wonder why we can't do the same thing in space.  Perhaps you've been to the mall and seen those coin funnels, where you drop a coin in and it goes round and round and round, spiralling into the center.  That's actually a REALLY good model of orbital motion, or the kind of motion satellites and planets follow.  The coin doesn't go in a straight line because of the shape of the funnel, rather it travels in a circle, because the circle is a level of constant gravitational energy.  Now if there was no friction between the coin and the surface of the funnel (or the air), it wouldn't ever spiral in--it would just go around and around in that circle forever.  But because of friction, the coin loses energy and spirals further and further down the funnel, losing gravitational energy and speeding up, ironically.  You see the same thing as a satellite loses orbital energy due to atmospheric drag--it speeds up, even though it's losing energy.

Well, the gravity of the Sun is what causes space to be "curved" gravitationally, much like the funnel.  Another way to imagine it is to imagine a trampoline.  When no one is sitting on it, if you roll a marble across it, it will basically go in a straight line.  But imagine putting a bowling ball in the middle of the trampoline and now rolling a marble.  It won't go in a straight line anymore but will follow a curve.  If you do it just right you can make a little solar system with the bowling ball representing the Sun and the marbles representing the planets.  Other than friction, the physics are basically the same.

So when it comes to Mars, imagine that we are on one marble (the Earth) spinning around the funnel, and we want to go to another marble (Mars) spinning further up the funnel.  The easiest way to get there is not a straight line, but rather a curve, and since both marbles are in constant motion, we have to leave Mars at the right time to return to the Earth.  That's why you need to stay on Mars for a year-and-a-half or so before you come back, so that Earth will actually be there when you return.

Go buy 50 cents worth of pennies and head down to the mall and roll them in that funnel, and you will probably get a better feel for orbital mechanics than you'll get from spending a week with the textbooks.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Davros on 12/30/2005 10:07 pm
I'd be in favor of this drive but the risk of enviormentalists trying to bring down the program makes me concerned about potential project cancellation. Is there a solution to this, such as a different holding and launch site for the nuclear element of the propulsion?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: realtime on 12/30/2005 10:27 pm
Quote
Davros - 30/12/2005  6:07 PM

I'd be in favor of this drive but the risk of enviormentalists trying to bring down the program makes me concerned about potential project cancellation. Is there a solution to this, such as a different holding and launch site for the nuclear element of the propulsion?
No, I'm afraid there is no cure for chronic ideologues short of brain transplant.  - sigh! -

;)  Just kidding.  Really.

Even the Greens are starting to accept that maybe, just maybe nuclear power is cleaner in the long run than fossil fuels.  Larson B might have been a turning point.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Bruce H on 12/30/2005 11:02 pm
Good point. They are starting to see the benefit of new forms of viable energy and should be 100 per cent behind our aims to find new fuels which are less harmful to the planet.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 12/30/2005 11:06 pm
Well, let's be careful in whose brains we plan to transplant.  The history of nuclear energy is not exactly a linear path to perfection.  That said, it is also important to note that there is an enormous diversity in the different types of reactors, even if the reactors built are not diverse.  There are some types of nuclear power that I would try to get built right next door to me, and there are other types that I would probably be out there protesting with the Greens and scaling the fences.  Reactors really are quite different.

The most important aspect of any reactor are its safety features, whether it is a space reactor or a terrestrial reactor.  The first and most important aspect is the temperature coefficient of reactivity.  That is a complicated nuclear way of saying, when the reactor gets hotter, does the rate of nuclear reactions increase?  This coefficient must always be negative, to as large a degree as possible.  A simple thought experiment helps understand why.  If the reactor gets hotter, and the temperature coefficient is positive, then the higher temperature leads to more nuclear reactions, which leads to higher temperatures, and so on---until the core disassembles.  This is what happened at Chernobyl.  That said, no Western reactor was ever built with a positive temperature coefficient, and now no reactor is the world is built with one.  They must always have negative temperature coefficients in all phases of operation and under any conceivable scenario.  That's not too hard to do with a water-cooled reactor, but with a liquid-metal reactor it's trickier.

The other key safety feature is the removal of decay heat.  After the reactor shuts down, about 5% of the heat generation capability is still present in decay heat.  This must always be removed under all scenarios.  The failure to remove decay heat is what led to a core meltdown at Three Mile Island-2.

The last safety feature is the minimization of excess reactivity--basically, you want to keep the amount of fuel in the core to just the level you need.  Very difficult to do in a space reactor with solid fuels.  This is a big problem in the launch safety analyses for space reactors---they're essentially non-radioactive at launch, but in the event of a launch accident, you don't want them to fall in the drink and go critical.

So lots of considerations to think about.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: To The Stars on 12/30/2005 11:43 pm
NTR is a good option for such transits as to Mars.

Looking forward to transit to the outer planets and one day to neighbouring stars, what are the future possibilities of new propulsion? even if they are still very much science fiction.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 12/31/2005 12:20 am
Quote
To The Stars - 30/12/2005  6:43 PM

NTR is a good option for such transits as to Mars.

Looking forward to transit to the outer planets and one day to neighbouring stars, what are the future possibilities of new propulsion? even if they are still very much science fiction.

Why do you say that NTR is a good option for flights to Mars?  Upon what do you base that opinion?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: To The Stars on 12/31/2005 12:26 am
On what has been said on another thread based on a previous report, starting about here:
http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=1086&start=81
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: simonbp on 12/31/2005 01:18 am
Quote
vanilla - 30/12/2005  7:20 PM

Why do you say that NTR is a good option for flights to Mars?  Upon what do you base that opinion?

Three reasons: Nuclear Thermal Propulsion is Efficient, Powerful, and (most importantly) Availible.

(For those who don't know, NTP involves acellerating a propellant by feeding it through an open-core nuclear reactor.)

The Efficiency of a rocket is typically defined by the specific impulse, or Isp, which is basically the exhaust velocity divided by 1 g (9.8 m/s^2). The Isp of chemical rockets theoretically max out at around 500 seconds (the Space Shuttle Main Engine is the best @ 453 sec, J-2 @ 436 sec), whereas the much higher engery output of a nuclear reactor allows Isp's on the order of 1000-2000 seconds (which was experimentally confirmed, see below).

The Power of a rocket is typically defined by its thurst, or the force it can exert onto the attached spacecraft or missile. Other high-Isp engines (such as electric/ionic thrusters and laser ablative rockets) typically have much lower thrusts than equal sized, but less efficient, chemical rockets. NTP fills that gap, allowing for thrusters that are both powerful and efficient.

Lastly, NTP engines are not pipe-dreams, but tangible fact. The idea has been around since the Manhatten Project (IIRC, Richard Feynman sold the patent for it to the US government for $1), and the first engines were built and test fired in the 1960's under Project NERVA. Von Braun actively promoted using them for a Mars follow-on to Apollo, but after that progam died, so did the funding for NTP research. The program was revived under Sean O'Keefe (who actually said in 2004 that "nuclear is our number-one priority"), and survives today at NASA Glenn and Marshall. The Huntsville group is active enough to have lugged the only surviving (unfired) NERVA engine from the US Space and Rocket Center out to the Arsenal and disassembled it!

Some math on the efficiency: getting from low earth orbit to a best-case Mars trajectory takes about 3.8 km/s delta v. A J-2 powered Mars ship thus needs a mass ratio of 2.43133, though a 2000 sec-Isp NTP Mars ship needs a ratio of just 1.21371, and less mass means less required funding...

Simon ;)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: JonClarke on 12/31/2005 02:19 am
NTR is a good option for Mars and elsewhere. There are just a few minor matters that need to be worked through first.

Developing a fully contained test facility (I doubt somehow that open airtests at the appropriately named Jackass Flats will be acceptable these days)

Eliminating volatilisation of the fission products into the exhaust (it is not polite anymore to rain radioactive strontium, caesium, iodine and other nasties across the landscape)

Demonstrating core containment during maximum credible accidents (including high velocity reentry).

Nuclear rocket engineers have shown a fairly cavalier attitude in the past (Orion, Kiwi, Timberwind anyone?).  They are going to have to work hard to show they are serious about environmental and safety concerns, and even harder to convince the public.  People are only just coming round to the idea that nuclear power stations are a good idea again.  Uncontained reactors spewing fission producs overhead isn't going to impres them.

The problem is that many of these solutions cost.  A high orbit start up (say 1200 km) requires an extra 0.5 km/s penalty over any HLV launching to that altiude over say 400 km.  A safe solar disposal orbit may require up to 2.5 km/s  Current NTRs that don't worry about containment issues are already 20 times heavier than chemical engines with significant thrust.  Once all of these things are taken into account the performance margin of an NTR is significantly eroded.

Oh, and an Isp of 2000 is a pipe dream for the foreseable future.  Figures of 900 to 950 are much more likely, only twice chemical fuels, seems to be the reasonable figure even fore the most advanced feasible design (ignoring fanciful gas core concepts).

Jon
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: braddock on 12/31/2005 02:35 am
But how to test?  What would the firing time for a Mars mission be?  I thought the longest any NERVA-project engine was fired, in the open with radioactive exhaust, for less than 60 minutes(?).  In whose backyard can we do a new round of testing?

I found the following in a July 2000 Space.com article, which actually features the same Mars transport picture as just appeared in the final ESAS report (so it must pre-date the BNTR report):

"For starters, the engine won't be test-fired in the open as NERVA was at Jackass Flats, Nevada. The new NTR engine could fire into a hole in the ground and the exhaust products would be caught by diatomaceous earth. Or, at the Idaho National Engineering Laboratory -- where the U.S. Navy tests new submarine reactors -- it could fire through a special filtration system that would trap fission products like xenon before the non-radioactive hydrogen exhaust is chemically burned."

http://www.space.com/businesstechnology/nuclear_power_000718.html

Also, the draft ESAS budget projection seemed to push serious nuclear propulsion research out of the budget until at least 2020.  Mars is gonna take a while I guess... :(
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: JonClarke on 12/31/2005 02:39 am
You don't need NTR to go to Mars, this has been shown over and over again.

It could be useful, if it were could be done safely and without a cost and performance penalty that out do its paper advantages.

Jon
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 12/31/2005 04:55 am
Quote
To The Stars - 30/12/2005  7:26 PM

On what has been said on another thread based on a previous report, starting about here:
http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=1086&start=81

Thank you for pointing me to that document, which I read with interest.  Being about seven years old, I am not sure how much that document represents current thinking on nuclear thermal propulsion, but their reliance on that Russian engine design did trouble me significantly, especially since it bears about as much resemblance to the work done on NERVA as a V-2 engine bears to the SSME.  NERVA engines were constructed of hexagonal graphite prisms, much like a bundle of pencils, with a hydrogen coolant channel right down the middle of the fuel element, much like the lead of a pencil.  This engine, on the other hand, uses zirconium hydride as the main moderating material of the core.  That is a bit of a surprise, since zirconium hydride will evolve its hydrogen gas at elevated temperatures.  Uranium-zirconium-hydride is used as the fuel in TRIGA test reactors, and has a number of attractive features, such as an excellent negative temperature coefficient of reactivity, but UZrH fuel is not intended for high temperatures, and an NTR is the highest-temperature nuclear core you will ever find.

Furthermore, I wonder about their Isp estimates since they assume a 2000 psi chamber pressure (from an expander cycle) and a 300:1 expansion ratio on the nozzle.  Those are fantastic numbers---certainly way beyond what we've ever gotten to with conventional expander-cycle engines.  For ground testing, the nozzle will have to be severely truncated in order have sufficient exhaust pressure to exhaust into a contained facility with cleanup.  This will drastically reduce the expansion ratio and hurt the Isp tremendously.  Even this could be not much of a concern if it wasn't for the fact that this is an expander-cycle engine, which relies on the heat transfer area of the chamber, throat, and nozzle to absorb the heat to drive the turbopumps.  Without that heat, insufficient energy will exist to reach the (significant) chamber pressure they list in that document.  Since the hydrogen itself will participate in the moderation of the reactor, having a non-flight-like chamber pressure will lead to non-flight-like operation of the reactor, and all the performance numbers obtained in testing will be questionable, to say the least.

I have a lot of trouble seeing how they will ever test this reactor in anything like a flight condition.  And I think it is extremely foolish to fly a nuclear thermal reactor without testing it properly and assessing the stability and integrity of the fuel elements after full power operation, especially if you wish to continue to operate it in a low-power mode for electric generation.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: simonbp on 12/31/2005 08:09 pm
As the son of test facility engineer, They'll Find A Way....

MSFC right now has a test facility consisting of a NTP engine with large heaing coils instead of fuel rods; thus allowing thermal testing without the pesky hard radiation. The new ESAS document released contains a reference to building a closed-loop testing stand in Nevada, which shouldn't be to hard considering the best thing to make it closed-loop (a vacuum chamber) is the same enviroment in which the rocket will be operating...

Also, as far as exhaust radiation, I recall an interview with a Glenn researcher who said that current NTP designs would release just about 2-4 rads over their 360-minute burn times (a dental x-ray is about 0.5 rad)....

Simon ;)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 12/31/2005 08:54 pm
Quote
simonbp - 31/12/2005  3:09 PM

As the son of test facility engineer, They'll Find A Way....

MSFC right now has a test facility consisting of a NTP engine with large heaing coils instead of fuel rods; thus allowing thermal testing without the pesky hard radiation. The new ESAS document released contains a reference to building a closed-loop testing stand in Nevada, which shouldn't be to hard considering the best thing to make it closed-loop (a vacuum chamber) is the same enviroment in which the rocket will be operating...

Simon ;)

I'm sorry, Simon, but the MSFC facility to which you refer is not a nuclear thermal test facility.  It is a small (100 kW thermal) electrically heated, heat-pipe cooled simulated reactor core.  The NTR engine in those documents is 335,000 kW thermal, and has a core power density of 5 MW/liter.  Even assuming you could match the core power density requirements, that is a factor of 3350:1 in the total power requirement.  To simply simulate the thermal power alone of the core would probably require dimming most of the lights in Huntsville, where you live.  Also, the key thing to test for in a nuclear reactor is its kinetics---how does it respond to transients, etc.  Even if you could do the testing with electrical heating, it would only be a static simulation, since electrical power systems can't change their power outputs by orders of magnitude in a fraction of a second, like a nuclear reactor can.  The closed-loop chamber referred to in the ESAS report also is NOT a vacuum chamber, but rather requires the engine to exhaust to a significant backpressure (~30 psi) in order to drive the effulent through the filters.  As I noted in the previous post, this will severely compromise the heat transfer, turbopump power, chamber pressure, and nuclear response of the system.  Even if we were allowed to exhaust into a vacuum chamber, such chambers don't stay vacuum very long when you're blowing 15,000 lbs of thrust of hydrogen into them.

Testing is a real problem, and I have read nothing in the ESAS document that allays my concerns.  MSFC has never tested a nuclear thermal engine, and it is not going to be like any engine they have ever tested before.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/01/2006 03:21 pm
Quote
Davros - 30/12/2005  5:07 PM

I'd be in favor of this drive but the risk of enviormentalists trying to bring down the program makes me concerned about potential project cancellation. Is there a solution to this, such as a different holding and launch site for the nuclear element of the propulsion?

There is definitely going to need to be a significant effort in educating the public further about nuclear power, and it would probably be best to begin by not lumping all things nuclear together, and followed by a healthy amount of apologies for the last sixty years of nuclear mistakes.  Open air testing of nuclear weapons, the drive of the AEC to produce weapons-grade plutonium and highly-enriched uranium at the exclusion of all else during the 50s and 60s, which has led to contamination problems in Washington and Colorado that persist to this day--these are examples of mistakes that need to be admitted.  The list could then continue with apologies for never developing a true management plan for high-level nuclear waste and the consequences of our wasteful approach to nuclear fuels, where less than 1% of the potential nuclear energy is extracted...you get the idea.

But I am firmly convinced that there are forms of nuclear energy that are very safe and efficient, even if we aren't currently exploring those forms.  That is why I am excited about the possibilities of space nuclear power, because if we are intelligent, we can do it RIGHT this time, and then apply what we learn to the nuclear problems on Earth.  One of the great things about a space system is that you put a premium on low mass, efficiency, self-reliance, and simplicity.  These requirements, if adhered to, will point towards very safe nuclear systems that do their job with a minimum of involvement.

The choice between a nuclear-electric system and a nuclear thermal system is significant.  One of the biggest distinguishing factors is the sheer power level involved.  A nuclear thermal system ranges in power level from 100s of megawatts to gigawatts of thermal power.  Indeed, the single largest nuclear reactor (in power rating) of ANY type was one of the NERVA cores tested in the 1960s, which had a core power rating of 5 gigawatts, thermal.  A reactor intended for nuclear-electric use, on the other hand, tends to be MUCH smaller.  The cores considered for robotic missions had core power ratings of hundreds of kilowatts, whereas a manned mission such as the one shown in the picture would have a core power of about 20 megawatts, thermal.  Not only are such reactors so much smaller, but they are contained reactors with no expectation for exhaust, unlike the nuclear thermal rocket.  They run in the same mode throughout the profile and do not attempt to switch modes between an open-cycle, hydrogen-cooled system to a closed-cycle, gas-cooled system.  Even after examining these documents, I'm still very unclear how they plan to do this.

I hope it is fairly well-known that a reactor that has not been operated has essentially no radioactivity at launch.  The uranium fuel of the reactor has a tremendously long half-life, in the billions of years, which means its activity (radiation emission) is incredibly small.  You can literally hold uranium fuel in your hands--you might want to wear gloves, but the dead skin in your hands will stop the small amount of (alpha-particle) radiation emission.  After the reactor reaches space and has been operated for a period of time, fission products (the result of fission) will build up.  They have very short half-lives and high activity.  Almost all the radiation emitted from a reactor comes from the decay of these fission products.  So fear of radiation during launch is not a big concern for a reactor.  Fear of the reactor falling in the water and going critical is.  Note that even in this scenario, a poorly designed reactor that fell in the water and went critical would not explode like a bomb....not even close.  It would boil some water and a small steam explosion would disassemble it.  The process would happen so fast that there would be very little time for any fission products to form and any significant radioactivity to be released.  The problem would be finding the pieces of the reactor on the bottom of the ocean.  Note that that was for a poorly-designed reactor--we must design reactors that do not go critical in a submersion launch accident.

So these are the types of information we need to tell the public about space nuclear power, in order to help them understand the risks and opportunities, and help them to understand how and why we should be doing this.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Rocket Nut on 01/01/2006 08:45 pm
Quote
vanilla - 1/1/2006  11:21 AM


There is definitely going to need to be a significant effort in educating the public further about nuclear power, and it would probably be best to begin by not lumping all things nuclear together, and followed by a healthy amount of apologies for the last sixty years of nuclear mistakes.  Open air testing of nuclear weapons, the drive of the AEC to produce weapons-grade plutonium and highly-enriched uranium at the exclusion of all else during the 50s and 60s, which has led to contamination problems in Washington and Colorado that persist to this day--these are examples of mistakes that need to be admitted.  The list could then continue with apologies for never developing a true management plan for high-level nuclear waste and the consequences of our wasteful approach to nuclear fuels, where less than 1% of the potential nuclear energy is extracted...you get the idea.

I guess I don't understand your need for mea culpa and apology.  We know a lot more than we did then.  Heck, I flew through nuclear clouds within minutes of detonation to bring back samples for analysis...we thought the radiation doses we got then were "safe".  We certainly know better now, but nobody owes me an apology or any compensation.  

Can't we just use the knowledge we have developed since the 50s and move on from here?

I certainly agree with your comments about educating the public.  We have made a lot of progress since the 50s and 60s and should use the knowledge we have gained from years of research and move forward.

Regards,

Larry
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/01/2006 09:30 pm
Quote
Rocket Nut - 1/1/2006  3:45 PM
I guess I don't understand your need for mea culpa and apology.  We know a lot more than we did then.  Heck, I flew through nuclear clouds within minutes of detonation to bring back samples for analysis...we thought the radiation doses we got then were "safe".  We certainly know better now, but nobody owes me an apology or any compensation.  

On the contrary, you are precisely the type of person I believe deserves an apology.  I do not know if you have suffered from cancer in your lifetime, but if you have or do, will you not wonder if your exposures had something to do with it?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Flightstar on 01/01/2006 11:38 pm
It is very important to note that nuclear options make for immediate diversity for power requirements, not only for propulsion and MTV power (I refer to Langley's comment about dualability), but for potential sustained energy at the target outpost. I gained an impression from the ESAS report that this dual target approach, alongside that of exploitation of Lunar, for example, resourses on more convential means, is what is being evaluated as the favored option for Mars transit.

Nuclear is still taboo for reasons rightly noted by vanilla, but given options presented, supporting this evalution stage and system intergration evaluation stage, appears to make sense to me.

Proceedures are much tighter than they have been previously, also.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Rocket Nut on 01/02/2006 12:01 am
Quote
vanilla - 1/1/2006  5:30 PM

Quote
Rocket Nut - 1/1/2006  3:45 PM
I guess I don't understand your need for mea culpa and apology.  We know a lot more than we did then.  Heck, I flew through nuclear clouds within minutes of detonation to bring back samples for analysis...we thought the radiation doses we got then were "safe".  We certainly know better now, but nobody owes me an apology or any compensation.  

On the contrary, you are precisely the type of person I believe deserves an apology.  I do not know if you have suffered from cancer in your lifetime, but if you have or do, will you not wonder if your exposures had something to do with it?

No, that is my point.  I was one who was directly harmed by those unrecognized dangers.  We were briefed on the known dangers at the time.  They didn't lie to us, they really thought we were being exposed to acceptable minimal doses of radiation.  We knew there was a risk.  We volunteered for the missions based on the perceived risk as it was known at the time.  In my opinion, nobody owes me anything because they and we acted upon the knowledge base that existed at the time.

We have learned so much since then that, in my opinion, we can and should move forward and not waste time apologizing for not understanding all the dangers that existed in decades past.   Learn from past mistakes, but don't dwell on those mistakes.  I worked with many scientists at Los Alamos and Lawrence Livermore Labs who were studying the effects of radiation exposure at that time.  I spent many hours in their scintillation chambers while they studied the effects of radiation on me and others who were flying the same missions.  Those scintillation chambers are a whole 'nother story, far more scary than flying through nuclear clouds.  We knew there were risks, just not how bad they were.

Just for the record, we were fairly well protected from Alpha and Beta radiation...maybe I'll scan some pictures taken in the 60s.  We were enclosed in heavy clothing from head to toe with tape covering the space between sleeves and gloves, etc.  We also passed through high volume showers on the way from the plane to the debriefing room.  (hah, the TSA has nothing on us...after the showers there were a lot of naked men standing with arms outstretched while they waved geiger-counters over us...kind of like airports here)...ah, the good old days...  Of course, the full pressure suit was also a great barrier to Alpha and Beta.

I guess I'm just being pragmatic about this.  Whenever the Government "apologizes" for anything, it costs millions or billions to support all the lawyers who swoop down on the "victims".  And yes, I would probably be one of those "victims".  I would rather that money be available for the space program.

I'm not trying to be argumentative...this is just my opinion...worth maybe 2 cents...

Regards,

Larry
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/02/2006 01:44 am
Quote
Rocket Nut - 1/1/2006  7:01 PM
I guess I'm just being pragmatic about this.  Whenever the Government "apologizes" for anything, it costs millions or billions to support all the lawyers who swoop down on the "victims".  And yes, I would probably be one of those "victims".  I would rather that money be available for the space program.

Very well, and I want you to know I have tremendous respect for the sacrifices you have personally made (and may yet make) for our country.  The point I was trying to make, and in retrospect realize that I did not make very well, is that for nuclear energy to be accepted in the future, I am personally of the opinion that there must be some degree of apology and admittance of past errors.  I think this step is essential in re/gaining the public's trust.  For decades we have said something that is basically not true--that nuclear power is as safe as it can be.  As a nuclear engineer, I can personally state that this is not a fact.  There are better--much better and safer ways to do nuclear power, that ironically are SAFER, SIMPLER, MORE EFFICIENT, and MORE ECONOMICAL than how we do it today.  The fact that we are not pursuing these options has little to do with safety or economics and had more to do with politics and bureaucratic inertia.  I feel passionate about the possibilities of space nuclear power because I believe this a mechanism to break out of our current nuclear stagnation.  I do not believe that reactor manufacturers will do it.  I do not believe that the Department of Energy or Naval Reactors will do it.  I hope that NASA might do it, but the Prometheus experience is not promising.

At this tenuous time, the answer (again in my opinion) is not to passively accept the 40-year old answer (NTR) that once again has been blessed as the path forward, but to push for the right solutions that have terrestrial benefit.  Do you know how many times they have decided to do nuclear thermal propulsion only to quit when they got into the real costs and diminished benefits of the technology?  Oh this would be time number 3 or 4.   Why do we think this will be any different?  If Griffin and his crew haven't solved any of the basic problems of NTR--and rather are pushing BNTR, which is substantially worse--then why do we think the result will change?

The nation, now more than ever, needs safe and economical sources of power.  The space nuclear program can be a catalyst to developing those forms of power, but the current push towards NTR will be little more than several more billion dollars wasted on testing and fuel forms that have no terrestrial benefit.  At the end of the process, they will say "nuclear" is too difficult, we give up.   Rather, let us assert a different vision for space nuclear development that can be built, can be tested, is reasonable, and has terrestrial benefit.  Our time is running out.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: realtime on 01/02/2006 05:42 am
Well, that was painful, yet strangely depressing.  Thank you for this thread.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: FransonUK on 01/02/2006 10:02 am
Quote
Rocket Nut - 1/1/2006  3:45 PM

I guess I don't understand your need for mea culpa and apology.  We know a lot more than we did then.  Heck, I flew through nuclear clouds within minutes of detonation to bring back samples for analysis...we thought the radiation doses we got then were "safe".  We certainly know better now, but nobody owes me an apology or any compensation.  

Regards,

Larry

I think that makes you a great person.

Too many people play it safe, live like a hermit and never do anything with their lives then fall down the stairs one day and break their necks.

There's risks everywhere. I was on the tube and took the train which was 10 minutes before the one Al Queda blew up. Makes you realise there's no point playing it safe, cause you never know.

I think you've more than done something great and exciting with your life and we shouldn't be moaning about what risks were taken, cause you'll outlive a lot of those people who eat nothing but fruit and veg, give loads of money to Greenpeace, eat Dophin friendly tuna, and never fly on a plane "cause they sometimes crash you know" ;)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: SimonShuttle on 01/02/2006 10:11 am
Quote
vanilla - 1/1/2006  8:44 PM

The nation, now more than ever, needs safe and economical sources of power.  The space nuclear program can be a catalyst to developing those forms of power, but the current push towards NTR will be little more than several more billion dollars wasted on testing and fuel forms that have no terrestrial benefit.  At the end of the process, they will say "nuclear" is too difficult, we give up.   Rather, let us assert a different vision for space nuclear development that can be built, can be tested, is reasonable, and has terrestrial benefit.  Our time is running out.

But isn't the space nuclear program designed for space travel, not terrestrial benefit? Why does there have to be a clause for this to be for terrestrial benefit? That I don't understand.

More over, can you give ideals on alternatives that are on the table that the general public might not be aware of.

Interesting thread!
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: SimonShuttle on 01/02/2006 10:12 am
Quote
vanilla - 1/1/2006  8:44 PM

Quote
Rocket Nut - 1/1/2006  7:01 PM
I guess I'm just being pragmatic about this.  Whenever the Government "apologizes" for anything, it costs millions or billions to support all the lawyers who swoop down on the "victims".  And yes, I would probably be one of those "victims".  I would rather that money be available for the space program.

Top man.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: FransonUK on 01/02/2006 10:19 am
Quote
SimonShuttle - 2/1/2006  5:12 AM

Quote
Rocket Nut - 1/1/2006  7:01 PM
I guess I'm just being pragmatic about this.  Whenever the Government "apologizes" for anything, it costs millions or billions to support all the lawyers who swoop down on the "victims".  And yes, I would probably be one of those "victims".  I would rather that money be available for the space program.

Top man.

This is the trap society is falling into.

I'm a lawyer (or soliciter as we're called in the UK) and there's a huge influx of law degrees graduates now finding jobs at accident insurance claim firms, where the whole world of rightful compansation is being turned on to its head.

Companies and governments simply aren't going to be taking an risks anymore, with violent schoolkids who attack their teachers getting huge compansation when the teacher clips them over the ear. Where window cleaners get huge compansation for falling off the third run of their ladder when it's wet.

I'm totally against this, despite the possibilities for lawyers to make fast money, as it's draining the talent of law into a level of call centers and robots pushing claim buttons on their PCs. That's not law.

If we had more people like Rocket Nut then maybe we'd take some risks and advance. The other way is to become a liberal society of people who wrap themselves up in cotton wool resulting in us never advancing.

For that very reason I hope the "Nuclear, doesn't sound very safe to me" thinkers never sway the current, best way, to get this job for Mars done.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Hotol on 01/02/2006 10:37 am
No harm in a bit of an armisist view on this. Push for the current best option while using that as a motivation to finding a better, safer, cheaper option.

Scientists shouldn't complain unless they have a viable alternative in the pipeline.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Rob in KC on 01/02/2006 03:29 pm
Quote
Hotol - 2/1/2006  5:37 AM

No harm in a bit of an armisist view on this. Push for the current best option while using that as a motivation to finding a better, safer, cheaper option.

Scientists shouldn't complain unless they have a viable alternative in the pipeline.

No argument from me there.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/02/2006 04:19 pm
Quote
SimonShuttle - 2/1/2006  5:11 AM
But isn't the space nuclear program designed for space travel, not terrestrial benefit? Why does there have to be a clause for this to be for terrestrial benefit? That I don't understand.

More over, can you give ideals on alternatives that are on the table that the general public might not be aware of.

NASA seems to be very fond of touting the "spinoffs" of space technology as a primary reason for space exploration...so why should there not be direct benefit from a space nuclear program?  Heaven knows we need the energy!

I read a book a few years ago that completely reshaped what I thought I knew about nuclear energy.  It was written in 1958 and was called "Fluid Fuel Reactors".  Before I read this book I hadn't even imagined that reactors could be built in any other way than using solid fuels.  And if they were, I thought, it must be some incredibly exotic system that operated at such high temperatures that no realistic material could contain it.  Much to my surprise, I found in reading this book that there were a number of uranium compounds that could be liquid at relatively reasonable temperatures.  Indeed, there were actually three totally different approaches to reactor design outlined in this book, all of them with the goal of building a thorium-fueled thermal breeder reactor.

One technique was to dissolve uranium sulfate into either normal (light) or preferably heavy water.  The reactor was called an aqueous homogenous reactor.  Imagine my surprise when I found out that two of these reactors were actually built!  They also had some incredible safety characteristics.  Because the water would expand when heated, which reduced neutron moderation, the reactor had a huge negative temperature coefficient.  Like a mass on a stiff spring, it was essentially impossible to get the reactor to have an "excursion" into a damaging region of operation.  Additionally, the decay heat (the heat generated by the decay of fission products, which remains even when the fission reaction has stopped) can be passively removed by draining the fuel into a different cooling configuration.  This is simply not possible with a solid-fueled reactor, which is why a failure in the pressure vessel is so seriously.  In addition, because the fuel was in a fluid, adding additional fuel as the reactor operated was easy, as was removing fission products during reactor operation.  Each of these steps is terribly difficult in a solid-fueled reactor--to "reprocess" solid fuel, you essentially have to chop it up, dissolve it in acid, and then separate everything chemically.  Basically you can only reprocess liquid fuel--if you have solid fuel you have to make it liquid--this reactor already had liquid fuel.

But despite all these advantages, the aqueous homogeneous reactor had a serious drawback.  By using water at the solvent/moderator, you were limited to rather low temperatures and high pressures by the characteristics of the water itself.  The next reactor in the book had all the advantages without this disadvantage--the molten fluoride reactor.

Most nuclear engineers are familiar with the role fluorides play in the "standard" nuclear cycle.  Mined uranium, which is in the form of uranium oxide, is converted to uranium tetrafluoride (UF4) by exposing it to fluorine gas.  The tetrafluoride, called "green salt" is then further exposed to more fluorine to form uranium hexafluoride (UF6), often called "hex" or "red salt".  UF6 is a gas at slightly higher than room temperature, and is used in gaseous diffusion or centrifuge plants to enrich uranium.  After enrichment, the enriched hex is converted back to uranium oxide and made into solid fuel for typical reactors.

UF4 has a very high melting temperature (1035 C), but when it is combined with other fluoride salts, the melting temperature is reduced dramatically.  The optimum solvent salts appear to be lithium fluoride and beryllium fluoride.  A combination of these salts with UF4 results in a salt that melts at 450 C.  But most importantly, the resultant fuel mixture can operate over a large temperature range at essentially ambient pressure.  That means that the large pressure vessel typical of most water reactors isn't needed.  That's a huge safety improvement and a big weight reduction item for a space reactor.  The molten-fluoride reactor retains all the safety features of the aqueous homogenous reactor (large negative temperature coefficient, passive decay heat removal, online fuel addition and fission-product removal) and adds more attractive features important for a space reactor:  low pressure operation, passive launch safety, and high temperature capability.

I'm frustrated that this reactor wasn't considered for Prometheus because I really think this reactor would have solved many of the basic problems they were running into, like fuel qualification, system complexity, and high-temperature heat rejection.  I still can't find a good technical reason why it wasn't being investigated--but now NASA is talking about pursuing NTR and I feel like we're slipping even further away from anything practical.  The issues of testing and qualifying NTR fuel make the fuel issues on Prometheus look like a walk in the park.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Chris Bergin on 01/02/2006 05:12 pm
Quote
vanilla - 2/1/2006  5:19 PM

Indeed, there were actually three totally different approaches to reactor design outlined in this book, all of them with the goal of building a thorium-fueled thermal breeder reactor.

One technique was to dissolve uranium sulfate into either normal (light) or preferably heavy water.  The reactor was called an aqueous homogenous reactor.  Imagine my surprise when I found out that two of these reactors were actually built!  They also had some incredible safety characteristics.  Because the water would expand when heated, which reduced neutron moderation, the reactor had a huge negative temperature coefficient.  Like a mass on a stiff spring, it was essentially impossible to get the reactor to have an "excursion" into a damaging region of operation.  Additionally, the decay heat (the heat generated by the decay of fission products, which remains even when the fission reaction has stopped) can be passively removed by draining the fuel into a different cooling configuration.  This is simply not possible with a solid-fueled reactor, which is why a failure in the pressure vessel is so seriously.  In addition, because the fuel was in a fluid, adding additional fuel as the reactor operated was easy, as was removing fission products during reactor operation.  Each of these steps is terribly difficult in a solid-fueled reactor--to "reprocess" solid fuel, you essentially have to chop it up, dissolve it in acid, and then separate everything chemically.  Basically you can only reprocess liquid fuel--if you have solid fuel you have to make it liquid--this reactor already had liquid fuel.

But despite all these advantages, the aqueous homogeneous reactor had a serious drawback.  By using water at the solvent/moderator, you were limited to rather low temperatures and high pressures by the characteristics of the water itself.  The next reactor in the book had all the advantages without this disadvantage--the molten fluoride reactor.


This is a superb thread - and also a heavy learning curve.

Thorium-fueled thermal breeder reactor, and more so the molten fluoride reactor - do we have any available web-based resources on these concepts for further learning? Also allow me to forward this thread to a couple of MSFC guys who are pretty savvy on such propulsion/energy concepts.

Thanks to Vanilla (and all) for some facinating insights.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/02/2006 05:36 pm
Quote
Chris Bergin - 2/1/2006  12:12 PM
Thorium-fueled thermal breeder reactor, and more so the molten fluoride reactor - do we have any available web-based resources on these concepts for further learning? Also allow me to forward this thread to a couple of MSFC guys who are pretty savvy on such propulsion/energy concepts.

If you make some space available on your FTP area, I will upload a number of documents related to the topic.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Chris Bergin on 01/02/2006 05:46 pm
Thanks. I've responded on potential routes on facilitating that by PM.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Orbiter Obvious on 01/02/2006 06:24 pm
Are those new forms available to be used in space transportation? Or are they like the Nuclear fission possibilities, which are meant to be about 20-30 years away?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Firestarter on 01/02/2006 06:58 pm
Quote
Orbiter Obvious - 2/1/2006  1:24 PM

Are those new forms available to be used in space transportation? Or are they like the Nuclear fission possibilities, which are meant to be about 20-30 years away?

And if not, that is too much a risk. Space travel should not be a test bed for Earth energy potential. It should be the other way around.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/02/2006 07:01 pm
Quote
Orbiter Obvious - 2/1/2006  1:24 PM

Are those new forms available to be used in space transportation? Or are they like the Nuclear fission possibilities, which are meant to be about 20-30 years away?

I believe this reactor could be used very effectively in space transportation...not as a nuclear thermal system, but as an heat source to generate electrical power to drive electric thrusters.  For systems like this, nuclear-electric systems, one of the most important overall parameters is the specific power, also called the "alpha".  It is a measurement of how many kilograms of reactor, power conversion, radiators, etc. it takes to generate a kilowatt of electricity.  The key to getting a good (low) alpha is to get the masses of the individual system components down--reactor, shield, power conversion, radiators, and so forth.  Surprisingly, the reactor is a rather small mass in the overall system, but it drives all the others, and the reactor really needs to have a high temperature capability.  This ripples through the power conversion system and allows waste heat to be rejected at higher temperatures, which leads to smaller radiators, since the effectiveness of the radiators is proportional to the fourth power of temperature.

The need for a high-temperature reactor is why typical terrestrial reactors, which are water-cooled in big pressure vessels, would not do well in space at all.  It is one of the reasons why I think the specific needs of the space reactor drive us in directions that will later lead to attractive terrestrial reactors.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: truebeliever on 01/02/2006 09:59 pm
Quote
Orbiter Obvious - 2/1/2006  2:24 PM

Are those new forms available to be used in space transportation? Or are they like the Nuclear fission possibilities, which are meant to be about 20-30 years away?

I have been following the threads of vanilla ice ( or is it just vanilla :) ). I have read Alvin Wienberg's  book, "The Life and Times of a Technological Fixer". He used to be the director of the DOE's Oak Ridge National Laboratory.  Great book. He talks about the development of the molten salt reactor (MSR). They built the first molten-salt reactor back in the 1950's just to see if it could be done. It ran for about a month and attained a then unheard of fuel temperature of 1133 K. They then built a second test reactor that ran for 5 years. Remarkable machine. They would run the reactor from Monday-Friday, and shut it down on Friday afternoon by dumping the fluid fuel into a storage tank. It would quickly cool down and solidify. On Monday morning, they would show up for work, turn on the electric heaters, remelt the salts, and pump the fuel back up into the sytem and restart the reactor. Pretty cool.

Weinberg called it " a pot, a pump and a pipe", in reference to the simplicity of the reactor itself.  

This is what NASA needs for space nuclear power. Something that is simple, cost effective and reliable.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Colby on 01/02/2006 10:12 pm

Vanilla, would you mind posting a little more background information on yourself? You already stated that you are a nuclear engineer and you gave us some other hints, but it would be very nice to see it all in one post. ;) Your knowledge is definately going to be an asset to this website!

Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/02/2006 10:43 pm
Quote
Colby - 2/1/2006  5:12 PM

Vanilla, would you mind posting a little more background information on yourself? You already stated that you are a nuclear engineer and you gave us some other hints, but it would be very nice to see it all in one post. ;) Your knowledge is definately going to be an asset to this website!


Um, that's probably not the best idea.  I'm not exactly working at a place that would smile on these kinds of comments.  Hence, my ability to post is rather connected to my anonymity.  I realize that that makes any information I post suspect, so I will try to document this information as much as possible.  If I can get some FTP storage on this site, I will upload a number of documents that will shed more light on many of these topics.

Sorry I can't say more, but I'm just a pretty vanilla-type person...
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/02/2006 11:03 pm
The Weinberg book is a very good one---its full title is "The First Nuclear Era:  The Life and Times of a Technological Fixer".  I ordered it on Amazon and really enjoyed it.

A good website about molten-fluoride reactors (more commonly called molten-salt reactors--I prefer the term molten-fluoride because there are also concepts for molten-chloride reactors) is Bruce Hoglund's:

http://home.earthlink.net/~bhoglund/
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Flightstar on 01/02/2006 11:04 pm
Colby, some of us are unable to indentify who we are, especially when working for NASA or a NASA contractor.

You'll see more NASA, USA etc. people on this forum than anywhere else on the open net as Chris owns the site and has a history of extreme confidentiality, so we're safe here. There's one USA worker on SDC who doesn't hide his indentity and is actually risking his job in the process, but appears to add false information in with true information to cover his own back, maybe.

I'd love to add a picture of myself with Atlantis, but that'll have to wait until my retirement, which isn't all that far away!
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: David AF on 01/02/2006 11:07 pm
A similar case with myself. I'm not NASA, but a Lt. Col in the USAF.  Websites are public access anywhere in the world, but as above, Chris is very highly briefed (he's military himself) on security, so this is a rare site to be free to talk on.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Colby on 01/02/2006 11:50 pm

I understand perfectly vanilla. I was mostly hoping for some more information that wouldn't identify you, although I'm not in your situation, so I'm not quite sure what that could be without risking your job. I am just very interested in aerospace engineering, but I also have a fascination with nuclear engineering, so your posts obviously caught my attention!

People like you, Flightstar, and David AF (and so many others) make this site truly unique, so do what you must so you can continue making this site one of the best!

Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/03/2006 01:43 am
Quote
Chris Bergin - 2/1/2006  12:12 PM
Thorium-fueled thermal breeder reactor, and more so the molten fluoride reactor - do we have any available web-based resources on these concepts for further learning?
http://www.nasaspaceflight.com/_docs/

I have uploaded the complete text of "Fluid Fuel Reactors" to this location.  The files of interest are titled "FFR_chap01.pdf" for the first chapter, and so forth.

Chapters 1-10 of the text deal with the aqueous homogeneous reactor, chapters 11-17 deal with the molten-fluoride (molten-salt) reactor, and chapters 18-25 deal with the liquid metal reactor, which was another fluid-fuel reactor concept where uranium would be dissolved in a mixture of lead and bismuth.  There are also three files that are indices for each section.  Altogether the book is about 47 MB in size.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Jamie Young on 01/03/2006 03:27 am
And there was me thinking the ESAS report was long! :) I'm going to have to book a week off school! ;)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: James Lowe1 on 01/03/2006 03:33 am
Thanks Vanilla. I just used Chris' login to see if I could place all the files you've uploaded into one seperate folder, to keep them in one place away from the videos etc. It won't let me, but if you know a way, then that might help as I'm sure we'd like to highlight this documentation in an area that is specific.

However, if not, I'm sure it's not a problem. It is hugely appreciated by all of us here that you've made such information available.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/03/2006 04:10 am
I tried the same thing during the upload, to make a separate folder, but it wouldn't let me....sorry!
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Chris Bergin on 01/03/2006 01:58 pm
Excellent, thanks for uploading this vast resource of information.

I'll be setting up a more direct way for people to download specific elements of the information at some point, inter-linking the information on this thread.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Mark Max Q on 01/03/2006 04:33 pm
Thanks Vanilla, I'm very interested in this subject and will read through the documents.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/03/2006 06:03 pm
Now that some documents on molten-fluoride reactors are available, I would like to further explain why this reactor could have some very attractive advantages as a space reactor, either for surface power or in a nuclear-electric vehicle.

During Prometheus, there were three basic reactor concepts studied.  They all used solid uranium fuel that was highly enriched in uranium-235.  In order to minimize the size of the reactor, they didn't slow down (moderate) the neutrons before fission....such types of reactors are called "fast" reactors, and there are only a handful of such reactors in operation today.  By running on fast neutrons and using highly-enriched fuel, they lacked two of the most important aspects in a typical reactor that give you a negative temperature coefficient:  neutron moderation and Doppler absorption.  When you have a reactor that uses low or moderate uranium-235 enrichments (2-20% roughly) you get a nice safety feature in the form of Doppler absorption.  This comes about because the other 98-80% of the uranium is uranium-238, which tends to absorb neutrons most of the time.  When the fuel gets hotter, the 238 gets more absorptive, tending to "drink" up neutrons and shut down the reaction, hence it is a big contributor to a negative temperature coefficient.  235 also gets more absorptive when it gets hotter, but its absorptions typically lead to fission, so using very-highly enriched fuel with little absorbing material present (238) makes your Doppler absorption turn from an effect that gives a negative temperature coefficient to an effect that gives a positive temperature coefficient.

The difference between the negative and positive temperature coefficients is extraordinarily important.  A negative coefficient is like having a marble in the bottom of a bowl.  If you displace the marble, it wants to roll back where it started.  It is "dynamically stable".  Now flip the bowl over and put the marble on the back of the bowl.  If you displace it, it will roll further and further away and off the bowl--it is "dynamically unstable."  There are experts in control theory who spend their whole careers figuring out how to stabilize dynamically unstable systems, like fighter aircraft, through active control.  You do the same sort of thing when you balance a ruler on your finger.  But try to hold still and the ruler falls over.

In a dynamically unstable reactor, with a positive temperature coefficient, you may only have a very, very brief amount of time to stabilize the reactor before it gets away on you and melts down or disassembles.  Unfortunately, there have been a few reactors built on Earth that were dynamically unstable, and they have needed very good control systems and very careful operation to control them.  No Western commercial reactor has ever been built that was dynamically unstable--it's just not allowed, and for good reason.

A solid-core, highly-enriched, fast reactor is almost certainly going to be dynamically unstable.  Now you better have an incredible control system onboard to keep it from melting down or disassembling, and if the reactor is out at the Moon or Jupiter, you will have to rely on that control system to be incredibly redundant, always work, and never fail, even in a terrible radiation environment.  I think that's a bit too much to ask for a space reactor.  Thus I think you must pursue a reactor that is dynamically stable.

I should note that this criticism does not apply to the NTR since the hydrogen in the core will lead to a moderated reactor, and the hydrogen will lead to a fairly strong negative coefficient.  An NTR should be dynamically stable in its neutronic operation, but the fast-spectrum, highly-enriched reactors that were investigated for Prometheus, I don't think so.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Justin Space on 01/03/2006 07:35 pm
Quote
vanilla - 3/1/2006  1:03 PM

The difference between the negative and positive temperature coefficients is extraordinarily important.  A negative coefficient is like having a marble in the bottom of a bowl.  If you displace the marble, it wants to roll back where it started.  It is "dynamically stable".  Now flip the bowl over and put the marble on the back of the bowl.  If you displace it, it will roll further and further away and off the bowl--it is "dynamically unstable."  There are experts in control theory who spend their whole careers figuring out how to stabilize dynamically unstable systems, like fighter aircraft, through active control.  You do the same sort of thing when you balance a ruler on your finger.  But try to hold still and the ruler falls over.

So the Doppler absorption soaks up neutrons to keep everything at a managable pace, keeping the 'bowl' the right way up, so that the "marble" stays where you want it, because it's aiding a negative coefficient? And thus the Doppler absorption is a safety barrier?

Too many neutrons, or neutrons speeding up out of control, your heading to positive temperature coefficients, and that's going to flip your bowl over and then you've got a battle on your hands in keeping the "marble" where you want it?

I hope I've got close to this. I've never touched on this subject before, but it's bloody facinating! :)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/03/2006 07:57 pm
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Justin Space - 3/1/2006  2:35 PM
So the Doppler absorption soaks up neutrons to keep everything at a managable pace, keeping the 'bowl' the right way up, so that the "marble" stays where you want it, because it's aiding a negative coefficient? And thus the Doppler absorption is a safety barrier?

Too many neutrons, or neutrons speeding up out of control, your heading to positive temperature coefficients, and that's going to flip your bowl over and then you've got a battle on your hands in keeping the "marble" where you want it?

I hope I've got close to this. I've never touched on this subject before, but it's bloody facinating! :)
What Doppler absorption does is it makes things that absorb neutrons absorb them better as they get hotter.  Uranium-238 (which is the abundant component of uranium) tends to absorb neutrons and turn into uranium-239, then decay to neptunium-239 and then to plutonium-239.  But these decays take a few days, so on the time scale of the fission process, all the U-238 does is drink up neutrons.  So when you have a fuel that has a lot of "resonance absorber" in it (which is nuclear-talk for a material that tends to absorb neutrons in resonances, which is another way to call Doppler [are you confused yet!]) then the Doppler really helps create a negative temperature coefficient, which leads to reactor stability.

The reason I point out U-238 is that it is the most common resonance absorber in a typical reactor core.  Your average terrestrial reactor has fuel that is probably 97% U-238, so this is a strong contributor to the negative coefficient.  Other resonance absorbers are thorium-232 or tungsten...most heavy elements are resonance absorbers.  Your average terrestrial reactor also has moderated neutrons, which is another big way to get a negative temperature coefficient.

U-235, on the other hand, tends to absorb a neutron and then fission, which sprays out a bunch more neutrons.  So if you have a core that is mostly U-235, like these space reactors I was telling you about, then the Doppler absorption (resonance absorption) tends to make the temperature coefficient positive, because higher temperature leads to more absorption leads to more fission, which leads to higher temperature....and so on....boom!

So the use of highly-enriched fuel, and the absence of resonance absorbers in the core, and no neutron moderation (fast reactors) lead to reactor designs with positive temperature coefficients....trouble, big big trouble.  Chernobyl happened because a reactor that was generally designed to have a negative coefficient got into an operating regime where it had a positive coefficient, and there was an explosion.  Western reactors are REQUIRED to have negative coefficients in ALL possible regimes.  The newer Russian reactors (VVERs) are like Western reactors in that regard.  The Chernobyl-type reactor (the RBMKs) I believe have all been decommisioned.

There are a number of things to make sure you get right in a reactor design, but I don't think there is any one more fundamental than the temperature coefficient of reactivity.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Avron on 01/04/2006 03:34 am
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vanilla - 3/1/2006  3:57 PM


U-235, on the other hand, tends to absorb a neutron and then fission, which sprays out a bunch more neutrons.  So if you have a core that is mostly U-235, like these space reactors I was telling you about, then the Doppler absorption (resonance absorption) tends to make the temperature coefficient positive, because higher temperature leads to more absorption leads to more fission, which leads to higher temperature....and so on....boom!


Is that one of the key reasons for using U-235 in bombs? The Positive coefficient drives the yield? more positive, the  bigger the bang? and does that also mean that U-238 cannot go bang?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/04/2006 04:25 am
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Avron - 3/1/2006  10:34 PM
Is that one of the key reasons for using U-235 in bombs? The Positive coefficient drives the yield? more positive, the  bigger the bang? and does that also mean that U-238 cannot go bang?
No, in a bomb your only goal is to make the reaction go supercritical, basically as fast as possible.  Designing a reactor is MUCH more difficult than designing a bomb, which is probably one of the reasons that the US built a bomb in 1945 but didn't get a civilian power reactor until 1957.  U-235 is fissile, at all neutron energies.  U-238 will actually fission, but only at very high neutron energies, which ironically, you typically find in a bomb!  Some bombs use U-235 or Pu-239 as the first stage, a fusion second stage where deuterium and tritium fuse to helium and neutrons, and then those high energy neutrons hit a third stage of U-238, which will fission under the intense energies of those fusion neutrons.

The negative temperature coefficient keeps your reactor exactly critical--you go subcritical, the reactor cools down, the reactor gets more reactive, and heats up again.  You go supercritical, even a little bit, the reactor gets hotter and less critical, cools down, and you're back where you want to be.  It's a beautiful feature, and it's what allows well-designed reactors to "follow the load"--they will adjust the power they produce to the power demand that they "feel" from the grid.

One of the ways German scientists messed up Hitler's attempts to develop the bomb was to tell him that you needed to moderate (slow down) the neutrons to make the bomb work.  It doesn't work--the bomb will disassemble before it blows up.  They knew it--but they didn't want Hitler to have to the bomb.  Aren't we glad they didn't?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/04/2006 04:34 am
I know I haven't explained yet where I'm going in these posts, and I feel like I'm writing a novel every time I post, so I'll give you an outline:

Why have people designed cores with all these unfavorable features I've been describing?  Because they're trying to get a long core lifetime for the reactor.

The molten-fluoride reactor can be refueled during operation, so you don't have to take the safety risks and yet you can still get the long lifetime.

Long lifetime is going to be very important for space nuclear reactors.  We don't run into this problem on Earth because we refuel reactors every year or so.

We can also use thorium as "future fuel" so that we don't have to take the risks of launching with extra fissile fuel (above and beyond that necessary to achieve initial criticality).  During the operation of the reactor we use extra neutrons to convert the thorium to uranium-233 which will then fuel the reactor.

This technology can allow us to "burn" thorium on Earth, the supply of which will last for tens of thousands of years...so long energy crisis!
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: JonClarke on 01/04/2006 11:55 am
Very informative Vanilla, thank you.

So a NTR is in principle inherently stage because of the hydrogen inflow, is that right?  What happens if there is a rupture to the hydrogen supplu to the reactor while it is operating?  Can it enter an unstable regime as a result, even if a shut down is attempted?

I have also read that cross coupling of neutrons raises problems with clustering NTRs.  In your view does this preclude the triple NTR stages suggested by studies such as those of Borowski, of the three side by side reactors of the old NERVA stages?

Jon
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/04/2006 01:01 pm
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JonClarke - 4/1/2006  6:55 AM
So a NTR is in principle inherently stage because of the hydrogen inflow, is that right?  What happens if there is a rupture to the hydrogen supplu to the reactor while it is operating?  Can it enter an unstable regime as a result, even if a shut down is attempted?

I have also read that cross coupling of neutrons raises problems with clustering NTRs.  In your view does this preclude the triple NTR stages suggested by studies such as those of Borowski, of the three side by side reactors of the old NERVA stages?

Jon

Yes, in my opinion, a well-designed NTR should be quite stable neutronically.  If the hydrogen flow is interuppted, the result would be a decrease in neutron moderation in the core, which would rapidly lead to a shut down.  But there is another problem that must be seriously considered.  I mentioned the critical importance of the temperature coefficient, but there is also the issue of decay heat removal as well.  When a reactor has been operating for awhile, the population of fission products builds up in the core.  Most of these are very unstable and decay quickly, emitting substantial amounts of heat.  In fact, when a typical reactor is operating, about 5% of the total heat output is coming from the decay of fission products.  The REALLY important thing to remember is, these fission products will keep making heat even AFTER the reactor completely shuts down fission!  So you always have to have a plan for decay heat removal--otherwise you will melt the core.

At Three Mile Island-2, they had an accident where water drained out the core.  Well the water was both the coolant and the moderator, and when the reactor lost the moderator, the fission totally shut down.  But the decay heat was still heating the fuel, and now there was no water to cool it, so the fuel began to melt and slump.  This is another unstable situation as well, because as the fuel melts and slumps, it blocks off the coolant passages so that even if you reinstate cooling, you won't be able to cool the fuel properly.

If an NTR lost its hydrogen flow during operation, the fission reaction would shut down but not the decay heat.  Borowski's designs account for the fact that you have to flow hydrogen through the core for awhile after you shut down for decay heat removal.  You may have heard of another NTR design looked at in the early 90s under a program called Timberwind--this was a "particle-bed" design, where instead of having fuel elements in channels, they were a bed of tiny particles.  This design is incredibly dangerous (and I have spoken to a number of people that worked the program) because any localized melting of the fuel particles tends to weld them together, and block coolant flow.  This leads to more melting and welding, and bam...you've fused the core and melted the reactor--not through fission heating, but through decay heat.

The triple NTR design is another one where the issue you brought up is exactly my concern.  Without good thick neutron shielding around each core (which is heavy and undesireable) the reactors will "communicate" neutronically with each other, simply through neutron leakage.  I think it will be really hard to not run them all at the same level, all the time, because if you try to run them at different power levels, the neutron leakage will tend to "drive" the other ones into being at the same level.  This could really get you in trouble if you had the scenario you postulated where hydrogen flow was interrupted to one reactor.  It would tend to want to shut down, but the neutron leakage from the other reactors would try to "drive" it and possibly overwhelm the negative coefficient, leading to a core melt.  This would be another problem that would be very difficult to test on the ground--the clustered operation of nuclear-thermal engines and their response to one another.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Rocket Ronnie on 01/04/2006 01:24 pm
But if the engine shuts down, is that the engine then useless? Can you "re-start" such engines?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: nacnud on 01/04/2006 01:54 pm
With a bimode NTR (thrust and/or power generation) could you use the power generation loops to get rid of the decay product heat?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/04/2006 02:21 pm
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nacnud - 4/1/2006  8:54 AM

With a bimode NTR (thrust and/or power generation) could you use the power generation loops to get rid of the decay product heat?

The bimodal NTR designs I have seen have had a rocket mode that is three orders of magnitude more powerful than the power generation mode.  With decay heat typically 5% of the rated power of the reactor, the decay heat would have to go down substantially before the heat levels would be in the range of the cooling capability of the power mode.  So I think you will have to use hydrogen, flowing it through the open core, for some time before you could do a transition.

From a presentation at the 2005 JPC in Tuscon I got these numbers:

Rocket mode:  317,000 kW (thermal)
Power mode:  500 kW (thermal)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: nacnud on 01/04/2006 02:26 pm
So even if the power mode is only 50% effeienct it is still 15 times too weak to cope with the decay heat problem. Oh well.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Stowbridge on 01/04/2006 03:43 pm
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vanilla - 4/1/2006  8:01 AM

The triple NTR design is another one where the issue you brought up is exactly my concern.  Without good thick neutron shielding around each core (which is heavy and undesireable) the reactors will "communicate" neutronically with each other, simply through neutron leakage.  I think it will be really hard to not run them all at the same level, all the time, because if you try to run them at different power levels, the neutron leakage will tend to "drive" the other ones into being at the same level.  This could really get you in trouble if you had the scenario you postulated where hydrogen flow was interrupted to one reactor.  It would tend to want to shut down, but the neutron leakage from the other reactors would try to "drive" it and possibly overwhelm the negative coefficient, leading to a core melt.  This would be another problem that would be very difficult to test on the ground--the clustered operation of nuclear-thermal engines and their response to one another.

That could indeed by an issue. I'll have to ask around and see what the conclusion is on adverting this problem.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/04/2006 03:55 pm
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Stowbridge - 4/1/2006  10:43 AM
That could indeed by an issue. I'll have to ask around and see what the conclusion is on adverting this problem.

My recommendation would be a thick neutron "curtain" on the side of the reactor vessel that "sees" the other reactors.  Boron, cadmium...any good neutron absorber will do.  The issue will be the mass of the curtain and the additional reduction in thrust/weight of the engine, which already has a pretty bad thrust/weight.  The neutron curtain might need active cooling as well, depending on the amount of heat deposited through neutron absorption.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: AndyMc on 01/04/2006 04:07 pm
All very interesting stuff. I see from what we are learning here that the 'energy crisis' is a figment of the medias imagination. Anyway back to space :)

Does anyone here have any thoughts on MITEE reactors? According to this site (http://www.nuclearspace.com/A_HEADNA.HTM) the hybrid electro-thermal engine has the greatetst potential.

Quote:
A hybrid electro-thermal engine in which electric power generated by expansion of hot high pressure H2 propellant in a turbine is used to further heat hot low pressure hydrogen from the reactor. The hybrid engine achieves temperatures of ~4000 K, with almost complete dissociation of the H2 propellant, and an lsp of ~l600 seconds.

Is this type of propulsion a valid proposition?


Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Tap-Sa on 01/04/2006 04:33 pm
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vanilla - 3/1/2006  5:21 PM

With decay heat typically 5% of the rated power of the reactor, the decay heat would have to go down substantially before the heat levels would be in the range of the cooling capability of the power mode.  


Doesn't the amount of decay heat depend on how long the reactor has been operating? NTR would be loaded with pure fuel and operated at max power a few hours at most. Doesn't that mean there would be much lower content of decaying byproducts than in typical terrestial reactor that's been up for months?

PS. Excellent thread!
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/04/2006 04:41 pm
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Tap-Sa - 4/1/2006  11:33 AM
Doesn't the amount of decay heat depend on how long the reactor has been operating? NTR would be loaded with pure fuel and operated at max power a few hours at most. Doesn't that mean there would be much lower content of decaying byproducts than in typical terrestial reactor that's been up for months?
Yes, you are correct, the decay heat will be a function of the burnup.  I couldn't give you a better number without doing some calculations, but another issue to consider is the core power density.  The numbers I've seen show a core power density of ~5000 kW/liter in the NTR core, whereas a terrestrial power reactor "burns" with a power density of 50-100 kW/liter.  So even though the NTR burn is only a few hours, it is very intense, and there will be significant fission product buildup and decay heat.  A 2-hour run at full power could be equivalent, in terms of fission products, to a 200-hr burn in a terrestrial reactor.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Tap-Sa on 01/04/2006 05:17 pm
Can you give any terrestial examples how long the 5% decay heat takes to cool down, are we talking minutes, hours or days here? I'm just wondering that if it takes, say, hours to cool from the 5% to manageable level then a great care has to be taken to purge H2 through the core at intervals/rate so that core temp remains at nominal thrusting level or overall Isp takes a hit.

The MSR technology sounds promising (and depressing, to think that TMI and Chernobyl might have been avoided if this technology would have become dominant but due to apparently nontechnical reasons this never happened). One thing about possible NEP usage though; handling even ordinary fluids in zero-g is always somewhat problematic, how about handling redhot fluid undergoing nuclear reactions? Is there any way to make to circulation system safe other than establish artificial gravity by rotation?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/04/2006 06:41 pm
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Tap-Sa - 4/1/2006  12:17 PM

Can you give any terrestial examples how long the 5% decay heat takes to cool down, are we talking minutes, hours or days here? I'm just wondering that if it takes, say, hours to cool from the 5% to manageable level then a great care has to be taken to purge H2 through the core at intervals/rate so that core temp remains at nominal thrusting level or overall Isp takes a hit.

I have a better image in my book that I'll try to scan in and post, but here's an image I found on the internet of the graph of decay heat reduction as a function of time for typical pressurized water reactors.  It's a little hard to read, and it's a log-log plot, but it shows that decay heat goes down by about an order of magnitude over the first year.  The lines plotted are fuel burn-ups in MWd/MTHM, which stands for megawatt-days per metric tonne of heavy metal.  I'm not sure what this would be for an NTR that ran for two hours, but you can see the general trends on the graph.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/04/2006 06:46 pm
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Tap-Sa - 4/1/2006  12:17 PM
The MSR technology sounds promising (and depressing, to think that TMI and Chernobyl might have been avoided if this technology would have become dominant but due to apparently nontechnical reasons this never happened). One thing about possible NEP usage though; handling even ordinary fluids in zero-g is always somewhat problematic, how about handling redhot fluid undergoing nuclear reactions? Is there any way to make to circulation system safe other than establish artificial gravity by rotation?

This might be a problem if the circulation system was based on gravitational processes (like natural convection), but it's not--it would be actively pumped.  A molten-fluoride reactor designed for space would pump the fluoride salt mixture through the graphite lattice (the moderator) of the core where it would undergo fission reactions and generate heat.  The hot fluid would then leave the "core" region and pass through a heat exchanger where it would heat the working fluid for the power conversion system, which could be a Brayton (gas-turbine), Rankine (vapor-cycle), Stirling, thermoelectric, or whatever.  Of course, judicious choice of the power conversion system is just as important for overall system performance as the reactor, but the reactor choice does not necessarily preclude any particular choice of power system.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/04/2006 07:37 pm
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Tap-Sa - 4/1/2006  12:17 PM
The MSR technology sounds promising (and depressing, to think that TMI and Chernobyl might have been avoided if this technology would have become dominant but due to apparently nontechnical reasons this never happened).

This is an excerpt from Weinberg's book that I mentioned earlier.  It should be noted that Weinberg ran Oak Ridge throughout the 50s and 60s and was a champion of molten-fluoride technology.  He also is the inventor (and holds the patent) on the pressurized water reactor, by far and away the most common type of reactor in the world.  It is interesting to note that the man who has the patent on PWRs is an advocate of another technology!

from Chapter 6:

How the Fast Breeder Won the Great Breeder Sweepstakes

As must be clear to the reader by now, the earliest ideas about commercial nuclear power were dominated by the mistaken belief that uranium was very scarce. Moreover, in America at least, electricity generated in conventional coal-fired plants was very cheap. Thus we could hardly imagine that nuclear energy based on burning scarce 235U could compete with coal, or if it could, that the necessary uranium ore would last long. As Eugene Wigner put it, the ultimate purpose of nuclear energy was not to replace coal when fossil fuel was abundant; rather it was to substitute for fossil fuel when the latter became scarce. Energy would then be very expensive: nuclear energy based on breeders would be an inexhaustible energy source, and its cost would be perhaps two times the present cost of fossil energy, rather than ten or fifteen times that cost. (Were energy cost to rise so steeply, a large share of our GNP would be spent for energy, and this would reduce our standard of living.)

About this time I wrote two essays on the role of the breeder. The first, "Power Breeding as a National Objective," appeared in Nucleonics in 1958 (vol. 16, no. 8, pp. 75-6): in this piece I argued that current economics alone should not be the sole basis for choosing which reactor system to pursue. Efficient use of the raw materials of nuclear energy—uranium and thorium—was equally important. Indeed it would eventually be more important than estimates of the cost of power. Once inexpensive ores were used up, nuclear power from nonbreeders would be very expensive. This general argument remains valid today, except for one point: if the capital cost of the breeder is too high, the price of uranium ore at which the breeder, with its more efficient utilization of ore, becomes competitive may be extremely high—say, $300 per kilogram. At this price, uranium resources are vast: the breeder can price itself out of business if its capital is too high. Unfortunately, this seems to be the case at the moment, but with Japan and France actively pursuing "power breeding as a national objective," capital costs may come down within the next fifty years.

The other essay, "Energy as an Ultimate Raw Material, or Burning the Rocks and Burning the Sea," appeared in 1959 in Physics Today (vol. 12, no. 11, p. 18). In this essay I speculated on the very long-range future-hundreds, even thousands, of years in the future. Where will our energy come from at that distant time when coal, oil, and natural gas have been used up? Solar energy is one obvious inexhaustible source. Another, if it works, could be controlled thermonuclear energy based on deuterium from the sea (thus "Burning the Sea"). My main point, however, was to stress what Phil Morrison and then Harrison Brown had already noticed: that the residual and all but infinite uranium and thorium in granite rocks could be burned with an energy yield larger than the energy required to mine and refine the ore—but only if breeders, which could burn nearly all the fertile material, are used. I spoke of "Burning the Rocks": the breeder, no less than controlled fusion, is an inexhaustible energy system. Up till then we had thought that breeders, burning 50% instead of 2% of the uranium, extended the energy derivable from fission "only" 25-fold. But, because the breeder uses its raw material so efficiently, one can afford to utilize much more expensive—that is, dilute—ores, and these are practically inexhaustible. The breeder indeed will allow humankind to "Burn the Rocks" to achieve inexhaustible energy!

Until then I had never quite appreciated the full significance of the breeder. But now I became obsessed with the idea that humankind's whole future depended on the breeder. For society generally to achieve and maintain a living standard of today's developed countries depends on the availability of a relatively cheap, inexhaustible source of energy. (As I write these words, I realize that until recently I tended to dismiss solar energy as too expensive, and fusion as probably infeasible. I really don't know whether this will always be the case.)

The breeder became central in my thinking about nuclear-energy development. And, with Glenn Seaborg's becoming the chair of AEC in 1960, the breeder acquired ever-increasing status with AEC—especially recognition as an essentially inexhaustible source of energy.

In 1962, the AEC issued a report to the president on civilian nuclear power. Lee Haworth, a superbly responsible physicist-administrator, was in charge of drafting the report. He projected a nuclear deployment by 2000 of about 700 gigawatts (compared with the actual deployment in 1993 of 102 gigawatts), which seemed at the time quite reasonable. Both the fast breeder based on the 239Pu-238U cycle and the thermal breeder based on the 233U-232Th cycle figured prominently in the report. Indeed, the report implied that both systems should be pursued seriously, including large-scale reactor experimentation. It particularly favored molten uranium salts for the thermal breeder. But the molten-salt system was never given a real chance. Although the AEC established an office labeled "Fast Breeder," no corresponding office labeled "Thermal Breeder" was established. As a result, the center of gravity of breeder development moved strongly to the fast breeder; the thermal breeder, as represented by the molten-salt project, was left to dwindle and eventually to die.

The fast-breeder project in the United States centered around the Clinch River Breeder, a 250-megawatt sodium-cooled breeder to be built in Oak Ridge by Westinghouse. But, by this time, objections to the breeder were being voiced, ostensibly because the breeder, with its coupled chemical reprocessing system, lent itself to the clandestine diversion of plutonium for nuclear weapons. But in my view the real aim of some of the more dedicated opponents of Clinch River was the extirpation of nuclear energy. The Clinch River Breeder was a handy and vulnerable target, particularly since it could not produce power at a competitive cost. And the opponents eventually won—Clinch River was killed in 1975.

Although the molten-salt system was never allowed to show its full capability as a breeder, a 233U-232Th thermal breeder was demonstrated in Admiral Rickover's Shippingport reactor. Operating with 233U fuel and a thorium blanket, this reactor actually demonstrated a breeding ratio of 1.03—i.e., for every 233U burned, 1.03 new 233U was produced. This accomplishment has gone unnoticed since the cost of power from Shippingport is much higher than from other sources. Whether, as cheap uranium becomes scarce, other reactors will be fueled with 233U and thorium remains to be seen. Thus, as Wigner once said, breeders may emerge from incremental improvements of existing light-water or heavy-water reactors, or may spring from entirely new technologies specifically designed for the breeder. As for fast uranium breeders, the latter path is being followed in France, Japan, India, and Russia. (The French fast breeder PHENIX has demonstrated a breeding ratio of 1.13.) But as for thermal thorium breeders, it seems that these will emerge from the existing nonbreeder LWR or CANDU rather than from molten-salt technology.

Why didn't the molten-salt system, so elegant and so well thought-out, prevail? I've already given the political reason: that the fast breeder arrived first and was therefore able to consolidate its political position within the AEC. But there was another, more technical reason. The molten-salt technology is entirely different from the technology of any other reactor. To the inexperienced, molten-salt technology is daunting. This certainly seemed to be Milton Shaw's attitude toward molten salts—and he after all was director of reactor development at the AEC during the molten-salt development. Perhaps the moral to be drawn is that a technology that differs too much from an existing technology has not one hurdle to overcome—to demonstrate its feasibility—but another even greater one—to convince influential individuals and organizations who are intellectually and emotionally attached to a different technology that they should adopt the new path. This, the molten-salt system could not do. It was a successful technology that was dropped because it was too different from the main lines of reactor development. But if weaknesses in other systems are eventually revealed, I hope that in a second nuclear era, the molten-salt technology will be resurrected.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Tap-Sa on 01/04/2006 08:10 pm
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vanilla - 4/1/2006  9:46 PM
 
This might be a problem if the circulation system was based on gravitational processes (like natural convection), but it's not--it would be actively pumped.  

There's the rub, pumping. In zero-g conditions you have to make sure that something keeps feeding the pump. Booster upper stages have special engines for propellant settling to initially feed the turbopumps and after ignition the acceleration takes care that propellant remains at the pump inlet while tank pressure pushes it into the pump. There's minute acceleration in NEP usage too but so small that it may not be enough to guarantee proper pump feeding. If the circulation path is completely void of gas pockets then of course the pump sort of feeds itself and there's no problem, but such condition is not easy to guarantee since there's a phase change during start-up, large temperature changes, formation of gaseous fission/decay products and so on. Some mechanism is needed to collect the gases and allow the fluid to expand and contract (somehow I think a rubber bladder won't cut it :)).

Not saying that this is a showstopper, just something to consider.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/04/2006 08:29 pm
Quote
Tap-Sa - 4/1/2006  3:10 PM
There's the rub, pumping. In zero-g conditions you have to make sure that something keeps feeding the pump. Booster upper stages have special engines for propellant settling to initially feed the turbopumps and after ignition the acceleration takes care that propellant remains at the pump inlet while tank pressure pushes it into the pump. There's minute acceleration in NEP usage too but so small that it may not be enough to guarantee proper pump feeding. If the circulation path is completely void of gas pockets then of course the pump sort of feeds itself and there's no problem, but such condition is not easy to guarantee since there's a phase change during start-up, large temperature changes, formation of gaseous fission/decay products and so on. Some mechanism is needed to collect the gases and allow the fluid to expand and contract (somehow I think a rubber bladder won't cut it :)).

Not saying that this is a showstopper, just something to consider.

It sure sounds like you know a lot more about pumps than me!  Those are all good points, some of which I hadn't considered.  I had thought that since there should be little or no gas in the system, that keeping fluid at the pump inlet shouldn't be a problem.  The formation of fission product gases should be rather small indeed--it is a very nice feature of fluid-fuel reactors that these gases come right out of solution--especially since xenon-135 is a very significant fission product poison.  Perhaps one approach that would help would be to launch the reactor with the salt already melted but not critical.  Then after achieving orbit, a small amount of uranium tetrafluoride could be dissolved in the salt sufficient to allow the reactor to go critical and begin power generation.  This situation would also alleviate any concern about the reactor "going in the drink" because if it did, the salt would freeze in the reactor loop and there would be no volume for the water to get into the core.  Fluoride salts, unlike chloride salts, are relatively insoluble in water.

Here's the role xenon-135 played in Chernobyl (since you're from Finland and probably particularly interested in this):  Chernobyl happened because they wanted to do a "safety test" of the reactor.  So they disabled many of the safety systems and ran the reactor as high power.  This built up a lot of xenon-135 in the core (which has a huge appetite for neutrons) and the reactor lost power and shut down.  They weren't able to finish their safety test.  The operators told the government official in charge of the test that they needed to wait about 18 hours for the xenon to decay away so they could restart the reactor (since Xe-135 has a 9-hour half-life).  But the government official was insistent that they finish the test that night, and told them to pull the control rods nearly all the way out of the reactor, to override the poisoning effects of the xenon.  This led to the accident.  Poor design, bad operation, and impatient people who didn't know any thing about reactors got a lot of people killed and poisoned.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Boris the Space Dog on 01/04/2006 08:44 pm
Damn, I don't know the first thing about all of this, but I can't stop reading it as it's all very interesting. Anyone else? I really don't have a question or a comment, but the PDFs are interesting and I think I'm learning something here :)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: ADC9 on 01/04/2006 11:57 pm
I have a question which may be stupid, so I apologize in advance.

I would assume that any nuclear waste product or still active material would be left in space. Are there any dangers of this, or would it be happily left to float in the vasteness of space for all time, losing its radioactivity over time? No side effects from such disposing of the material in space?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/05/2006 12:40 am
Quote
ADC9 - 4/1/2006  6:57 PM
I would assume that any nuclear waste product or still active material would be left in space. Are there any dangers of this, or would it be happily left to float in the vasteness of space for all time, losing its radioactivity over time? No side effects from such disposing of the material in space?

Well, you certainly wouldn't want it to reenter the Earth's atmosphere, especially if the core vessel was not designed to survive the reentry intact.  The dispersal of fission products in the atmosphere, especially strontium-90 and cesium-137, would not be good.  Let's not forget that back in the 50s and 60s, open-air testing of bombs released comparatively enormous amounts of fission products directly into the atmosphere...that was really really not good.

That said, unless you went out to the spent core in space, and ate it or hugged it or something, it's not going to pose any danger out there.  In about 300 years, the fission products will decay to background levels of radiation.  The radiation hazards of the Sun and galactic cosmic radiation are a much bigger danger.

An aside, but I find it very interesting that the same politicians in Nevada who oppose the construction of the Yucca Mountain nuclear waste dump because they are afraid that radioactive materials might be released 10,000 years from now also want the administration to resume underground testing of nuclear weapons at the Nevada Test Site, which weapons would release all of their fission products into fractured rock, immediately.  No logic there.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/05/2006 01:14 am
Quote
Tap-Sa - 4/1/2006  12:17 PM
Can you give any terrestial examples how long the 5% decay heat takes to cool down, are we talking minutes, hours or days here? I'm just wondering that if it takes, say, hours to cool from the 5% to manageable level then a great care has to be taken to purge H2 through the core at intervals/rate so that core temp remains at nominal thrusting level or overall Isp takes a hit.


I put two gifs on the website (decayheat1.gif and decayheat2.gif)--this information will help calculate the effects of decay heat removal.

CLICK HERE FOR decayheat1 DOWNLOAD (http://www.nasaspaceflight.com/_docs/decayheat1.gif)

CLICK HERE FOR decayheat2 DOWNLOAD (http://www.nasaspaceflight.com/_docs/decayheat2.gif)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Stowbridge on 01/05/2006 01:43 pm
Thanks, interesting images.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Tap-Sa on 01/05/2006 02:30 pm
Quote
vanilla - 5/1/2006  4:14 AM
 
I put two gifs on the website

Thanks! It looks like that even after short period of operation and one hour cooling there's still over one percent decay heat left.

About inactive reactor/uranium fuel accidentally falling in to the sea, that's like coal to Newcastle. Seawater contains ~3ppb uranium as is. That may not seem much but a cubic kilometer of seawater contains three tonnes of U. And there are quite many cubic kms in the seas ;) Japanese are working on methods to extract uranium from the sea using some special fibres.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: JonClarke on 01/06/2006 08:31 pm
Vanilla, you wrote: "a well-designed NTR should be quite stable neutronically."  I agree.  The question is, were the numerous NTRs stable?

In the Borowski design (which I understand is based on a Russian concept) how much propellant is expended cooling the reactor after the initial shut down?  Does this contribute to the dV of the burn, or is it effectively lost?  

Yes, I have heard of Timberwind, and what I heard gave me the willies.  A suborbital test flight over  Antarctica  with a upper atmosphere start up and an impact point somewhere in the Indian ocean was bad enough.  Wasn't there also the idea of dumping and recharging the core after each burn to reduce the effect of fission products poisoning the reaction?

Thanks for the info on neutron cross-coupling. How much shielding would be required to stop the problem?  

I am also interested in two other issues, whether containment during a maximum credible  accident (say a ruptured core entering the atmosphere at 14 km/s) is actually possible and whether the problem of exhaust radioactivity can be solved.

My interest in this is two fold.  Much as NTR is superfically attractive from an Isp perspective, I can't see one flying unless the safety issues are addressed to a very high level of confidence.  The literature I have seen skirts round most of these.  I am also wondering what the actual performance of an NTR is over a chemical stage, once the extra mass of incidentals like shielding, containment, and propellant needed for safe disposal orbit have been factored in.

Thanks

Jon
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Martin FL on 01/06/2006 10:36 pm
A far less educated question, but there's a problem with the start of development of a NTR or any form of nuclear propulsion engine. That is if there's any form of an accident, or close call involving nuclear, even at domestic level, public opinion could be affected to the point it has political wieght, and then you've got a potential cancellation of the program.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: David AF on 01/07/2006 12:08 am
Quote
Martin FL - 6/1/2006  5:36 PM

A far less educated question, but there's a problem with the start of development of a NTR or any form of nuclear propulsion engine. That is if there's any form of an accident, or close call involving nuclear, even at domestic level, public opinion could be affected to the point it has political wieght, and then you've got a potential cancellation of the program.

Yes, but we won't get anywhere with "but what if's".
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/07/2006 04:23 am
Quote
JonClarke - 6/1/2006  3:31 PM

Vanilla, you wrote: "a well-designed NTR should be quite stable neutronically."  I agree.  The question is, were the numerous NTRs stable?

I do not have the actual temperature coefficient of reactivity of the NERVA reactors, so I cannot categorically state that I KNOW they were stable, but I anticipate that they were stable based on the role the hydrogen propellant would play in the neutron moderation of the core.  The graphite in the fuel structure, certainly, provided most of the moderation, but I would anticipate that the hydrogen was a non-trivial contributor, probably sufficient to determine the neutronic stability.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/07/2006 04:27 am
Quote
JonClarke - 6/1/2006  3:31 PM
In the Borowski design (which I understand is based on a Russian concept) how much propellant is expended cooling the reactor after the initial shut down?  Does this contribute to the dV of the burn, or is it effectively lost?  

Tap-Sa, do you have any calculations on this?  I can safely say that the cooldown hydrogen probably contributes very little to the DV, for two reasons.  First is that the hydrogen is probably run into the chamber at essentially the tank pressure, which would not be sufficient chamber pressure to choke the flow in the throat and actually get thrust from expansion in the nozzle.

Second is that the vehicle will be climbing out of the gravity well very quickly after the completed trans-Mars injection burn.  As it moves further away from perigee, the beneficial effects of any DV will diminish rapidly due to gravity losses.  Propulsion forces generated during cool-down will probably prove a nuisance to the system, requiring course correction maneuvers.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/07/2006 04:41 am
Quote
JonClarke - 6/1/2006  3:31 PM

I am also interested in two other issues, whether containment during a maximum credible  accident (say a ruptured core entering the atmosphere at 14 km/s) is actually possible and whether the problem of exhaust radioactivity can be solved.

My interest in this is two fold.  Much as NTR is superfically attractive from an Isp perspective, I can't see one flying unless the safety issues are addressed to a very high level of confidence.  The literature I have seen skirts round most of these.  I am also wondering what the actual performance of an NTR is over a chemical stage, once the extra mass of incidentals like shielding, containment, and propellant needed for safe disposal orbit have been factored in.
Protecting from a 14 km/s reentry is going to be very tough.  That said, even if we were flying NTRs (which I don't think we should) this issue would not cause me to lose much sleep at night--assuming of course that we are talking about a returning Mars vehicle and not some cislunar shuttle (which Borowski has proposed in the past).  The cislunar vehicle really would trouble me, but the Mars vehicle would probably be targeted for Earth avoidance, and the CEV would separate several days before Earth return and do a correction DV to get on an intercept course.

Your comments about "what are the real benefits" is a very valid one, and one I have attempted to quantify myself.  If you believe the Isp (~950 seconds) everything looks great.  But remember that NERVA only got to ~825 seconds, and they had to do a lot of very "non-flight-like" things to the reactor core to get it to hold together even at those relatively modest core temperatures (~2700 K).  But hey, let's give them 950 seconds for a minute...the engine runs on hydrogen and hydrogen alone...so all the propellant suffers from the low bulk density of hydrogen (71 kg/m3).  That's a really big propellant tank.  And hydrogen needs to be kept at 20 K.  That's REALLY cold...LOX at 90 K looks balmy in comparison.  So you have to have lots of multi-layer insulation to keep it cold, but MLI is based on radiation heat transfer...it only works when you're in vacuum, so what about the losses on the pad and during ascent.  Is it inside a shroud?  Are you purging the interior?  I mean, all cryo stages have similar issues, but this is a LOT of hydrogen, and Borowski's schemes want to keep it liquid for YEARS at a time in deep space.   Can it be done?  Maybe, but it's not trivial.

There is also the deleterious effect of the low thrust/weight on the engine.  This means you have to make a longer burn arc, or to keep gravity losses down, two burns.  One burn gets you from LEO to an highly-elliptical orbit, then you come around again and burn to interplanetary injection.  Another ding to tally up when comparing systems.

When I've looked at it, even if you believe all the NTR performance numbers, the chemical stage is about half again heavier than the NTR stage, not a factor of two.  When you throw in a more realistic value of Isp for the engine (~825 seconds) the difference really shrinks.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: braddock on 01/07/2006 01:00 pm
Can you compare those NTR parameters to Nuclear Electric Propulsion?...isp, engine mass, thrust?

Also, how do the two technologies scale as ranges increase to Jupiter or Pluto?  If there is a significant H2 boiloff during transit, does that make NTR only friendly for "short" hops and returns to and from Mars?

I kinda think an article on this stuff would be fantastic, although I can't speak for Chris.
I've learned a ton in this thread, and it has nudged me to learn more about reactor theory.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Avron on 01/07/2006 02:03 pm
Quote
braddock - 7/1/2006  9:00 AM

Can you compare those NTR parameters to Nuclear Electric Propulsion?...isp, engine mass, thrust?

Also, how do the two technologies scale as ranges increase to Jupiter or Pluto?  If there is a significant H2 boiloff during transit, does that make NTR only friendly for "short" hops and returns to and from Mars?

I kinda think an article on this stuff would be fantastic, although I can't speak for Chris.
I've learned a ton in this thread, and it has nudged me to learn more about reactor theory.


I would like to see a lot more analysis on risks and options to cover them... like dumping cores and how would that fly in terms of risks to the people of earth..

This has been one hell of a fantastic thread, have learned a lot...
Many thanks to vanilla for the 'tour' and providing so much easy to follow feedback to all the questions.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Tap-Sa on 01/07/2006 03:07 pm
Quote
vanilla - 7/1/2006  7:27 AM

Quote
JonClarke - 6/1/2006  3:31 PM
In the Borowski design (which I understand is based on a Russian concept) how much propellant is expended cooling the reactor after the initial shut down?  Does this contribute to the dV of the burn, or is it effectively lost?  

Tap-Sa, do you have any calculations on this?

No, sorry, I'm not familiar with this design (have I missed something in the thread(s) or should that be general knowledge? :))

Google to the rescue! Found this paper (http://trajectory.grc.nasa.gov/aboutus/papers/AIAA-93-4170.pdf) by Borowski & co in which 3% of available propellant is allocated to cooldown.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/07/2006 03:17 pm
Quote
Avron - 7/1/2006  9:03 AM

Quote
braddock - 7/1/2006  9:00 AM
I kinda think an article on this stuff would be fantastic, although I can't speak for Chris.
I've learned a ton in this thread, and it has nudged me to learn more about reactor theory.
This has been one hell of a fantastic thread, have learned a lot...
Many thanks to vanilla for the 'tour' and providing so much easy to follow feedback to all the questions.
Thanks for the kind words, but we've only just gotten started on reactor theory and its implications.  The message I've been wanting to write now for several days, and haven't gotten around to, is why are people messing around with solid-fueled reactors?  The short answer---I don't know.  I keep scratching my head why solid-fuel is used instead of fluid-fuel...they are vastly inferior in safety (because there is no way to passively remove decay heat) and lack versatility, since you can't refuel them or remove fission products (especially the gaseous xenon-135, which has so much to do with reactor control).

Now before I talked about all the risks inherent in trying to run a highly-enriched, solid-core, fast-spectrum nuclear reactor.  It's logical to ask, why the heck would good engineers propose such an unstable beast---surely they know these things, right?  Well, yes, but there is another factor that drives them (in their solid-fuel paradigm) to pursue doing it that way.  And that factor is long core life.  See, a space reactor, especially one for a mission like JIMO, needs to last 15 years or more.  How do you get a reactor to last that long?

Well, perversely for a solid-core reactor, there's no risk of running out of fuel, yet the basic problem is that you're trying not to run out of fuel.  Paradox?  Let me try to explain.  In one sense, the reactor never runs out of fuel because even at the end of the reactor's operation, only a small fraction of the fissile material has been consumed--the vast majority is still there.  But on the other sense, you have to load the reactor up with so much fuel because as it runs, and as the fission products build up, they will increasing poison the reaction, consuming neutrons that otherwise would have gone to fission.  So you must have enough fuel in the reactor so that even after 15-30 years of operation, and a core carrying a lot of fission products that are gobbling up neutrons, you still have enough fuel to maintain criticality.

To do this, you have to load the core up, in the beginning, with WAY more fuel than it needs initially.  Now this is a great safety risk because a core with too much fuel wants to go supercritical and disassemble.  So to keep it near criticality, you have to load the core up with a bunch of other stuff that will gobble up all the extra neutrons in the beginning, and then slowly "burn" off as the fission products build up and you need to start conserving the neutrons.  Sound wasteful and complicated?  Yup, it is, but that's what you have to do with solid-core reactors.  Typically the "burnable poison" material is gadolinium in the fuel, or boron (in the form of boric acid) dissolved in the water.  This stuff will drink up neutrons when there are too many at the beginning of core operation, and then if you do it just right, they will "burn out" over time, so that there's enough neutrons to maintain criticality when the fission product poisons have really built up later in core operation.

This is how the Navy runs their long-lived reactors.  See, a pressurized-water reactor is a massive beast, with a pressure vessel designed to hold 3000 psi water.  It's a 9-inch thick steel vessel, and weighs a lot.  They basically build the rest of the ship around it.  So getting fuel in and out of the core is not desireable...you would have to take the ship apart to get at the core.  So they need a core that runs the lifetime of the vessel (~30 years).  So they use highly-enriched fuel and have lots of burnable poisons in the core (gadolinium) to they can run that long.  A typical terrestrial reactor gets refueled every year (they replace about 1/3 of the fuel in the core and move the rest around) so they don't use gadolinium in the fuel.  Their excess reactivity is a lot smaller, and they control it with boron, in the form of boric acid, in the water coolant.

Doing it this way has some real risks, though.  In 2002, the Davis-Besse nuclear plant in Toledo Ohio was found to have suffered severe corrosion of its steel pressure vessel from boric acid.  The acid (which is a weak acid) had managed to eat through 6 inches of steel.  Had it gone much longer, it would have eaten through enough of the vessel that the internal pressure of the water (~3000 psi) would have caused a rupture of the vessel.  The water would have drained out of the core.  Fission would have stopped, but decay heat would have led to a core meltdown.  Even at Three-Mile-Island, the pressure vessel had not failed (they had mistakenly drained the core of water) and they were able to refill the vessel with water.  At Davis-Besse, had the vessel ruptured, they would not have been able to get water back into the vessel and cool the fuel since it would just drain out the breach.

Scary stuff.  All of these are non-problems in molten-fluoride reactors, because they don't need any excess reactivity.  You can refuel them anytime you want.  No burnable poisons, boric acid, gadolinium, and you get all the safety features we need in a reactor.  I'll talk more about this in the next post.  Here's a story on the Davis-Besse almost accident.

http://www.pennlive.com/news/patriotnews/index.ssf?/news/tmi/stories/nationsaging.html
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/07/2006 04:37 pm
Quote
Tap-Sa - 7/1/2006  10:07 AM

Google to the rescue! Found this paper (http://trajectory.grc.nasa.gov/aboutus/papers/AIAA-93-4170.pdf) by Borowski & co in which 3% of available propellant is allocated to cooldown.
Wow, there it is on page 11.  The NTR stage for trans-lunar injection has a mass of 101 tonnes, the chemical stage has a mass of 155 tonnes.  Half again more heavy, just like I said.  You know, it's just my opinion, but that's really not "better enough" to justify all the cost and risk, both technical, progammatic, political, and environmental.  And Borowski is assuming an Isp of 900 seconds, core temperatures of 2900 K, and all the other magic performance numbers of the Russian NTR engines.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: STS Tony on 01/08/2006 12:09 am
>The NTR stage for trans-lunar injection has a mass of 101 tonnes, the chemical stage has a mass of 155 tonnes.<

Still pretty damn heavy :(
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: JonClarke on 01/08/2006 09:01 am
Thanks Vanilla for your insights.  Yes NERVA XE only ran at 850 sec, although I believe the Russians demonstrated 910 on their equivalent, the RD 0410.

Anothering that NTR enthusiasts never mention is that by the time we start specifically planning to go to mars chemical EDRs will be fully dveloped and tested operation hardware for lunar missions.  NTRs and their dacilities wil have to be developed from a theoretical base that will be decades old.

What do you see as the application of moltern salt reactors in space?  Surface power sources?  NEP?

Cheers

Jon
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/08/2006 02:11 pm
Quote
JonClarke - 8/1/2006  4:01 AM
What do you see as the application of moltern salt reactors in space?  Surface power sources?  NEP?

I think surface power is an obvious application, with NEP a very promising possibility.  One of the complaints I have heard levied against the molten-fluoride reactor as a space reactor is that it is considered "exotic" and has never been studied for space applications before.  Yes, to a nuclear community that has been "born and raised" on solid fuel, it probably does seem exotic.  And yes, it has not been studied for space power before.  But to that I would hasten to point out that the molten-fluoride reactor has actually been built (twice) and operated for a period of five years, whereas all the other space power reactors concepts (solid-fuel, lithium-cooled, fast spectrum....SP-100; solid-fueled, heat-pipe-cooled, fast-spectrum....LANL's "SAFE"; and the solid-fueled, helium-xenon-cooled, fast-spectrum reactor) are all paper concepts, they have never been built or operated before.

We've had liquid-metal cooled reactors, but they were cooled by sodium and did not have nearly the fuel enrichment of the proposed SP-100 design.  We've never had a real heat-pipe cooled reactor with that fuel type, and we've had gas-cooled reactors, but they've had graphite fuel and a thermal neutron spectrum, plus they ran at lower temperatures.  There's just not really any "tailwind" to speak of from terrestrial reactors to help us too much in space.

One of the biggest costs any space reactor will face is "fuel qualification".  When you use solid fuel, especially fuel that is intended for deep burnup, you need to test it out in a real reactor, and expose it to the neutron flux that it will see over its operational lifetime.  Then you need to remove the fuel, which will be extremely radioactive due to the buildup of fission products, and examine it in a hot cell to see how the fuel has degraded.  Has it swollen, cracked, have the crystal structures of the uranium changed and how?...all this has to be ascertained.  And it's very expensive to qualify fuel.  The talk I saw at the JPC was given by an individual who works for BWXT, which makes specialty nuclear fuels for the Navy, and he said that the fuel qualification would take about five years.

So there's a big incentive to use fuels that have already been qualified, but all of these options have temperature constraints that lead to rather unattractive space reactors.  This was part of the story on JIMO.

Qualifying a fluoride fuel is a piece of cake in comparison, and the basic reason is that the fuel does not need to maintain any structural integrity as it operates.  It's a fluid, so it has no strength.  In solid fuels, gaseous fission products build up and create internal pressure on the fuel element--they have to leave a "gap" in the fuel for all the gaseous fission products--in a fluid fuel, they just come right out of solution.  The ionically-bonded structure of the fluoride salts makes them impervious to radiation damage...you can pound them forever with radiation and it just doesn't make any difference, whereas the covalently-bonded solid fuels with swell and crack due to lattice displacements, displacements shifting to grain boundaries, gas accumulation, and crystalline structural changes.

To qualify a fluid fuel, you go put a little salt in a capsule and you go put it in a high-flux reactor like the HFIR at ORNL.  There you can give it the equivalent neutron flux it will see in a decade in a few months, and then pull it out and examine its chemical properties.  It's very easy and they did it over and over again when they were testing different fluoride salts for reactors at ORNL back in the 1960s.

I really want to get into the choice of power conversion system soon, because that has about as much to do with the attractiveness of your nuclear system as the reactor selection does.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: JonClarke on 01/08/2006 08:01 pm
Weren't the Russian reactors that powered the US-A series of radar reconnaissance satellites liquid metal cooled as well?  I know they were much smaller (less than 10 kW) than the ones being proposed for future power applications.

I also understand that there are concerns with some of these which have been in orbit for over 20 years, and apparently have started to leak droplets of molten metal.  

That raises another issue with respect to future surface power applications - how do we deal with old reactors on the surface of Mars or the moon, ensuring the reactors don't leak over time, so the bases become ringed by spent reactors and a plethora leaking nasties?  especially on Mars, whethy they could blow round in the wind.

Jon
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: JonClarke on 01/08/2006 08:01 pm
Weren't the Russian reactors that powered the US-A series of radar reconnaissance satellites liquid metal cooled as well?  I know they were much smaller (less than 10 kW) than the ones being proposed for future power applications.

I also understand that there are concerns with some of these which have been in orbit for over 20 years, and apparently have started to leak droplets of molten metal.  

That raises another issue with respect to future surface power applications - how do we deal with old reactors on the surface of Mars or the moon, ensuring the reactors don't leak over time, so the bases become ringed by spent reactors and a plethora leaking nasties?  especially on Mars, whethy they could blow round in the wind.

Jon
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Tap-Sa on 01/08/2006 08:09 pm
Quote
JonClarke - 8/1/2006  11:01 PM

I also understand that there are concerns with some of these which have been in orbit for over 20 years, and apparently have started to leak droplets of molten metal.  
 

Yes, the Russian RORSATs (http://en.wikipedia.org/wiki/RORSAT). After operation they ejected their core to graveyard orbit at 950km and NaK coolant leaked in the process.

A Space.com article (http://www.space.com/news/mystery_monday_040329.html) with more details.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/09/2006 12:45 am
Probably worth mentioning a basic difference between liquid metals and molten-fluorides.  Liquid metals are extremely chemically reactive.  Lithium, sodium, potassium metals will all burn in water--that's how desperate they are to oxidize.  A fluoride salt on the other hand, like lithium fluoride or sodium fluoride, is totally chemically stable.  The alkali metal (lithium, sodium) has given up its electron to the halogen (fluorine) and they are ionically bonded.  They will not react chemically because they is not another chemical state where they are more stable.  That's a very important consideration for testing, handling, and launch accident contingencies.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: JonClarke on 01/09/2006 05:55 am
Indeed.  Imagine the interaction between droplets of liquid potassium or lithium and a water-rich icy regolith charged with superoxides.....

Could be quite spectacular from a safe distance!  

Jon
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/10/2006 01:01 am
In trying to describe the benefits of a molten-fluoride reactor, I would be hard-pressed to surpass these paragraphs from chapter 11 of "Fluid Fuel Reactors", written in 1958.

"Fluoride-salt mixtures suitable for use in power reactors have melting points in the temperature range 850 to 900°F and are sufficiently compatible with certain nickel-base alloys to assure long life for reactor components at temperatures up to 1300°F. Thus the natural, optimum operating temperature for a molten-salt-fueled reactor is such that the molten salt is a suitable heat source for a modern steam power plant. The principal advantages of the molten-salt system, other than high temperature, in comparison with one or more of the other fluid-fuel systems are (1) low-pressure operation, (2) stability of the liquid under radiation, (3) high solubility of uranium and thorium (as fluorides) in molten-salt mixtures, and (4) resistance to corrosion of the structural materials that does not depend on oxide or other film formation.

"The molten-salt system has the usual benefits attributed to fluid-fuel systems. The principal advantages over solid-fuel-element systems are (1) a high negative temperature coefficient of reactivity, (2) a lack of radiation damage that can limit fuel burnup, (3) the possibility of continuous fission-product removal, (4) the avoidance of the expense of fabricating new fuel elements, and (5) the possibility of adding makeup fuel as needed, which precludes the need for providing excess reactivity. The high negative temperature coefficient and the lack of excess reactivity make possible a reactor, without control rods, which automatically adjusts its power in response to changes of the electrical load. The lack of excess reactivity also leads to a reactor that is not endangered by nuclear power excursions."

That, in a nutshell, is why it would make a fantastic space power system.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: nacnud on 01/10/2006 01:36 am
That sounds almost too good! :)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: braddock on 01/10/2006 02:02 am
How could excess fission products be removed?  Would you require an on-board centrifuge or some-such?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/10/2006 02:38 am
Quote
braddock - 9/1/2006  9:02 PM

How could excess fission products be removed?  Would you require an on-board centrifuge or some-such?
The really troublesome fission products are the gaseous fission products, krypton and xenon, and specifically xenon-135.  Xenon-135 has a huge appetite for neutrons, and its accumulation in nuclear fuel really affects the reactor control--especially high-flux reactors (like NTRs) with a high core power density.  In a fluid-fuel reactor like a molten-fluoride reactor, gaseous fission products come out of solution almost immediately and could be vented directly to space.  This alone improves the neutron economy and controllability of the reactor substantially.

As far as the rest of the fission products, they fall in two classes--those that form stable fluorides, like barium and strontium, and those that don't form fluorides but stay "noble"...these are called the "noble metals" and tend to plate out on cooler surfaces in the reactor.  In the event that the core is drained (which would be done in a terrestrial molten-fluoride reactor for passive decay heat removal) a low-level gas flow through the reactor loop would remove decay heat from the noble metals that had plated out.  In a space molten-fluoride reactor, there would be little point in draining the core (and no gravity to drain it) so decay heat removal would be handled with the fluorides in the core.

For a space molten-fluoride reactor such as would be used on a NEP vehicle shuttling to and from Mars, I would just let the fission products accumulate in the core during the mission, and then when the vehicle returned to its Earth base (which would probably be at one of the Lagrange points) you could flush the whole core salt mixture into a vessel and replace the core salts with fresh salt.  Removing the uranium-233 fuel from the old salt would be easy--fluorinate it and it will turn to gaseous uranium hexafluoride, which will come out of solution.  Then this 233UF6 could be reduced in the new fresh core salt to return it to uranium-tetrafluoride.

The old core salts, which would consist at this point of lithium fluoride, beryllium fluoride, and fission product fluorides, could either be disposed in total, or the LiF and BeF2 (which would constitute the bulk of the salt) could be distilled from the salt mixture through high-temperature distillation.  Such a process was actually used on the salts of the MSRE to separate the base salts (LiF, BeF2) from a mixture that included fission products.  In the end, you have a highly concentrated mixture of fission product fluorides that can be disposed and will decay to background levels of radiation in ~300 years.

So two processes, fluorination and distillation, both of which were successfully demonstrated on real molten-fluoride reactors, are sufficient to recover all nuclear fuel and cleanse the original base salts of fission products.  All of this is made possible because you are dealing with a fluid reactor fuel rather than a solid fuel.

Here are two images of the salt recycle process developed for terrestrial molten-fluoride reactors at ORNL in the mid-1960s.  Not all of these steps would necessarily be a part of a space reactor system--depending on the level of recycle desired you would include or omit some.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Tap-Sa on 01/10/2006 06:21 pm
Couple questions (again :)):

1) Are the gaseous fission products/poisons also the major contributors of the decay heat?

2) In the diagrams there appears to be two different salt circuits in the reactor, core and blanket. Why this setup? I'm guessing that while the core is critical and glowing excess neutrons the blanket loop isn't critical but captures these neutrons transmutating Thorium into 233U, IOW breeding?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: JonClarke on 01/10/2006 07:31 pm
What's the half life of the gasesous fisson products, xenon and krypton?  How much are actually produced?

Jon
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/10/2006 07:38 pm
Quote
Tap-Sa - 10/1/2006  1:21 PM
1) Are the gaseous fission products/poisons also the major contributors of the decay heat?

2) In the diagrams there appears to be two different salt circuits in the reactor, core and blanket. Why this setup? I'm guessing that while the core is critical and glowing excess neutrons the blanket loop isn't critical but captures these neutrons transmutating Thorium into 233U, IOW breeding?

No, the gaseous fission products are not significant contributors to decay heat...not to say they are trivial also, but removing all the gaseous fission products does not drastically reduce your decay heating load.

This is an example of a two-fluid molten-fluoride reactor, with a core salt (LiF-BeF2-233UF4) and a blanket salt (LiF-BeF2-ThF4).  You're right on on the breeding.  The blanket absorbs the excess neutrons from the core (typically 2.3 neutrons are produced per absorption in 233U, but only 1 is required to sustain the chain reaction) and thorium will transmute to thorium-233, then rapidly decay to protactinium-233.  233Pa is a tricky thing, because it has a high neutron absorption cross-section and a 27-day half-life.  You don't want it to absorb a neutron before it decays to 233U, otherwise it will become 234Pa, and then decay to 234U, which is not fissile.

So here is another advantage of the molten-fluoride concept--you can isolate protactinium (because it is chemically different from thorium) and let it decay outside the reactor.  Once it has decayed to 233U, you can remove the uranium-233 from the mix through fluoridation (to UF6), and then add the new 233U to the core salt.  This diagram shows protactinium isolation from the blanket salt followed by fluorination to extract 233U.  Protactinium isolation isn't required (and probably wouldn't be done in space reactor) but if you don't isolate protactinium, you want to keep the neutron flux levels in the blanket down to keep from "frying" all the 233Pa you make before it has time to decay.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/10/2006 07:47 pm
Quote
JonClarke - 10/1/2006  2:31 PM

What's the half life of the gasesous fisson products, xenon and krypton?  How much are actually produced?
Boy, there's a bunch of different isotopes of xenon and krypton, some with half-lives of seconds and minutes, xenon-135 has a 9-hour half-life, krypton-85 has a ten-year half life.  There's also the stable isotopes of xenon and krypton that other decay chains decay into, and those gases build-up as well.  About 20% of the fission products are gaseous or pass through a gaseous state, I believe.  Atomic masses 84-90 either terminate on krypton or pass through a krypton state during their decay.  Atomic masses 129-138 either terminate on xenon or pass through xenon during their decay.

A fair amount of fission products can be removed from fluid-fuel reactors simply by virtue of these gaseous states.  It's a nice feature, especially because of the ability to remove xenon-135.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/11/2006 02:00 pm
One would intuitively suppose that fission yields two fission products of roughly equal mass, but that is not the case.  Fission almost always comes from an asymmetric mass distribution, which leads to two fission fragments of different masses.  Uranium needs substantially more neutrons than protons to maintain its quasi-stability (evidenced by its long half-life).  After fission, the fragments have too many neutrons and need to lose them to acheive stability.  Since fission products don't alpha-decay (which removed two protons and two neutrons) they beta-decay (which functionally changes a neutron into a proton and emits an electron).  Thus, typically after a few beta decays, a fission product reaches its stable condition.  This happens surprisingly fast...for most atomic masses the process is complete in a day or two.  Only a few have really long half-lives.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: truebeliever on 01/12/2006 02:21 am
In reference to removing the fission products, I saw a picture of Dr. Alvin Weinberg of ORNL standing next to several small metal cansiters of U-235 that had been in the operating MSRE reactor. It may have been in his book mentioned previously. They chemically removed all the U-235 and replaced it with U-233 and restarted the reactor. Weinberg remarked about how the U-235 he was next to had just been in the reactor several days previous. They must have had very efficient chemical recovery procedures. In his book he mentions that removal of the fission products should be just as easy as removing the U-235. A typical light water reactor only uses a small fraction (several percent) of its uranium inventory. The "waste" that is to be sent to Yucca mountain still has a lot of good fuel left in it. With a molten salt you would only remove the "ashes" so to speak, and put a small log back in to keep it going.

It would seem to me that by removing the fission produts during operation and disposing of them by sending them on a safe trajectory to the sun, you could reuse such a reactor many times for a Mars NEP mission. One thing I dislike about NTR is that once you use it, you have to throw it away.

Does anyone know it they did multiple restarts on any of the NERVA engines tested at Rocky Flats?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/12/2006 06:16 pm
Concerning the removal of uranium fuel (through fluorination) from a molten-fluoride reactor, here is an excerpt from the paper "Experience with the Molten-Salt Reactor Experiment" written by Paul Haubenreich and J.R. Engel of Oak Ridge National Labs, and published in the journal "Nuclear Applications and Technology" in February 1970.


"The shutdown on March 26, 1968 was the end of nuclear operation with 235U.  For several months, preparations had been underway to strip the uranium from the fuel salt and replace it with 233U. The uranium was to be removed on-site by the fluoride volatility process. For the 233U addition, equipment had been set up in the nearby Thorium-Uranium Recycle Facility (TURF) at ORNL to produce half a cubic foot of UF4-LiF eutectic containing 35 kg of 233U. (This uranium contained ~229 ppm 232U, which made it so radioactive as to practically prohibit any other use.

"In the first month of the shutdown the core specimens were replaced, and the fuel piping and vessels were surveyed using remote gamma-ray spectroscopy to determine the distribution of fission products. At the same time necessary maintenance was completed. This included repair of two heaters from the primary heat exchanger, rodding out the fuel off-gas line at the pump bowl and fishing (unsuccessfully) for mother sample capsule that had been accidentally dropped into the pump bowl. Attention then focused on the salt processing system where the system for disposal of excess fluorine was as yet untried.

"In the preparations for uranium recovery, difficulties were encountered in obtaining the required efficiency in disposal of fluorine by reaction with SO2 gas, and eventually the process was changed to reaction in a caustic solution. Processing of the flush-salt charge started on August 1; eleven days later the 6 kg of uranium in the salt had been fluorinated out as UF6 and collected on sodium fluoride pellets, corrosion-product fluorides had been reduced to filterable metals, and the salt had been filtered on its return to the reactor. Fluorination of the fuel salt to recover the 218 kg of uranium present took only 47 h of fluorine sparging over a 6-day period. Corrosion products were reduced and filtered in another 10 days. Two cubic feet of the stripped carrier salt, still loaded with fission products, was left in the processing vessel for a future test of a vacuum distillation process. The remainder was returned to the reactor.

"Loading of 233U began immediately. The bulk of the uranium required for criticality was loaded into the carrier salt through equipment attached temporarily to one of the drain tanks. Cans of frozen eutectic, containing up to 7 kg 233U each, were lowered into the hot tank where the salt melted and poured into the carrier salt. Neutron multiplication measurements were made with the salt in the core after the addition of 21, 28, and 33 kg uranium. After the last of these, the addition equipment was removed and the cell was closed and leak-tested before the addition of the remaining 400 g required for criticality. This amount was added in capsules through the sampler-enricher and on October 2 the MSRE was first critical with 233U fuel. Capsule additions were continued and on October 8, U.S. Atomic Energy Commission Chairman Seaborg took the reactor power to 100 kW for the first time after ceremonies marking the world’s first power operation with 233U fuel. Over the next month another 1.7-kg U was added while the control rods were calibrated and the temperature and concentration coefficients of reactivity were measured."
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/13/2006 12:32 pm
Is there interest in continuing this discussion and getting into the topic of power conversion systems?  Even though thermodynamics can seem daunting, I think I can describe the basic concepts in a way that most can understand?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Tap-Sa on 01/13/2006 02:37 pm
Quote
vanilla - 13/1/2006  3:32 PM

Is there interest in continuing this discussion and getting into the topic of power conversion systems?  

You bet there is! I've been looking for the conversion topic because in many discussions it gets totally forgotten. People talk only about reactor this reactor that while that's only half way to electricity.

Here's a first little question on the topic: has any other conversion method seen actual flight than thermocoupling?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/13/2006 03:45 pm
Quote
Tap-Sa - 13/1/2006  9:37 AM
Here's a first little question on the topic: has any other conversion method seen actual flight than thermocoupling?

No, only thermoelectrics have flown in space.  The US only flew one reactor (SNAP 10A) back in April 1965, and has flown a couple of dozen RTGs (which are not reactors), and they all used thermoelectric power conversion, with conversion efficiencies ranging from 1% up to about 5%.  Fairly inefficient devices whose primary virtue is no moving parts and long life.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/13/2006 05:45 pm
OK, power conversion.  Power conversion is basically the process where you convert random thermal energy into directed energy (like work, shaft power, or electricity).  Natural processes always move towards higher states of disorder, so it doesn't seem intuitive that you could take random thermal energy and convert it into a more highly ordered state.  And you can't.  But what you can do is convert some of the random energy into ordered energy with the penalty of having even more disorder in the end than you started with.

Confused?  Yeah I was too when I first took thermodynamics.  But believe me it's a lot worse when you use the technical terms for all these concepts (entropy, enthalpy, irreversability, isentropic heat engines) than when you explain it in more basic principles.

A long time ago there was a very very clever French guy named Sadi Carnot who wanted to know what the maximum amount of ordered energy was that you could extract from disordered energy.  I don't know how he figured this out, but he figured out that if you have two reservoirs of thermal (heat) energy, at two different temperatures, that nature put a limit on how much useful (ordered) energy you can extract from them.

So if you imagine having a basin of boiling water, and another basin of ice water, Carnot said that you could actually extract some of the thermal energy from the hot water, in an ordered form, by running a heat engine between those two reservoirs of temperature.  The key was the fact that they were at different temperatures.  If you dumped both basins of water into a bigger one and all the water mixed and was at one temperature, all the energy is still there, but there is no way to get it out.

The equation he came up with is incredibly simple:  the maximum theoretical efficiency of energy conversion that can be achieved is equal to 1 - the temperature of the low-temp reservoir divided by the temperature of the high-temp reservoir.

So back to our water analogy, the maximum amount of energy that could be extracted would be equal to 1 - 0 deg C/100 deg C...100% right?  Wrong...we have to use the absolute temperature scale, the Kelvin scale.  Now do it again.... 1- 273K/373K = 26.8%.  And that is the absolute best you can do.

So Carnot's equation says that to extract more ordered energy from two reservoirs of energy, you want the high-temp one to be as hot as possible, and the low-temp one to be as cold as possible.  Well, the background temperature of space is about 4K, so we should be all set, right?  Just about any temperature you choose will have high conversion efficiency.  Try room temperature:  1- 4K/295K = 98.6% conversion efficiency.

Well, that's right, and it's wrong...and the reason is radiation heat transfer, which I will get to in the next post.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Tap-Sa on 01/13/2006 08:38 pm
Quote
vanilla - 13/1/2006  8:45 PM
 
So if you imagine having a basin of boiling water, and another basin of ice water, Carnot said that you could actually extract some of the thermal energy from the hot water, in an ordered form, by running a heat engine between those two reservoirs of temperature.  The key was the fact that they were at different temperatures.  If you dumped both basins of water into a bigger one and all the water mixed and was at one temperature, all the energy is still there, but there is no way to get it out.

Allow me to play demon's advocate: what about Maxwell's Demon (http://en.wikipedia.org/wiki/Maxwell_demon)? I've read of attempts to build some sort of rectifiers (involving nanotechs, CNTs etc) that would turn ambient heat into electric current. Is this hopeless crackpot science?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: nacnud on 01/14/2006 09:19 am
No, it just shrinks the size of the bowls a lot ;)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/14/2006 03:09 pm
I'd never read about Maxwell's Demon before, but as a general rule, going up against the laws of thermodynamics is a losing proposition.  Doesn't stop thousands of fools each year from trying, however.

OK, back to power conversion.  I ended the previous post saying that space is cold and that to get good conversion efficiency we want the hot side hot and the cold side cold, as much as possible.  So every seems good.  But no.

The basic problem is rejecting all the heat that doesn't get converted to useful (ordered) energy.  Let's say we have a power conversion system that converts heat to electricity at 30% (which is pretty darn good).  Well, we still have to eliminate the other 70% of the heat at the lower temperature.  There are three ways to move thermal energy (heat)--conduction, convection, and radiation.  Conduction is when two bodies are in contact with one another and heat is transferred directly.  It's the way heat moves through solid bodies.  When you grab a cold rail with your bare hand, you get a quick lesson in conductive heat transfer.  Problem is there's nothing to conduct to in free space.  Same with convection, which is based on a fluid moving past a surface and picking up heat.  No, the only way to get rid of heat in space is radiation.

Radiation heat transfer is a very tricky concept to understand (I spent a lot of time in thermodynamics trying to understand it, but I'm slower than the average guy).  When you stand out in the Sun you feel hotter than when you step under some shade.  The air is the same temperature, right?  Yes, but when you're in the Sun the Sun is heating you up directly via radiation.  It's the same reason that you can feel so hot sitting next to a campfire that you hold your hand between your face and the campfire.  The campfire isn't heating the air between you and the fire--it's heating you via radiation.

Everything radiates heat, all the time.  In space, if more heat is getting radiated to you than you are radiating, you will heat up until you are in balance.  If you are radiating more heat than you're absorbing or generating, you will cool down until you are in balance.  If you are radiating the same amount of energy that you are absorbing and/or generating, you will stay at the same temperature.  Radiation heat transfer is the reason why the question "what temperature is it in space?" has no meaning.  The answer is more like "well, what color are you?"  If you put two objects of the same shape in the same point in space with the same orientation, depending on their color (and their radiative properties) they will assume different temperatures.

The amount of heat radiated by a body is proportional to the FOURTH power of its temperature.  So an object at 200 K and another object at 400 K don't differ in radiative heat transfer by a factor of 2, but by a factor of 16!  And this is why as you try to radiate heat at lower and lower temperatures, it becomes progressively more difficult.  So a spacecraft that wants to radiate heat away at 4K isn't going to be able to get rid of any heat this way.  It has to radiate at a higher temperature.  Nature will heat or cool a spacecraft to the point where its radiative heat transfer is balanced.  It might not be balanced at a temperature you want!  This is one of the reasons that spacecraft designers spend so much time on thermal design.

Remember Apollo 13?  They were going to the Moon and everything was fine, the spacecraft was at a pleasant temperature, and then they had their accident and they had to turn off all their systems.  The spacecraft got very cold.  But it was still pointed at the Sun, right?  Why didn't that warm it up?  Because the spacecraft had been designed to radiate all the heat away that the crew and computers and systems would generate when they were all operating, and when they shut everything off, all that heat that it would normally radiate away wasn't there anymore.  The spacecraft assumed a new temperature where heat in and heat out would balance.  Unfortunately for the crew that temperature was much too cold for comfort.

So a basic challenge for a power system designer is the need to reject heat at a high temperature, so as to reduce the area of radiators required...but that need runs right up against the thermodynamic principles described by Carnot, which say you want to reject heat at as low a temperature as possible to get maximum conversion efficiency.  So these two requirements are at opposition to one another, and the mark of a good power system (reactor, power conversion, radiator) is how well these conflicting requirements have been balanced.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Tap-Sa on 01/15/2006 12:53 pm
Sorry if I sidetrack again a little but since we are talking about thermodynamics; As you said the efficiency of heat engine gets better the bigger the temperature difference between hot and cold sink is. How about cooling, moving heat from cold sink to hot sink. Is the efficiency reverse to the heat engine case, the bigger the difference the more inefficient the cooling is? I ask because cryogenic cooling may be a necessity if we are to store LH2 in orbital fuel depots and other long time missions (LH2 in CEV SM?!).
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/15/2006 01:55 pm
Quote
Tap-Sa - 15/1/2006  7:53 AM

Sorry if I sidetrack again a little but since we are talking about thermodynamics; As you said the efficiency of heat engine gets better the bigger the temperature difference between hot and cold sink is. How about cooling, moving heat from cold sink to hot sink. Is the efficiency reverse to the heat engine case, the bigger the difference the more inefficient the cooling is? I ask because cryogenic cooling may be a necessity if we are to store LH2 in orbital fuel depots and other long time missions (LH2 in CEV SM?!).

Absolutely right.  Running a refrigeration system consumes energy, but you can actually remove more heat from the colder object than is consumed in the refrigerator.  But the refrigerator wants the temperature difference to be as SMALL as possible.  The larger the temperature difference, the more energy it takes to remove heat from the colder object.  For cryogenic cooling of something like LH2, this becomes very very difficult, especially considering that you will have to reject waste heat through radiation at low temperatures, with the inefficiency of radiators previously mentioned.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/15/2006 05:08 pm
The need to liquefy hydrogen gives me great pause both for long-duration NTR operations (utilization of the engine beyond the TMI burn), and for LH2/LOX ISRU on the Moon.  I have little doubt that the hydrogeneous material detected by Lunar Prospector is water ice, and I have little doubt we can get this water out of the regolith, electrolyze it to gaseous oxygen and gaseous hydrogen, and even liquefy the oxygen.  But liquefying the hydrogen--that will be rather difficult, because the heat that must be extracted from the gaseous hydrogen to get it to liquefy not only is substantial, but must be radiated to space at very very low temperatures.  Hydrogen liquefies at 20K, and if we were to reject heat at 100K, even with a perfectly emissive radiator to deep space, it would still be able to radiate only (1.0)*(5.67e-8 W/(m^2*K^4))*(100K)^4 = 5.6 watts per square meter.  That is a very very slow rate of cooling.

As an aside, hydrogeneous material (probably water) was detected on the Moon through the moderation of neutrons bouncing off the lunar surface--the same process that goes on in the moderating material in a nuclear reactor.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/16/2006 02:53 am
OK, once more into the world of thermodynamics we plunge.  I should hasten to mention to those who haven't had thermodynamics that I'm really skipping a lot of steps here.  You typically spend the entire first semester just getting the basic principles down, like conservation of energy, enthalpy, phase change, entropy, and so forth.  Not until the second semester do you typically get into power cycles.  And that's where we're plunging right in.

OK, Carnot's ideal power conversion cycle is designed to convert the maximum fraction of disordered thermal energy, at a high temperature, to ordered energy (shaft power, electricity, etc) by rejecting the remainder of the energy to another disordered thermal energy reservoir at lower temperature.  The real trick about the Carnot cycle is that, in theory, this fraction of ordered energy can be extracted without any increase in total entropy (disordered states).

That said, you can't really do it in real life.  But ideal cycles are very useful because they provide a benchmark, a line in the sand that can't be beaten.  And so you can figure out just about how good your cycle is by comparing it to the ideal cycle and seeing how close you get.  For instance, there's a concept called OTEC (ocean thermal-electric conversion) that's based on running a heat engine between the warm ocean waters at the surface and the deep cold waters thousands of feet down.  Let's say you got 3% conversion efficiency...you might be somewhat disappointed, but then if you checked the Carnot efficiency between these two water temperatures and it said that the maximum conversion efficiency was 4%, you'd be feeling like you did pretty darn good.

So it is with any power cycle.  We benchmark against the Carnot cycle, knowing we can never do any better than that.  One of the basic principles of the Carnot cycle is that heat should be added isothermally (at one temperature) and heat should be rejected isothermally.  Well, that kind of flies in the face of reality.  If you add heat to a gas or a liquid, it will increase in temperature, and then obviously that heat addition process is not isothermal (constant temperature).  But nature has given a little trick for adding or removing heat without changing temperature--phase change.

You stick a thermometer in a pot of water and put it on the stove.  If you're at sea level, the temperature of the water will keep climbing until it reached 100C, and then it will begin to boil.  But it won't get any hotter.  In fact, the temperature will not increase at all--but the water will all boil away.  Similarly, if you cool the water it will get down to 0C, but it won't get any colder--it will turn to ice.  So phase change is a very useful way to make an efficient power conversion cycle.

A power cycle based on the phase change of the working fluid is called the Rankine cycle.  It is commonly used in coal and nuclear power plants, with water as the working fluid.  The basic limitation of the Rankine cycle is that working fluid itself.  It only has a certain temperature range over which it can operate as a liquid, so by choosing your working fluid, in a sense, you're choosing your temperature range.  But within that limited temperature range, the Rankine cycle can do very well indeed, and come pretty close to the Carnot ideal.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: realtime on 01/16/2006 05:48 am
Quote
vanilla - 15/1/2006  1:08 PM

The need to liquefy hydrogen gives me great pause both for long-duration NTR operations (utilization of the engine beyond the TMI burn), and for LH2/LOX ISRU on the Moon.  I have little doubt that the hydrogeneous material detected by Lunar Prospector is water ice, and I have little doubt we can get this water out of the regolith, electrolyze it to gaseous oxygen and gaseous hydrogen, and even liquefy the oxygen.  But liquefying the hydrogen--that will be rather difficult, because the heat that must be extracted from the gaseous hydrogen to get it to liquefy not only is substantial, but must be radiated to space at very very low temperatures.  Hydrogen liquefies at 20K, and if we were to reject heat at 100K, even with a perfectly emissive radiator to deep space, it would still be able to radiate only (1.0)*(5.67e-8 W/(m^2*K^4))*(100K)^4 = 5.6 watts per square meter.  That is a very very slow rate of cooling.

As an aside, hydrogeneous material (probably water) was detected on the Moon through the moderation of neutrons bouncing off the lunar surface--the same process that goes on in the moderating material in a nuclear reactor.
Hydrogen is liquefied using refrigerants and compression/expansion loops, not through direct radiation.  If we're talking about an industrial operation here, then the necessary refrigerants will be boosted from Earth or an alternate method will be developed that can take advantage of the extreme cold.  With the temperature at the lunar poles only a few tens of kelvins, once you produce the stuff it should be relatively easy to keep it from boiling off.

Indeed, if H3 mining operations ever become an economic reality, most of them won't be located at the poles.  Large amounts of regolith will need processing and plenty of H3 will probably be found at lower latitudes, associated with ilmenite deposits.  When H3 is shipped Earthside, it will be in liquid form.  They'll use a process that does not require polar temperatures.

There are a lot of very smart people working this problem and there's some lead time before the technology must be ready to go.  I find it hard to believe that this is a problem that won't be solved in time.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/16/2006 01:02 pm
Quote
realtime - 16/1/2006  12:48 AM
Hydrogen is liquefied using refrigerants and compression/expansion loops, not through direct radiation.  If we're talking about an industrial operation here, then the necessary refrigerants will be boosted from Earth or an alternate method will be developed that can take advantage of the extreme cold.  With the temperature at the lunar poles only a few tens of kelvins, once you produce the stuff it should be relatively easy to keep it from boiling off.

There are a lot of very smart people working this problem and there's some lead time before the technology must be ready to go.  I find it hard to believe that this is a problem that won't be solved in time.
Hydrogen is liquefied in a cascade process, typically using liquid nitrogen to remove heat at about 80-100K.  But all the heat is ultimately rejected at ambient temperatures of about 300 K, typically to river water.  On the Moon there will be no such resource, nor in deep space.  Direct radiation is the ONLY way to get rid of heat.  If we choose to ramp the heat all way up to 300K before rejecting it, it will take much more electrical energy but less radiator area.  If we reject at lower temperatures, radiator area will be increased but power consumption will be reduced.  But don't delude yourself, it will not be easy.

It is tempting to think that on the lunar surface, at the poles, that the heat could be rejected through direct conduction to the regolith, but the ceramic nature of lunar soils and its porous structure makes it an excellent insulator rather than a thermal conductor.

There's no one working on this problem right now, realtime, and physics are physics.  There's far too many space projects that have begun with willful ignorance of the challenge of the physical problem before them and burned through billions in futile attempts to wish their technical problems away.  The number of smart people working on a problem has nothing to do with the basic mechanisms involved.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: realtime on 01/16/2006 05:35 pm
Quote
vanilla - 16/1/2006  9:02 AM

Quote
realtime - 16/1/2006  12:48 AM
Hydrogen is liquefied using refrigerants and compression/expansion loops, not through direct radiation.  If we're talking about an industrial operation here, then the necessary refrigerants will be boosted from Earth or an alternate method will be developed that can take advantage of the extreme cold.  With the temperature at the lunar poles only a few tens of kelvins, once you produce the stuff it should be relatively easy to keep it from boiling off.

There are a lot of very smart people working this problem and there's some lead time before the technology must be ready to go.  I find it hard to believe that this is a problem that won't be solved in time.
Hydrogen is liquefied in a cascade process, typically using liquid nitrogen to remove heat at about 80-100K.  But all the heat is ultimately rejected at ambient temperatures of about 300 K, typically to river water.  On the Moon there will be no such resource, nor in deep space.  Direct radiation is the ONLY way to get rid of heat.  If we choose to ramp the heat all way up to 300K before rejecting it, it will take much more electrical energy but less radiator area.  If we reject at lower temperatures, radiator area will be increased but power consumption will be reduced.  But don't delude yourself, it will not be easy.

It is tempting to think that on the lunar surface, at the poles, that the heat could be rejected through direct conduction to the regolith, but the ceramic nature of lunar soils and its porous structure makes it an excellent insulator rather than a thermal conductor.

There's no one working on this problem right now, realtime, and physics are physics.  There's far too many space projects that have begun with willful ignorance of the challenge of the physical problem before them and burned through billions in futile attempts to wish their technical problems away.  The number of smart people working on a problem has nothing to do with the basic mechanisms involved.

If we are to be honest with ourselves we must admit that we know little of the details of what ISRU engineering will look like in ten or fifteen years.  We can make guesses, that's all.  Some actually might point out challenges to be overcome and some may coincide with gross trends.  

But to definitively say it can only be done one way and no other way is possible?  And to make such an open-ended statement without experimental data?  That's not good science.

Many are the self-satisfied and venerable scientists who have allowed arrogant declarations to pass their lips only to be proven publicly and spectacularly wrong.  At best, they become ridiculous footnotes in science history.  At worst, they are vilified for standing in the way of scientific advancement.  Unless you wish to join their ranks, you should consider opening your mind to other possibilities.  They might surprise and delight you.

Real engineers use their imaginations.  For instance, the heat could be used to melt the lunar ice, or rejected into underground cisterns of water used by the miners, or used to run Sterling engines, or converted to power and stored in flywheels or monster caps and transmitted to outposts Darkside.  All of these examples likely have some fatal flaw; they are meant only to illustrate that there are a multiplicity of solutions if one is willing to use cross-disciplinary, unconventional, and non-linear means.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/17/2006 01:01 am
Yes, realtime, every single one of your ideas is fatally flawed.  And I don't have to build them to know that because they violate thermodynamics.  So before you insult my abilities as an engineer, why don't you spend some time with a thermodynamics and heat transfer text going over some basic principles:

1.  energy is always conserved (first law)
2.  energy is always degraded (second law)
3.  heat always flows from hotter to colder
4.  heat flows because of a temperature difference through conduction, convection, or radiation

I've already spent too much of my career watching people waste billions on ideas that won't work and can be shown to fail with a pencil and paper.  But you seem to want to wish for the impossible.  Be my guest.  But don't cry when non-physical fantasies don't come true.  Let's go to the Moon and build a hydrogen liquefier without a realistic way to reject waste heat and see how long it operates.  Oops, we'll probably have burned through a few billion by the time we figured out that a realistic thermodynamic analysis would have saved us a lot of time and trouble.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: braddock on 01/17/2006 01:24 pm
This thread has had over 2400 reads by an audience of hundreds; not everyone is going to take a parametric physics-based view of the world, at least not without the possibility of reasonable consideration of creative engineering solutions.

So, how do you size these radiators once you choose an exhaust heat temperature?  How does the temperature effect radiator mass and length?  I assume the higher temperature would require a thicker radiator for heat conduction?  Or does the higher temperature actually need less mass because the heat is radiated faster?  Or is there some balancing point?  Can you shade the sun-side of the radiators, or do you have to keep them oriented so that they are parallel with the direction towards the sun?

Many people on here believe that nuclear-in-space loses most benefits because of the radiator mass.  Where is the balance?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Chris Bergin on 01/17/2006 01:57 pm
I've got zero tolerance policy for people showing disrespect. Post deleted and some comments deleted.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: truebeliever on 01/18/2006 02:16 am
The size of the radiators is simply determined by Q''' = sigma * Area * ( Trad ^4 - T space ^4). Usually Tspace ~ 0. Note the sigma is the Stefan-Boltzmann constant. Therein lies the problem: if you reduce the temperature at which you reject the heat (ie T rad), the area needed for the same amount of heat rejection Q'' INCREASES by a factor of 2 ^ 4 = 16. Thi s i s why the liquid metal rankine cycle looks so attractive. It rejects heat at approximately 900K versus 425K for a Brayton, hence the radiator area is REDUCED by a factor of 16. Unfortunately, NASA killed the NRA on this work.

Vanilla has a very good point about NTR's. I had not given much thought to the heat rejection for keeping the hydrogen liquified. Even if it is only several hundred watts, if it has to reject at a very low temperature (I.e < 200-300 K), then these radiators could get very large. Typical values for radiator masses are given in paper studies as 10-15 kg/m2. Present day radiator techology is in the 20-30 kg/m2 range. You can see that it does not take much surface area to get a pretty hefty mass increase. The reason they are so heavy is that you need structural support so they don't flex, protection against micrometorites, etc.

The NTR's will also need fairly good size radiator to remove the decay heat after shutdown. NEP also needs this during operation and during shutdown. This  was one of the problems with JIMO. The reactor was  probably one of the easier parts of the vehicle. The other technologies: power conversion, high temperature radiators, long life ion engines were much less developed than the reactor. After all, we have been building small submarine and research reactors for almost 40 years now. The cost of the mission was also large due to the high cost of doing business with the DOE and Naval Reactors. They tend to view space reactor development as a "jobs'" program rather than being interested in NASA's mission. NASA should do it in-house. It might be cheaper, if you could get good technical people on it, and keep the empire builders away.

If HQ thought JIMO was too expensive, wait until they get some REAL world NTR development cost estimates, and I don't mean the BS numbers coming out of GRC.   You can't go to MARS with an NTR until you have a surface power reactor, but that has been killed as well.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: CuddlyRocket on 01/18/2006 05:54 am
Quote
vanilla - 17/1/2006  2:01 AM

Let's go to the Moon and build a hydrogen liquefier without a realistic way to reject waste heat and see how long it operates.
If we were thinking of electrolysing the theorised ice that may have collected in permanently shaded craters at the lunar poles, then some of the heat would be useful in melting the ice (you can only electrolyse water). As for removing the rest of the heat, I would have thought that the rocks of a permanently shaded crater would be a good heat sink.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: braddock on 01/18/2006 12:13 pm
Quote
truebeliever - 17/1/2006  10:16 PM

The size of the radiators is simply determined by Q''' = sigma * Area * ( Trad ^4 - T space ^4).
...
Present day radiator techology is in the 20-30 kg/m2 range. You can see that it does not take much surface area to get a pretty hefty mass increase. The reason they are so heavy is that you need structural support so they don't flex, protection against micrometorites, etc.

If it is only the matter of the exposed radiating area, though, can the radiators just be an extended sheet of foil?  What dictates the thickness?  Is it just so that the radiator system has the thermal mass to absorb short bursts of excess heat?

For a lunar base, I could almost imagine the need to develop some sort of thermally conductive in-situ "lunar concrete"...pave a few football fields of the surface with concrete and place your reactor/processing center/LH2 refrigerator in the middle?  Make the concrete massive enough to absorb excess heat in the long day, and radiate it at night.  But dreaming only gets so far.... :)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: JonClarke on 01/18/2006 08:40 pm
people tend to overlook radiators when rapsodising about nuclear power in space.  As a rough rule of thumb a reactor will produce 10-20 times as much heat as electricity.  So the 100 kW reactor that is touted as a baseline for Moon and Mars applications will need to be able to shed 1-2 mW of heat.

One argument that is used against solar power for Mars applications is the problem of dust.  But thjis will also be a major problem with radiators.  Depending on how hot the radiators get, you may start inducing physiochemical changes in the dust on them as well.

Jon
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/21/2006 09:57 pm
Quote
vanilla - 7/1/2006  10:17 AM

Doing it this way has some real risks, though.  In 2002, the Davis-Besse nuclear plant in Toledo Ohio was found to have suffered severe corrosion of its steel pressure vessel from boric acid.  The acid (which is a weak acid) had managed to eat through 6 inches of steel.  Had it gone much longer, it would have eaten through enough of the vessel that the internal pressure of the water (~3000 psi) would have caused a rupture of the vessel.  The water would have drained out of the core.  Fission would have stopped, but decay heat would have led to a core meltdown.  Even at Three-Mile-Island, the pressure vessel had not failed (they had mistakenly drained the core of water) and they were able to refill the vessel with water.  At Davis-Besse, had the vessel ruptured, they would not have been able to get water back into the vessel and cool the fuel since it would just drain out the breach.

Scary stuff.  All of these are non-problems in molten-fluoride reactors, because they don't need any excess reactivity.  You can refuel them anytime you want.  No burnable poisons, boric acid, gadolinium, and you get all the safety features we need in a reactor.  I'll talk more about this in the next post.  Here's a story on the Davis-Besse almost accident.

http://www.pennlive.com/news/patriotnews/index.ssf?/news/tmi/stories/nationsaging.html

http://www.toledoblade.com/apps/pbcs.dll/article?AID=/20060121/NEWS02/60121001/-1/RSS

A record $28 million fine has been levied against the utility for falsifying information about the inspection of the reactor.  This boric acid corrosion is caused by the use of boron for reactivity control, something you have to do in solid-core reactors because there's no way to add fuel during operation--you have to start out with more fuel than you need and "waste" neutrons through boron absorption throughout the fuel cycle (typically a year).  This is not a problem in molten-fluoride reactors because of the ability to add fuel thoughout operation, eliminating the need for excess reactivity.  This same capability has very big implications for space reactors as well, because solid-core space reactors cannot be refueled throughout their expected operational life.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/27/2006 09:22 pm
Quote
vanilla - 16/1/2006  8:02 AM

Hydrogen is liquefied in a cascade process, typically using liquid nitrogen to remove heat at about 80-100K.  But all the heat is ultimately rejected at ambient temperatures of about 300 K, typically to river water.  On the Moon there will be no such resource, nor in deep space.  Direct radiation is the ONLY way to get rid of heat.  If we choose to ramp the heat all way up to 300K before rejecting it, it will take much more electrical energy but less radiator area.  If we reject at lower temperatures, radiator area will be increased but power consumption will be reduced.  But don't delude yourself, it will not be easy.
Here is a process diagram of a hydrogen refrigeration system that uses the Claude cycle, which is a variation of Kapitza's groundbreaking work using expanders to extract work from the pressurized gas (thus cooling it isentropically) rather than simply using a Joule-Thomson valve to get isenthalpic cooling.  Kapitza's real breakthrough was to recognize that most of the stream wouldn't be liquefied anyway, and by running most of the stream through the expander rather than the J-T valve, he could get much more efficient cross-current cooling of the smaller stream that would go through the J-T valve (of which some fraction would be liquefied).  Kapitza's breakthrough worked so well that for years it was considered "black magic" rather than fantastic engineering borne of a fundamental grasp of thermodynamics.

The image is from page 99 of Randall Barron's book "Cryogenic Systems".
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: simonbp on 01/28/2006 12:38 am
So, the basic problem with NTP is that both the LH2 propellant and the reactor itself have massive cooling requirements?

Simon ;)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/28/2006 01:13 am
I would say that the issues with hydrogen boiloff would probably preclude the use of the NTR engine beyond the trans-Mars injection (TMI) burn.  Note that the JSC studies that included NTR (DRM 1.0 and 3.0) did not use the NTR beyond TMI.  Borowski's studies at GRC, trying to maximize the importance of the NTR, use the engine for TMI, Mars orbit insertion, and trans-Earth injection.  He claims the bimodal mode will produce the power to drive the cryocoolers to keep the hydrogen refrigerated.  I do not think he understands the magnitude of the problem, especially since when I asked NASA's best mind in "zero-boiloff" cryogen storage if hydrogen could be stored over three years in heliocentric space, he laughed in my face and told me I must not be serious.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: simonbp on 01/28/2006 07:28 pm
So, a good configuration would be Nuclear/LH2 Trans-Mars Injection, Aerobraking for capture, and Chemical (with in-situ propellants or otherwise) for Trans-Earth Injection?

(Meaning the architecture hinted at in the ESAS report)

Simon ;)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/28/2006 09:41 pm
Quote
simonbp - 28/1/2006  2:28 PM

So, a good configuration would be Nuclear/LH2 Trans-Mars Injection, Aerobraking for capture, and Chemical (with in-situ propellants or otherwise) for Trans-Earth Injection?

(Meaning the architecture hinted at in the ESAS report)

Simon ;)
Here we have the paradox of the nuclear thermal rocket.  If you go through the cost and development hell to build it, then you want to use it for as many mission phases as possible, like Borowski's work.  But there are very good reasons to eschew its use beyond the trans-Mars injection burn, the primary concern being hydrogen storage.  Borowski, somewhat naively in my opinion, proposes to attack the hydrogen storage problem through active refrigeration.  But that requires that the NTR now be a "bimodal" NTR, capable of power generation as well. which is well beyond any design or concept put forward during the NERVA program.  Even Borowski is left proposing some speculative Russian engine which relies on a core configuration, cooling channels, and fuel forms we've never even manufactured.

The slippery slope...  Well, as you know, I think the development of even a simple NTR engine is going to be a financial, political, and technical nightmare for very little real improvement over chemical.  Now with this L2-based architecture work I've discovered, this seems to put even more of a nail in the NTR coffin.  For by starting from this high-energy location, the remaining amount of DV needed to be supplied by the engine at TMI is in the 600-800 m/s range instead of the 3800-4000 m/s range.  With so little DV, it's really hard for an NTR to show a radical improvement over a chemical stage, especially one that might be supplied by lunar propellants.  And that really reduces the last justification for the NTR engine.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: simonbp on 01/29/2006 01:46 am
Quote
vanilla - 28/1/2006  4:41 PM
 For by starting from this high-energy location, the remaining amount of DV needed to be supplied by the engine at TMI is in the 600-800 m/s range instead of the 3800-4000 m/s range.  With so little DV, it's really hard for an NTR to show a radical improvement over a chemical stage, especially one that might be supplied by lunar propellants.  And that really reduces the last justification for the NTR engine.

But how do you get to that high-energy (which I assume means GEO+) orbit in the first place? IIRC the major problem with NEP (beyond the huge number of launches required) was the ~20 month trip from LEO to GEO, mostly through the van Allen belts...

Personally, I like the idea of simple, high thrust, single use nuclear thermal stages that be used to chuck large payloads on a Trans-Mars trajectory from LEO, essentially a nuclear S-IVB...

Simon ;)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/29/2006 02:48 am
Quote
simonbp - 28/1/2006  8:46 PM

But how do you get to that high-energy (which I assume means GEO+) orbit in the first place? IIRC the major problem with NEP (beyond the huge number of launches required) was the ~20 month trip from LEO to GEO, mostly through the van Allen belts...

Personally, I like the idea of simple, high thrust, single use nuclear thermal stages that be used to chuck large payloads on a Trans-Mars trajectory from LEO, essentially a nuclear S-IVB...

Simon ;)
I think the MXER tether concept looks very attractive for throwing payloads to a trans-lunar injection, with a chemical stage being the second place option.  These are high-thrust systems that will transfer payloads in just a few days.  I have no doubt a tether system would be simpler and cheaper to build than an NTR, which will be neither simple or cheap.

If you did have a NEP vehicle, starting directly from L2, your departure spiral would only be a few days before you would be on your heliocentric trajectory.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Tap-Sa on 01/29/2006 01:08 pm
Quote
vanilla - 28/1/2006  12:22 AM
 
Here is a process diagram of a hydrogen refrigeration system that uses the Claude cycle, which is a variation of Kapitza's groundbreaking work using expanders to extract work from the pressurized gas (thus cooling it isentropically) rather than simply using a Joule-Thomson valve to get isenthalpic cooling.  

This sent me to googling more info about JT valve, couldn't find much. Is it simply a pressure relief valve or is there something more special in it? I bumped into this paper (http://www.mpptech.com/techpp/joule-thomson.PDF) that promotes two-phase turbines as JT valves for added efficiency.

Found also a nice summary (http://www.mars-lunar.net/Hydrogen%20Storage/Hydrogen.pdf) about hydrogen storability. A monthly boil-off rate of 3-7% for passive tanks seems doable. I'm surprised that most of the calculations assume ~300K vacuum temperature. Is this the temperature that an object doing BBQ-rolling near Earth assumes?

A peculiar tidbit about hydrogen; if it's let into expander at room temperature it heats up instead of cooling down (negative Joule-Thompson effect (http://en.wikipedia.org/wiki/Joule-Thompson_effect)). Therefore precooling with LN2 is necessary to make further cooling work.

There's also the problem of different forms of H2 molecules, para and ortho. Something to do with atom spins. Most hydrogen at room temp wants to be ortho-form but nearly all would be para-form at boiling point. Conversion from ortho to para is exothermic. I've read from several sources that some catalyst is used to convert the form during hydrogen liquefaction, does anyone have any more details about this?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/29/2006 01:37 pm
Quote
Tap-Sa - 29/1/2006  8:08 AM
does anyone have any more details about this?

I would highly recommend Randall Barron's book, Cryogenic Systems, which covers each of these topics in excellent detail.  If you have a basic background in thermodynamics, you will probably be able to do quite well in the book.  It's a little expensive, but here it is both at Amazon and Alibris:

At Amazon (http://www.amazon.com/gp/product/0195035674/qid=1138544637/sr=2-1/ref=pd_bbs_b_2_1/002-9041506-0749601?s=books&v=glance&n=283155)

At Alibris (http://www.alibris.com/search/search.cfm?qwork=1415666&wtit=cryogenic%20systems&matches=15&qsort=r&cm_re=works*listing*title)

I will try to pull some excerpts from the book covering the topics you asked about.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/29/2006 01:49 pm
Quote
Tap-Sa - 29/1/2006  8:08 AM
This sent me to googling more info about JT valve, couldn't find much. Is it simply a pressure relief valve or is there something more special in it? I bumped into this paper (http://www.mpptech.com/techpp/joule-thomson.PDF) that promotes two-phase turbines as JT valves for added efficiency.
Wow!  We were always taught in thermodynamics that you needed very high "quality" in your vapor as you went through the expansion turbine, otherwise the blades would be blasted away by condensed particles.  If someone has figured out a way to solve that problem, there is enormous money to be made in the vapor-turbine industry, which is basically every steam turbine plant on Earth.  The ease and efficiency of liquefaction processes would be dramatically increased as well.  Remarkable work.

On the other hand, it doesn't really fix the basic problem I see with active refrigeration of the hydrogen, namely the enormous radiator resulting from the very low heat rejection temperature.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Stowbridge on 01/29/2006 05:10 pm
This is all very interesting. Is there a starting point to learn about nuclear propulsion so that some of this makes sence to those of us that don't have degrees in this?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Chris Bergin on 01/29/2006 05:13 pm
Hold that thought. I want to give all of this a section of its own. Bare with me as it'll be tonight all being well. That sort of question would deserve a thread of its own.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/29/2006 05:26 pm
Quote
Stowbridge - 29/1/2006  12:10 PM

This is all very interesting. Is there a starting point to learn about nuclear propulsion so that some of this makes sence to those of us that don't have degrees in this?

I would recommend buying or checking out from the library Space Nuclear Power (http://www.amazon.com/gp/product/0894640003/sr=1-1/qid=1138558435/ref=pd_bbs_1/002-9041506-0749601?%5Fencoding=UTF8) which was written back in 1985 but still has some of the best information about the different space power and propulsion concepts.  Unfortunately, not much has changed in the space nuclear world since 1985, so this book's still pretty up to date.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: no_bull on 01/31/2006 01:37 am
I second that "Space Nuclear Power" is a very good book. It is still very relevant.
Nothing has changed much from the SEI days :(

Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/31/2006 03:03 am
Quote
Tap-Sa - 29/1/2006  8:08 AM
There's also the problem of different forms of H2 molecules, para and ortho. Something to do with atom spins.

From Barron's book "Cryogenic Systems", pages 47-50.

2.13. Hydrogen
Liquid hydrogen has a normal boiling point of 20.3 K (36.5°R) and a density at the normal boiling point of only 70.79 kg/m3 (4.42 lb/ft3). The density of liquid hydrogen is about one-fourteenth that of water; thus, liquid hydrogen is one of the lightest of all liquids. Liquid hydrogen is an odorless, colorless liquid that alone will not support combustion. In combination with oxygen or air, however, hydrogen is quite flammable. Experimental work (Cassutt et al. 1960) has shown that hydrogen-air mixtures are explosive in an unconfined space in the range from 18 to 59 percent hydrogen by volume.

Natural hydrogen is a mixture of two isotopes: ordinary hydrogen (atomic mass = 1) and deuterium (atomic mass = 2). Hydrogen gas is diatomic and is made up of molecules of H2 and HD (hydrogen deuteride) in the ratio of 3200:1. A third unstable isotope of hydrogen exists, called tritium; however, it is quite rare in nature because it is radioactive with a short half life.

One of the properties of hydrogen that sets it apart from other substances is that it can exist in two different molecular forms: ortho-hydrogen and para-hydrogen. The mixture of these two forms at high temperatures is called normal hydrogen, which is a mixture of 75 percent ortho-hydrogen and 25 percent para-hydrogen by volume. The equilibrium (catalyzed) mixture of o-H2 and p-H2 at any given temperature is called equilibrium-hydrogen (e-H2). The equilibrium concentration of p-H2 in e-H, as a function of temperature is given in Table 2.7. At the normal boiling point of hydrogen (20.3 K or 36.5°R), equilibrium hydrogen has a composition of 0.20 percent o-H2 and 99.80 percent p-H2. One could say that it is practically all para-hydrogen.

The distinction between the two forms of hydrogen is the relative spin of the particles that make up the hydrogen molecule. The hydrogen molecule consists of two protons and two electrons. The two protons possess spin, which gives rise to angular momentum of the nucleus, as indicated in Fig. 2.15. When the nuclear spins are in the same direction, the angular momentum vectors for the two protons are in the same direction. This form of hydrogen is called ortho-hydrogen. When the nuclear spins are in opposite directions, the angular-momentum vectors point in opposite directions. This form of hydrogen is called para-hydrogen.

Deuterium can also exist in both ortho and para forms. The nucleus of the deuterium atom consists of one proton and one neutron, so that the high-temperature composition (compositon of normal deuterium) is two¬thirds ortho-deuterium and one-third para-deuterium. In the case of deuterium, p-D2 converts to o-D, as the temperature is decreased, in contrast to hydrogen, in which o-H2 converts to p-H2 upon decrease of temperature. The hydrogen deuteride molecule does not have the symmetry that hydrogen and deuterium possess; therefore, HD exists in only one form.

As one can see from Table 2.7, if hydrogen gas at room temperature is cooled to the normal boiling point of hydrogen, the o-H2 concentration decreases from 75 to 0.2 percent; that is, there is a conversion of o-H2 to p-H2 as the temperature is decreased. This changeover is not instantaneous but takes place over a definite period of time because the change is made through energy exchanges by molecular magnetic interactions. During the transition, the original o-H2 molecules drop to a lower molecular-energy level. Thus the changeover involves the release of a quantity of energy called the heat of conversion. The heat of conversion is related to the change of momentum of the hydrogen nucleus when it changes direction of spin. This energy released in the exothermic reaction is greater than the heat of vaporization of liquid hydrogen, as one can see from the tabulated values of the heat of conversion and heat of vaporization shown in Appendix D.2.

When hydrogen is liquefied, the liquid has practically the room-temperature composition unless some means is used to speed up the conversion process. If the unconverted normal hydrogen is placed in a storage vessel, the heat of conversion will be released within the container, and the boil-off of the stored liquid will be considerably larger than one would determine from the ordinary heat inleak through the vessel insulation. Note that the heat of conversion at the normal boiling point of hydrogen is 703.3 kJ/kg (302.4 Btu/lbm) and the latent heat of vaporization is 443 kJ/kg (190.5 Btu/lbm). The conversion process evolves enough energy to boil away approximately I percent of the stored liquid per hour, so the reaction would eventually result in much of the stored liquid being boiled away. For this reason, a catalyst is used to speed up the conversion so that the energy may be removed during the liquefaction process before the liquid is placed in the storage vessel.

These two forms of hydrogen have different specific heats because of the different weighting of the energy levels of the two forms, as indicated in Fig. 2.16. Because of this difference in specific heats, other thermal and transport properties are also affected. For example, para-hydrogen gas has a higher thermal conductivity than ortho-hydrogen gas because of the higher specific heat of p-H2.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/31/2006 08:51 pm
Quote
Tap-Sa - 29/1/2006  8:08 AM

Conversion from ortho to para is exothermic. I've read from several sources that some catalyst is used to convert the form during hydrogen liquefaction, does anyone have any more details about this?
More from "Cryogenic Systems" pages 93-103.  Note that he mentions that an efficient hydrogen liquefaction system on Earth takes roughly 11 MW of electrical power per kilogram/second of liquid hydrogen generated.


LIQUEFACTION SYSTEMS FOR NEON, HYDROGEN, AND HELIUM

3.15. Precooled Linde-Hampson system for neon and hydrogen

Because of its simplicity, the Linde-Hampson system is quite desirable for small-scale liquefaction plants. We have seen, however, that the basic Linde-Hampson system with no precooling would not work for neon, hydrogen, or helium because the maximum inversion temperature for these gases is below ambient temperature. With the precooled system, the temperature of the gas entering the basic Linde-Hampson part of the liquefier can be lowered below ambient temperature by choosing the correct fluid to precool the system.

In principle, any fluid that has a triple-point temperature below that of the maximum inversion temperature of neon or hydrogen could be used as a precoolant. Checking Table 2.6, we see that such fluids would include fluorine, oxygen, air, methane, argon, and nitrogen. The first four can be ruled out from a practical consideration because of their explosion hazard. Argon would be a possibility; however, it is generally more expensive than liquid nitrogen. This leaves liquid nitrogen as the choice for the precoolant for hydrogen- and neon-liquefaction systems.

A liquid-nitrogen-precooled Linde-Hampson system is shown schematically in Fig. 3.24. For small laboratory liquefiers, the nitrogen-liquefaction subsystem would be replaced by a small storage vessel from which liquid nitrogen could be withdrawn and passed through the precooling bath, and the vapor discnarged through the three-channel heat exchanger to the atmosphere. For large-scale systems, an economic study should be made to determine whether a separate nitrogen-liquefaction plant should be used or not.

From our discussion in Sec. 3.6, we observed that the liquid yield for the precooled Linde-Hampson system could be improved by lowering the temperature at the entrance to the cold heat exchanger (point 4 in Fig. 3.24). This can be accomplished easily in the hydrogen- or neon-liquefaction system by lowering the pressure in the liquid-nitrogen bath. Because the liquid nitrogen boils in the precoolant bath, a reduction in pressure lowers the boiling-point temperature or the bath temperature. There is a practical limit to this process of lowering the bath temperature, however. At 63.2 K (113.7°R) liquid nitrogen solidifies under its own vapor pressure (this is the triple point to nitrogen), and further reduction in pressure results in solid nitrogen in the bath instead of liquid. Good thermal contact is difficult to achieve between solid nitrogen and the heat-exchanger walls because a layer of vapor forms between the solid and the exchanger walls. This phenomenon limits the precoolant bath temperature to values above 63.2 K. The precoolant boil-off parameter z/y is shown in Fig. 3.25 as a function of temperature of the precoolant bath, assuming a reversible system.

3.16. Claude system for hydrogen or neon

The Claude system does not depend primarily on the expansion valve to produce low temperatures. Therefore, the system discussed in Sec. 3.9 may be used for hydrogen or neon without modification. The performance is improved, however, if a liquid-nitrogen precooling bath is used with the Claude system, as shown in Fig. 3.26. With the liquid-nitrogen precooling, the Claude system for hydrogen production has a figure of merit 50 to 75 percent higher than that of the precooled Linde-Hampson system.

3.17. Helium-refrigerated hydrogen-liquefaction system

An auxiliary helium-gas refrigerator can be used to condense hydrogen or neon, as shown in Fig. 3.27. In this system, hydrogen or neon is compressed, precooled by a liquid-nitrogen bath to reduce the helium-refrigerator work requirement, and finally condensed by heat exchange with cold helium gas. The helium refrigerator is a modified Claude system in which the gas is not liquefied but is still colder than liquid hydrogen. The helium is compressed, precooled in the liquid-nitrogen bath, and expanded in an expander to produce the low temperatures.

An advantage of the helium-refrigerated system is that relatively low pressures can be used. The compressor size can be reduced (although two compressors are required) and the pipe thickness can be reduced, in comparison with that required for higher pressures. The hydrogen or neon need be compressed only to a pressure high enough to overcome the irreversible pressure drops through the heat exchangers and piping in an actual system. Pressure from 300 kPa (3 atm) to 800 kPa (8 atm) is usually adequate for the hydrogen loop. The system is relatively insensitive to the pressure level of the helium refrigerator. For a helium-gas pressure of 1 MPa (10 atm), work requirements of approximately 11,000 kJ/kg liquefied (26,000 Btu/lb.) can be realized for a practical system, or a figure of merit of 0.11, which includes the work required to produce the liquid nitrogen.

3.18. Ortho-para-hydrogen conversion in the liquefier

We saw in Sec. 2.13 that hydrogen can exist in two different forms-parahydrogen and ortho-hydrogen. The ortho-para concentration in equilibrium hydrogen depends upon the temperature of the hydrogen. Near room temperature, the composition is practically 75 percent ortho-hydrogen and 25 percent para-hydrogen, whereas at the normal boiling point of hydrogen, the equilibrium composition is almost all para-hydrogen. When hydrogen gas is passed through a liquefaction system, the gas does not remain in the heat exchangers long enough for the equilibrium composition to be established at a particular temperature. The result is that the fresh liquid has practically the room-temperature ortho-para composition and will, if left alone in the liquid receiver, undergo the exothermic reaction there. The changeover from ortho- to para-hydrogen involves a heat of conversion that is greater than the heat of vaporization of para-hydrogen; therefore, serious boil-off losses will result unless measures are taken to prevent it. This is a problem peculiar to hydrogen-liquefaction systems that must be solved in any efficient system.

A catalyst may be used to speed up the conversion process, while the heat of conversion is absorbed in the liquefaction system before the liquid is stored in the liquid receiver. Because the heat of conversion results in an increase in liquid evaporated, it is advantageous to carry out as much of the conversion in the liquid-nitrogen bath as possible. The nitrogen is much less costly to produce than the liquid hydrogen. Note from Table 2.7 that the equilibrium composition at temperature near 70 K (126°R), corresponding to liquid nitrogen boiling under vacuum, is approximately 55 to 60 percent para-hydrogen. Thus if the conversion is complete at this temperature, the energy released in the liquid receiver is reduced by almost one-half.

Two possible arrangements for ortho-para conversion are shown in Fig. 3.28. In the first arrangement, the hydrogen is passed through the catalyst in the liquid-nitrogen bath, expanded through the expansion valve into the liquid receiver, and drawn through a catalyst bed before passing into a storage vessel. The hydrogen that is evaporated due to the heat of conversion flows back through the heat exchanger and furnishes additional refrigeration to the incoming stream. The second arrangement is similar to the first one, except that the high-pressure stream is divided into two parts before the expansion valve. One part is expanded through an expansion valve and flows through a catalyst bed immersed in a liquid-hydrogen bath; the converted hydrogen is passed to a storage vessel. The other part of the high-pressure stream is expanded through another expansion valve into the liquid receiver to furnish refrigeration for the catalyst bed; the vapor is passed back through the heat exchanger to cool down the incoming gas. The second arrangement allows approximately 20 percent higher liquid-hydrogen yields compared with the first arrangement.

Some of the catalysts that have proved effective are (1) hydrous ferric oxide, (2) chromic oxide on alumina particles, (3) charcoal and silica gel, and (4) nickel-based catalyst. Of these, hydrous ferric oxide is the most active; that is, a relatively small volume of catalyst is required to produce practically complete conversion to the equilibrium composition. The conversion process is speeded up for any of the catalysts if they are ground into fine pellets, which offer a larger surface area per unit volume than do large chunks of material.

Certain impurities will "poison" the catalysts or severely reduce their effectiveness (Scott et al. 1964). Methane, carbon monoxide, and ethylene act as temporary poisons, whereas chlorine, hydrogen chloride, and hydrogen sulfide permanently decrease the catalyst activity. It is important to remove these materials from the hydrogen feed stream before they enter the liquefier.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 02/01/2006 01:43 pm
Quote
Stowbridge - 29/1/2006  12:10 PM

This is all very interesting. Is there a starting point to learn about nuclear propulsion so that some of this makes sence to those of us that don't have degrees in this?
I have been looking for some basic material in nuclear energy for some time, that might help those who would like to learn more about the basics of nuclear energy.  I found an old book in a secondhand bookstore that had what I was looking for, and I've put some chapters that are relevant to nuclear energy up for your enjoyment.  This book really focuses on the conventional, terrestrial, light-water reactor, but that is still a good starting point to then branch out and understand some of the unique requirements of space nuclear power, and the possibilities of molten-fluoride reactors.

We have to know where we are to know where we could go.

Nuclear Fuels (http://www.nasaspaceflight.com/_docs/steam_chap19.pdf)

Principles of Nuclear Fission (http://www.nasaspaceflight.com/_docs/steam_chap20.pdf)

Nuclear Steam Supply Systems (http://www.nasaspaceflight.com/_docs/steam_chap21.pdf)

Nuclear Installations for Electric Utilities (http://www.nasaspaceflight.com/_docs/steam_chap23.pdf)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: PlanetStorm on 02/01/2006 11:16 pm
>Found also a nice summary about hydrogen storability. A monthly boil-off rate of 3-7% for passive tanks seems doable. I'm surprised that most of the calculations >assume ~300K vacuum temperature. Is this the temperature that an object doing BBQ-rolling near Earth assumes?


I don't understand the 300K vacuum temperature either. Isn't this the temperature that a "black" body would attain if in equilibrium with solar radiation? (and so why the Earth's surface temperature is what it is?) If so, just silvering the tanks should reduce the temperature by a large fraction. Equally, a heat-shield (as used in the Spitzer telescope) would provide shadow and hence much lower ambient temperature.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 02/02/2006 12:45 am
Quote
PlanetStorm - 1/2/2006  6:16 PM

>Found also a nice summary about hydrogen storability. A monthly boil-off rate of 3-7% for passive tanks seems doable. I'm surprised that most of the calculations >assume ~300K vacuum temperature. Is this the temperature that an object doing BBQ-rolling near Earth assumes?


I don't understand the 300K vacuum temperature either. Isn't this the temperature that a "black" body would attain if in equilibrium with solar radiation? (and so why the Earth's surface temperature is what it is?) If so, just silvering the tanks should reduce the temperature by a large fraction. Equally, a heat-shield (as used in the Spitzer telescope) would provide shadow and hence much lower ambient temperature.
Well, temperature in space is kind of a strange thing to understand...it has everything to do with your radiative properties and what is shining on you.  "Silvering" the tank is essentially equivalent to using MLI (multi-layer insulation) which is what these tanks would be wrapped in anyway.  It basically does the same thing as the Spitzer shield--reflects all the sunlight off the tank.

But that's the problem--it's not PERFECTLY reflective, and some energy gets absorbed as heat.  MLI is designed to minimize this as much as possible, but some heat always makes it through.  That's probably where the 3-7% boiloff per month estimate comes from.  Obviously, with that kind of boiloff rate, there wouldn't be much hydrogen to use six months later when you get to Mars or 24 months later when you want to leave.

I'm beginning to see the beauties of methane and ISRU....

The Earth's temperature is also based on radiative equilibrium--the Earth radiates as a blackbody at some surprisingly low temperature, 255K (source: Terraforming (http://www.amazon.com/gp/product/1560916095/sr=1-1/qid=1138844040/ref=pd_bbs_1/002-9041506-0749601?%5Fencoding=UTF8), pg 65) but the greenhouse effect has everything to do with why the planet is inhabitable.  Except now we're letting the greenhouse effect get too strong.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: simonbp on 02/03/2006 04:57 am
Quote
vanilla - 1/2/2006  7:45 PM

The Earth's temperature is also based on radiative equilibrium--the Earth radiates as a blackbody at some surprisingly low temperature, 255K (source: Terraforming (http://www.amazon.com/gp/product/1560916095/sr=1-1/qid=1138844040/ref=pd_bbs_1/002-9041506-0749601?%5Fencoding=UTF8), pg 65) but the greenhouse effect has everything to do with why the planet is inhabitable.  Except now we're letting the greenhouse effect get too strong.

I had to calculate that ~255K value on an Intro to Astrophysics test last year; it's not that hard to do as you just have to find the solar flux over the cross-sectional area of the earth, divide it by half the surface area, and plug the resulting irradiance into the Stefan-Boltzmann law to get the blackbody temperature...

Incidentily, most of the "greenhouse gases" on earth are water vapour...

Simon ;)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 02/04/2006 01:52 am
Quote
simonbp - 2/2/2006  11:57 PM

I had to calculate that ~255K value on an Intro to Astrophysics test last year; it's not that hard to do as you just have to find the solar flux over the cross-sectional area of the earth, divide it by half the surface area, and plug the resulting irradiance into the Stefan-Boltzmann law to get the blackbody temperature...

Incidentily, most of the "greenhouse gases" on earth are water vapour...

Simon ;)
Ah, a man with whom I can discuss the beauties and barriers of thermodynamics... :)
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: PlanetStorm on 02/04/2006 10:54 am
Quote

I had to calculate that ~255K value on an Intro to Astrophysics test last year; it's not that hard to do as you just have to find the solar flux over the cross-sectional area of the earth, divide it by half the surface area, and plug the resulting irradiance into the Stefan-Boltzmann law to get the blackbody temperature...


Is it is really that clear cut? The result that you get from that direct energy balance calculation depends on the value you use for the Earth's emissivity. If you assume black body, then ~255K comes out, but really you should then be allowing for spin, unequal day/night suface temperature, and heat transfer around the Earth. If you assume Earth acts more like a mirror (shiny reflective oceans, ice caps etc), you get something very different. In fact, a perfect mirror implies T= 0K!.

So I guess I am still missing why this is so difficult a problem. A well placed mirror placed between a tank and the sun, together with radiators on the shadow-side of the mirror that are angled to radiate away from the tank ought to give very good protection. I think the Spitzer heat shield is designed this way, and that allows for very low operating temperates doesn't it? I mean Spitzer "looks" in the far finfra-red, so is must be very low templerature itself, otherwise any signal would be swamped.




Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 02/26/2006 02:23 pm
Quote
vanilla - 31/1/2006  3:51 PM

For a helium-gas pressure of 1 MPa (10 atm), work requirements of approximately 11,000 kJ/kg liquefied (26,000 Btu/lb.) can be realized for a practical system, or a figure of merit of 0.11, which includes the work required to produce the liquid nitrogen.
Note that this is equivalent to saying that it takes 11 MEGAWATTS of electrical power for each kilogram per second of hydrogen liquefied, in a highly-efficient system on Earth.  That is a lot of electrical power.  Makes me seriously wonder/doubt that a future hydrogen economy on Earth will use liquid hydrogen as the favored form of hydrogen.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 03/23/2006 04:36 pm
We all took a bit of a detour in this thread into hydrogen storage and issues related to that (no one more detoured than myself) but I would like to get a discussion on nuclear going again if possible.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Tap-Sa on 03/23/2006 06:19 pm
The detour began from power conversion cycles. What's the 'final word' on that? Rankine, Brayton, Stirling, other?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Avron on 03/24/2006 03:04 am
Quote
vanilla - 23/3/2006  12:36 PM

We all took a bit of a detour in this thread into hydrogen storage and issues related to that (no one more detoured than myself) but I would like to get a discussion on nuclear going again if possible.

Time for a serious detour on the Fusion end of the spectrum...:)

Had a look at the Jet site today... help me please with "Breakdown" (I think ) and ignition (other than the factor of five in temp requirements)...

1) Ignition from what I understand is the point where there is no need to inject more external energy into the system - i.e. self sustaining reaction
2) How do they get rid of the He.. I understand that it leads to "contamination" and thus cooling.
3) How do you get energy out of the closed system... free neutrons?  How?

So many questions...

Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Mark Max Q on 03/25/2006 04:42 pm
Sorry if this has been answered, but how do you 'start' a nuclear propulsion engine?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 03/25/2006 04:56 pm
Quote
Mark Max Q - 25/3/2006  11:42 AM

Sorry if this has been answered, but how do you 'start' a nuclear propulsion engine?
To start a nuclear thermal engine (which is what I think you were asking about) you rotate the control drums into a position where they're no longer inhibiting criticality (you've got the neutron-absorbing sides pointed away from the core).  Remember that criticality is where you are producing in each generation of fission the same number of neutrons that you produced in the last generation.  Criticality can occur at any power level, from a billionth of a watt to a billion watts.

Then you start flowing hydrogen through the "combustion" chamber (even though there's no combustion there).  The hydrogen increases the moderation of the neutrons and enhances the probability of fission, causing the engine to go supercritical and the power levels to increase rapidly.  When the power level reaches a point where it begins to heat the hydrogen, the negative feedback effect begins to take place that will control the engine.  When the core temperature is at the proper level (for the given hydrogen flow rate) the neutron multiplication will go from supercritical back to critical and the engine will be at its operating condition.

To shut down the engine, a reduction in hydrogen flow rate will reduce neutron moderation, which will make the engine go subcritical and cause power levels to fall rapidly, until the engine is effectively shut down.  You'll still need to flow a little hydrogen to remove decay heat from the fuel, but the flow rate will be much smaller than at full power operation.

It seems strange to think you can start and stop a nuclear reaction with a hydrogen valve, but indeed you can.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: PlanetStorm on 03/25/2006 05:51 pm
Quote
Avron - 23/3/2006  10:04 PM

Quote
vanilla - 23/3/2006  12:36 PM

We all took a bit of a detour in this thread into hydrogen storage and issues related to that (no one more detoured than myself) but I would like to get a discussion on nuclear going again if possible.

Time for a serious detour on the Fusion end of the spectrum...:)

Had a look at the Jet site today... help me please with "Breakdown" (I think ) and ignition (other than the factor of five in temp requirements)...

1) Ignition from what I understand is the point where there is no need to inject more external energy into the system - i.e. self sustaining reaction
2) How do they get rid of the He.. I understand that it leads to "contamination" and thus cooling.
3) How do you get energy out of the closed system... free neutrons?  How?

So many questions...


Breakeven (or break-even) is the point at which there is as much energy being liberated via the fusion reaction as is required to be input via external heating mechanisms (whether this is from current, microwave, laser, or whaterver) in order to maintain the temperature of the plasma so that the fusion reaction can continue. As you rightly said, "ignition" is the point at which the reaction becomes self-sustaining, with energy liberated by fusion matching the energy loss due to cooling, etc, of the plasma. The difference between the two points is basically down to thermodynamic efficiency (only a small fraction of the energy generated in a better-than-break-even reactor can be recycled - most is simply lost as waste heat). So break-even (which has been achieved) is a moral victory, but as Vanilla poiinted out on another thread, it is ignition (which hasn't been achieved) that represents the point where useful, excess power is generated.

There is no good way to get the energy out of JET. JET is a purely experimental device. It was designed to test ways to control a very high temperature plasma (high temperature plasmas are very "wriggly"), using magnetic containment in a toroidal, D-shape geometry. It never needed any means of getting the energy out because it was not designed as a generator, not to mention there was very little energy to be got! (JET only just made break-even after all). For me, the problem of how to get the energy out is the biggest stumbling blocks for fusion, and one where ITER will hopefully make some serious progress. I haven't kept up with recent developments in this area so can't add much more on this question, sorry.

To answer your other question though, it is possible to filter out He because it has a different charge-to-mass ratio (0.5) than H (1). Assuming that both components are at the same temperature, He will consequently have a smaller "orbit" than H about the magnetic field lines. With detailed control over the gradients in the magnetic field, it is possible to make He "drift" outwards or inwards relative to the H, and this is the basic principle of one variety of filtering mechanisms.

Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: PlanetStorm on 03/25/2006 05:59 pm

when I was talking about charge-to-mass ratios, I should have said that I was using a unit system where the electron charge is taken to be 1, and the proton mass is also taken to be 1. Hence the hydrogen charge to mass ratio is 1/1=1, whereas for He it is 2/4 = 0.5
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Avron on 03/27/2006 05:54 am
Quote
PlanetStorm - 25/3/2006  1:59 PM


when I was talking about charge-to-mass ratios, I should have said that I was using a unit system where the electron charge is taken to be 1, and the proton mass is also taken to be 1. Hence the hydrogen charge to mass ratio is 1/1=1, whereas for He it is 2/4 = 0.5


thanks for the answers...  so can I understand that He can be "scraped" off the plasma?  Wonder if Vanilla had any ideas on how one gets energy out of the the Fusion system?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: David BAE on 03/27/2006 09:12 pm
How is the American public perception on nuclear propulsion nowadays?

Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 03/27/2006 10:24 pm
Quote
Avron - 26/3/2006  11:54 PM
thanks for the answers...  so can I understand that He can be "scraped" off the plasma?  Wonder if Vanilla had any ideas on how one gets energy out of the the Fusion system?
As I mentioned before, the bulk of the energy (80%) of a D-T reaction comes off as 14.7 MeV neutrons, which immediately escape the plasma and impact the first wall.  Because of the need to breed new tritium, and the fact that not every neutron strikes a fertile lithium-6 nucleus to form tritium, typically the first wall is made of lithium-6 and beryllium (which acts as a neutron multiplier through its n,2n reaction).

Unsurprisingly, lithium and beryllium fluoride salts have been considered as the first wall of a fusion reactor.  The heat deposited by the neutrons would heat the salt, which could then give up its heat to a closed-cycle gas turbine and produce electricity at ~50% efficiency, much like the molten-fluoride fission reactors I've mentioned.  In fact, lithium and beryllium fluorides were used as the solvent for the Molten-Salt Reactor Experiment (MSRE) in the 1960s.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Avron on 03/28/2006 05:01 am
Quote
vanilla - 27/3/2006  5:24 PM

Quote
Avron - 26/3/2006  11:54 PM
thanks for the answers...  so can I understand that He can be "scraped" off the plasma?  Wonder if Vanilla had any ideas on how one gets energy out of the the Fusion system?
As I mentioned before, the bulk of the energy (80%) of a D-T reaction comes off as 14.7 MeV neutrons, which immediately escape the plasma and impact the first wall.  Because of the need to breed new tritium, and the fact that not every neutron strikes a fertile lithium-6 nucleus to form tritium, typically the first wall is made of lithium-6 and beryllium (which acts as a neutron multiplier through its n,2n reaction).

Unsurprisingly, lithium and beryllium fluoride salts have been considered as the first wall of a fusion reactor.  The heat deposited by the neutrons would heat the salt, which could then give up its heat to a closed-cycle gas turbine and produce electricity at ~50% efficiency, much like the molten-fluoride fission reactors I've mentioned.  In fact, lithium and beryllium fluorides were used as the solvent for the Molten-Salt Reactor Experiment (MSRE) in the 1960s.


Somehow I missed that answer, thanks..
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 03/28/2006 01:42 pm
Quote
Avron - 27/3/2006  11:01 PM
Somehow I missed that answer, thanks..
I'm sorry, I must be going senile and forgetting which threads I said which things on...

http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=1913&start=7
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kraisee on 04/10/2006 09:10 pm
I haven't commented here yet, because a lot of this discussion is out of my experience base.   But I've been following along over the months and after comments elsewhere, I'm going to ask a critical question about all this.

Assuming funding were made available tomorrow, what are the practical implications for actually creating a real-world reactor, like the ones being described here, for a future Mars mission?

NTP/NEP/whatever.

All of the above even!


What do you predict the development and testing times will be?

How much money is that going to soak up?

How big (physical size and mass) is such a reactor likely to be, in order to power a spacecraft on a multi-year long voyage to Mars?   Further, how much fuel would it require at the start of the mission?   If it were re-usable, would it require re-fuelling at Mars or just when it finally returns?

What checks and maintenance would have to be performed routinely between missions?

What protection systems are available for a crew in relatively close proximity to such a reactor?

What are the likely failure modes for such a propulsion system?

Would a complete, independant backup system be advisable in case of failure of the primary?   Or would an alternative design be recommendable as a truly independant form of propulsion in case the main engine(s) were to fail?

What forms of maintenance will such a system require at various stages of an actual mission, and how much more difficult are such procedures going to be made given the fact that any external work will have to be performed by astronauts on EVA's?   And what radiological risks are there to any of these procedures?

These are all issues which I, as a lowly outsider, can identify will have to be solved before any such system is ever going to fly us to Mars.   I'm interested in learning what the solutions you guys envisage are to these sorts of issues?

Ross.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Jim on 04/10/2006 10:04 pm
JIMO was going to be a "pathfinder" for all future reactors.  KSC infrastructure costs were over a billion.  That was just to process the spacecraft and reactor (in separate buildings).  Security costs are unreal.  The movement of the fissionable material is classifed, so it was going to be almost like processing an NRO payload.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Jamie Young on 04/11/2006 02:49 am
Couldn't such a mission be set up in some remote area? Or does it have to be launched from the infrasture of KSC/Cape etc?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Jim on 04/11/2006 02:55 am
Quote
Jamie Young - 10/4/2006  10:49 PMCouldn't such a mission be set up in some remote area? Or does it have to be launched from the infrasture of KSC/Cape etc?

Define remote area

If it were to fly, it has to fly from where the rockets are launched.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: lmike on 04/11/2006 09:34 am
In addition to what Jim mentioned about the complexities and costs.  JIMO's reactor is the tiny blob at the far end of the radiator truss: http://www.jpl.nasa.gov/images/spacecraft/jimo-092004-browse.jpg  Also, AFAIK, Xenon production (the working propulsion matter in the Promethius) is a contentious issue and is very limited and costly worldwide, it's also used in anaesthesia.  Then there were the huge radiators (all with piping, and deployment actuators), the truss, the power converters, the thrusters with highvoltage grids, and well... finally the science payload bay.  Not that I wouldn't love to see it go to Jupiter, but...
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Seattle Dave on 04/12/2006 03:58 pm
JIMO was very interesting, but rose to 10billion in costs, correct?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: publiusr on 04/12/2006 07:05 pm
It was also going to be huge. I doubt Delta IV heavy could lift it except in segments. Folks wanted to have a hydrogen upper stage kick it out of Earth moon before they pulled the rods.  Might as well go all chemical if you are going to do that.
Oh wel, one more payload for CaLV in the future.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/12/2006 05:25 pm
Quote
Star-Drive - 11/5/2006  11:12 PM

In reference to nuclear thermal rockets (NTR) not being good for thrust, just Isp, I have to point out the USAF Timberwind work in the late 1980s that used particle bed fission reactors that obtained projected full scale thrust to weight (T/W) ratios of 25 to 1 or better and these results were backed with ground component testing.  And then you add LOX agmentation as an afterburner to the hydrogen flow in the nozzle downstream of the throat and the MSFC studies AND TESTS have shown that SEA LEVEL T/W ratios of 75-to-1 are obtainable as well.  Last time I looked, the SSME's T/W of 73-to-1 in vacuum doesn't give it any advantage in power over the Timnberwind/LOX Augmented NTR approach and the SSME's Isp is nowhere close to the Timberwind's 950 to 1,000 seconds.  What held back these particle bed NTR designs was NOT their performance, it was the political will to build anything with the word NUCLEAR attached.  
Timberwind is one of the most insane nuclear thermal ideas ever conceived.  The particle-bed reactor is dynamically unstable, and this too was confirmed in tests.  Basically, it achieves such good performance by having zillions of these little uranium carbide particles, and thus tons of surface area per unit volume.  The problem is, if there's any local melting, the particles tend to fuse together.  This blocks off further cooling, which leads to more heating, more fusing, less cooling, and wham--in a few seconds you've fused the core together and can't cool it and the reactor becomes a glowing ball.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/12/2006 05:29 pm
Quote
publiusr - 12/4/2006  2:05 PM

It was also going to be huge. I doubt Delta IV heavy could lift it except in segments. Folks wanted to have a hydrogen upper stage kick it out of Earth moon before they pulled the rods.  Might as well go all chemical if you are going to do that.
Oh wel, one more payload for CaLV in the future.
It didn't have to be this big--they should have spent much more time trying to get an attractive specfic mass (kilograms per kilowatt electric) in the reactor system rather than just running off trying to launch a reactor.  It would be like saying, "I'm going to build a LOX/hydrogen rocket no matter what the Isp".  You start out thinking it's going to be 450 seconds, but then it's 400, then 300, then 200...at what point should you stop and say, "It should be better than this and I'm doing something wrong if the performance is this far off..."

This never happened with JIMO...the program was sold with a reactor performance of 30 kg/kWe, and by the time it was killed it was well over 100 kg/kWe.  Mostly due to bad engineering decisions.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/13/2006 07:55 pm
The most basic thing I would have done differently (if I had been in charge of Prometheus) would have been to pursue a liquid-fluoride reactor as the heat source and a potassium-Rankine power conversion system for electrical generation.  The K-Rankine PCS would have allowed heat rejection at about 900 K, instead of 400-500K as was baselined for the helium Brayton cycle they chose.  Since the effectiveness of the radiator scales with the FOURTH power of temperature, rejecting at roughly twice the temperature means that you only need 1/16th the radiator area to reject the heat.  Since the K-Rankine PCS would have an efficiency of roughly 20%, vs. the 25% efficiency they expected for the He-Brayton PCS, the effect would have been about 1/10th the radiator area necessary for the same electrical power generation.

The other major advantage would be to use the liquid-fluoride reactor.  Apart from the fact that it has actually been built and operated (unlike all the other JIMO reactor concepts), it can easily be developed, fueled, operated, and controlled.  It basically controls itself, and very well at that.  The twitchy fast-spectrum reactors considered for JIMO needed VERY fast, very responsive reactor control systems to keep the reactor from turning itself into a glowing blob in a few seconds.

Between the two of these advances, I think the "alpha" (specific mass) that could be achieved would be significantly better than the 30 kg/kWe goal (for an electrical power rating of 100 kWe) and that would have had dramatic effects of the mission architecture, trip time, launch mass, and science payload that could have been carried.  Plus the reactor would be very extensible to future, multi-megawatt power needs, like a human NEP mission to Mars.
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Andy L on 06/01/2006 02:31 pm
This thread is going to be very valuable as a resource when we change our focus from the moon to Mars. Is it fair to say that our only viable option for manned exploration past the moon will be nuclear?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: mong' on 06/01/2006 02:43 pm
not necessarily.
we can go to mars with high performance chemical engines, and probably venus too.
nuclear will be useful for sustained exploration/settlement (i.e: frequent reuspply, trade,..) and going farther
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Pete at Edwards on 08/30/2006 12:32 am
Quote
mong' - 1/6/2006  9:30 AM

not necessarily.
we can go to mars with high performance chemical engines, and probably venus too.
nuclear will be useful for sustained exploration/settlement (i.e: frequent reuspply, trade,..) and going farther

And it's all about duration over velocity, although the two are relevant to each other!
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mauk2 on 08/30/2006 06:25 am
Why not use molten salt for the radiator, too?

http://www.ornl.gov/~webworks/cppr/y2001/pres/122984.pdf

Just as a note, Dr. Forsberg and crew state in that paper that liquid metals are not compatible with carbon structures, and that is not strictly true, liquid tin is perfectly compatible with carbon, as is proved by every piece of float glass on the planet. :D

Carbon is not compatible with traditional liquid metal coolants. :)

As for launch safety?  Well, it'd be easily possible to launch the salt as a frozen solid lump and thaw it on orbit.  Keeping it in a single lump might be a chore, though.  The stuff is quite benign at STP, though.

Potassium metal, on the other hand?  Nosirree bob.  Not nice.  Tin would be much better.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 08/30/2006 12:52 pm
Quote
mauk2 - 30/8/2006  1:12 AM

Potassium metal, on the other hand?  Nosirree bob.  Not nice.  Tin would be much better.

Potassium is chosen because of its vapor dome characteristics (boiling and condensation in the range of 800-1400 K).  Tin doesn't boil until 2270 C.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mauk2 on 08/30/2006 05:53 pm
Hrrrrrrm.

Good point.

I don't know of many likely alternatives that boil in that range, either.  Maybe organics?   I know PCB's got looked at as reactor coolants, but the dioxin issues would likely be a concern....   I like tin for such uses because it's so benign.   I mean, "tin cans"?  :)   Doesn't get much safer than that!
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: Rob in KC on 09/05/2006 10:43 pm
Is there an alternative to both chemical and nuclear?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: TyMoore on 09/05/2006 11:13 pm
1) Solar Sails--but they're big and slow. No propellant consumption though...

2) Magneto plasma sails--interesting concept that basically uses an ion engine as a plasma contactor and a strong magnetic field to create an artificial magnetosphere around the space craft that the solar wind pushes against.  Should work in theory, but I'd guess you'd get pretty marginal (very, very modest) performance--still very little propellant consumed.

3) Maybe a large electrodynamic tether to interact with the solar magnetic field--still, you'd need a gigantic tether and you would probably get pretty small performance. Also, how do you steer the thing?
Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 09/06/2006 03:16 am
Quote
Rob in KC - 5/9/2006  5:30 PM

Is there an alternative to both chemical and nuclear?

Dr. Robert Forward had the idea to use momentum-exchange tethers around the Earth and Mars to throw payloads back and forth, like planetary jai-alai.  Considering that both chemical, nuclear thermal, and aerocapture all do their maneuvers deep in a planetary gravity well, it's not inconceivable that a tether might be used to do the same thing as well.

Here's a paper (http://www.tethers.com/papers/MERITT.pdf), and another one (http://www.tethers.com/papers/InterplanetaryTetherTrnsprt.pdf).
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mauk2 on 09/06/2006 09:28 pm
The issues with solar sails, magneto plasma sails, and tethers in most deployments are that they all assume we have already achieved Earth Orbit.    Unfortunately, that first step is a doozy. :(

Now, it is POSSIBLE to use a tether to reach Earth Orbit, in the Space Elevator concept, but it requires very, very high tensile strengths, that are proving difficult to achieve.  CNT materials have a lot of promise, but not in lengths measured in the tens of thousands of meters, never mind tens of thousands of kilometers.  Not to mention, how do we power a cable car up 20+ thousand miles of track into a vacuum?  This turns out to be a fairly major issue.

That said, those papers Vanilla links are quite good stuff, if only we hadd a way to get to Earth Orbit in a significant way.

Dr. Birch has some very provocative ideas using so-called tension-compression members, an idea which would require fairly large breakthroughs to make work, but still seems a bit more likely to me  than "beanstalk-strength" cables.  While Dr. Birch is a trifle eccentric, his ideas certainly have considerable merit.  At least as much as the Space Elevator advocates out there. :)

Here's his page with links to his papers:

http://www.paulbirch.net/

The 1982 papers called "Orbital Rings and Jacob's Ladders" are the ones you want.  Very provocative stuff, and as the papers will make clear, very much real.  To put it mildly, Dr. Birch is a smart dude who is comfortable with his maths.  :D


However, in practical, real-world terms, there is really nothing besides chemical and nuclear, and the energy density possible with chemicals, barring LARGE breakthroughs, is extremely limiting.  You're pretty much left with nuclear.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 09/06/2006 10:19 pm
Quote
mauk2 - 6/9/2006  4:15 PM

The issues with solar sails, magneto plasma sails, and tethers in most deployments are that they all assume we have already achieved Earth Orbit.    Unfortunately, that first step is a doozy. :(
Tethers don't assume that you've reached Earth orbit.  Dr. Forward had many ideas for tethers that would catch suborbital payloads, supplying 50% or more of the orbital energy required to achieve LEO, and even more orbital energy than that, since the tether usually released the payload into a high-energy elliptical orbit.

Hypersonic Airplane Space Tether Orbital Launch System (http://www.tethers.com/papers/HASTOLAIAAPaper.pdf)

Eagle Sarmont of Lockheed also had similar ideas, except his tether concept was hanging rather than spinning:

Earth Orbiting Elevator (http://www.affordablespaceflight.com/home.html)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mauk2 on 09/07/2006 12:39 am

Hiya vanilla! :)

The point I'm getting at is, how do you put up the tethers in the first place? :)   I mean, even that fancy multi-stage tether had a mass fraction of several times the effective payload, and the more realistic version had mass fractions of over a hundred. (See bottom of page 5 for that bit.)

If your design payload is 15,000 kilos, that's what, 1500 tons you need in orbit?  :)  How you gonna get all that up there?   And yes, I know you can theoretically bootstrap your way up.  The idea of bootstrapping your way there is as ludicrous for this notion as it is for every other notion it's suggested for.   Columbus did not start out with canoes.  :D

On top of THAT, the best mass fractions assume you have a Mach15 plane to catch the bottom end of that tether with.  We don't have many of those laying around that I know of.  Not to mention, do YOU wanna try that rendezvous with the skyhook at Mach15?  Eeesh!

Now, don't get me wrong, I really enjoy Dr. Forwards ideas, and he has some real doozies, it's just that there HAS to be a better way.

And indeed, there is, once we finally decide to stop being scared of it. :)

Now, that being said, those papers are full of awesome info, and I'm gonna be saving them both!  :)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: publiusr on 10/17/2006 10:51 pm
I lament his death.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 10/19/2006 06:13 pm
Quote
vanilla - 30/8/2006  7:35 AM

Quote
mauk2 - 30/8/2006  1:12 AM

Potassium metal, on the other hand?  Nosirree bob.  Not nice.  Tin would be much better.

Potassium is chosen because of its vapor dome characteristics (boiling and condensation in the range of 800-1400 K).  Tin doesn't boil until 2270 C.

For a good paper on the subject, go to

http://libcat.ornl.gov/F

log on as guest, and look up a paper called

Comparison of 1-, 2-, and 3-loop systems for nuclear turbine-generator space power plants of 300 kW to 5 MW of Electrical Output. (ORNL-TM-1366)

I'd post a link but it doesn't seem to work well that way.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mauk2 on 10/19/2006 11:03 pm
Ooooh, now THAT's a nice resource!  I wonder how many of the old ORNL papers are tucked away in that archive? :D

Here's a more recent survey paper I found after we poked at this earlier:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890013609_1989013609.pdf

There's several papers in the references section, I may have to see if I can locate some of the originals using that tool.  Most excellent stuff!  :)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Orbiter Obvious on 10/23/2006 03:47 am
Bottom line, on a rating of 1 to 10. How likely will we see ships using this for exploration in the next 50-100 years?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: TyMoore on 10/23/2006 02:29 pm
Personally I'd love to say 100%, but the way things are going: 25-35%
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: GuessWho on 03/15/2007 04:52 am
Vanilla - I just found this thread and have spent some time reviewing NEP/NTP thread.  Interesting discussion but I have a couple of clarification questions for you.

What operating temperature are you assuming for a molten flouride salt reactor?  I seem to recall a statement about operating at 850-900 degF.  Is this correct?

Why would a molten salt reactor be allowed to vent fission products when all other concepts are prohibited from doing so?  Since this impacts fuel pin design for ceramic fuels that are required to contain fission gases (via gas plenums, cladding strength, etc.), relaxing that requirement would reduce the design complexity for solid fuel reactor concepts.

How do you know what the molten salt reactor core goemetry is in zero-g?  Given the typical behavior of fluids under zero-g/micro-g conditions, your core geometry is unknown, thus your reactor control systems are indeterminate.  How do you get around this issue?

Title: RE: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 02/27/2008 02:18 pm
My, my how the pendulum swings away from nuclear power and propulsion.

Why I remember only a few years ago when I read full-page ads in Space News every week from the aerospace primes extolling in breathless language how CRUCIAL nuclear propulsion was to the future of space exploration...

NASA Offers Pre-screening of Principal Investigator Revised Requirements for New Frontiers Opportunity (http://www.spaceref.com/news/viewsr.html?pid=27162)

Quote
This NF-3 AO will solicit only missions that do not require nuclear sources for power generation or propulsion.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: GuessWho on 02/28/2008 04:24 am
The New Frontiers AO is talking about radioisotope power sources, specifically MMRTG or ASRG systems.  Given the current low supply of PuO2 for these devices, the remaining fuel is being set aside for MSL, a potential Discovery-class demonstration of 2 ASRG's, and 1 or 2 Flagship missions that will require radioisotope power systems.  They are not referencing fission-based nuclear power.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: GI-Thruster on 04/06/2009 05:09 pm
I'm disappointed this thread died the way it did.  I know some of it was removed and I was breathless waiting as Vanilla was about to explain his phase change radiator.

Vanilla, would you like to take a stab at finishing?  I've always thought phase change was a great phenomena to exploit in heat exchange.  How are you going to use phase change with moving fluid?  Obviously, you'd need between liquid and solid.  I don't see how to work that.  Would you have a slurry?

Also, can you or someone else explain the difficulties in using regolith as propellant for NTR?  If there were a way to burn rocks, we might be having a very different NEP/NTR discussion.  After all, if you have to throw something out the back, best thing is rocks. . .they're everywhere we want to go.

Finally, is there a way to benefit from all the nice stability and high temperature advantages of the molten-floride concept in an NTR?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/06/2010 05:39 pm
What operating temperature are you assuming for a molten flouride salt reactor?  I seem to recall a statement about operating at 850-900 degF.  Is this correct?

About 1000K would be the operating temperature for the reactor.

Why would a molten salt reactor be allowed to vent fission products when all other concepts are prohibited from doing so?

Because in deep space no one cares about gaseous fission products.  They decay quickly anyway.

How do you know what the molten salt reactor core goemetry is in zero-g?  Given the typical behavior of fluids under zero-g/micro-g conditions, your core geometry is unknown, thus your reactor control systems are indeterminate.  How do you get around this issue?

Well, you keep void space low in the reactor design, ideally nothing.  So the space in the core is either graphite moderator or fluid salt, so there's not much ambiguity in the geometry of the core.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: neilh on 05/06/2010 09:22 pm
A recent slide by Laurie Leshin (deputy associate adminstrator for ESMD) noted a potential date of 2020 for a nuclear thermal propulsion demo as part of the Enabling Technology Development and Demonstration program:

http://forum.nasaspaceflight.com/index.php?topic=21474.msg585771#msg585771
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/06/2010 09:35 pm
Nuclear thermal propulsion is a waste of time if you're trying to improve payload performance.  See my analysis here:

http://selenianboondocks.com/2010/02/pf-expressions-example/

and here

http://selenianboondocks.com/2010/02/payload-fraction-example-proof/
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/06/2010 09:39 pm
Nuclear thermal propulsion is a waste of time if you're trying to improve payload performance.  See my analysis here:

http://selenianboondocks.com/2010/02/pf-expressions-example/

and here

http://selenianboondocks.com/2010/02/payload-fraction-example-proof/
I remember reading that. What about for a lander that can refuel via in-situ propellant? I generally prefer NTR over NEP (for the inner solar system) because a NTR has a convenient place to dump all the heat, whereas the Nuclear electric still needs a giant, deployable structure to dump heat (might as well use a solar array, if in the inner solar system!!!).
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/06/2010 09:48 pm
Seriously, someone tell me exactly why nuclear electric is used instead of solar electric for a Mars Transfer Vehicle? All the analysis I've seen and done seems to point to solar electric as being far, far more mature, lower complexity, lower cost, and higher performance (given the same development) as nuclear for a Mars mission.

I mean, I'm not afraid of nukes at all. But WHAT is the huge draw for nuclear-electric for a MTV vs solar?

It's almost to the point that anytime a serious chance at a human Mars mission occurs, someone throws "nuclear" into the architecture to ensure it never gets past the drawing board and so doesn't need to be funded....

PS, I don't think nuclear electric research is bad for outer-planets missions (and I think it should be funded for that), but for Mars, it just doesn't make sense to me!
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/06/2010 10:12 pm
I generally prefer NTR over NEP (for the inner solar system) because a NTR has a convenient place to dump all the heat, whereas the Nuclear electric still needs a giant, deployable structure to dump heat (might as well use a solar array, if in the inner solar system!!!).
That's probably a NEP/Brayton design.  Brayton's have lower heat rejection temperatures and much higher radiator areas.  If we look at a NEP/Rankine the situation improves considerably, and the radiator area is far smaller than a solar array.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/06/2010 10:16 pm
I mean, I'm not afraid of nukes at all. But WHAT is the huge draw for nuclear-electric for a MTV vs solar?

There is potential for it to be a whole lot smaller than a solar array for the same mission.  About two years ago I was working on a huge parametric analysis of this problem which unfortunately I have not finished.  Perhaps I should revisit it to get some of the answers to the questions that you ask.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/06/2010 10:41 pm
Here's the thing, though, Kirk, solar cells are already at the performance level needed for a SEP MTV. Commercially. They just need to be scaled up. Sure, you would want to improve them at least by 50% in specific power just to make it worth your while, but they actually would be fine the way they are and could be improved in specific power by about two orders of magnitude if you really wanted to.

Nuclear power has only been demonstrated in space at less than one tenth the specific power that commercial solar arrays can do now. I mean, sure I think you could get a smaller radiator and improve performance considerably by dumping heat at a higher radiator temperature, but these are just areas where there has been virtually no real hardware for decades.

Part of the big problem is that nuclear only works at specific powers competitive with solar when it is done at a very large scale. This is partly because shielding requirements don't increase much when reactor power increases, but this also means that there's a HUGE barrier to actually developing this for human missions. SEP has been demonstrated on multiple deep space spacecraft and is even used commercially. It has been making progress via multiple routes to increase specific power. Thin-film, concentrating triple-junction, or just thinner wafers can increase specific power to where it's needed (~270W/kg is what is listed for some MTV concepts like STCAEM SEP (http://www.astronautix.com/craft/stcemsep.htm)... Ultraflex is at about 225W/kg).  There's such a sharp contrast there between what is doable and what is powerpoint. And for the inner solar system, we're not even sacrificing performance to choose solar over nuclear, either.

EDIT:
(It should be noted that with modern 40% efficient concentrating solar arrays, one half the area would be needed for the same power compared to the 1991 STCAEM SEP study...)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/06/2010 10:47 pm
My analysis was intended to figure out just what "good enough" was for both solar and nuclear.  Even with comparable values of alpha (kg/kWe) solar still had the 1/r2 disadvantage, as well as the issues around pointing those huge arrays, and they are huge.

You might be surprised to know that I spend a great deal of my working day dealing with solar cells for spacecraft, so their performance is not a metric I am unfamiliar with.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/06/2010 11:03 pm
My analysis was intended to figure out just what "good enough" was for both solar and nuclear.  Even with comparable values of alpha (kg/kWe) solar still had the 1/r2 disadvantage, as well as the issues around pointing those huge arrays, and they are huge.
Wouldn't you still want to point the radiators parallel to the sunshine? Or doesn't it matter with high-temperature radiators?

At Mars, it's still only 50% diminished, but solar often has alpha to burn. Further out (like Jupiter), and the 1/r² really kills your specific power.

Quote

You might be surprised to know that I spend a great deal of my working day dealing with solar cells for spacecraft, so their performance is not a metric I am unfamiliar with.
Good to know!

Also, are you taking into account shielding and structures for both nuclear and solar, etc, in your parametric studies? What about fission fuel consumption, transmutation effects on unshielded metals, etc? How possibly could such a reactor be maintained and/or refueled after each mission, or would the reactor/radiator/etc be expendable?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/06/2010 11:14 pm
I was basically just going to trade the concepts against each other based on values of alpha, assuming that you had "book-kept" in that metric those things you asked about.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/06/2010 11:22 pm
I was basically just going to trade the concepts against each other based on values of alpha, assuming that you had "book-kept" in that metric those things you asked about.
So two questions you were trying to find the answers to were:

What value of alpha is adequate for nuclear and solar? (depends on what Isp you choose, doesn't it?)

and

How easy (money or time or uncertainty) is it for each value of alpha to be achieved for nuclear and solar? (This seems like a quite important question, more important than the first one, which can be guesstimated easily.)

Other relevant questions:
How easy is it to reuse the architecture and how robust it is to failure? (Many NEP proposals assume redundant power, right? this affects minimum mission size, and perhaps alpha)

How is maintenance or repair done in case of failure during the mission?

Is there synergy with other projects? (commercial satellite power, other deep space missions, tugs for GSO/cislunar, SBSP, etc)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: nooneofconsequence on 05/06/2010 11:42 pm
I keep arguing for SEP to get off the ground (literally) even though also do not believe in anything but NEP for MTV propulsion.

If we do NEP, it will likely be a joint Russian - US venture where we do the propulsion and they do the generation - it wouldn't even surprise me that the parts would be launched separately, with reactor / fissile / propulsion riding separate LV's from multiple countries.

NEP scales, so much so that if you want to get there faster just scale some more. If you want more provisions, use the same on cargo only on a much longer transit. This is the current thinking.

SEP economics aren't as accepted as I think they should be, and remember I'm an advocate of SEP. Absolutely no one takes seriously SEP as a scalable alternative to NEP. So instead they think of SEP in a "bottom up" way - something small that can be used/useful ... to get it onto roadmaps. But they don't because they can't find a small mission where the economics "work" ... I'm constantly told this.

SEP and NEP need a small SEP mission *badly*. Something like a resupplyable Orbital Express as a logistical component.

Once you get SEP flying as some kind of logistical reliable service ... both NEP and SEP become much more realistic. Then the proven economics sort out the scalability / usability of each - scalability can only be determined in practice.

In other areas solar often gets a short shrift. Still think that the MER's should have been flown again in place of MSL (we pushed too hard/fast to "move on" to nuclear and missed the opportunity to fly) to other locations with other instrumentation (yes it would have pushed mass limits given that increased dV need for the less close opposition). The MER's have demonstrated that solar operation is incredibly cheap / long lasting - that nuclear can do more (drive at night, not need intensive power management driving exploration schedules) I'm wary of the returns on.

But don't run down nuclear to favor solar - they are intimately related, and we'll very probably use both interchangeably.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/06/2010 11:51 pm
I'm just a little frustrated that SEP is not included in recent Design Reference Architectures, when it's actually been used for unmanned missions already and the technology situation has actually improved considerably since two decades ago, instead of stagnating nuclear has.

I like nuclear, though, but for inner solar system.... the Sun is a great reactor!
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: nooneofconsequence on 05/07/2010 12:02 am
Chris,

I agree.

NEP gets attention because it only can surpass chemical given scaling.

SEP doesn't get attention because if you threaten NEP people fall back to chemical - not SEP.

No one does NEP because "fissile fear". So we go back to chemicals ...

SEP? SEP? What do we do with that? You want to send megawatt arrays back and forth to Mars? Yeah sure ...

Oh, and BTW concentration cells have a really bad name. They were tried by Hughes (and Boeing) on comsats, and they degraded in performance on-orbit.
When your bird gets underpowered because you drank the new tech Koolaid its very embarrassing ... someone else gets the contract for the follow on sat that replaces YOU ... this happened. We believe it was a materials issue BTW.

edit:
added Boeing
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: A_M_Swallow on 05/07/2010 12:35 am
There is solar thermal dynamic for use in the Van Allen Belts.  Use concentrated sun light to heat salt and then the same generators as nuclear.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/07/2010 01:01 am
...
Oh, and BTW concentration cells have a really bad name. They were tried by Hughes (and Boeing) on comsats, and they degraded in performance on-orbit.
When your bird gets underpowered because you drank the new tech Koolaid its very embarrassing ... someone else gets the contract for the follow on sat that replaces YOU ... this happened. We believe it was a materials issue BTW.

edit:
added Boeing
Good to know about the concentration cells. I have been scouring the internets in search of more information about concentrating solar for space, and I haven't found anything past about 2006 or 2007, which really made me frustrated and curious, since it seemed like such a good solution.

I did see some attempts at a work-around for the radiation issue by coating the lenses with rad-resistant glass coating of some sort, then cracking the glass coating so it'd be flexible. Supposedly, it worked well and didn't cause that much of an efficiency drop-off, but it struck me as a sort of amateurish hack. Then, after 2007, nothing. Entech doesn't even really do space solar anymore (they've transitioned to terrestrial).

BTW, who manufactured the degraded concentrating arrays? Was it in-house? I would like to see more information to see if it's an inherent problem with the approach or if it is solvable (I suspect the latter, since I still see proposals talking about it, but that's not always a good predictor of workability!).

Luckily, there are quite a few other approaches to high specific power solar arrays.

BTW, consider the fact that we now know many of the formerly-unforeseen problems with different solar arrays. What unforeseen problems lurk for nuclear-electric that will similarly cost us billions to solve? Nuclear-electric is much more complex.

A word on the scaling issues: Nuclear-electric is ultimately limited by area, just like solar. Once nuclear gets to the point where material limits are encountered, the radiator area must scale linearly with power (actually, worse than that because heat must travel further for a larger array, therefore encounters more resistance Edit: not worse if you have distributed reactors). Solar must always scale bigger area (although efficiency has made very helpful gains over the decades), but can be made exceedingly thin.

In fact, there are other similarities between nuclear-electric and solar-electric... there is active research into "thermophotovoltaics" which convert infrared ("heat") light into electricity very efficiently and using the same basic physics as photovoltaics, plus a few other tricks thrown in. So, the same mechanism for converting to electricity for both solar and nuclear.

The advantage for solar (which have always used photovoltaics) is that you don't have to lug around the heat source. The advantage for nuclear is you can carry around the heat source.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: nooneofconsequence on 05/07/2010 02:27 am
Good to know about the concentration cells. I have been scouring the internets in search of more information about concentrating solar for space, and I haven't found anything past about 2006 or 2007, which really made me frustrated and curious, since it seemed like such a good solution.
It was a really, really bad experience.
Quote
I did see some attempts at a work-around for the radiation issue by coating the lenses with rad-resistant glass coating of some sort, then cracking the glass coating so it'd be flexible. Supposedly, it worked well and didn't cause that much of an efficiency drop-off, but it struck me as a sort of amateurish hack. Then, after 2007, nothing. Entech doesn't even really do space solar anymore (they've transitioned to terrestrial).
The materials were extensively tested for long term exposure to the expected environment. Yes thoroughly qualified, as with any such hardware.

But as we all know, unplanned for effects occur - especially after long term operation. Many promising technologies have not worked out as replacements for others.

It may be the case that it wasn't the concentrators but the cells - perhaps thermal or coupled thermal/radiation processes (possibly BLEO).
Quote
BTW, who manufactured the degraded concentrating arrays? Was it in-house? I would like to see more information to see if it's an inherent problem with the approach or if it is solvable (I suspect the latter, since I still see proposals talking about it, but that's not always a good predictor of workability!).
I think Boeing acquired concentrators around which they made arrays - its been 15 years since I was involved with this. They went on to spin out the technology into another subsidiary.

The inherent problem is in qualifying the technology given the past failure to do so. You don't have to do lenses - some concentrators use cylinders - one from PARC uses a miniature Cassegrain optics systems on GaAs high temperature cells. Beats me why they don't experiment on the ISS with these ... not exactly GSO environment though...

Quote
Luckily, there are quite a few other approaches to high specific power solar arrays.
Do not underestimate the trouble in qualifying any of them.
Quote
BTW, consider the fact that we now know many of the formerly-unforeseen problems with different solar arrays. What unforeseen problems lurk for nuclear-electric that will similarly cost us billions to solve? Nuclear-electric is much more complex.
Yes and no. Nuclear already is in a hostile environment, so actual operation in space is considered easier! It is more that the need for nuclear can be supplied by proven chemicals or solar that reduces the need to go nuclear - you can get most done without it.

Here's the nightmare - you get it operating, something bad happens, you can't gain control of the situation, ... and somehow it contaminants. So its not like concentrated solar where you don't know the processes ... but that you can't trust that you'll always retain necessary control end to end. We don't want fissile materials falling out of orbit. 

Quote
A word on the scaling issues: Nuclear-electric is ultimately limited by area, just like solar. Once nuclear gets to the point where material limits are encountered, the radiator area must scale linearly with power (actually, worse than that because heat must travel further for a larger array, therefore encounters more resistance). Solar must always scale bigger area (although efficiency has made very helpful gains over the decades), but can be made exceedingly thin.
A micrometeorite strike on a radiator is meaningless. A strike on a solar could take out significant capacity. Also, radiators can be made very, very mass effective - you can create near infinite surface area materials with high thermal conductivity. In the race between dissipating (emissivity) and PV power conversion (selective absorptivity) , radiators win hands down wrt to mass, volume and area.
Quote
In fact, there are other similarities between nuclear-electric and solar-electric... there is active research into "thermophotovoltaics" which convert infrared ("heat") light into electricity very efficiently and using the same basic physics as photovoltaics, plus a few other tricks thrown in. So, the same mechanism for converting to electricity for both solar and nuclear.
A guy at Berkeley came up with the ability to do this with cheap organic compounds very effectively. It may also be the case that other uses of nuclear power may be more effective than NEP for propulsion.
Quote
The advantage for solar (which have always used photovoltaics) is that you don't have to lug around the heat source. The advantage for nuclear is you can carry around the heat source.
No sir. You get heat on the PV's to but they come with own radiator.

The advantage for solar is the sun itself. And that we can build an cost effective logistical system out of it immediately, creating a path for NEP/SEP, then other nuclear.

Boy what a long post.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Solman on 05/09/2010 10:22 pm



 Solar Thermal.
I still marvel that it is routinely left out of the discussion

 If you have a solar thermal rocket you of necessity, have a large concentrator mirror or two. The PV concentrator arrays had small arrays of concentrators with the solar cells only an inch or two from the small concentrator. . The fouling from the concentrators that caused Boeing problems is not really possible if the concentrator type PV is located at the focus of a large - tens of meters say-mirror. The support structure for the mirror can double as radiator for the PV.
 Put a thermal rocket at the focus and it can use almost anything as propellent. It can operate at 1200 sec. Isp or higher. By using a series of perigee thrusts and a lower Isp final one, STR can switch to SEP and beat an NTR to Mars. Heck they even have a propellent depot already there in GEO - dead comsats weighing in at millions - well okay a couple million at least of pounds. Put concentrator cells at the focus and run a MPD or other electric propulsion system. Put a solar furnace at the focus and ISRU gets a lot easier.

Sol
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/09/2010 11:49 pm
Solar Thermal.
I still marvel that it is routinely left out of the discussion

We leave it out because it has piddly acceleration and will never go anywhere.  Plus it uses hydrogen that trashes any Isp advantage it might have.  See the analysis I referenced earlier relative to NTR to see why hydrogen propellant is such a bad idea.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: A_M_Swallow on 05/10/2010 01:08 pm
Solar Thermal.
I still marvel that it is routinely left out of the discussion

We leave it out because it has piddly acceleration and will never go anywhere.  Plus it uses hydrogen that trashes any Isp advantage it might have.  See the analysis I referenced earlier relative to NTR to see why hydrogen propellant is such a bad idea.

Medium sized thrust is available from solar thermal, it just needs big mirrors - about 1.3 kW/m2.  Solar thermal has a higher Isp then chemical (600s - 900s) and a higher thrust that most electrical propulsion.

Argon can be used instead of hydrogen.
http://www.uigi.com/argon.html (http://www.uigi.com/argon.html)
Argon
boiling point –302.6°F (–185.9°C) 
melting point –308.8°F (–199.3°C)
gas density 1.7837 kg/m3
liquid density 535.6 kg/m3
latent heat of vaporisation 162.3 kJ/kg
gas specific heat 0.523 kJ/kg oC
liquid specific heat 1.078 kJ/kg oC
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Solman on 05/10/2010 05:00 pm
Solar Thermal.
I still marvel that it is routinely left out of the discussion

We leave it out because it has piddly acceleration and will never go anywhere.  Plus it uses hydrogen that trashes any Isp advantage it might have.  See the analysis I referenced earlier relative to NTR to see why hydrogen propellant is such a bad idea.


 Why do you make the assumption that solar thermal us limited to hydrogen propellent? One of its main advantages as I see it is that it can use almost anything as propellent including the aluminum and lithium in dead comsats. Lithium is used in MPD no?

 As to the "piddly" thrust, this is largely dependent on the specific power of the concentrator and of course the particular propellent used. By working up to a highly elliptical orbit using a series of 1 ft. per second or so 1000 sec. perigee thrusts for example from LEO, it can take advantage of its higher Isp than NTR (at least solid core) for that part of the delta V and then increase mass flow rate (lowering Isp) on the final thrust to get as much velocity change near perigee as it can. It would then switch to concentrator PV powered MPD or other electric propulsion using lithium or some other propellent.

 I'd bet although I can;t prove it; that it could then catch up to and pass a NTR powered craft. At the least it;d be competitive. I have a paper copy of an old study by Kraft Ehricke in which the transit time for some Mars missions is reduced by dipping inside the orbit of Venus and using the twice as intense sunlight to enhance a STR thrust.

 Also, in these times the two to three orders of magnitude cost difference and much greater time to develop for NTR vs. ST/EP is crucial I think. One is cheap, fast, versatile and even alreadt has its own propellent depot so you save all those bucks on HLV development.

 NTR is unobtanium and NEP is a very slow boat made of unobtanium.

 Sol

Sol
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: A_M_Swallow on 05/10/2010 06:20 pm
The other advantage of solar thermal is that the concentrators are not damaged by the Van Allen Belts.  Solar cells lose about 20% of their power.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/10/2010 06:31 pm
The other advantage of solar thermal is that the concentrators are not damaged by the Van Allen Belts.  Solar cells lose about 20% of their power.

Not universally true. It depends on the solar cell. Also, annealing can do wonders for restoring beginning-of-life performance for some solar cells.

Also, the concentrator itself may degrade, which would effect solar thermal greatly.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Patchouli on 05/10/2010 06:44 pm
The more I look at the the more NTR engines such as Triton seem to be the best option for near term manned deep space flight.
http://www.engineeringatboeing.com/dataresources/AIAA-2004-3863.pdf

I sugest first testing them on lunar ferries such as the 1994 LANTR proposal.
This is how we should return to the Moon so we can learn how to travel to Mars ,the Asteroids and beyond.
http://www.nss.org/settlement/moon/LANTR.html

Lets face it most other high ISP engines just require too much time to gain speed.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: A_M_Swallow on 05/10/2010 06:46 pm
Also, the concentrator itself may degrade, which would effect solar thermal greatly.

Aluminium foil is resistant to degrading as a mirror.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/10/2010 06:50 pm
Also, the concentrator itself may degrade, which would effect solar thermal greatly.

Aluminium foil is resistant to degrading as a mirror.
The plastic substrate may degrade. Using aluminum foil as its own substrate would add a lot of weight.

I'm just saying that degradation is still an issue that should be considered even with solar thermal. As I said, with annealing, solar cells can be kept basically at full beginning-of-life performance, though in-situ annealing probably has never been demonstrated (though neither has solar-thermal).
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/10/2010 07:17 pm
Why do you make the assumption that solar thermal us limited to hydrogen propellent?

Because anything else has such lousy Isp that it's not worth doing.  Even with the Isp you can get from hydrogen it's still not worth doing.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Solman on 05/11/2010 04:11 pm
Why do you make the assumption that solar thermal us limited to hydrogen propellent?

Because anything else has such lousy Isp that it's not worth doing.  Even with the Isp you can get from hydrogen it's still not worth doing.

 I think we must be mis-communicating. I should have emphasized the use of resources already in GEO and beyond (and perhaps some in LEO ) beyond just saying that they can use dead comsats for propellent. Surely you agree that using aluminum at what 300 sec.? does make sense if that aluminum is already in orbit. Also lithium is present in at least modest quantities in those comsats and it should have an Isp better than chemical's best. Other possibilities include methane and ammonia. As for hydrogen; are you saying that it can't be used for the initial escape from LEO? STR's only need it to last a week or two and use it up over that time with most of it used in the first few days. After that lithium MPD would take over.

Sol
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Solman on 05/11/2010 04:31 pm
Also, the concentrator itself may degrade, which would effect solar thermal greatly.

Aluminium foil is resistant to degrading as a mirror.
The plastic substrate may degrade. Using aluminum foil as its own substrate would add a lot of weight.

I'm just saying that degradation is still an issue that should be considered even with solar thermal. As I said, with annealing, solar cells can be kept basically at full beginning-of-life performance, though in-situ annealing probably has never been demonstrated (though neither has solar-thermal).

 Actually, degradation can be a good thing. As the inventor of " Inflation Insituform with Degradable Elements" for forming structures in space, I think it can be the means to truly low mass concentrators far beyond anything currently proposed. The concentrator is itself a thin layer of aluminum or sodium laid down of a plastic backing that gradually degrades upon exposure to UV ( they're selling trash bags that do this). The support structure that forms the shape of the concentrator contains composites with resins that harden on exposure to UV. The concentrator layer may be perforated with holes smaller than visible light wavelengths when it is made on Earth so that its mass may be reduced tenfold. A conservative WAG is 100KW/kg, maybe better.

NTR is unobtanium

First ST/EP


Sol
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/11/2010 05:08 pm
NTR is unobtanium

No argument there.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/17/2010 08:33 pm
NTR is unobtanium

No argument there.

Can you explain why NTRs are unobtanium?  Or am I misunderstanding?

We've already built successful NTRs.  We're currently using these successful NTRs as reference designs.  We currently have people quietly working on them.  We could, if we so desired, launch a NERVA- or Pewee-derivative right now with no new technology development, strictly engineering.  What problem is so dire that you use the term "unobtanium"?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/17/2010 08:57 pm
No Dave, we haven't built successful NTRs.  We've fired engines on the ground in non-representative configurations.  None of those engines would even be considered for flight today.  Even if they were, their performance doesn't "buy" them on to the flight.  To get something that "buys" its way on, you need vastly higher Isps, and that requires materials development that is frankly unobtainium and probably always will be if it involves testing here on the ground in the terrestrial ecosystem.  See my referenced analysis of NTR performance vs. a chemical stage.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/17/2010 09:43 pm
No Dave, we haven't built successful NTRs.  We've fired engines on the ground in non-representative configurations.  None of those engines would even be considered for flight today.  Even if they were, their performance doesn't "buy" them on to the flight.  To get something that "buys" its way on, you need vastly higher Isps, and that requires materials development that is frankly unobtainium and probably always will be if it involves testing here on the ground in the terrestrial ecosystem.  See my referenced analysis of NTR performance vs. a chemical stage.
Also, a good, roughly linear metric of engine performance that includes propellant density that compromises pure Isp and density-Isp is this:
(from here: http://www.gkllc.com/lit/misc/LLNL-Single_Stage_to_Orbit_Mass_Budgets_Derived_from_Propellant_Density_Isp-1996.pdf )

(bulk density)*Isp^2

liquid hydrogen: .071kg/liter
hydrolox: .3587kg/liter
kerolox: 1.03kg/liter

NTR Isp: 1000s
hydrolox: 460s
kerolox: 340s

NTR:
dens*Isp^2=71000*s^2*kg/liter

hydrolox:
dens*Isp^2=75900*s^2*kg/liter

kerolox:
dens*Isp^2=119068*s^2*kg/liter

Now, this doesn't take into full account the heavy shielding and reactor weight, but it does give you a good explanation of why NTRs aren't so great (and partly why many countries continue to use kerolox), and why in order to really make them worthwhile when using a low-density fuel like liquid hydrogen, Isp needs to be raised substantially, like to 1500s. Of course, the actual trade-off between Isp and prop density (and dry mass and T/W of the engine) depends on the actual requirements of the mission, and changes substantially whether we require a high T/W because we're launching while at rest in a deep gravity well or if we're just floating around in orbit and want to speed up our mission transit time.

Also, remember that large tank sizes are clumsy logistically and are (obviously) more difficult to fabricate than a smaller tank, thus making them more expensive. Since aerospace-grade hardware is so expensive (especially compared to the market cost of propellant), it makes sense to minimize dry weight, in which hydrolox does poorly and NTR does quadruply poorly (only liquid hydrogen plus atrocious T/W plus shielding).
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/18/2010 06:14 pm
No Dave, we haven't built successful NTRs.  We've fired engines on the ground in non-representative configurations. 

You've redefined 'successful' AND 'unobtanium' in a way that's incompatible with my definitions.  To me, 'successful' means that we developed the technologies sufficiently well to build nuclear rockets.  'Unobtanium' means 'cannot under any circumstances be obtained', i.e., impossible.  While you may think of NTRs as 'difficult', they are certainly not 'unobtanium'.

NERVA/EST and NERVA/XE were both tested configured completely with flight components in downward configurations and in near-vacuum conditions.  To quote Harry Finger (who headed NASA SNPO), "The total run time on the XE engine was 115 minutes and included twenty-eight starts to power operation. The XE test series was significant in that it confirmed that a nuclear rocket engine was suitable for space flight application and was able to operate at a specific impulse twice that of chemical rocket system. All engine feasibility issues were successfully addressed, no component or systems issues were observed, and the development of a flight nuclear rocket system could proceed with confidence."

Quote
None of those engines would even be considered for flight today.
 

Of course not.  The next step would have been the Pre-Flight Test Reactor, and the one after that the first flight prototype, had they been deployed as-scheduled by 1975.  They weren't, but all subsequent serious solid-core NTR studies are based on NERVA or Pewee derivatives (except in the CIS).

Quote
Even if they were, their performance doesn't "buy" them on to the flight.  To get something that "buys" its way on, you need vastly higher Isps, and that requires materials development that is frankly unobtainium and probably always will be if it involves testing here on the ground in the terrestrial ecosystem.  See my referenced analysis of NTR performance vs. a chemical stage.

Last night, I went and saw your referenced analysis, and built a spreadsheet, and played.  I enjoyed your articles.  But you used an Isp of 850, while the current Pewee-based models all use Isps of 925, which is considered to be conservative enough to be 'safe'.  You also used a T/W of 3, and current models all use T/W of at least 5 (I actually seem to remember that Pewee was 6).

Plugging a new Isp and T/W of 5 into the spreadsheet I got multipliers of

       3800  4000  4200  4400
0.2   1.39   1.42   1.44   1.47
0.4   1.30   1.33   1.35   1.37
0.6   1.22   1.23   1.25   1.27
0.8   1.12   1.14   1.15   1.17
1.0   1.03   1.04   1.05   1.06

Several of the NERVA tests ran slush hydrogen, which improves the numbers a bit.  Going to composite tanks improves the numbers a bit as well.  There are other tradeoffs, like using the NTR to generate vehicle power and refrigeration for boil-off.

It's not unreasonable to plug in higher numbers for Isp; I've surveyed what's available and I tend to think that with modern materials we're capable of attaining at least 1100.

If we thought the way that you're suggesting, Kirk, we never would have developed travel to LEO.  Any technology that has a very long-term payoff is generally unsuitable for business.  If the payoff is large enough, though, it's advantageous for government to step up and do the heavy lifting as a proxy for the society that will benefit from it much later.  In this case, basic physics says that we're going to need nuclear thermal propulsion.  In order to really travel around the solar system, we need to be up at least around Isp of 2500.  So what?  We swallow hard, aggregate our tax dollars, and get 'er done.  The government ends up paying a lot for the initial Mars mission, and then private industry eventually picks up the technology and begins using it. 

Decades later, when NTRs and travel around the solar system are commonplace, the government investment is considered well worth it.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/18/2010 06:30 pm
"Nuclear" is not the same as "magic."

The Tri-modal nuclear engines like "Triton" are new designs, but have only 3.6 T/W in nuclear mode and an Isp of 911s. Better than NERVA, but they aren't these magical 1200s Isps. And does this include substantial shielding for a manned spacecraft?

The most realistic and mature designs have lower performance.

EDIT: It should also be noted than Sorensen is no enemy of nuclear power in general. He writes the http://energyfromthorium.com/ blog.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/18/2010 07:57 pm
"Nuclear" is not the same as "magic."

Certainly not.  But "difficult" is not the same as "unobtanium".  As more time passes nuclear begins to seem more and more difficult to us, for no real reason other than that the generation that built NTRs is beginning to disappear.

Nuclear rockets bring with them their own engineering issues, not least of which is that we don't yet know how to keep neutrons from going where they're not wanted.  But these have been *engineering* issues, not science or technology issues, since at least as far back as 1966.

There is not enough energy in the chemical bond to get us around the solar system.  Period.  End of story.  We can go to LEO on chemical rockets, but as soon as we want to service a lunar base or take supplies elsewhere doing it without using nuclear energy becomes ridiculous.  Anybody in their right mind would grant that nuclear is stuck in an early stage right now; it's stuck in the 1960's, until we pull it out, dust it off and start using it.  When we start using it, we'll extract higher Isps.

Quote
The Tri-modal nuclear engines like "Triton" are new designs, but have only 3.6 T/W in nuclear mode and an Isp of 911s. Better than NERVA, but they aren't these magical 1200s Isps. And does this include substantial shielding for a manned spacecraft?

Liquid hydrogen is excellent shielding, but an accumulator tank for shielding is another thing that has to be included in calculations for manned nuclear spacecraft.

Quote
The most realistic and mature designs have lower performance.

Yes, but anybody who knows what's available defines lower performance as "below 1000 Isp".  There are plenty of reasonable ways to get Isps above 1000.  Neither the USA nor the Russians have stopped working on NTRs since 1972, although work certainly slowed down after NERVA ended.

Quote
EDIT: It should also be noted than Sorensen is no enemy of nuclear power in general. He writes the http://energyfromthorium.com/ blog.

In 1946, Luis Alvarez wrote in a flight magazine that nuclear rockets would be so heavy that they would be impossible to build at T/W ratios approaching 1.  In 1952, the assistant director of Livermore (forgot his name) figured out how to do it.  In 1955, when Rover started, it still seemed very difficult, but by 1959, Los Alamos was firing rockets.  Along the way they basically invented the graphite industry, pioneered cryogenics, characterized several elements (like carbon) that had never been well-characterized, figured out that you could lubricate a turbopump with hydrogen, invented several big chunks of the electronics and sensor industry, pioneered robotics, and not least of all invented a lot of nuclear physics.

The reason they did all of that was primarily because the Democrats invented the Space Race to gain control of the country, and secondarily because some of those Democrats, like Clinton P. Anderson and LBJ, really and truly believed in space exploration.  They had been told by scientists like Leo Szilard and Glenn Seaborg that the only way we would explore the solar system would be with the energy available from splitting or fusing atoms.  And they didn't even know then how radioactive space is.

Now we know that human beings can't endure long periods of time in space without huge amounts of shielding, and we have also learned that Congress won't fund huge missions with durations longer than the public attention span (which is growing shorter every day).  Ultimately, for economy, for practicality, for feasibility, the energy necessary for moving around the solar system is too large to come from chemical bonds.  Seaborg and Szilard and Bussard are still right, and however long we wait we still have to get good at nuclear rockets. 

The good news is, it's just engineering.  The problems that Kirk points out are valid ones, sort of (he uses dated numbers and acts as if the problems are unsolvable) but we're at a point now (and have been) where we build, and then just refine, refine, refine.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/18/2010 08:16 pm
"Nuclear" is not the same as "magic."

Certainly not.  But "difficult" is not the same as "unobtanium".  As more time passes nuclear begins to seem more and more difficult to us, for no real reason other than that the generation that built NTRs is beginning to disappear.
...
My point wasn't that nuclear thermal rockets are impossible to build. Not at all. It's just that they really don't buy you that much compared to high performance hydrolox. I mean, suppose they double your payload to Mars orbit for the same IMLEO. Big deal!

Prop depots are far more important, because they allow you to potentially lower costs to orbit to one-tenth current costs (and because they allow you to pre-position propellant using slower, more efficient trajectories and/or propulsion systems)

Even 1000s isn't that great. We need 2000s Isp to really make a huge difference, and there is no WAY nuclear thermal rockets will be capable of that, simply because the materials required don't exist. For that, we need electric propulsion.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: isa_guy on 05/18/2010 08:42 pm
I am not an expert like you guys so i have two questions related to the future prospects of the nuclear thermal rocket . First is there some way  to get more than  1000 isp  from a solid core NTR . Second  (quite of topic )  is it possible to develop a gas core NTR ( which may produce a Specific impulse of about  3000-5000 sec ) in the 10 to 20 years time frame so its could be ready for the new nasa's proposed  manned mission to mars in the 2030's ?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: tnphysics on 05/18/2010 08:48 pm
2000sec Isp IS possible with a nuclear lightbulb, but that is immature tech.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: 93143 on 05/18/2010 08:52 pm
The Tri-modal nuclear engines like "Triton" are new designs, but have only 3.6 T/W in nuclear mode and an Isp of 911s.

That's a tri-modal design with electric power production capability.

A Dumbo design could do ten times better on T/W.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/18/2010 08:53 pm
2000sec Isp IS possible with a nuclear lightbulb, but that is immature tech.
The materials for that don't exist. I used to be a big fan of that idea, too, but I don't see how the "glass" of the light bulb could possibly survive for long. I mean, we're talking about something far hotter than the surface of the sun, plus neutrons flying around everywhere and transmuting everything... even if the unobtainium "glass" could at first survive the inferno, it would be transmuted into something else that absorbs UV and causes the "glass" to vaporize.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/18/2010 08:56 pm
"Nuclear" is not the same as "magic."

Certainly not.  But "difficult" is not the same as "unobtanium".  As more time passes nuclear begins to seem more and more difficult to us, for no real reason other than that the generation that built NTRs is beginning to disappear.
...
My point wasn't that nuclear thermal rockets are impossible to build. Not at all. It's just that they really don't buy you that much compared to high performance hydrolox. I mean, suppose they double your payload to Mars orbit for the same IMLEO. Big deal!

Depending on the mission, that's a pretty big deal.  That can mean the difference between six years and six months.

Quote
Prop depots are far more important, because they allow you to potentially lower costs to orbit to one-tenth current costs (and because they allow you to pre-position propellant using slower, more efficient trajectories and/or propulsion systems)

The original meaning of "Space Transportation System" (STS) included shuttles for Earth-LEO, space stations with propellant depots in several different orbits, and nuclear tugs for both logistical and interplanetary missions.  I wouldn't say "far more important", though; I'd say they're just another part of the system.

Quote
Even 1000s isn't that great. We need 2000s Isp to really make a huge difference, and there is no WAY nuclear thermal rockets will be capable of that, simply because the materials required don't exist. For that, we need electric propulsion.

I don't think 2000s is all that impossible either, given a couple of decades' development and some clever engineering.  Much beyond that and hopefully we'll have decent SBNRs to power them EPs you like, along with the requisite football-field-sized radiators.  But for impulse density and efficiency, there's currently no substitute for NTRs.  That may change the next time a new invention comes along, but for now, that's where we are.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/18/2010 09:24 pm
In the inner solar system, there's no substitute for solar electric propulsion:
proven ( http://dawn.jpl.nasa.gov ), cheap, safe (by everyone's standard), lightweight, and high performance (electric propulsion has the highest Isp). It can be used on the largest scales and on the smallest scales. No fission fuel to replace. No extra shielding required. No transmutation. No latent radioactivity means it can be serviced by astronauts halfway through the mission in an emergency:
http://www.youtube.com/watch?v=YW8lcQaicfw

It is capable of specific powers of 1000W/kg (or potentially even 10,000W/kg) for the power source (see: http://en.wikipedia.org/wiki/IKAROS ) or greater, but certainly good enough even with the only 200W/kg specific power you can get right now ( http://www.aec-able.com/arrays/arrayperform.html and http://www.ncsu.edu/kenan/ncsi/pdf/ARO_Energy2_Lyons.pdf and http://www.aec-able.com/corpinfo/Resources/ABLE%20PVSC-TF%20v%20tems%20RevA.1.pdf).

The only place nuclear is required is in the outer solar system where there's not enough sunshine or on the surface where night lasts too long or the environment interferes.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: tnphysics on 05/18/2010 09:25 pm

Quote
Even 1000s isn't that great. We need 2000s Isp to really make a huge difference, and there is no WAY nuclear thermal rockets will be capable of that, simply because the materials required don't exist. For that, we need electric propulsion.

I don't think 2000s is all that impossible either, given a couple of decades' development and some clever engineering.  Much beyond that and hopefully we'll have decent SBNRs to power them EPs you like, along with the requisite football-field-sized radiators.  But for impulse density and efficiency, there's currently no substitute for NTRs.  That may change the next time a new invention comes along, but for now, that's where we are.

How?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/19/2010 02:13 am

Quote
Even 1000s isn't that great. We need 2000s Isp to really make a huge difference, and there is no WAY nuclear thermal rockets will be capable of that, simply because the materials required don't exist. For that, we need electric propulsion.

I don't think 2000s is all that impossible either, given a couple of decades' development and some clever engineering.  Much beyond that and hopefully we'll have decent SBNRs to power them EPs you like, along with the requisite football-field-sized radiators.  But for impulse density and efficiency, there's currently no substitute for NTRs.  That may change the next time a new invention comes along, but for now, that's where we are.

How?

I've thought about it enough to have some ideas.  Without getting too specific, I think there are plenty of areas that could be addressed:  fuel rod tensile strength, non-refractory ultra high-temperature compounds, very high pressure cores, zirconium hydride infusion, transpiration cooling.   Like I said, it would take at least a couple of decades, but I actually think there's a lot of room for improvement in the current state-of-the-art.  I don't agree at all with the folks who think a 3000K core temp is anywhere near the temperature ceiling of a solid core NTR.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: sandrot on 05/19/2010 03:11 am
What do you do when you run out of coolant (hydrogen used for propulsion) on a nuclear lightbulb?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/19/2010 03:19 am
What do you do when you run out of coolant (hydrogen used for propulsion) on a nuclear lightbulb?

Scram!
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Patchouli on 05/19/2010 03:50 am
Scramming a nuclear light bulb is fairly easy you just send the UF6 into a boron lined holding vessel.

Easier then the nuclear light bulb is the nuclear saltwater rocket but this cannot be used in LEO because it's exhaust is radioactive.
Good news once in you're at least 1200km up you could use and the exhaust products would be at escape velocity.

The best near term propulsion for crewed vehicles though is the solid core NTR as it's exhaust is just hot hydrogen.

As for remarks it's ISP is not high enough even 900sec would be a game changer.

A 900 sec ISP would allow a transfer craft to fly from LEO to the moon and return to LEO on a single refueling.
Single stage  from LEO to LLO back to LLO.
It also can go to the moon in just 24 to 36 hours.
http://www.nss.org/settlement/moon/LANTR.html

As for concerns on radiation shielding that was solved decades ago during the XB-36H program.
A spacecraft would have it's nuclear engines much farther back then the reactor in the XB-36H and the hydrogen tanks would make an excellent radiation shield.
http://en.wikipedia.org/wiki/Convair_X-6


Solar is still the best for cargo as it's cheaper but it's no good for live cargo that cares how long the trip through the Van Allen belts takes or would prefer to have a 150 day vs a 220 day trip to Mars.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: sandrot on 05/19/2010 03:58 am
And does SCRAM preclude a restart?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: isa_guy on 05/19/2010 04:33 pm
 What about this study on the gas (vapor) core reactor technology made by University of Florida with NASA's funding several years ago http://ams.cern.ch/AMS/ETB/Appendix%20D-Anghaie.pdf , i know the professor that managed this program at the University of Florida  has been accused of fraud  http://www.floridatoday.com/content/blogs/space/2009/02/nasa-feds-investigating-uf-prof.shtml , but the concept of gas core reactors was also investigated  by others like the russians for example http://pdf.aiaa.org/preview/CDReadyMASM07_1064/PV2007_35.pdf . Chang-diaz also advocated to use this type of reactor technology as a power source ( and in particular the concept  that has been studied in the University of Florida ) for his proposed 200 MWe VASIMR powered Mars transfer vehicle .  My questions is IF this concept is viable could NASA ( or DOE ) develop  it in about 10 to 20 years so its may be used for the NASA's proposed  manned mission to mars in the 2030's .
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/19/2010 04:45 pm
The best near term propulsion for crewed vehicles though is the solid core NTR as it's exhaust is just hot hydrogen.

As for remarks it's ISP is not high enough even 900sec would be a game changer.

What did you mean to say here?  I'm afraid I didn't understand your sentence.

NASA's current reference designs sit at 925 seconds.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/19/2010 06:31 pm
NASA's current reference designs sit at 925 seconds.

That is a fantasy value.  Just because Stan Borowski of GRC tells Bret Drake of JSC to use that number in a mission study doesn't make it real.  Stan gets it from a very speculative guess about Russian tricarbide fuel that has never been made in the US and certainly never tested.

But hey, I used to run Mars mission studies and would take Stan's numbers at face value.  I learned the hard way not to.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 05/19/2010 07:31 pm
NASA's current reference designs sit at 925 seconds.

That is a fantasy value.  Just because Stan Borowski of GRC tells Bret Drake of JSC to use that number in a mission study doesn't make it real.  Stan gets it from a very speculative guess about Russian tricarbide fuel that has never been made in the US and certainly never tested.

But hey, I used to run Mars mission studies and would take Stan's numbers at face value.  I learned the hard way not to.

Are you familiar with the capabilities of tungsten-cermet reactors and their possible nuclear thermal rocket (NTR) derivatives?   Since we would have to develop these NTRs practically from scratch again, we might as well go with a fuel system with some performance legs that can reach the 925 seconds Isp performance you speak of and perhaps beyond.   Especially if we went with the Dumbo high power design approach to the NTR core.

See attached INL presentation and the below URL.

http://www.usra.edu/cs/csnr
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: isa_guy on 05/19/2010 07:40 pm
NASA's current reference designs sit at 925 seconds.

That is a fantasy value.  Just because Stan Borowski of GRC tells Bret Drake of JSC to use that number in a mission study doesn't make it real.  Stan gets it from a very speculative guess about Russian tricarbide fuel that has never been made in the US and certainly never tested.

But hey, I used to run Mars mission studies and would take Stan's numbers at face value.  I learned the hard way not to.
Speculative ? and what about RD0410 http://www.kbkha.ru/?p=8&cat=11&prod=66 .
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Archibald on 05/19/2010 08:45 pm
The RD 0410 is precisely the basis of Borowski work of the last twenty years. Borowski and others went to Russia in 1992, and found that the soviets had their own NERVA program - the said
RD 0410.
Amid the things more or less tested was the famed tricarbide fuel
Elements of the thing have been tested separately, but not the whole reactor.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/19/2010 09:48 pm
NASA's current reference designs sit at 925 seconds.

That is a fantasy value.

Sheesh, Kirk.  That's a pretty strong statement.  Might you be willing to temper your sentence to, "I don't agree with that value"?

Quote
Just because Stan Borowski of GRC tells Bret Drake of JSC to use that number in a mission study doesn't make it real.  Stan gets it from a very speculative guess about Russian tricarbide fuel that has never been made in the US and certainly never tested.

Goodness knows, the Russians don't have any credibility when it comes to rocketry.  I wonder whether Bill Emrich has done tricarbides at NTREES.  Or what Y-12 decided, exactly, about tricarbides, back in 1970.  Paul Wagner here in Albuquerque seems to feel strongly that they had made a great deal of progress with the last of the materials tests in the Nuclear Furnace.

Regardless, even if Kirk Sorensen doesn't believe any research on NTRs up until May, 2010 has revealed anything of note, it doesn't matter.  There isn't enough energy in the chemical bond to take humans around the solar system.  There aren't any other technologies to take humans around the solar system without football-field-sized radiators or impractical amounts of radiation exposure, much less the travel times that make the Dutch East India Company look like FedEx.  We're left with NTRs.

Kirk, I'm sure you'll be out here for NETS 2011 in February, so you can yell and shake your fist at all the folks in the NTR track.  In the mean time, though, remember that these forums will eventually show up in Google, and having the wrong person read them might get a bunch of people's research defunded, some of which might be research you believe in.  Nuclear is nuclear is nuclear, to a politician with an agenda, and the power reactors might get thrown out with the NTRs on the strength of Kirk Sorensen's untempered statements.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/19/2010 09:50 pm
The Sun is even better than nuclear fission: it's nuclear fusion.  :P  ;D
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/19/2010 09:53 pm
Kirk, I'm sure you'll be out here for NETS 2011 in February, so you can yell and shake your fist at all the folks in the NTR track.

No, after attending the NTR session at JPC 2005 I gave up once and for all on the NTR community.

In the mean time, though, remember that these forums will eventually show up in Google, and having the wrong person read them might get a bunch of people's research defunded, some of which might be research you believe in.  Nuclear is nuclear is nuclear, to a politician with an agenda, and the power reactors might get thrown out with the NTRs on the strength of Kirk Sorensen's untempered statements.

Nobody cares what I think.  Trust me on that.  If anyone was listening to my opinion on NTRs they would have gotten defunded a long time ago.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/19/2010 09:55 pm
The Sun is even better than nuclear fission: it's nuclear fusion.  :P  ;D

No, fission is better.  It's done with uncharged particles at low energies.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/19/2010 10:01 pm
The Sun is even better than nuclear fission: it's nuclear fusion.  :P  ;D

No, fission is better.  It's done with uncharged particles at low energies.
I agree, but I was being somewhat facetious.
(When I look at all the complicated steps researchers go through to try to achieve break-even fusion energy production--using exotic bred fuels like tritium or even He3--it makes me appreciate the beauty of the fission chain reaction... one neutron enters, more than one leave... and it uses a pretty plentiful fuel, too, if you do it right... of course, look who I'm talking to. :) )
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/19/2010 10:10 pm
Kirk, I'm sure you'll be out here for NETS 2011 in February, so you can yell and shake your fist at all the folks in the NTR track.

No, after attending the NTR session at JPC 2005 I gave up once and for all on the NTR community.

Perhaps you can just think nasty thoughts from the power tracks, then.  There's some good work being done on small surface reactors for exploration, and I'm quite sure they'd benefit from you showing up with some thorium papers.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/19/2010 10:32 pm
Kirk, I'm sure you'll be out here for NETS 2011 in February, so you can yell and shake your fist at all the folks in the NTR track.

No, after attending the NTR session at JPC 2005 I gave up once and for all on the NTR community.

Perhaps you should devote a little bit of time toward fixing what you perceive as the problems with the NTR.

I have a question for this discussion.  I've wondered whether it might be possible to pinch the output of an NTR to pick up a few fusions to increase the power output.  I figured the output temperature of the hydrogen is roughly an order of magnitude off what it needs to be, but there's plenty of energy available to drive it.  Standing wave?  Magnetic pinch?  Stupid idea?  Anybody have any thoughts?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/19/2010 10:48 pm
Kirk, I'm sure you'll be out here for NETS 2011 in February, so you can yell and shake your fist at all the folks in the NTR track.

No, after attending the NTR session at JPC 2005 I gave up once and for all on the NTR community.

Perhaps you should devote a little bit of time toward fixing what you perceive as the problems with the NTR.

I have a question for this discussion.  I've wondered whether it might be possible to pinch the output of an NTR to pick up a few fusions to increase the power output.  I figured the output temperature of the hydrogen is roughly an order of magnitude off what it needs to be, but there's plenty of energy available to drive it.  Standing wave?  Magnetic pinch?  Stupid idea?  Anybody have any thoughts?
Stupid idea. It's not one order of magnitude off, it's at least 3 or 4. And hydrogen (i.e. protium) doesn't fuse well with its self, so even at tens of millions of degrees, power-production density is not much better than the human metabolic rate... obviously not very good for a rocket engine. ;)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/19/2010 11:00 pm
Kirk, I'm sure you'll be out here for NETS 2011 in February, so you can yell and shake your fist at all the folks in the NTR track.

No, after attending the NTR session at JPC 2005 I gave up once and for all on the NTR community.

Perhaps you should devote a little bit of time toward fixing what you perceive as the problems with the NTR.

I have a question for this discussion.  I've wondered whether it might be possible to pinch the output of an NTR to pick up a few fusions to increase the power output.  I figured the output temperature of the hydrogen is roughly an order of magnitude off what it needs to be, but there's plenty of energy available to drive it.  Standing wave?  Magnetic pinch?  Stupid idea?  Anybody have any thoughts?
Stupid idea. It's not one order of magnitude off, it's at least 3 or 4. And hydrogen (i.e. protium) doesn't fuse well with its self, so even at tens of millions of degrees, power-production density is not much better than the human metabolic rate... obviously not very good for a rocket engine. ;)

Hmmm.  Whups.  That IS four orders of magnitude.  Ever think something that sounds entirely reasonable until somebody points out that it's stupid?  Er, thanks.  I'll just hang my head here.

"When you are a Bear of Very Little Brain, and you Think of
Things, you find sometimes that a Thing which seemed very
Thingish inside you is quite different when it gets out into
the open and has other people looking at it."
-- A.A. Milne
   The House at Pooh Corner
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/19/2010 11:06 pm
...
Hmmm.  Whups.  That IS four orders of magnitude.  Ever think something that sounds entirely reasonable until somebody points out that it's stupid?  Er, thanks.  I'll just hang my head here.
...
Don't be so down. On the upside, luckily fusion ISN'T so easy, otherwise the first atomic bomb test may have fused the whole atmosphere. ;)
Good day!
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: tnphysics on 05/19/2010 11:35 pm

Quote
Even 1000s isn't that great. We need 2000s Isp to really make a huge difference, and there is no WAY nuclear thermal rockets will be capable of that, simply because the materials required don't exist. For that, we need electric propulsion.

I don't think 2000s is all that impossible either, given a couple of decades' development and some clever engineering.  Much beyond that and hopefully we'll have decent SBNRs to power them EPs you like, along with the requisite football-field-sized radiators.  But for impulse density and efficiency, there's currently no substitute for NTRs.  That may change the next time a new invention comes along, but for now, that's where we are.

How?

I've thought about it enough to have some ideas.  Without getting too specific, I think there are plenty of areas that could be addressed:  fuel rod tensile strength, non-refractory ultra high-temperature compounds, very high pressure cores, zirconium hydride infusion, transpiration cooling.   Like I said, it would take at least a couple of decades, but I actually think there's a lot of room for improvement in the current state-of-the-art.  I don't agree at all with the folks who think a 3000K core temp is anywhere near the temperature ceiling of a solid core NTR.

How hot do you need to get to dissociate H2?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/19/2010 11:40 pm

Quote
Even 1000s isn't that great. We need 2000s Isp to really make a huge difference, and there is no WAY nuclear thermal rockets will be capable of that, simply because the materials required don't exist. For that, we need electric propulsion.

I don't think 2000s is all that impossible either, given a couple of decades' development and some clever engineering.  Much beyond that and hopefully we'll have decent SBNRs to power them EPs you like, along with the requisite football-field-sized radiators.  But for impulse density and efficiency, there's currently no substitute for NTRs.  That may change the next time a new invention comes along, but for now, that's where we are.

How?

I've thought about it enough to have some ideas.  Without getting too specific, I think there are plenty of areas that could be addressed:  fuel rod tensile strength, non-refractory ultra high-temperature compounds, very high pressure cores, zirconium hydride infusion, transpiration cooling.   Like I said, it would take at least a couple of decades, but I actually think there's a lot of room for improvement in the current state-of-the-art.  I don't agree at all with the folks who think a 3000K core temp is anywhere near the temperature ceiling of a solid core NTR.

How hot do you need to get to dissociate H2?

About 3000K.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/20/2010 01:06 am
Perhaps you should devote a little bit of time toward fixing what you perceive as the problems with the NTR.

No.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: jimgagnon on 05/20/2010 06:49 am
Reading through the thread I was a little surprised to see no mention of another nuclear engine option for interplanetary travel: the fission fragment engine. Rodney A. Clark and Robert B. Sheldon have proposed a dusty plasma design that they feel could produce exhaust velocities of 3% - 5% the speed of light with efficiencies up to 90%, and achieve over 1,000,000 sec Isp.

Here's their paper on the design:
  http://www.rbsp.info/rbs/RbS/PDF/aiaa05.pdf

I know it's not as mature as some of the other designs but their engine does have that elegant simplicity you like to see in a solution. Is it worthy of consideration beside the other designs proposed so far?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: tnphysics on 05/21/2010 01:03 am
Yes for in-space. No for launch (not enough thrust).
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/21/2010 04:31 am
I like the fission fragment engine concept. Neat idea because (if you could make it work) it really would make Oort-cloud missions possible, perhaps even exploring a near-by Brown Dwarf (if there are any) if you're ambitious.

It's pretty far in the future, though. I would still like to see some research for it. It does obey the laws of physics (for the most part) and it almost allows interstellar travel without the typical multiple-human-civilizations-power-and-funding-level of many other interstellar propulsion methods.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: jimgagnon on 05/21/2010 04:08 pm
Yes for in-space. No for launch (not enough thrust).

Of course. Actually, since it is an open nuclear reactor, I wouldn't operate this thing anywhere inside the Van Allen belts. I'm just a little surprised that it doesn't receive more attention; if it could be built zipping around the solar system would be lots of fun with three or four of 'em strapped to your tail.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: isa_guy on 05/21/2010 09:21 pm
And what about the architecture used for the HOPE ( Human Outer Planet Exploration ) study ,  http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040010797_2004001506.pdf , and one with a nuclear electric multi-mw MPD thrusters ( also for the HOPE study ) http://trajectory.grc.nasa.gov/aboutus/papers/STAIF-2003-177.pdf , also what are the prospects that the HOPE concept could be useful for a future manned missions to mars . Second what would be the mass of a 20-30 MWe molten-salt or a liquid metal space reacors that could power nuclear electric Mars Transfer Vehicle with a human crew  to mars and beyond  ?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/22/2010 04:16 pm
Here's an example of some of the comparative trajectory analyses I have done to assess the performance of NEP and SEP vehicles relative to one another.  This particular case was an Earth-Mars transfer mission.

354 trajectories executed...
Optimal NEP Departure MJD:   61332.0  10/18/2026    JV: 4.006 W/kg
Optimal SEP Departure MJD:   61343.0  10/29/2026    JV: 6.488 W/kg

                     NEP VEHICLE_________________________________________________________________
ALPHA  SPPWR  TRSFR      MJD  DATE        ISP      POWER    OPTJC   PROPF   POWRF   PAYLF   TANKF
kg/kW   W/kg   days                       sec        kWe     W/kg   mp/m0   ms/m0   mn/m0   mt/mo
  2.0  500.0  300.0  61330.0  10/16/2026  3000.0  23.307    5.903  0.2278  0.0466  0.7142  0.0114
  3.0  333.3  300.0  61330.0  10/16/2026  3000.0  20.490    5.403  0.2338  0.0615  0.6930  0.0117
  4.0  250.0  300.0  61330.0  10/16/2026  3000.0  18.840    5.134  0.2389  0.0754  0.6738  0.0119
  5.0  200.0  300.0  61330.0  10/16/2026  3000.0  17.706    4.965  0.2435  0.0885  0.6558  0.0122
  6.0  166.7  300.0  61330.0  10/16/2026  3000.0  16.901    4.857  0.2475  0.1014  0.6388  0.0124
  7.0  142.9  300.0  61330.0  10/16/2026  3000.0  16.257    4.781  0.2512  0.1138  0.6224  0.0126
  8.0  125.0  300.0  61330.0  10/16/2026  3000.0  15.761    4.730  0.2546  0.1261  0.6066  0.0127
  9.0  111.1  300.0  61330.0  10/16/2026  3000.0  15.343    4.693  0.2579  0.1381  0.5912  0.0129
 10.0  100.0  300.0  61330.0  10/16/2026  3000.0  14.992    4.668  0.2609  0.1499  0.5761  0.0130
 11.0   90.9  300.0  61330.0  10/16/2026  3000.0  14.691    4.651  0.2638  0.1616  0.5614  0.0132
 12.0   83.3  300.0  61330.0  10/16/2026  3000.0  14.433    4.642  0.2665  0.1732  0.5470  0.0133
 13.0   76.9  300.0  61329.0  10/15/2026  3000.0  14.174    4.636  0.2695  0.1843  0.5328  0.0135
 14.0   71.4  300.0  61329.0  10/15/2026  3000.0  13.960    4.635  0.2721  0.1954  0.5188  0.0136
 15.0   66.7  300.0  61329.0  10/15/2026  3000.0  13.787    4.637  0.2744  0.2068  0.5050  0.0137
 16.0   62.5  300.0  61329.0  10/15/2026  3000.0  13.627    4.641  0.2767  0.2180  0.4914  0.0138
 17.0   58.8  300.0  61329.0  10/15/2026  3000.0  13.482    4.648  0.2789  0.2292  0.4779  0.0139
 18.0   55.6  300.0  61329.0  10/15/2026  3000.0  13.338    4.657  0.2812  0.2401  0.4646  0.0141
 19.0   52.6  300.0  61329.0  10/15/2026  3000.0  13.219    4.667  0.2833  0.2512  0.4514  0.0142
 20.0   50.0  300.0  61329.0  10/15/2026  3000.0  13.103    4.679  0.2854  0.2621  0.4383  0.0143
 21.0   47.6  300.0  61329.0  10/15/2026  3000.0  13.000    4.691  0.2873  0.2730  0.4253  0.0144

                     SEP VEHICLE_______________________
ALPHA  SPPWR  TRSFR      MJD  DATE        ISP      POWER    OPTJC   PROPF   POWRF   PAYLF   TANKF
kg/kW   W/kg   days                       sec        kWe     W/kg   mp/m0   ms/m0   mn/m0   mt/m0
  2.0  500.0  300.0  61346.0  11/01/2026  3000.0  33.316    8.768  0.2388  0.0666  0.6827  0.0119
  3.0  333.3  300.0  61345.0  10/31/2026  3000.0  29.404    8.170  0.2470  0.0882  0.6524  0.0124
  4.0  250.0  300.0  61345.0  10/31/2026  3000.0  27.147    7.882  0.2540  0.1086  0.6247  0.0127
  5.0  200.0  300.0  61344.0  10/30/2026  3000.0  25.638    7.726  0.2600  0.1282  0.5988  0.0130
  6.0  166.7  300.0  61344.0  10/30/2026  3000.0  24.559    7.641  0.2654  0.1474  0.5740  0.0133
  7.0  142.9  300.0  61344.0  10/30/2026  3000.0  23.729    7.598  0.2703  0.1661  0.5501  0.0135
  8.0  125.0  300.0  61344.0  10/30/2026  3000.0  23.076    7.580  0.2747  0.1846  0.5270  0.0137
  9.0  111.1  300.0  61344.0  10/30/2026  3000.0  22.534    7.581  0.2789  0.2028  0.5044  0.0139
 10.0  100.0  300.0  61344.0  10/30/2026  3000.0  22.092    7.594  0.2827  0.2209  0.4822  0.0141
 11.0   90.9  300.0  61344.0  10/30/2026  3000.0  21.706    7.617  0.2864  0.2388  0.4605  0.0143
 12.0   83.3  300.0  61344.0  10/30/2026  3000.0  21.379    7.646  0.2899  0.2565  0.4391  0.0145
 13.0   76.9  300.0  61344.0  10/30/2026  3000.0  21.084    7.681  0.2933  0.2741  0.4180  0.0147
 14.0   71.4  300.0  61344.0  10/30/2026  3000.0  20.837    7.717  0.2963  0.2917  0.3971  0.0148
 15.0   66.7  300.0  61344.0  10/30/2026  3000.0  20.621    7.756  0.2992  0.3093  0.3765  0.0150
 16.0   62.5  300.0  61344.0  10/30/2026  3000.0  20.397    7.803  0.3024  0.3264  0.3561  0.0151
 17.0   58.8  300.0  61344.0  10/30/2026  3000.0  20.214    7.847  0.3052  0.3436  0.3359  0.0153
 18.0   55.6  300.0  61344.0  10/30/2026  3000.0  20.047    7.892  0.3079  0.3608  0.3159  0.0154
 19.0   52.6  300.0  61344.0  10/30/2026  3000.0  19.878    7.943  0.3107  0.3777  0.2960  0.0155
 20.0   50.0  300.0  61344.0  10/30/2026  3000.0  19.730    7.993  0.3134  0.3946  0.2763  0.0157
 21.0   47.6  300.0  61344.0  10/30/2026  3000.0  19.597    8.043  0.3159  0.4115  0.2567  0.0158
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/22/2010 05:39 pm
Here's an example of some of the comparative trajectory analyses I have done to assess the performance of NEP and SEP vehicles relative to one another.  This particular case was an Earth-Mars transfer mission.

354 trajectories executed...
Optimal NEP Departure MJD:   61332.0  10/18/2026    JV: 4.006 W/kg
Optimal SEP Departure MJD:   61343.0  10/29/2026    JV: 6.488 W/kg

                     NEP VEHICLE_________________________________________________________________
ALPHA  SPPWR  TRSFR      MJD  DATE        ISP      POWER    OPTJC   PROPF   POWRF   PAYLF   TANKF
kg/kW   W/kg   days                       sec        kWe     W/kg   mp/m0   ms/m0   mn/m0   mt/mo
  2.0  500.0  300.0  61330.0  10/16/2026  3000.0  23.307    5.903  0.2278  0.0466  0.7142  0.0114
  3.0  333.3  300.0  61330.0  10/16/2026  3000.0  20.490    5.403  0.2338  0.0615  0.6930  0.0117
  4.0  250.0  300.0  61330.0  10/16/2026  3000.0  18.840    5.134  0.2389  0.0754  0.6738  0.0119
  5.0  200.0  300.0  61330.0  10/16/2026  3000.0  17.706    4.965  0.2435  0.0885  0.6558  0.0122
  6.0  166.7  300.0  61330.0  10/16/2026  3000.0  16.901    4.857  0.2475  0.1014  0.6388  0.0124
  7.0  142.9  300.0  61330.0  10/16/2026  3000.0  16.257    4.781  0.2512  0.1138  0.6224  0.0126
  8.0  125.0  300.0  61330.0  10/16/2026  3000.0  15.761    4.730  0.2546  0.1261  0.6066  0.0127
  9.0  111.1  300.0  61330.0  10/16/2026  3000.0  15.343    4.693  0.2579  0.1381  0.5912  0.0129
 10.0  100.0  300.0  61330.0  10/16/2026  3000.0  14.992    4.668  0.2609  0.1499  0.5761  0.0130
 11.0   90.9  300.0  61330.0  10/16/2026  3000.0  14.691    4.651  0.2638  0.1616  0.5614  0.0132
 12.0   83.3  300.0  61330.0  10/16/2026  3000.0  14.433    4.642  0.2665  0.1732  0.5470  0.0133
 13.0   76.9  300.0  61329.0  10/15/2026  3000.0  14.174    4.636  0.2695  0.1843  0.5328  0.0135
 14.0   71.4  300.0  61329.0  10/15/2026  3000.0  13.960    4.635  0.2721  0.1954  0.5188  0.0136
 15.0   66.7  300.0  61329.0  10/15/2026  3000.0  13.787    4.637  0.2744  0.2068  0.5050  0.0137
 16.0   62.5  300.0  61329.0  10/15/2026  3000.0  13.627    4.641  0.2767  0.2180  0.4914  0.0138
 17.0   58.8  300.0  61329.0  10/15/2026  3000.0  13.482    4.648  0.2789  0.2292  0.4779  0.0139
 18.0   55.6  300.0  61329.0  10/15/2026  3000.0  13.338    4.657  0.2812  0.2401  0.4646  0.0141
 19.0   52.6  300.0  61329.0  10/15/2026  3000.0  13.219    4.667  0.2833  0.2512  0.4514  0.0142
 20.0   50.0  300.0  61329.0  10/15/2026  3000.0  13.103    4.679  0.2854  0.2621  0.4383  0.0143
 21.0   47.6  300.0  61329.0  10/15/2026  3000.0  13.000    4.691  0.2873  0.2730  0.4253  0.0144

                     SEP VEHICLE_______________________
ALPHA  SPPWR  TRSFR      MJD  DATE        ISP      POWER    OPTJC   PROPF   POWRF   PAYLF   TANKF
kg/kW   W/kg   days                       sec        kWe     W/kg   mp/m0   ms/m0   mn/m0   mt/m0
  2.0  500.0  300.0  61346.0  11/01/2026  3000.0  33.316    8.768  0.2388  0.0666  0.6827  0.0119
  3.0  333.3  300.0  61345.0  10/31/2026  3000.0  29.404    8.170  0.2470  0.0882  0.6524  0.0124
  4.0  250.0  300.0  61345.0  10/31/2026  3000.0  27.147    7.882  0.2540  0.1086  0.6247  0.0127
  5.0  200.0  300.0  61344.0  10/30/2026  3000.0  25.638    7.726  0.2600  0.1282  0.5988  0.0130
  6.0  166.7  300.0  61344.0  10/30/2026  3000.0  24.559    7.641  0.2654  0.1474  0.5740  0.0133
  7.0  142.9  300.0  61344.0  10/30/2026  3000.0  23.729    7.598  0.2703  0.1661  0.5501  0.0135
  8.0  125.0  300.0  61344.0  10/30/2026  3000.0  23.076    7.580  0.2747  0.1846  0.5270  0.0137
  9.0  111.1  300.0  61344.0  10/30/2026  3000.0  22.534    7.581  0.2789  0.2028  0.5044  0.0139
 10.0  100.0  300.0  61344.0  10/30/2026  3000.0  22.092    7.594  0.2827  0.2209  0.4822  0.0141
 11.0   90.9  300.0  61344.0  10/30/2026  3000.0  21.706    7.617  0.2864  0.2388  0.4605  0.0143
 12.0   83.3  300.0  61344.0  10/30/2026  3000.0  21.379    7.646  0.2899  0.2565  0.4391  0.0145
 13.0   76.9  300.0  61344.0  10/30/2026  3000.0  21.084    7.681  0.2933  0.2741  0.4180  0.0147
 14.0   71.4  300.0  61344.0  10/30/2026  3000.0  20.837    7.717  0.2963  0.2917  0.3971  0.0148
 15.0   66.7  300.0  61344.0  10/30/2026  3000.0  20.621    7.756  0.2992  0.3093  0.3765  0.0150
 16.0   62.5  300.0  61344.0  10/30/2026  3000.0  20.397    7.803  0.3024  0.3264  0.3561  0.0151
 17.0   58.8  300.0  61344.0  10/30/2026  3000.0  20.214    7.847  0.3052  0.3436  0.3359  0.0153
 18.0   55.6  300.0  61344.0  10/30/2026  3000.0  20.047    7.892  0.3079  0.3608  0.3159  0.0154
 19.0   52.6  300.0  61344.0  10/30/2026  3000.0  19.878    7.943  0.3107  0.3777  0.2960  0.0155
 20.0   50.0  300.0  61344.0  10/30/2026  3000.0  19.730    7.993  0.3134  0.3946  0.2763  0.0157
 21.0   47.6  300.0  61344.0  10/30/2026  3000.0  19.597    8.043  0.3159  0.4115  0.2567  0.0158


Very interesting. What do all those acronyms stand for and what do the variables mean?

MJD? JV?

I am very interested in this. What did you use for the simulation? Are you willing to share the source code?

Thanks! ;)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/22/2010 06:26 pm
Very interesting. What do all those acronyms stand for and what do the variables mean?

MJD? JV?

I am very interested in this. What did you use for the simulation? Are you willing to share the source code?

MJD is modified Julian date, a more convenient way to work with dates in astrodynamics codes.  The corresponding calendar date is to the right.  JV is a parameter used in the low-thrust trajectory simulation code (in this case, Chebytop) to optimize a variable-thrust trajectory to the given boundary conditions.  I wrote a Java front-end for the Chebytop code and used it to run these cases.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/22/2010 06:49 pm
What is the "specific power" listed above as SPPWR, then? the specific power for the whole spacecraft including propellant? dry? dry but not including tanks? only counting thrusters and power source (and what structure is required for power)?

Another really relevant question is the thruster's own specific power. VASIMR currently is no better than ~250W/kg while existing ion thrusters like used on Dawn are worse. In other words, it doesn't matter much if we have a 10,000W/kg power source or a 1000W/kg power source because the thruster is so much heavier. What are the specific powers of the various new electric thrusters out there? Is there any way to get a really high specific power for an electric thruster (i.e. >10kW/kg) while keeping a reasonable efficiency (this may provide some really interesting optimization possibilities with swinging close to the sun to increase solar array specific power)? Also relevant is the specific power of the power conditioning electronics and its efficiency (and thus inversely how much heat is generated).

Also, did you just calculate for roughly the same trajectory, but then seeing how the different specific powers gave different mass fractions?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/23/2010 07:30 am
Is it true that MPD thrusters may be capable of ~100kW/kg? Or even 10kW/kg?

It sounds like such thrusters, while being less efficient (less than 50% efficient currently), may make a lot of sense if you have really high specific power solar arrays (like 1kW/kg). Heck, it may even make sense to make a close swing-by of Venus, not just for a Venus gravity assist, but to temporarily double the specific power of the solar array (plus a double Oberth effect from Venus and the Sun). Swing-by even closer (like Mercury) may be difficult because you have to shed orbital velocity, but it could allow ten times the specific power for solar arrays than they would be at Earth, allowing even higher Isp.

Or, if someone makes a magic lightweight supercapacitor or battery, it may even allow these "low-thrust" engines to take advantage of the Oberth effect by only doing short, intense burns during periapsis. Doubt this would make sense, since 1kW/kg solar arrays would be much lighter for the same power as typical aerospace fuel cells.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Proponent on 05/23/2010 08:26 am
What is the "specific power" listed above as SPPWR, then?

Looks to me like it's the reciprocal of alpha.

What is OPTJC?  Optimal... something or other?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Proponent on 05/25/2010 05:56 am
Nuclear thermal propulsion is a waste of time if you're trying to improve payload performance.  See my analysis here:

http://selenianboondocks.com/2010/02/pf-expressions-example/

and here

http://selenianboondocks.com/2010/02/payload-fraction-example-proof/

I find those arguments pretty convincing in the context of earth-escape scenarios.  In addition there's the fact that NTR's higher specific impulse means it will have a longer burn time than a chemical departure stage.  That in turn means larger gravity losses, which make NTR even less attractive.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: 93143 on 05/25/2010 06:45 am
Nuclear thermal propulsion is a waste of time if you're trying to improve payload performance.  See my analysis here:

http://selenianboondocks.com/2010/02/pf-expressions-example/

and here

http://selenianboondocks.com/2010/02/payload-fraction-example-proof/

I find those arguments pretty convincing in the context of earth-escape scenarios.  In addition there's the fact that NTR's higher specific impulse means it will have a longer burn time than a chemical departure stage.  That in turn means larger gravity losses, which make NTR even less attractive.

Only if you assume a T/W ratio that even the NERVA project more than doubled in what was to be their final design...  I believe NTR has a lot more developmental potential than you're giving it credit for.  Something like the Dumbo design, for instance, could dramatically improve the T/W...

Also, the RL-10 in that example is unrealistically light for the Isp assigned to it; the B-2 nozzle extension is actually quite heavy...  not that it matters much; the big influence is the NTR T/W...

It strikes me that a stage designed for more delta-V (for instance, a zero-boiloff stage designed to burn at both ends of a high-energy trajectory) could benefit quite a bit from NTR...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/25/2010 01:51 pm
Only if you assume a T/W ratio that even the NERVA project more than doubled in what was to be their final design...  I believe NTR has a lot more developmental potential than you're giving it credit for.  Something like the Dumbo design, for instance, could dramatically improve the T/W...

Fine, double the T/W and see what difference it makes.  I've given you all the equations to find out.  The hydrogen is just as bad if not worse than the lousy T/W.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mmeijeri on 05/25/2010 01:54 pm
Fine, double the T/W and see what difference it makes.  I've given you all the equations to find out.  The hydrogen is just as bad if not worse than the lousy T/W.

How about hydrazine or ammonia? I've been playing with Propep a bit, but I'm getting inexplicable numbers that even vary with the amount of propellant used, not just with the mass fractions.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/25/2010 02:11 pm
How about hydrazine or ammonia? I've been playing with Propep a bit, but I'm getting inexplicable numbers that even vary with the amount of propellant used, not just with the mass fractions.

The Isp will be lousy.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: tnphysics on 05/25/2010 04:10 pm
Actually, you can get 857 seconds Isp, if all of the hydrogen dissociates. Water/Methane in 1:1 molar ratio works almost as well.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/25/2010 04:17 pm
That's the issue that NTR has:
Since modern chemical rocket engines are often limited to internal temperature by practical materials, NTR engines will be similarly limited (no matter how much "extra" energy they can provide from fission) except for the fact that they can use only hydrogen if they want to, instead of hydrolox. Hydrogen has the worst density of almost any liquid, while also having to be stored deeply cryogenically. This low density increases tank mass.

Might as well store the energy in the chemical bonds of your propellant since you still need propellant.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mmeijeri on 05/25/2010 04:23 pm
You could still do better than chemical, or boost the performance of lower performing propellants, even monopropellants, somewhat like a hydrazine resistojet. Imagine the thrust and density of hydrazine combined with a specific impulse that is significantly better than LOX/LH2. I'm not saying there definitely is a niche, but there might be.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: tnphysics on 05/25/2010 04:46 pm
That's the issue that NTR has:
Since modern chemical rocket engines are often limited to internal temperature by practical materials, NTR engines will be similarly limited (no matter how much "extra" energy they can provide from fission) except for the fact that they can use only hydrogen if they want to, instead of hydrolox. Hydrogen has the worst density of almost any liquid, while also having to be stored deeply cryogenically. This low density increases tank mass.

Might as well store the energy in the chemical bonds of your propellant since you still need propellant.

You can get above 3000K in an NTR-enough to dissociate the chemical bonds of H2.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/25/2010 04:47 pm
That's the issue that NTR has:
Since modern chemical rocket engines are often limited to internal temperature by practical materials, NTR engines will be similarly limited (no matter how much "extra" energy they can provide from fission) except for the fact that they can use only hydrogen if they want to, instead of hydrolox. Hydrogen has the worst density of almost any liquid, while also having to be stored deeply cryogenically. This low density increases tank mass.

Might as well store the energy in the chemical bonds of your propellant since you still need propellant.

You can get above 3000K in an NTR-enough to dissociate the chemical bonds of H2.
Really? Who has done this?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: 93143 on 05/25/2010 06:50 pm
Fine, double the T/W and see what difference it makes.  I've given you all the equations to find out.  The hydrogen is just as bad if not worse than the lousy T/W.

The engine mass in your example (second link) is 74% of the stage dry mass.  Cutting it in half saves 50% more than the total tankage mass.

With an advanced design, a T/W of 20 is probably achievable. (I've seen people claim anywhere from 60 to over 100 for Dumbo, and 30 for Timberwind...)  This results in a total engine mass of roughly 2 mT not counting thrust structure, leading to a payload fraction of about 0.54 within the same mass envelope.  That's a 36% boost to payload mass versus the old T/W, and 42% versus the chemical stage (with a 437 kg penalty to the chemical stage payload for the high-Isp nozzle extensions).

I haven't cranked through it, but I suspect that this gives the NTR a lot more headroom for increasing delta-V without having to nerf the stage T/W.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/25/2010 06:52 pm
I haven't cranked through it, but I suspect that this gives the NTR a lot more headroom for increasing delta-V without having to nerf the stage T/W.

Feel free to "crank" it.  Then you can quote numbers instead of opinions.  I've done a lot of work to make it simple for you.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mmeijeri on 05/25/2010 06:55 pm
Really? Who has done this?

Playing with Cpropep-Web (http://rocketworkbench.sourceforge.net/equil.phtml), it seems you would have to go to really high temperatures to dissociate most of the H2, close to the upper limit for Propep. You'd need gas-core NTR for that, which seems like a long, long way off.

Unfortunately the web interface doesn't allow you to calculate Isp for a given temperature, so you'd have to calculate the equilibrium conditions and then calculate Isp from that by hand/calculator/spreadsheet. This makes it hard to see how close you could get.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mmeijeri on 05/25/2010 06:57 pm
Feel free to "crank" it.  Then you can quote numbers instead of opinions.  I've done a lot of work to make it simple for you.

I'm curious about the T/W. What is the limiting factor? Is it just the density of the LH2 or would total power of the reactor be a limitation if you used a denser working fluid?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/26/2010 03:51 am
That's the issue that NTR has:
Since modern chemical rocket engines are often limited to internal temperature by practical materials, NTR engines will be similarly limited (no matter how much "extra" energy they can provide from fission) except for the fact that they can use only hydrogen if they want to, instead of hydrolox. Hydrogen has the worst density of almost any liquid, while also having to be stored deeply cryogenically. This low density increases tank mass.

Might as well store the energy in the chemical bonds of your propellant since you still need propellant.

If what you wrote was true, copper wouldn't be such a great material for rocket engines.

And if what you wrote was true, NTRs would be limited to 450 seconds.

The extra energy of which you speak is the reason that chemical rockets are basically limited to near-Earth destinations, and nuclear rockets are not.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/26/2010 04:03 am
That's the issue that NTR has:
Since modern chemical rocket engines are often limited to internal temperature by practical materials, NTR engines will be similarly limited (no matter how much "extra" energy they can provide from fission) except for the fact that they can use only hydrogen if they want to, instead of hydrolox. Hydrogen has the worst density of almost any liquid, while also having to be stored deeply cryogenically. This low density increases tank mass.

Might as well store the energy in the chemical bonds of your propellant since you still need propellant.

If what you wrote was true, copper wouldn't be such a great material for rocket engines.

And if what you wrote was true, NTRs would be limited to 450 seconds.
Hydrolox still needs oxygen. The reason hydrogen works as a good propellant is because of its low mass. I.E. for the same temperature, hydrogen molecules are moving faster. NTRs can use pure hydrogen instead of needing a bunch of oxygen. So, NTRs can get a higher Isp (basically, exhaust velocity) for the same operating temperature compared to a hydrolox engine. ...at the expense of lower thrust (for the same turbopump capacity) and larger tank size.
Quote
The extra energy of which you speak is the reason that chemical rockets are basically limited to near-Earth destinations, and nuclear rockets are not.
Totally opposite, actually. Chemical rockets are by far the most popular pick for missions well beyond Earth (the only other rocket used being the solar electric thruster), while nuclear rockets are only used (on a test-stand) on Earth. ;)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/26/2010 04:27 am
That's the issue that NTR has:
Since modern chemical rocket engines are often limited to internal temperature by practical materials, NTR engines will be similarly limited (no matter how much "extra" energy they can provide from fission) except for the fact that they can use only hydrogen if they want to, instead of hydrolox. Hydrogen has the worst density of almost any liquid, while also having to be stored deeply cryogenically. This low density increases tank mass.

Might as well store the energy in the chemical bonds of your propellant since you still need propellant.

If what you wrote was true, copper wouldn't be such a great material for rocket engines.

And if what you wrote was true, NTRs would be limited to 450 seconds.
Hydrolox still needs oxygen. The reason hydrogen works as a good propellant is because of its low mass. I.E. for the same temperature, hydrogen molecules are moving faster. NTRs can use pure hydrogen instead of needing a bunch of oxygen. So, NTRs can get a higher Isp (basically, exhaust velocity) for the same operating temperature compared to a hydrolox engine. ...at the expense of lower thrust (for the same turbopump capacity) and larger tank size.
Quote
The extra energy of which you speak is the reason that chemical rockets are basically limited to near-Earth destinations, and nuclear rockets are not.
Totally opposite, actually. Chemical rockets are by far the most popular pick for missions well beyond Earth (the only other rocket used being the solar electric thruster), while nuclear rockets are only used (on a test-stand) on Earth. ;)

And that would be the reason that humans don't travel beyond LEO.  The disconnect in this discussion is that people who want human spaceflight beyond LEO look at NTRs as the only serious choice currently available.  Folks who would rather explore robotically can rely on chemical or solar-powered thrusters.  Our goals are completely different.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/26/2010 04:50 am

If what you wrote was true, copper wouldn't be such a great material for rocket engines.

And if what you wrote was true, NTRs would be limited to 450 seconds.
Hydrolox still needs oxygen. The reason hydrogen works as a good propellant is because of its low mass. I.E. for the same temperature, hydrogen molecules are moving faster. NTRs can use pure hydrogen instead of needing a bunch of oxygen. So, NTRs can get a higher Isp (basically, exhaust velocity) for the same operating temperature compared to a hydrolox engine. ...at the expense of lower thrust (for the same turbopump capacity) and larger tank size.
Quote
The extra energy of which you speak is the reason that chemical rockets are basically limited to near-Earth destinations, and nuclear rockets are not.
Totally opposite, actually. Chemical rockets are by far the most popular pick for missions well beyond Earth (the only other rocket used being the solar electric thruster), while nuclear rockets are only used (on a test-stand) on Earth. ;)

And that would be the reason that humans don't travel beyond LEO.  The disconnect in this discussion is that people who want human spaceflight beyond LEO look at NTRs as the only serious choice currently available.  Folks who would rather explore robotically can rely on chemical or solar-powered thrusters.  Our goals are completely different.
Not true for me. In fact, it's the other way around: I support chemical and solar-electric propulsion precisely because I want to see beyond Earth orbit human exploration as soon as possible and for as low cost as possible.

I acknowledge that when humans ever go to the orbit of Jupiter or beyond, they surely will be using nuclear propulsion of some kind (probably nuclear-electric, especially because the trip will take a while any prop method you choose and electric propulsion can use that extra time to accelerate propellant to ever high velocities, allowing 10,000+s Isps to make sense).
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/26/2010 05:31 am
And that would be the reason that humans don't travel beyond LEO.  The disconnect in this discussion is that people who want human spaceflight beyond LEO look at NTRs as the only serious choice currently available.  Folks who would rather explore robotically can rely on chemical or solar-powered thrusters.  Our goals are completely different.
Not true for me. In fact, it's the other way around: I support chemical and solar-electric propulsion precisely because I want to see beyond Earth orbit human exploration as soon as possible and for as low cost as possible.

I acknowledge that when humans ever go to the orbit of Jupiter or beyond, they surely will be using nuclear propulsion of some kind (probably nuclear-electric, especially because the trip will take a while any prop method you choose and electric propulsion can use that extra time to accelerate propellant to ever high velocities, allowing 10,000+s Isps to make sense).

If you can think of a way to carry humans around the solar system with nuclear-electric without killing them first with radiation exposure and subsequently with old age, then, uh, more power to you.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: isa_guy on 05/26/2010 10:56 am
Solar system ? You probably meant up to the martian orbit because beyond that solar power becomes useless.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 05/26/2010 11:32 am
If you can think of a way to carry humans around the solar system with nuclear-electric without killing them first with radiation exposure and subsequently with old age, then, uh, more power to you.

Apparently you are grossly ignorant of the state of technology. Nuclear Electric includes VASIMR plasma propulsion powered by nuclear reactors. A 200 MW full scale VASIMR with two reactors can make the Earth-Mars run in 39 days on a fully loaded reference mission. Thats vastly faster than chemical OR straight nuclear propulsion.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/26/2010 01:42 pm
Solar system ? You probably meant up to the martian orbit because beyond that solar power becomes useless.

You didn't read my whole post, did you? Go back and read it.

And besides:

http://en.wikipedia.org/wiki/Dawn_(spacecraft) (http://en.wikipedia.org/wiki/Dawn_(spacecraft)) (mission to Vesta and Ceres--beyond Mars, using solar-electric propulsion and solar arrays for power)

http://en.wikipedia.org/wiki/Juno_(spacecraft) (http://en.wikipedia.org/wiki/Juno_(spacecraft)) (mission to Jupiter, using solar arrays for power instead of the usual RTG at that distance)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/26/2010 01:44 pm
Apparently you are grossly ignorant of the state of technology. Nuclear Electric includes VASIMR plasma propulsion powered by nuclear reactors. A 200 MW full scale VASIMR with two reactors can make the Earth-Mars run in 39 days on a fully loaded reference mission. Thats vastly faster than chemical OR straight nuclear propulsion.

Apparently you're grossly ignorant of realities rather than claims.  The power supply that makes these claims even remotely believable is far beyond the state-of-the-art.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: isa_guy on 05/26/2010 02:18 pm
Apparently you are grossly ignorant of the state of technology. Nuclear Electric includes VASIMR plasma propulsion powered by nuclear reactors. A 200 MW full scale VASIMR with two reactors can make the Earth-Mars run in 39 days on a fully loaded reference mission. Thats vastly faster than chemical OR straight nuclear propulsion.

Apparently you're grossly ignorant of realities rather than claims.  The power supply that makes these claims even remotely believable is far beyond the state-of-the-art.
Also Apparently the guy that stand behind the vapor core reactor concept ( that may provide those 200 MWe ) has been accused of fraud http://abcnews.go.com/US/wireStory?id=8960673 , so we can forget from sending people to mars in a gigantic 200 MWe VASIMR  powered NEP MTV.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/26/2010 03:18 pm
Solar system ? You probably meant up to the martian orbit because beyond that solar power becomes useless.

Well, no, I didn't.  I specifically said "carry humans", and I don't take SEP seriously as a way to carry humans from Earth to Mars.  IMO, the collectors get a bit too large to be practical when you factor in shielding, radiators and logistics.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/26/2010 03:28 pm
If you can think of a way to carry humans around the solar system with nuclear-electric without killing them first with radiation exposure and subsequently with old age, then, uh, more power to you.

Apparently you are grossly ignorant of the state of technology. Nuclear Electric includes VASIMR plasma propulsion powered by nuclear reactors. A 200 MW full scale VASIMR with two reactors can make the Earth-Mars run in 39 days on a fully loaded reference mission. Thats vastly faster than chemical OR straight nuclear propulsion.

Dr. Chang-Diaz is a kind, intelligent and highly-personable guy, but the VASIMR claims are a bit out of this world (I seem to be highly prone to bad puns on this forum). 

Let's take your 200 MW full-scale VASIMR running all out at 30% efficiency for that 39-day mission.  That's a 667 MW reactor running with a 467 MW radiator.  Maybe someone else here knows how to calculate exactly how big that radiator needs to be, and what its mass is; I don't.  I do know that your mass just blew off the charts, and so did your 39-day reference mythion.  Apologies in advance for any numbers I've got wrong here, but you get the idea.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/26/2010 03:44 pm
Solar system ? You probably meant up to the martian orbit because beyond that solar power becomes useless.

Well, no, I didn't.  I specifically said "carry humans", and I don't take SEP seriously as a way to carry humans from Earth to Mars.  IMO, the collectors get a bit too large to be practical when you factor in shielding, radiators and logistics.
?
Shielding isn't strictly required like for nuclear, radiators are going to be rather small for solar electric since the solar arrays are their own radiators, and what "logistics" are you referring to?

If we are comparing electric propulsion to nuclear thermal propulsion, the Isps are an order of magnitude different, so nuclear thermal is far more sensitive to mass increases than electric propulsion is.

If we are comparing solar electric to nuclear electric, solar electric has a higher specific power in the inner solar system (if you give the same dev money to each), the only metric that matters.

I don't take you seriously when you make claims which are not backed up by anything like real evidence.

And, PS, the VASIMR runs at least at 60% efficiency, and higher efficiency (like 80%) is being developed. If you can live with lower efficiency and lower lifetime, then a MPD thruster is what you want, since it is incredibly light while being capable of megawatts of power.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: 93143 on 05/26/2010 04:28 pm
Let's take your 200 MW full-scale VASIMR running all out at 30% efficiency for that 39-day mission.  That's a 667 MW reactor running with a 467 MW radiator.  Maybe someone else here knows how to calculate exactly how big that radiator needs to be, and what its mass is; I don't.  I do know that your mass just blew off the charts, and so did your 39-day reference mythion.

That 39-day reference mission assumes an 800 mT reactor system IIRC.

For minimum radiator size, it's probably better to have the cold side at about 76% of the hot side temperature.  Assuming the core operates at 900 K (probably not water-cooled, but there are options), that gives you a cold-side temperature of ~680 K.  Assuming 90% cycle efficiency (Stirling?), this gives you a thermal efficiency of 22%.  So to get 200 MWe, you need to dump 709 MWth.

With the radiators at 680 K, this means you need about 65,000 m^2.  Even at 10 kg/m^2 (which I think is probably conservative), that's 650 mT, leaving 150 mT for the power system.

That's a fairly loose analysis, but anyway I suspect the radiator mass is already factored into the total system mass.  800 mT is a lot of equipment...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/26/2010 04:45 pm
And, PS, the VASIMR runs at least at 60% efficiency, and higher efficiency (like 80%) is being developed.

Prove it.  I've had many qualified electric propulsion experts tell me that VASIMR will have lousy efficiency.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/26/2010 04:53 pm
And, PS, the VASIMR runs at least at 60% efficiency, and higher efficiency (like 80%) is being developed.

Prove it.  I've had many qualified electric propulsion experts tell me that VASIMR will have lousy efficiency.
I'm not married to it. I think VASIMR has certainly gotten more than its share of hype.

I believe you.

VASIMR's efficiency increases with Isp (like most ion thrusters). It's not the most efficient thruster. HiPeP is more efficient, and thus may be a better choice (_especially_ since it may reduce need to reject heat, depending on where the inefficiencies are). From what I've heard, VASIMR is around 65% efficient, while HiPEP is around 80%.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/26/2010 05:58 pm
Solar system ? You probably meant up to the martian orbit because beyond that solar power becomes useless.

Well, no, I didn't.  I specifically said "carry humans", and I don't take SEP seriously as a way to carry humans from Earth to Mars.  IMO, the collectors get a bit too large to be practical when you factor in shielding, radiators and logistics.
?
Shielding isn't strictly required like for nuclear,

No extra shielding for longer trip times?

Quote
radiators are going to be rather small for solar electric since the solar arrays are their own radiators,

Whups, sorry, I remember going back and adding "radiators" but I shouldn't have.

Quote
and what "logistics" are you referring to?

No extra food, water, incidentals for longer trip times?  Larger ship?  Extra shielding?  Other changes in mass that go along with a slower mission?

Quote
If we are comparing electric propulsion to nuclear thermal propulsion, the Isps are an order of magnitude different, so nuclear thermal is far more sensitive to mass increases than electric propulsion is.

Sure, but moving from a fast Mars mission to a slow Mars mission should bring some pretty big increases in mass.  IIRC mass at least doubles.  I dunno, maybe that doesn't matter if you're using the ITN and you have cheap launch.

The von Braun-era (Space Transportation System) plan was to have three or four NTR tugs parked around 200 km that could grab a ship and take it to wherever its destination might be.  Refueling at orbital depots, they'd be good for several missions.

Quote
If we are comparing solar electric to nuclear electric, solar electric has a higher specific power in the inner solar system (if you give the same dev money to each), the only metric that matters.

I'm assuming a number of around 2kw/m^2?

Quote
I don't take you seriously when you make claims which are not backed up by anything like real evidence.

Fair enough.

Quote
And, PS, the VASIMR runs at least at 60% efficiency, and higher efficiency (like 80%) is being developed. If you can live with lower efficiency and lower lifetime, then a MPD thruster is what you want, since it is incredibly light while being capable of megawatts of power.

IIRC the efficiency varies down to around 30% at higher thrusts.  I don't remember where I read that, so you don't have to take it seriously.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/26/2010 06:26 pm
SEP doesn't have necessarily a longer trip time compared to NTR. Where are you getting the information from, and what are its assumptions? I was assuming opposition-class missions for both SEP and NTR (400-500 days total mission time, same time for each).

Specific power is measured in watts/kg and is an important metric when comparing different electric-propulsion technologies. (it's the inverse of "alpha") Solar is currently at around 175W/kg for ultraflex panels (or greater... 220W/kg) and Japan is demonstrating one method of getting over 1000W/kg and specific powers greater than 4000W/kg have been demonstrated in the lab (which would translate to over 1000W/kg deployed), with increases in efficiency enabling over 10,000W/kg in the next couple decades. For solar, specific power depends on distance from the sun, which is why solar isn't typically used on the outer planets, though great strides have been made in increasing solar array specific power is allowing solar-powered missions to Jupiter, with Japan planning an solar-electric ion-thruster-propelled mission there in the late 2010s.

Nuclear electric may be capable of 350W/kg for very high-performance liquid-metal-cooled reactors. But such reactors require a lot of shielding, which means they only make sense above a certain size (and, I would say, only for outer-planets missions...).

Very few nuclear reactors have ever flown into space, and those were far, far lower in specific power compared to solar arrays.

The only US reactor flown into orbit ( http://en.wikipedia.org/wiki/SNAP-10A )used an inefficient (but reliable) thermal-to-electric converter and so the unshielded reactor only produced less than 2Watts/kg (for a total power of 500 Watts). That's right, it only had the performance of 1% of solar arrays today and actually lower than most RTGs. And that was for an unshielded reactor. Obviously, a modern, larger nuclear reactor could be far more efficient and powerful, but so could a high-efficiency thin-film solar array.

And, like RTGs, solar arrays can function for many decades without refueling, whereas a nuclear reactor will need to be refueled if operated continuously for years at high-power.

I don't oppose development of nuclear-electric power because I think it'd be great to see manned missions to Saturn, but solar-electric is here, now, and works GREAT for the inner solar system. Probably even better than nuclear-electric.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/26/2010 06:28 pm
Let's take your 200 MW full-scale VASIMR running all out at 30% efficiency for that 39-day mission.  That's a 667 MW reactor running with a 467 MW radiator.  Maybe someone else here knows how to calculate exactly how big that radiator needs to be, and what its mass is; I don't.  I do know that your mass just blew off the charts, and so did your 39-day reference mythion.

That 39-day reference mission assumes an 800 mT reactor system IIRC.

For minimum radiator size, it's probably better to have the cold side at about 76% of the hot side temperature.  Assuming the core operates at 900 K (probably not water-cooled, but there are options), that gives you a cold-side temperature of ~680 K.  Assuming 90% cycle efficiency (Stirling?), this gives you a thermal efficiency of 22%.  So to get 200 MWe, you need to dump 709 MWth.

With the radiators at 680 K, this means you need about 65,000 m^2. 

So, just to get my brain around it, that's about a dozen American football fields.  Huh.  Not large enough to make a good sail...

Quote
Even at 10 kg/m^2 (which I think is probably conservative), that's 650 mT, leaving 150 mT for the power system.

That's interesting.  What's a reasonable thickness if it were aluminum?  1 mm?  So around 2.7 kg/m^2 plus structure, pipes, seals, etc.?  Seems like the moments would be pretty huge.  You'd have to be very careful not to let that number grow too much, and I can see why the rest of the power system might be a lot of pumps.

I'm personally a little intimidated by the thought of making something like that work and attempting to assemble it in orbit, but maybe some people wouldn't be.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mmeijeri on 05/26/2010 06:28 pm
Chris, are you taking the supporting structure into account, or is this just the mass of the solar cells themselves? For large structures and therefore for high total power this makes a lot of difference.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/26/2010 07:11 pm
Chris, are you taking the supporting structure into account, or is this just the mass of the solar cells themselves? For large structures and therefore for high total power this makes a lot of difference.

Commercially available space-qualified solar cells themselves (not including structure) have been up around ~500 W/kg for a few years, now (see here: http://www.emcore.com/solar_photovoltaics/space_solar_cells ), whereas ultra-flex is around 175W/kg to 220W/kg (that includes structure). Thin-film is over 4000W/kg right now in the lab, and increases in efficiency and reductions in substrate mass can increase that further (10,000W/kg for thin substrates).

The support structure will be most of the weight. But even that weight can be reduced significant if you can live with really low fundamental frequencies. This is more like designing a solar sail than designing a traditional rigid solar array.

So, yes, I am taking support structures into account when I talk about 200-1000W/kg being reasonable values for a big solar array (200W/kg for non-demo missions launched right now, 1kW/kg for demo missions). I really think 1kW/kg is what we should shoot for because it's certainly doable with current technology and current engineering. Rapid improvements in solar array technology over the last couple decades (and continuing) have made this possible.

For a very large solar array, like in the megawatts range, we will have to be content with the minor control issues that low stiffness arrays will present. That's about the only fundamental difficulty I see (well, and if we travel through the Van Allen belts... but thin-film cells are actually very robust to radiation and even then can be annealed to basically full, beginning-of-life power... they just need to be heated up to around 100 or 200 C for a few hours and they will heal the radiation damage... this actually happens at room temperature a little bit).


PS: I don't think a 200MW power source makes sense, nuclear or solar. Even one tenth of that would be PLENTY for a full, short ~450 day surface mission with the typical big lander.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: JasonAW3 on 05/26/2010 09:49 pm
Chris, are you taking the supporting structure into account, or is this just the mass of the solar cells themselves? For large structures and therefore for high total power this makes a lot of difference.

Commercially available space-qualified solar cells themselves (not including structure) have been up around ~500 W/kg for a few years, now (see here: http://www.emcore.com/solar_photovoltaics/space_solar_cells ), whereas ultra-flex is around 175W/kg to 220W/kg (that includes structure). Thin-film is over 4000W/kg right now in the lab, and increases in efficiency and reductions in substrate mass can increase that further (10,000W/kg for thin substrates).

The support structure will be most of the weight. But even that weight can be reduced significant if you can live with really low fundamental frequencies. This is more like designing a solar sail than designing a traditional rigid solar array.

So, yes, I am taking support structures into account when I talk about 200-1000W/kg being reasonable values for a big solar array (200W/kg for non-demo missions launched right now, 1kW/kg for demo missions). I really think 1kW/kg is what we should shoot for because it's certainly doable with current technology and current engineering. Rapid improvements in solar array technology over the last couple decades (and continuing) have made this possible.

For a very large solar array, like in the megawatts range, we will have to be content with the minor control issues that low stiffness arrays will present. That's about the only fundamental difficulty I see (well, and if we travel through the Van Allen belts... but thin-film cells are actually very robust to radiation and even then can be annealed to basically full, beginning-of-life power... they just need to be heated up to around 100 or 200 C for a few hours and they will heal the radiation damage... this actually happens at room temperature a little bit).


PS: I don't think a 200MW power source makes sense, nuclear or solar. Even one tenth of that would be PLENTY for a full, short ~450 day surface mission with the typical big lander.

     Thin film solar panels on inflatable support structure structure.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/26/2010 09:52 pm
Chris, are you taking the supporting structure into account, or is this just the mass of the solar cells themselves? For large structures and therefore for high total power this makes a lot of difference.

Commercially available space-qualified solar cells themselves (not including structure) have been up around ~500 W/kg for a few years, now (see here: http://www.emcore.com/solar_photovoltaics/space_solar_cells ), whereas ultra-flex is around 175W/kg to 220W/kg (that includes structure). Thin-film is over 4000W/kg right now in the lab, and increases in efficiency and reductions in substrate mass can increase that further (10,000W/kg for thin substrates).

The support structure will be most of the weight. But even that weight can be reduced significant if you can live with really low fundamental frequencies. This is more like designing a solar sail than designing a traditional rigid solar array.

So, yes, I am taking support structures into account when I talk about 200-1000W/kg being reasonable values for a big solar array (200W/kg for non-demo missions launched right now, 1kW/kg for demo missions). I really think 1kW/kg is what we should shoot for because it's certainly doable with current technology and current engineering. Rapid improvements in solar array technology over the last couple decades (and continuing) have made this possible.

For a very large solar array, like in the megawatts range, we will have to be content with the minor control issues that low stiffness arrays will present. That's about the only fundamental difficulty I see (well, and if we travel through the Van Allen belts... but thin-film cells are actually very robust to radiation and even then can be annealed to basically full, beginning-of-life power... they just need to be heated up to around 100 or 200 C for a few hours and they will heal the radiation damage... this actually happens at room temperature a little bit).


PS: I don't think a 200MW power source makes sense, nuclear or solar. Even one tenth of that would be PLENTY for a full, short ~450 day surface mission with the typical big lander.

     Thin film solar panels on inflatable support structure structure.
The problem with inflatable structures is that for thin, lightweight inflatables, gas gradually escapes, I believe. Inflatables can work good for deployment, though.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Solman on 05/27/2010 12:46 am
Chris, are you taking the supporting structure into account, or is this just the mass of the solar cells themselves? For large structures and therefore for high total power this makes a lot of difference.

Commercially available space-qualified solar cells themselves (not including structure) have been up around ~500 W/kg for a few years, now (see here: http://www.emcore.com/solar_photovoltaics/space_solar_cells ), whereas ultra-flex is around 175W/kg to 220W/kg (that includes structure). Thin-film is over 4000W/kg right now in the lab, and increases in efficiency and reductions in substrate mass can increase that further (10,000W/kg for thin substrates).

The support structure will be most of the weight. But even that weight can be reduced significant if you can live with really low fundamental frequencies. This is more like designing a solar sail than designing a traditional rigid solar array.

So, yes, I am taking support structures into account when I talk about 200-1000W/kg being reasonable values for a big solar array (200W/kg for non-demo missions launched right now, 1kW/kg for demo missions). I really think 1kW/kg is what we should shoot for because it's certainly doable with current technology and current engineering. Rapid improvements in solar array technology over the last couple decades (and continuing) have made this possible.

For a very large solar array, like in the megawatts range, we will have to be content with the minor control issues that low stiffness arrays will present. That's about the only fundamental difficulty I see (well, and if we travel through the Van Allen belts... but thin-film cells are actually very robust to radiation and even then can be annealed to basically full, beginning-of-life power... they just need to be heated up to around 100 or 200 C for a few hours and they will heal the radiation damage... this actually happens at room temperature a little bit).


PS: I don't think a 200MW power source makes sense, nuclear or solar. Even one tenth of that would be PLENTY for a full, short ~450 day surface mission with the typical big lander.

     Thin film solar panels on inflatable support structure structure.
The problem with inflatable structures is that for thin, lightweight inflatables, gas gradually escapes, I believe. Inflatables can work good for deployment, though.

 Hence you need inflation insituform in which upon inflation, composite structural elements with UV catalized resins harden upon exposure to sunlight. In addition, unneeded elements can be made of plastic that degrades upon exposure to UV such as the type currently being marketed for landfill degrading trash bags.

 In the newly proposed SEP system, the concentrators are small and integrated into the solar panel. Separating the solar concentrator from the concentrator type PV can allow concentrations of 500 suns according to the Spectrolab website; which should result in greatly increased specific power and modest efficiency increase over the 12 sun setup proposed.  This would require a larger radiator than the proposed SEP OTV but the concentrator support structure can be designed to also serve this function.

 The drawback to thin film solar as compared to having a separate large solar concentrator is that you are limited to the production of electricity (and at about 10% efficiency compared with about 40% for concentrator type cells ) whereas the concentrator can power a solar thermal rocket or solar furnace for ISRU and double as radiator, high baud antenna, radio telescope, radar, etc.
 Thin film will be 4x as large for a given amount of electricity produced. Solar thermal at about 60% efficiency compared with thin film/ion at about 6% overall, can produce 10X the thrust at a given Isp and take advantage of the Oberth effect to have an effective Isp of 1600 to 2400 sec. Okay I'm fudging here - just going on the assumption that perigee thrusts as opposed to a powered spiral for LEO to escape, will give a roughly 2X advantage.
 The solar thermal engine is low in mass as compared with electric and when all is said and done, the savings in propellent mass fraction vs. thin film solar electric may not even exist for LEO to escape for OTV's with modest payload capability. Even if this is not the case, having the option of perigee thrusts leads to a less than 2 week time to earth departure as compared with the projected 6 months for the currently proposed SEP system.

First ST/EP - solar thermal/electric propulsion

Sol
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/28/2010 02:44 am
SEP doesn't have necessarily a longer trip time compared to NTR. Where are you getting the information from, and what are its assumptions? I was assuming opposition-class missions for both SEP and NTR (400-500 days total mission time, same time for each).

Sorry, but I've been trying to get my head around this the last couple of days while I made a feeble attempt to earn a living.  The rule of thumb for NEP vs NTR is that NEP uses twice the delta-V that NTR does (Zubrin, Deban) due to gravity losses.  That generally means that NEP missions are going to be twice the duration of NTR missions. OTOH, w/o improvements in Isp, NEP currently makes more sense for longer missions, not shorter ones.  In other words, right now you'd stick to NEP for Jupiter out to Pluto, and NTP for Mars, asteroids and inner planets.  Bussard had a little table for NTRs; Isp of 1000 gets you lunar colonies and Mars missions, 1500 Mars colonies and asteroids, 2000 Jupiter, 2500+ the rest of the solar system.

So substitute solar for nuclear, and it seems to me that the inert fraction goes up, not down, with no commensurate increase in Isp.  What am I missing?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/28/2010 02:58 am
SEP doesn't have necessarily a longer trip time compared to NTR. Where are you getting the information from, and what are its assumptions? I was assuming opposition-class missions for both SEP and NTR (400-500 days total mission time, same time for each).

Sorry, but I've been trying to get my head around this the last couple of days while I made a feeble attempt to earn a living.  The rule of thumb for NEP vs NTR is that NEP uses twice the delta-V that NTR does (Zubrin, Deban) due to gravity losses.  That generally means that NEP missions are going to be twice the duration of NTR missions. OTOH, w/o improvements in Isp, NEP currently makes more sense for longer missions, not shorter ones.  In other words, right now you'd stick to NEP for Jupiter out to Pluto, and NTP for Mars, asteroids and inner planets.  Bussard had a little table for NTRs; Isp of 1000 gets you lunar colonies and Mars missions, 1500 Mars colonies and asteroids, 2000 Jupiter, 2500+ the rest of the solar system.

So substitute solar for nuclear, and it seems to me that the inert fraction goes up, not down, with no commensurate increase in Isp.  What am I missing?
You're missing the fact that solar has a larger specific power than nuclear out to Mars and its minimum size can be smaller, whereas with nuclear, you need to at least lug around a shadow shield.

From what I've seen, optimistic nuclear power would be ~350W/kg, whereas we could easily build a solar array will 1000W/kg (500W/kg at Mars). From here: http://www.astronautix.com/craft/stcemsep.htm , the solar electric powered spacecraft for this opposition-class mission has the lowest IMLEO of any of the four architectures listed (NEP, NTR, SEP, or cryogenic chemical with aerocapture), and we can probably do better than the assumptions for that mission (specific power more like 250W/kg including structure for that architecture but we can do 1000W/kg).

Since a big solar array would certainly cost less to develop than a couple of big nuclear reactors (with liquid metal cooling, giant radiators, two reactors for redundancy), you could afford more IMLEO (even though you require less) so that translates into greater delta-v.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/28/2010 03:29 am
From what I've seen, optimistic nuclear power would be ~350W/kg, whereas we could easily build a solar array will 1000W/kg (500W/kg at Mars).

Easily?

And I thought NTR's were hard to park.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/28/2010 03:54 am
From what I've seen, optimistic nuclear power would be ~350W/kg, whereas we could easily build a solar array will 1000W/kg (500W/kg at Mars).

Easily?

And I thought NTR's were hard to park.
At least three SEP-propelled interplanetary spacecraft have flown, and some solar-electric rcs systems for comm sats are even commercially available. IKAROS is demonstrating high-specific-power thin-film solar power right now. "Easily" means it can easily make the leap from powerpoint to production if given money like the other SEP spacecraft like Deep Space 1 and Dawn. Only one US nuclear reactor has ever flown, and it had horrible performance. A high performing nuclear reactor could be demonstrated, but likely not without many billions of dollars... And who knows how many tens of billions for a megawatt-class system that's human rated. There's a lot of solar array you can buy for that much money.

Oh, and no additional political push-back either, as irrational as that is.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: 93143 on 05/28/2010 06:55 am
The rule of thumb for NEP vs NTR is that NEP uses twice the delta-V that NTR does (Zubrin, Deban) due to gravity losses.  That generally means that NEP missions are going to be twice the duration of NTR missions.

...no, how does that follow?

Sure, you spend more time getting out of the gravity well, but the much higher Isp means you can add lots of delta-V and do a high-energy transfer, or even (if the Isp is high enough) do a continuous-thrust trajectory, where you accelerate halfway and then turn around and decelerate.

So electric propulsion can actually get you where you want to go in less overall time, if it's far enough away (Mars is, the moon isn't).
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 05/28/2010 07:24 am
SEP doesn't have necessarily a longer trip time compared to NTR. Where are you getting the information from, and what are its assumptions? I was assuming opposition-class missions for both SEP and NTR (400-500 days total mission time, same time for each).

Sorry, but I've been trying to get my head around this the last couple of days while I made a feeble attempt to earn a living.  The rule of thumb for NEP vs NTR is that NEP uses twice the delta-V that NTR does (Zubrin, Deban) due to gravity losses.  That generally means that NEP missions are going to be twice the duration of NTR missions. OTOH, w/o improvements in Isp, NEP currently makes more sense for longer missions, not shorter ones.  In other words, right now you'd stick to NEP for Jupiter out to Pluto, and NTP for Mars, asteroids and inner planets.  Bussard had a little table for NTRs; Isp of 1000 gets you lunar colonies and Mars missions, 1500 Mars colonies and asteroids, 2000 Jupiter, 2500+ the rest of the solar system.

So substitute solar for nuclear, and it seems to me that the inert fraction goes up, not down, with no commensurate increase in Isp.  What am I missing?

You are missing VASIMR, which ranges from 5,000-30,000 sec Isp depending on the thrust level, while NTP is stuck around 800-1000 sec.

Yes, NTP is great for lunar missions, maybe even NEOs. But Mars and beyond, VASIMR is far more useful. While it is true that NTP allows for dV optimal Hohmann transfer orbits, and NEP has to spiral out of LEO, NEP, once out of earth orbit, can continue thrusting indefinitely and so can reach a higher velocity and go much further in less time.

A 200MW VASIMR reference mission to Mars can get there in 39 days, but takes 15 of those days to get out of Earth orbit.

A NTP is going to get you out of Earth orbit and into an escape trajectory in a matter of minutes, but you'll be floating in that trajectory the rest of the way and will ultimately take a lot longer to get to Mars.

Since, at least as far as NASA missions are concerned, the moon is off the table for future missions, then it stands to reason that NEP is going to be the propulsion of choice.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: isa_guy on 05/28/2010 09:23 am
What could be the mass of current state of the art fission space reactors ( like molten-salt or liquid-metal ) that may power multi MW  electric thrusters ( like MPD and VASIMR )  that could cut the trip time to mars compare to NTR and chemical propulsion .
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/28/2010 12:35 pm
A 200MW VASIMR reference mission to Mars can get there in 39 days, but takes 15 of those days to get out of Earth orbit.

No it doesn't.  That's a fantasy, and repeating it doesn't make it any more true.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 05/29/2010 06:56 pm
A 200MW VASIMR reference mission to Mars can get there in 39 days, but takes 15 of those days to get out of Earth orbit.

No it doesn't.  That's a fantasy, and repeating it doesn't make it any more true.

It is hardly a fantasy.

This is a reference mission that Chang-Diaz stands behind. Are you calling this astronaut and the VASIMR developer a liar? Are you calling the other five NASA coauthors of these papers liars?

http://spaceflight.nasa.gov/shuttle/support/researching/aspl/reference/2000_3756.pdf
http://www.physorg.com/news186397741.html
http://dma.ing.uniroma1.it/users/bruno/Petro.prn.pdf
http://spaceflight.nasa.gov/shuttle/support/researching/aspl/reference/f_wsc02.pdf

Now, care to rescind your accusations?

Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 05/29/2010 07:03 pm
Come on, mlorrey. Do you really think a 200MW gas-core reactor is a reasonable proposition, when one-tenth that power would be plenty for a 450-day Mars mission?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 05/29/2010 07:21 pm
Come on, mlorrey. Do you really think a 200MW gas-core reactor is a reasonable proposition, when one-tenth that power would be plenty for a 450-day Mars mission?

A 450 day mission to Mars when 400 of that 450 days is spent in zero g and subject to radiation hazards in interplanetary space is extremely risky to the lives of astronauts. Interplanetary radiation is far higher than in LEO.

Nor would that be in one single reactor. It would be in 2,3 or 4 reactors for redundancy and safety. You'd spend 1/5th the amount of time in space and thus have a hope of having kids and not dying of cancer.

It also allows you to spend a lot more time on Mars doing science. You can still do a 450 day mission, but spend 370 of those days doing actual science on Mars rather than a month or so.

A 450 day mission really is incredibly unrealistic beyond the radiation hazards, don't forget the logistical issues. Each astronaut needs about 10 kgs of consumables per day. So you need to plan not only for enough supplies storage space for 4500 kg of consumables per astronaut, but enough waste storage to handle a huge amount of sewage, recycling, etc. that makes that sort of a mission a much more complex undertaking than a much shorter duration mission using higher power propulsion.

You have the same environmental risks putting 200MW of nuke in orbit as you do putting 20MW in orbit, but you don't have all the logistical problems of supporting humans for 450 days. You could do the same 50 days of science on Mars with 80 days transit back and forth and have 1/4 the logistics issues.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/29/2010 08:01 pm
The rule of thumb for NEP vs NTR is that NEP uses twice the delta-V that NTR does (Zubrin, Deban) due to gravity losses.  That generally means that NEP missions are going to be twice the duration of NTR missions.

...no, how does that follow?

Sure, you spend more time getting out of the gravity well, but the much higher Isp means you can add lots of delta-V and do a high-energy transfer, or even (if the Isp is high enough) do a continuous-thrust trajectory, where you accelerate halfway and then turn around and decelerate.

So electric propulsion can actually get you where you want to go in less overall time, if it's far enough away (Mars is, the moon isn't).

From "Design and Optimization of Low-Thrust Gravity-Assist Trajectories to Selected Planets" (Debban, McConaghy, Longuski, 2002) and "The Application of Nuclear Power and Propulsion for Space Exploration Missions" (Zubrin, Sulmeisters, 1992):

                                           Mission         ∆V       TOF
                                                            (km/s)   (yrs)
Deban/McConaghy/Longuski   Pluto NEP      13.4       19
Zubrin/Sulmeisters                Pluto NTR       6.52      16
Zubrin/Sulmeisters                Pluto NTR      12.9       10

These studies favor NTRs for inner planet missions and NEPs for outer planet missions.  They illustrate the rule of thumb for mission planners that NTR's have approximately half the ∆V of NEP systems for the same time of flight.

The attraction of a VASIMR, should the plasma containment issues be smoothed out, is the variable impulse.  HIPEP, for instance, sits at 80% efficiency.  An NTR's efficiency, for perspective, is 98%.  Whatever's leftover has to be dumped some other way, usually via radiators.

By the way, just to get the numbers out there, a 75,000-pound-thrust NERVA, which I would characterize as a small-to-medium rocket engine, is approximately 1500 MW.  The 500MW Pewee derivatives now used in NTR reference missions are roughly the same thrust as an RL-10  (NTRs don't have a problem being clustered).  Looking at the enormous amount of power generated by a rocket engine, you can see why talking about making the power ratings of electric thrusters match those of rockets is somewhat unrealistic.  Rocket numbers are even more impressive when you look at how small and light they are for what they accomplish. 

Electric propulsion has its strengths and weaknesses, and so do NTRs, chemical rockets and everything else.  There are plenty of other options; slinging an NTR with a tether is a cool idea.  I saw a proposal the other day for a giant graphene trebuchet.  I hadn't thought of that one.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 05/29/2010 08:06 pm
It is hardly a fantasy.

This is a reference mission that Chang-Diaz stands behind. Are you calling this astronaut and the VASIMR developer a liar? Are you calling the other five NASA coauthors of these papers liars?

I love it how when an engineer uses a number that another engineer thinks is hopelessly unrealistic, sideline observers like you (mlorrey) like to characterize it as a "liar" type thing, as if we're before a court of law or something.  When Franklin did this 2000 paper, I was on the phone with him regularly about the VASIMR study.  I was the one running the larger study of which VASIMR was a subcomponent.  So don't act like you're telling me something you know more than me about.  I know things about VASIMR that never have and never will get to see the light of day, precisely for the reason that FCD is an astronaut and politically powerful and says things that higher-ups like to hear.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/29/2010 08:24 pm
A 200MW VASIMR reference mission to Mars can get there in 39 days, but takes 15 of those days to get out of Earth orbit.

No it doesn't.  That's a fantasy, and repeating it doesn't make it any more true.

It is hardly a fantasy.

This is a reference mission that Chang-Diaz stands behind. Are you calling this astronaut and the VASIMR developer a liar? Are you calling the other five NASA coauthors of these papers liars?

http://spaceflight.nasa.gov/shuttle/support/researching/aspl/reference/2000_3756.pdf
http://www.physorg.com/news186397741.html
http://dma.ing.uniroma1.it/users/bruno/Petro.prn.pdf
http://spaceflight.nasa.gov/shuttle/support/researching/aspl/reference/f_wsc02.pdf

Now, care to rescind your accusations?

I don't think accusing the main author of leaving some important bits out is at all an indictment of everyone listed on a paper.  The coauthors on these papers for the most part just had some of their calculations included.

The general consensus among EP guys is that VASIMR has gotten away from itself.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 05/29/2010 10:26 pm
It is hardly a fantasy.

This is a reference mission that Chang-Diaz stands behind. Are you calling this astronaut and the VASIMR developer a liar? Are you calling the other five NASA coauthors of these papers liars?

I love it how when an engineer uses a number that another engineer thinks is hopelessly unrealistic, sideline observers like you (mlorrey) like to characterize it as a "liar" type thing, as if we're before a court of law or something.  When Franklin did this 2000 paper, I was on the phone with him regularly about the VASIMR study.  I was the one running the larger study of which VASIMR was a subcomponent.  So don't act like you're telling me something you know more than me about.  I know things about VASIMR that never have and never will get to see the light of day, precisely for the reason that FCD is an astronaut and politically powerful and says things that higher-ups like to hear.

Well rather than acting obnoxious and arrogant, how about backing up your claims with some facts and some references? I don't know you from adam.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: isa_guy on 05/30/2010 11:14 am
The former head (Professor Samim Anghaie) of the team at university of florida that worked on this concept of 200 MWe gas(vapor) core reactor   http://ams.cern.ch/AMS/ETB/Appendix%20D-Anghaie.pdf , which Chang diaz used for his study ( to power 200 MWe VASIMR thrusters ) has been accused of fraud  http://abcnews.go.com/Business/wireStory?id=8961162 :) .
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 05/30/2010 02:58 pm
The former head (Professor Samim Anghaie) of the team at university of florida that worked on this concept of 200 MWe gas(vapor) core reactor   http://ams.cern.ch/AMS/ETB/Appendix%20D-Anghaie.pdf , which Chang diaz used for his study ( to power 200 MWe VASIMR thrusters ) has been accused of fraud  http://abcnews.go.com/Business/wireStory?id=8961162 :) .

I read up a little bit on what's happening with Anghaie and I don't think the fraud charges affect his engineering work, for the most part.  There is some evidence that he illegally subcontracted some of the work he was supposed to have done himself, and my understanding is that in some cases he used Russian subcontractors.  I wonder whether he might not have appointed himself savior of the Russian nuclear rocket program.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 06/02/2010 02:25 pm
I'd like to explore something that Kirk might have some input on.

I think that forty years ago liquid core NTRs were basically too difficult.  Since then, materials technology has improved a great deal, especially in the field of carbon and its various allotropes.  I'm wondering about the concept of a thorium-based liquid NTR.  The two ideas I've kicked around while out walking dogs this morning both basically had the core material in the center with a few layers of carbon passages carrying liquid hydrogen on the exterior, surrounded in turn by a layer of beryllium.  One configuration is annular, the other linear.

Any thoughts?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/02/2010 09:47 pm
It is hardly a fantasy.

This is a reference mission that Chang-Diaz stands behind. Are you calling this astronaut and the VASIMR developer a liar? Are you calling the other five NASA coauthors of these papers liars?

I love it how when an engineer uses a number that another engineer thinks is hopelessly unrealistic, sideline observers like you (mlorrey) like to characterize it as a "liar" type thing, as if we're before a court of law or something.  When Franklin did this 2000 paper, I was on the phone with him regularly about the VASIMR study.  I was the one running the larger study of which VASIMR was a subcomponent.  So don't act like you're telling me something you know more than me about.  I know things about VASIMR that never have and never will get to see the light of day, precisely for the reason that FCD is an astronaut and politically powerful and says things that higher-ups like to hear.

Well rather than acting obnoxious and arrogant, how about backing up your claims with some facts and some references? I don't know you from adam.

Kirk & Mike:

Trying to hand-wave orbital mechanics without all the givens can be fraught with errors.  Here at JSC a small group of interested parties were curious about whether Franklin Chang Diaz (FCD)'s 200 MWe, 39-day transit to Mars VASIMR claims were correct, or him just blowing smoke.  So after running to ground Franklin’s mission assumptions for this 39-day example, we had two different programmers in our group run the orbital simulations for such a ONE-WAY mission, which required aerobraking at Mars for the small 22mT manned lander payload, assuming the use of FDR's stated 200 MWe VASIMR, (thruster eff.= 60% with a variable Isp= 3,000 sec to 30,000 sec), with the powerplant and VASIMR dry specific mass alpha = 0.50 kg/kWe to see if this example could close.  It did for both orbital programming approaches we used, so given THESE mission assumptions and limitations, including the crew being stranded on the Mars surface while their mothership is parked in Mars orbit under autopilot control with dry tanks ~45 days after the crew lands, Franklin's 39-day transit time claim is doable.  I wouldn’t want to fly it though, but hey that’s just me!

Paul March
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: A_M_Swallow on 06/02/2010 11:05 pm
The return propellant can be sent using a slower tug.  The argon or hydrogen propellant could also be refined on Mars, the ascent stage being sent in advance.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/02/2010 11:10 pm
A 39-day trip could be made easier if the crew is launched to the MTV right as the MTV reached Earth escape velocity (or slightly after).

And yeah, you would want to pre-position just about everything you could (I think you ought to preposition lots of equipment at Mars's surface and Mars orbit no matter what architecture you choose), since otherwise your spacecraft would just be way too big (and 200MW wouldn't even be enough) for such a fast trip.

But I think such a fast trip is definitely unnecessary. Even stretching it to 80-days of transit would greatly decrease power requirements.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: A_M_Swallow on 06/03/2010 12:40 am
Has the viability of a 4 month trip to Mars possibly using 12MW solar arrays VASIMR been checked?

Ref:
Mars missions
http://www.adastrarocket.com/aarc/ToMars (http://www.adastrarocket.com/aarc/ToMars)

Solar cell mass
http://www.spaceflightnow.com/news/n1006/01vasimr (http://www.spaceflightnow.com/news/n1006/01vasimr)
"The Pentagon and Boeing Co. are developing a next-generation solar array aimed at reaching a weight-to-power ratio of 7 kilograms per kilowatt, according to the Defense Advanced Research Projects Agency."
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 06/03/2010 01:23 am
Has the viability of a 4 month trip to Mars possibly using 12MW solar arrays VASIMR been checked?

Ref:
Mars missions
http://www.adastrarocket.com/aarc/ToMars (http://www.adastrarocket.com/aarc/ToMars)

Solar cell mass
http://www.spaceflightnow.com/news/n1006/01vasimr (http://www.spaceflightnow.com/news/n1006/01vasimr)
"The Pentagon and Boeing Co. are developing a next-generation solar array aimed at reaching a weight-to-power ratio of 7 kilograms per kilowatt, according to the Defense Advanced Research Projects Agency."

What sized vehicle and payload? The test unit being installed on ISS in the next few years isn't sufficient to keep the station in orbit with 100kw of power available to it. You're going to need to declare some mission particulars if you want to use that equipment.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/03/2010 01:45 am
Has the viability of a 4 month trip to Mars possibly using 12MW solar arrays VASIMR been checked?

Ref:
Mars missions
http://www.adastrarocket.com/aarc/ToMars (http://www.adastrarocket.com/aarc/ToMars)

Solar cell mass
http://www.spaceflightnow.com/news/n1006/01vasimr (http://www.spaceflightnow.com/news/n1006/01vasimr)
"The Pentagon and Boeing Co. are developing a next-generation solar array aimed at reaching a weight-to-power ratio of 7 kilograms per kilowatt, according to the Defense Advanced Research Projects Agency."
The basic concept of an opposition-class mission to Mars with 10MW worth of SEP is viable, according to this:
http://www.astronautix.com/craft/stcemsep.htm
(although the robots servicing the array sounds like a kind of dumb idea... just make the array have enough EOL power to not require such servicing)

The 7kg/kW is a pretty easy goal to hit. 200W/kg (5kg/kW) can easily be done, and 1kW/kg is certainly possible with efficient thin-film arrays and solar-sail-like construction techniques.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: A_M_Swallow on 06/03/2010 02:00 am
Has the viability of a 4 month trip to Mars possibly using 12MW solar arrays VASIMR been checked?

Ref:
Mars missions
http://www.adastrarocket.com/aarc/ToMars (http://www.adastrarocket.com/aarc/ToMars)

Solar cell mass
http://www.spaceflightnow.com/news/n1006/01vasimr (http://www.spaceflightnow.com/news/n1006/01vasimr)
"The Pentagon and Boeing Co. are developing a next-generation solar array aimed at reaching a weight-to-power ratio of 7 kilograms per kilowatt, according to the Defense Advanced Research Projects Agency."

What sized vehicle and payload? The test unit being installed on ISS in the next few years isn't sufficient to keep the station in orbit with 100kw of power available to it. You're going to need to declare some mission particulars if you want to use that equipment.

Using the Ad Astra page I linked to:
Crew lander (61 mT Payload)
31 mT Habitat
13.5 mT Areoshell
16.3 mT Descent System

Solar arrays 12,000 kW * 7 kg/kW = 84,000 kg = 84 mT

The mass of the core including VASIMR thrusters, RCS and fuel tanks must be around somewhere.  Permitting calculation of the propellant mass.

Edit : correct units and mass of solar arrays
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: stealthyplains on 06/03/2010 05:20 am
What sized vehicle and payload? The test unit being installed on ISS in the next few years isn't sufficient to keep the station in orbit with 100kw of power available to it. You're going to need to declare some mission particulars if you want to use that equipment.

Isn't the test unit operated in pulsed mode because the station doesn't have enough power for a continuous run?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: 93143 on 06/03/2010 09:17 pm
Solar cell mass
http://www.spaceflightnow.com/news/n1006/01vasimr (http://www.spaceflightnow.com/news/n1006/01vasimr)
"The Pentagon and Boeing Co. are developing a next-generation solar array aimed at reaching a weight-to-power ratio of 7 kilograms per kilowatt, according to the Defense Advanced Research Projects Agency."
Solar arrays 12,000 kW / 7 kW/kg = 1,715 kg or 1.8 mT

12,000 kW * 7 kg/kW = 84,000 kg or 84 mT
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: A_M_Swallow on 06/03/2010 09:45 pm

12,000 kW * 7 kg/kW = 84,000 kg or 84 mT

Correction applied.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/04/2010 02:06 pm
with the powerplant and VASIMR dry specific mass alpha = 0.50 kg/kWe to see if this example could close.

Yeah, that's the magic number right there.  That's not doable.  And without that power supply all the rest of the argument falls apart.  FCD is disingenuous for continuing to quote that number.  I've asked him to his face why he does it, and he waves his hands and looks away from me and claims that he gets it from Samim Anghaie and that I should believe him.  Which I don't.  But FCD leaves the public and the NASA leadership with the impression that it is "VASIMR" that can do this when it's really "VASIMR and a magic power source" that does this.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: isa_guy on 06/04/2010 04:00 pm
with the powerplant and VASIMR dry specific mass alpha = 0.50 kg/kWe to see if this example could close.

Yeah, that's the magic number right there.  That's not doable.  And without that power supply all the rest of the argument falls apart.  FCD is disingenuous for continuing to quote that number.  I've asked him to his face why he does it, and he waves his hands and looks away from me and claims that he gets it from Samim Anghaie and that I should believe him.  Which I don't.  But FCD leaves the public and the NASA leadership with the impression that it is "VASIMR" that can do this when it's really "VASIMR and a magic power source" that does this.
Chang-diaz believed to a guy that has been accused of fraud  http://abcnews.go.com/Business/wireStory?id=8961162 , hmm no offence but it  looks like a conspiracy :) .
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/04/2010 04:05 pm
with the powerplant and VASIMR dry specific mass alpha = 0.50 kg/kWe to see if this example could close.

Yeah, that's the magic number right there.  That's not doable.  And without that power supply all the rest of the argument falls apart.  FCD is disingenuous for continuing to quote that number.  I've asked him to his face why he does it, and he waves his hands and looks away from me and claims that he gets it from Samim Anghaie and that I should believe him.  Which I don't.  But FCD leaves the public and the NASA leadership with the impression that it is "VASIMR" that can do this when it's really "VASIMR and a magic power source" that does this.
An MPD thruster could have an alpha this low, though its low efficiency means you need an even bigger power source (and it'd probably be glowing a dull red).

A really lightweight solar-power-sail could theoretically have such an alpha.

Doesn't seem worth it, though. Just learn to live in space for 400+ days. The Russians have already demonstrated that.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: isa_guy on 06/04/2010 05:51 pm
Why 400 days  shouldn't the NEP/SEP propulsion system shorten the trip time to mars ( compare to NTR or chemical propulsion ) ?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/04/2010 05:58 pm
Why 400 days  shouldn't the NEP/SEP propulsion system shorten the trip time to mars ( compare to NTR or chemical propulsion ) ?
Yeah, or it could allow you to bring more mass. Or have greater margins. Or both.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 06/04/2010 11:08 pm
with the powerplant and VASIMR dry specific mass alpha = 0.50 kg/kWe to see if this example could close.

Yeah, that's the magic number right there.  That's not doable.  And without that power supply all the rest of the argument falls apart.  FCD is disingenuous for continuing to quote that number.  I've asked him to his face why he does it, and he waves his hands and looks away from me and claims that he gets it from Samim Anghaie and that I should believe him.  Which I don't.  But FCD leaves the public and the NASA leadership with the impression that it is "VASIMR" that can do this when it's really "VASIMR and a magic power source" that does this.
Chang-diaz believed to a guy that has been accused of fraud  http://abcnews.go.com/Business/wireStory?id=8961162 , hmm no offence but it  looks like a conspiracy :) .

Lets see, they committed "Fraud" because the work was done on campus and in Russia rather than at NE facilities?

There's been some iffy prosecutions lately by the government, they recently tried to go after a pshrink at Dartmouth Hitchcock who did work at the VA across the river and managed some grants that he happened to do the work on and paid himself (shockers!). The jury found him innocent and said afterwards the governments case was baseless.

Part of the problem with these contracting prosecutions is that the investigators and prosecutors get a cut of the money and assets recovered, which encourages fake prosecutions.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/05/2010 02:28 am
I don't really care if Samim Anghaie is a crook but his 0.5 kg/kWe number is a fantasy.  Why FCD builds his VASIMR sales case on that number when all other reputable electric-propulsion researchers have rejected it (even though it makes their thrusters look incredible too) is beyond my understanding.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mmeijeri on 06/05/2010 02:36 am
Why FCD builds his VASIMR sales case on that number when all other reputable electric-propulsion researchers have rejected it (even though it makes their thrusters look incredible too) is beyond my understanding.

Because he sees VASIMR as a precursor to fusion powered propulsion and hopes to get that precursor funded? Not the most honest sales strategy...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/05/2010 04:40 am
The best supportable dry mass specific mass figure for the current VASMIR and a Polywell based aneutronic fusion reactor in the 100 MWe output power class was 1.70 kg/kWe plus structure + Tanks + H2 Propellant.  That doesn't make Franklin's 39 day one-way to Mars trip time doable, but it does allow pushing a manned 52 mT payload from Earth GEO to Mars Phobus orbit, stay for 30 days around Mars, and then go back to Earth GEO all in just under 12 months round trip time.  To me that makes a lot of Mars mission senarios very doable as compared to the current 900 day (30 monts) Mars round trip times still baselined by NASA. 
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/05/2010 04:44 am
The best supportable dry mass specific mass figure for the current VASMIR and a Polywell based aneutronic fusion reactor in the 100 MWe output power class was 1.70 kg/kWe plus structure + Tanks + H2 Propellant.  That doesn't make Franklin's 39 day one-way to Mars trip time doable, but it does allow pushing a manned 52 mT payload from Earth GEO to Mars Phobus orbit, stay for 30 days around Mars, and then go back to Earth GEO all in just under 12 months round trip time.  To me that makes a lot of Mars mission senarios very doable as compared to the current 900 day (30 monts) Mars round trip times still baselined by NASA. 
Those longer Mars missions are Conjunction-class missions, though. Though they have a long trip time, they stay at Mars for over a year, potentially getting more done. I hope we can quickly transition from Opposition-class 400-500 day missions to those Conjunction-class 900 day missions. Heck, rotate crew every couple years. But that's probably dreaming.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: KelvinZero on 06/05/2010 09:27 am
Hey, just another throwaway idea..

As I understand it, the biggest technical problem with NTR is that you cannot make the gas arbitrarily hotter than a chemical rocket because you are still limited by the materials of the chamber which must contain this heat.

However, as I understand it, initially this heat is in the form of extremely hot fragments of nuclei and gamma rays. I imagine these have so much energy that they ionise whatever they bang into, and the results of these collisions might also have enough energy to ionise whatever they bang into.

So if you could use magnetic fields to separate out these ions before their energy is defused into mere heat, you could use them for thrust immediately and at the same time greatly reduce the amount of heat you have to deal with, because you have expelled it while it was still in a low entropy state.

In short I guess I am talking about a nuclear powerplant/ VASIMR hybrid, where the powerplant produces the usual amount of heat for electricity, but also is a source of hot ions which do not count towards this heat and are fed directly into the VASIMR
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/05/2010 01:54 pm
The best supportable dry mass specific mass figure for the current VASMIR and a Polywell based aneutronic fusion reactor in the 100 MWe output power class was 1.70 kg/kWe plus structure + Tanks + H2 Propellant.  That doesn't make Franklin's 39 day one-way to Mars trip time doable, but it does allow pushing a manned 52 mT payload from Earth GEO to Mars Phobus orbit, stay for 30 days around Mars, and then go back to Earth GEO all in just under 12 months round trip time.  To me that makes a lot of Mars mission senarios very doable as compared to the current 900 day (30 months) Mars round trip times still baselined by NASA. 

Those longer Mars missions are Conjunction-class missions, though. Though they have a long trip time, they stay at Mars for over a year, potentially getting more done. I hope we can quickly transition from Opposition-class 400-500 day missions to those Conjunction-class 900 day missions. Heck, rotate crew every couple years. But that's probably dreaming.

You have to remember that neither the Russians nor the USA have ANY long term (greater than a week) human flight experience outside of the Earth's geomagnetic field.  The proposed long term stays (~900 days) in the cosmic ray shooting gallery and the radiation issues associated with solar flare events that is encapsulated in interplanetary journeys have many crew safety question marks still unanswered.  So longer missions to Mars poses more risk to the crew than NASA is willing to take at the moment.  Shorter stay and trip times are better if you don't want dead or dying crew members coming back to Earth...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/05/2010 02:03 pm
Hey, just another throwaway idea..

As I understand it, the biggest technical problem with NTR is that you cannot make the gas arbitrarily hotter than a chemical rocket because you are still limited by the materials of the chamber which must contain this heat.

However, as I understand it, initially this heat is in the form of extremely hot fragments of nuclei and gamma rays. I imagine these have so much energy that they ionise whatever they bang into, and the results of these collisions might also have enough energy to ionise whatever they bang into.

So if you could use magnetic fields to separate out these ions before their energy is defused into mere heat, you could use them for thrust immediately and at the same time greatly reduce the amount of heat you have to deal with, because you have expelled it while it was still in a low entropy state.

In short I guess I am talking about a nuclear powerplant/ VASIMR hybrid, where the powerplant produces the usual amount of heat for electricity, but also is a source of hot ions which do not count towards this heat and are fed directly into the VASIMR


Sorry Kelvin, but 99.9XXX% of the fission fragments from a Nuclear Thermal Rocket (NTR) stay in the nuclear fuel matrix that makes up the NTR fission core.  That's especially true for the tungsten cermet based NTR cores being proposed by Glenn Research Center (GRC) for use in their proposed NTR for the JSC/Glenn 2009 Mars Referece Mission-5 case.  And gammas don't ionize low density (compared to liquids) gases very well either.  In other words, NTRs work best heating their chosen propellants, but not in making electricity, except in small amounts.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/05/2010 02:46 pm
Hey, just another throwaway idea..

As I understand it, the biggest technical problem with NTR is that you cannot make the gas arbitrarily hotter than a chemical rocket because you are still limited by the materials of the chamber which must contain this heat.

However, as I understand it, initially this heat is in the form of extremely hot fragments of nuclei and gamma rays. I imagine these have so much energy that they ionise whatever they bang into, and the results of these collisions might also have enough energy to ionise whatever they bang into.

So if you could use magnetic fields to separate out these ions before their energy is defused into mere heat, you could use them for thrust immediately and at the same time greatly reduce the amount of heat you have to deal with, because you have expelled it while it was still in a low entropy state.

In short I guess I am talking about a nuclear powerplant/ VASIMR hybrid, where the powerplant produces the usual amount of heat for electricity, but also is a source of hot ions which do not count towards this heat and are fed directly into the VASIMR

I wouldn't classify that as a throw-away idea. It sounds like a great idea to me!

You've just described the fission-fragment rocket. ;)
http://en.wikipedia.org/wiki/Fission-fragment_rocket
Quote
The fission-fragment rocket is a rocket engine design that directly harnesses hot nuclear fission products for thrust, as opposed to using a separate fluid as working mass. The design can, in theory, produce very high specific impulses while still being well within the abilities of current technologies.

Quote
With exhaust velocities of 3 % - 5 % the speed of light and efficiencies up to 90 %, the rocket should be able to achieve over 1,000,000 sec Isp.

Very fascinating idea. I think we may well use something like this to reach the stars, if we ever do.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Hop_David on 06/05/2010 04:17 pm
... but his 0.5 kg/kWe number is a fantasy.

kWe is kilowatt electricity?

What's the best plausible kg/kWe number?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: PurduesUSAFguy on 06/05/2010 05:06 pm
The SP-100 was designed to provide 2.3mW (thermal) and 100kW (electrical) and had a design mass of 4000 kilograms. That works out to 40 kilograms a kilowatt.

If Project Prometheus would have been completed double that mass efficiency is likely possible. Still 20kg/kWe is a far cry from what the VASIMR is envisioning.

I'll grant you that there are scaling efficiencies to be had with larger reactors, but still, 0.5kg/kWe is a pipe dream.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/05/2010 05:08 pm
The SP-100 was designed to provide 2.3mW (thermal) and 100kW (electrical) and had a design mass of 4000 kilograms. That works out to 40 kilograms a kilowatt.

If Project Prometheus would have been completed double that mass efficiency is likely possible. Still 20kg/kWe is a far cry from what the VASIMR is envisioning.

I'll grant you that there are scaling efficiencies to be had with larger reactors, but still, 0.5kg/kWe is a pipe dream.

Which is why for a Mars mission, we should use solar power!!! Save nuclear for the outer planets.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: PurduesUSAFguy on 06/05/2010 05:21 pm
The SP-100 was designed to provide 2.3mW (thermal) and 100kW (electrical) and had a design mass of 4000 kilograms. That works out to 40 kilograms a kilowatt.

If Project Prometheus would have been completed double that mass efficiency is likely possible. Still 20kg/kWe is a far cry from what the VASIMR is envisioning.

I'll grant you that there are scaling efficiencies to be had with larger reactors, but still, 0.5kg/kWe is a pipe dream.

Which is why for a Mars mission, we should use solar power!!! Save nuclear for the outer planets.

I have to say I disagree, I think NTRs are the best near term solution, I don't think an apples to apples comparison of equivalent system masses will compare favourably for SEP vs NTRs.

Also for Mars, surface nuclear power will be vital to ISRU, which in my view is the key to making humans to Mars viable in the near term. (or at least not too distant future)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/05/2010 05:33 pm
The SP-100 was designed to provide 2.3mW (thermal) and 100kW (electrical) and had a design mass of 4000 kilograms. That works out to 40 kilograms a kilowatt.

If Project Prometheus would have been completed double that mass efficiency is likely possible. Still 20kg/kWe is a far cry from what the VASIMR is envisioning.

I'll grant you that there are scaling efficiencies to be had with larger reactors, but still, 0.5kg/kWe is a pipe dream.

Which is why for a Mars mission, we should use solar power!!! Save nuclear for the outer planets.

I have to say I disagree, I think NTRs are the best near term solution, I don't think an apples to apples comparison of equivalent system masses will compare favourably for SEP vs NTRs.
Of the Opposition-class Mars architectures listed here, SEP has the lowest IMLEO of any of the four architectures (Chemical/aerocapture, NTR, NEP, SEP) while also being closest to full reusability:
Quote
The solar electric propulsion (SEP) Mars transfer concept was the only non-nuclear advanced propulsion option in the STCAEM study. It offered advantages of the lowest IMLEO of the four reference vehicles.
http://www.astronautix.com/craft/stcemsep.htm

Also for Mars, surface nuclear power will be vital to ISRU, which in my view is the key to making humans to Mars viable in the near term. (or at least not too distant future)
Agreed! However, the power needed for ISRU is orders of magnitude different than that needed for electric propulsion. Also, the Martian atmosphere is available for cooling. And, the power requirements are actually low enough that advanced "RTG"s (either thermophotovoltaic or advanced sterling) could be used, though we'd need to restart making Pu-238. A couple advantages of RTGs over a fission power plant is that the radiation level is FAR less at all stages, allowing the power plant to be close to the astronauts if required (an RTG was on the Apollo Lunar Lander, for instance) while also providing continuous power for decades.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: PurduesUSAFguy on 06/05/2010 05:55 pm
Interesting, I wasn't really aware of the STCAEM study. I wonder what the transfer times for a conjunction class mission would be.

Could you do a conjunction class mission with the long spiral out and spiral down times associated with SEP?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/05/2010 06:29 pm
Interesting, I wasn't really aware of the STCAEM study. I wonder what the transfer times for a conjunction class mission would be.

Could you do a conjunction class mission with the long spiral out and spiral down times associated with SEP?
Spiral times aren't an issue if the crew (in, say, an Orion or Dragon or Soyuz spacecraft) does a rendezvous with the MTV right as it's finished spiraling out of Earth orbit. The crew also departs the MTV in the lander just as the MTV approaches Mars (not a really high velocity reentry, though, because the MTV reverses thrust about half-way through). The MTV spirals in to Mars orbit while the crew is doing their mission on the surface. On the way back, the crew does a direct entry at Earth, so there's no spiral needed there, either.

This was to be the same for both NEP and SEP, I believe.

And an advantage of both these high-Isp architectures is that they have lots of mass margin and are not sensitive at all to mass increases (could even take two landers along if you wanted to).

The specific power for SEP is around 250 W/kg in this architecture (including structure), or an alpha of about 4kg/kW. That's within spitting distance of current solar power (Ultraflex supposedly can do over 200W/kg with the right photovoltaic circuits), what's needed is a far greater scale than what's been demonstrated so far, although efficient thin-film solar arrays could probably have a specific power (including structure) of 1kW/kg (alpha of 1kg/kW), especially if you're willing to live with low fundamental frequencies for your structure. (EDIT: An alpha of .5kg/kW is definitely possible for solar arrays, but it may take awhile for the efficiency of thin-film cells to approach this alpha.)

I think that a better architecture would include EML1/2 rendezvous, high Mars orbit rendezvous, low Mars orbit rendezvous, and Mars surface rendezvous with pre-placing assets at all those locations (along with carbon monoxide/LOX ISRU). That would be complicated, yes, but it means your spaceship can be far smaller and it sets the stage for long-term Martian exploration while also fitting well with incremental steps and "flexible path." Also, it means your lander can be FAR smaller, around only twice as big as the Apollo LM. You could even use fully propulsive Martian EDL for the crew, if you wanted to (a full Mars ascent vehicle can reach orbit from the surface, but it can also reach the surface from orbit... entirely propulsively).
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kkattula on 06/05/2010 06:59 pm
Of the Opposition-class Mars architectures listed here, SEP has the lowest IMLEO of any of the four architectures (Chemical/aerocapture, NTR, NEP, SEP) while also being closest to full reusability:
Quote
The solar electric propulsion (SEP) Mars transfer concept was the only non-nuclear advanced propulsion option in the STCAEM study. It offered advantages of the lowest IMLEO of the four reference vehicles.
http://www.astronautix.com/craft/stcemsep.htm


Well yeah, because it used aerobraking into Mars orbit. NTR used propulsive braking, and left the MTV in a much lower orbit. Also:

SEP total trip time was 550 days compared to 420 for NTR
Surface time 20 days compared to 30 for NTR

Artificial gravity during transit was relatively easy for NTR, not so for SEP.

It's all about the details, not just the smallest IMLEO.

Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/05/2010 07:32 pm
Of the Opposition-class Mars architectures listed here, SEP has the lowest IMLEO of any of the four architectures (Chemical/aerocapture, NTR, NEP, SEP) while also being closest to full reusability:
Quote
The solar electric propulsion (SEP) Mars transfer concept was the only non-nuclear advanced propulsion option in the STCAEM study. It offered advantages of the lowest IMLEO of the four reference vehicles.
http://www.astronautix.com/craft/stcemsep.htm


Well yeah, because it used aerobraking into Mars orbit. NTR used propulsive braking, and left the MTV in a much lower orbit. Also:
It doesn't use aerobraking. Anyways, so what if it does?
SEP total trip time was 550 days compared to 420 for NTR[/quote]True, but if you used a lower Isp for the thrusters, you could lower that time (at the expense of more IMLEO).[/quote]
Surface time 20 days compared to 30 for NTR

Artificial gravity during transit was relatively easy for NTR, not so for SEP.

It's all about the details, not just the smallest IMLEO.
[/quote]I agree that that architecture isn't the best possible one. The point is that we already have demonstrated SEP. It just needs to be bigger.

Combined with pre-placement of infrastructure (especially on the surface of Mars), a far less massive MTV could be used, thus far less power and a smaller array. ...while also making Martian EDL easier.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Solman on 06/05/2010 08:07 pm
Of the Opposition-class Mars architectures listed here, SEP has the lowest IMLEO of any of the four architectures (Chemical/aerocapture, NTR, NEP, SEP) while also being closest to full reusability:
Quote
The solar electric propulsion (SEP) Mars transfer concept was the only non-nuclear advanced propulsion option in the STCAEM study. It offered advantages of the lowest IMLEO of the four reference vehicles.
http://www.astronautix.com/craft/stcemsep.htm


Well yeah, because it used aerobraking into Mars orbit. NTR used propulsive braking, and left the MTV in a much lower orbit. Also:
It doesn't use aerobraking. Anyways, so what if it does?
SEP total trip time was 550 days compared to 420 for NTR
True, but if you used a lower Isp for the thrusters, you could lower that time (at the expense of more IMLEO).[/quote]
Surface time 20 days compared to 30 for NTR

Artificial gravity during transit was relatively easy for NTR, not so for SEP.

It's all about the details, not just the smallest IMLEO.
[/quote]I agree that that architecture isn't the best possible one. The point is that we already have demonstrated SEP. It just needs to be bigger.

Combined with pre-placement of infrastructure (especially on the surface of Mars), a far less massive MTV could be used, thus far less power and a smaller array. ...while also making Martian EDL easier.
[/quote]

 You know, I realize this goes outside orthodoxy, but there is this thing called solar thermal/electric propulsion. By using a series of perigee thrusts it can use hydrogen propellant at 800 to 1200 sec. Isp to get from LEO to escape in under 2 weeks and can then move concentrator type PV near its focus at that point to power an MPD or other electric propulsion, Proving that it would be faster than NTR is beyond me but just for laughs why don't one of you guys try it - .1 kg/KWthermal for the concentrator or is that too optimistic? a little over 1kg/KW for the cells?

Sol
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/05/2010 09:05 pm
Of the Opposition-class Mars architectures listed here, SEP has the lowest IMLEO of any of the four architectures (Chemical/aerocapture, NTR, NEP, SEP) while also being closest to full reusability:
Quote
The solar electric propulsion (SEP) Mars transfer concept was the only non-nuclear advanced propulsion option in the STCAEM study. It offered advantages of the lowest IMLEO of the four reference vehicles.
http://www.astronautix.com/craft/stcemsep.htm


Well yeah, because it used aerobraking into Mars orbit. NTR used propulsive braking, and left the MTV in a much lower orbit. Also:
It doesn't use aerobraking. Anyways, so what if it does?
SEP total trip time was 550 days compared to 420 for NTR
True, but if you used a lower Isp for the thrusters, you could lower that time (at the expense of more IMLEO).
Quote
Surface time 20 days compared to 30 for NTR

Artificial gravity during transit was relatively easy for NTR, not so for SEP.

It's all about the details, not just the smallest IMLEO.
I agree that that architecture isn't the best possible one. The point is that we already have demonstrated SEP. It just needs to be bigger.

Combined with pre-placement of infrastructure (especially on the surface of Mars), a far less massive MTV could be used, thus far less power and a smaller array. ...while also making Martian EDL easier.

 You know, I realize this goes outside orthodoxy, but there is this thing called solar thermal/electric propulsion. By using a series of perigee thrusts it can use hydrogen propellant at 800 to 1200 sec. Isp to get from LEO to escape in under 2 weeks and can then move concentrator type PV near its focus at that point to power an MPD or other electric propulsion, Proving that it would be faster than NTR is beyond me but just for laughs why don't one of you guys try it - .1 kg/KWthermal for the concentrator or is that too optimistic? a little over 1kg/KW for the cells?

Sol
[/quote]A concentrator would have about the same specific power as a solar array. You can deposit solar cells only a few microns thick on mylar-like material, the same sort of thing you'd use as a concentrator. Solar-thermal is too low performance to be worth it, though.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/06/2010 12:40 am
The SP-100 was designed to provide 2.3mW (thermal) and 100kW (electrical) and had a design mass of 4000 kilograms. That works out to 40 kilograms a kilowatt.

If Project Prometheus would have been completed double that mass efficiency is likely possible. Still 20kg/kWe is a far cry from what the VASIMR is envisioning.

I'll grant you that there are scaling efficiencies to be had with larger reactors, but still, 0.5kg/kWe is a pipe dream.

Internal numbers I saw had Prometheus at >100 kg/kWe when they cancelled the program.  The program was sold to NASA administration at ~20 kg/kWe, which made the application of NEP much more attractive.  As the alpha headed south, so did the applicability of the technology to interplanetary missions.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/06/2010 12:41 am
I have to say I disagree, I think NTRs are the best near term solution, I don't think an apples to apples comparison of equivalent system masses will compare favourably for SEP vs NTRs.

For robotic missions, NTRs will lose horribly to other forms of propulsion simply on development costs.  For human missions, they just lose badly instead of horribly.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/06/2010 12:44 am
The SP-100 was designed to provide 2.3mW (thermal) and 100kW (electrical) and had a design mass of 4000 kilograms. That works out to 40 kilograms a kilowatt.

If Project Prometheus would have been completed double that mass efficiency is likely possible. Still 20kg/kWe is a far cry from what the VASIMR is envisioning.

I'll grant you that there are scaling efficiencies to be had with larger reactors, but still, 0.5kg/kWe is a pipe dream.

Internal numbers I saw had Prometheus at >100 kg/kWe when they cancelled the program.  The program was sold to NASA administration at ~20 kg/kWe, which made the application of NEP much more attractive.  As the alpha headed south, so did the applicability of the technology to interplanetary missions.
What about SEP?

I see great promise in SEP, mainly because if solar sails work well (not a given), then SEP should be able to have very impressive alphas. And solar power has already made enormous strides in the last few decades when it comes to alpha.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/06/2010 12:44 am
SEP total trip time was 550 days compared to 420 for NTR
Surface time 20 days compared to 30 for NTR

Artificial gravity during transit was relatively easy for NTR, not so for SEP.

It's all about the details, not just the smallest IMLEO.

Comparisons like this are somewhat meaningless because they are very opportunity-specific.  There's nothing magic that makes a 100 or 130 day difference between propulsion systems.  It's all about what payload fraction you'll accept and your development costs.  Quoting mission times is a disingenuous trick commonly used by FCD to push his propulsion system, and it's nearly meaningless.

Except when quoting 40-day transits, when it goes from being meaningless to utterly fantasy-based.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/06/2010 12:45 am
I see great promise in SEP, mainly because if solar sails work well (not a given), then SEP should be able to have very impressive alphas.

Solar sails have nothing to do with SEP.  Solar sails are based on Mylar and solar reflective pressure, not on the capture and conversion of solar energy for electricity for a propulsion system.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/06/2010 12:47 am
I see great promise in SEP, mainly because if solar sails work well (not a given), then SEP should be able to have very impressive alphas.

Solar sails have nothing to do with SEP.  Solar sails are based on Mylar and solar reflective pressure, not on the capture and conversion of solar energy for electricity for a propulsion system.
I totally understand that. However, solar sails can also use thin-film solar cells instead of just metalized mylar. Behold, IKAROS:
http://www.jspec.jaxa.jp/e/activity/ikaros.html

EDIT: Designing a solar array in this manner could allow an alpha an order of magnitude smaller than even very high performance solar arrays like UltraFlex (which have an alpha of around 7kg/kW). Of course, this is at the expense of very low structural fundamental frequencies and all the control issues involved with that.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: KelvinZero on 06/06/2010 01:19 am
I wouldn't classify that as a throw-away idea. It sounds like a great idea to me!

You've just described the fission-fragment rocket. ;)
http://en.wikipedia.org/wiki/Fission-fragment_rocket

Ah, thanks.. I hadn't spent much time looking at nuclear engines, I think I had seen the name but not looked it up because I had confused it with one of the variations of NTR.

My idea I guess was exactly the fission fragment rocket, except I was imagining that most of the ions came from some material (coolant or shielding or neutron moderator) surrounding the fissile material and being hit by gamma rays, neutrons or fission fragments, and then any particles that they in turn hit, all being good candidates until the energy drops below the point where particles are ionised.

I guess the key is that most (all?) energy from fission, whether initially from gamma rays, neutrons or larger fragments, is momentarily in the form of fast moving ions before it diffuses into mere heat.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 06/06/2010 10:57 pm
I have to say I disagree, I think NTRs are the best near term solution, I don't think an apples to apples comparison of equivalent system masses will compare favourably for SEP vs NTRs.

For robotic missions, NTRs will lose horribly to other forms of propulsion simply on development costs.  For human missions, they just lose badly instead of horribly.

We differ here, Kirk.  We've already spent the money on development costs where NTRs are concerned. 

It's not like we're starting from scratch; we already have well-documented NTR designs that have run at full power and have hours of testing.  If you want better Isp than 865 seconds, the last-tested USA fuel, and you don't trust anything the Russians say, fine.  Further fuel rod testing would be electric, first, and then probably progress to verification in a small NF-sized reactor in a scrubber, which is not an expensive proposition. 

The next step for NERVA-derived flight prototypes would be run-up to a few hundred degrees on the ground and then testing in space.  That was the plan for the first flight prototype in 1969, and there's no need for the plan to change 40 years later. 

Quite frankly, NTRs have been tested far more than any other advanced option discussed on this forum (and are at a far more advanced level of development) when it comes to impulse levels high enough to take humans anywhere.  Every other propulsion type I see on this forum is based on extrapolation and speculation.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/07/2010 12:26 am
865 sec is not nearly enough Isp to justify putting an NTR on any deep space mission.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mmeijeri on 06/07/2010 12:32 am
865 sec is not nearly enough Isp to justify putting an NTR on any deep space mission.

What if you throw time to initial operational capability into the mix? I think that chemical + prepositioning propellant with SEP is the solution that could be operational soonest. That could be used soon, say for landing probes on Mars in areas that are currently inaccessible. Might NTR be faster than NEP for manned spacecraft? It would only have to be competitive with chemical in that case. I'm specifically thinking of water as a working fluid: it could give you slightly higher Isp than LOX/LH2 and much greater density. As an added bonus it doesn't even require cryogenic propellant transfer, though that may not be much of an argument if it requires NTR. Would this help with T/W?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/07/2010 12:34 am
I have to say I disagree, I think NTRs are the best near term solution, I don't think an apples to apples comparison of equivalent system masses will compare favourably for SEP vs NTRs.

For robotic missions, NTRs will lose horribly to other forms of propulsion simply on development costs.  For human missions, they just lose badly instead of horribly.

We differ here, Kirk.  We've already spent the money on development costs where NTRs are concerned. 

It's not like we're starting from scratch; we already have well-documented NTR designs that have run at full power and have hours of testing.  If you want better Isp than 865 seconds, the last-tested USA fuel, and you don't trust anything the Russians say, fine.  Further fuel rod testing would be electric, first, and then probably progress to verification in a small NF-sized reactor in a scrubber, which is not an expensive proposition. 

The next step for NERVA-derived flight prototypes would be run-up to a few hundred degrees on the ground and then testing in space.  That was the plan for the first flight prototype in 1969, and there's no need for the plan to change 40 years later. 

Quite frankly, NTRs have been tested far more than any other advanced option discussed on this forum (and are at a far more advanced level of development) when it comes to impulse levels high enough to take humans anywhere.  Every other propulsion type I see on this forum is based on extrapolation and speculation.
More tested? SEP is already operational! Has been for years.
http://en.wikipedia.org/wiki/Dawn_(spacecraft) (http://en.wikipedia.org/wiki/Dawn_(spacecraft))
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/07/2010 12:44 am
It's not like we're starting from scratch;

We are starting from scratch, because all the people who did that work are dead and gone, and the work was done under different environmental and launch safety considerations than today.  We're starting at square one, and you can take five minutes (maybe 15 if you're slow) to look at the Isp and T/W of an NTR and figure out if it's worth the effort to develop in the first place.

It's not.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 06/07/2010 02:54 am
It's not like we're starting from scratch;

We are starting from scratch, because all the people who did that work are dead and gone,


No they're not, surprisingly enough.  I talked to Harry Finger a few days ago, and I'll talk to Frank Durham this week.  I'll probably talk to Paul Wagner in the next couple of weeks.  Stan Gunn's still around.  There are plenty of people still alive.  They're in their eighties and nineties, but they're still kicking.

But moreover, there's an active group of people up at Los Alamos who have carefully archived the Rover documentation and maintain relationships with the remaining Rover folks.  A lot of it has reappeared on the DOE website over the past few months.

Quote
and the work was done under different environmental and launch safety considerations than today. 

The environmental and launch safety considerations during the testing have nothing to do with the data.  Physics is physics, data is data.  Examining what was actually developed, most current launch safety considerations have not changed substantially since the period when NRX or Pewee were designed.  Further, both were designed and tested under more stringent conditions than most chemical rockets ever had to meet.  Only one rocket program has ever been asked to meet a goal of 10 hours of operation and 70 restarts.  NRX met that goal.

Quote
We're starting at square one, and you can take five minutes (maybe 15 if you're slow) to look at the Isp and T/W of an NTR and figure out if it's worth the effort to develop in the first place.

I used your methods, but real numbers, and published the numbers right here in this forum.  It was obviously worth the effort. 

Quote
It's not.

There are a lot of highly qualified people remaining, several of them AAAS Fellows like Harry Finger, who feel the opposite.  They're not stupid people, Kirk. 

I don't understand your emotional problem with NTRs.  There are reasonable and unreasonable objections, and yours are obviously unreasonable.  I think you should get over it and figure out how to do it with thorium.  That would be a real contribution.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/07/2010 03:00 am
This has absolutely zero to do with thorium.

Show me these "real numbers" you used and the results you got from them.  If you used 850 sec Isp and a T/W of 4 then there's no way you could claim it was worth the effort.

My "emotional problem" is the sorrow I would feel about seeing billions of dollars of my and others taxpayer funds wasted on this costly and non-improvement approach to space propulsion.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 06/07/2010 03:18 am
This has absolutely zero to do with thorium.

Show me these "real numbers" you used and the results you got from them.  If you used 850 sec Isp and a T/W of 4 then there's no way you could claim it was worth the effort.

My "emotional problem" is the sorrow I would feel about seeing billions of dollars of my and others taxpayer funds wasted on this costly and non-improvement approach to space propulsion.

NON IMPROVEMENT??? Over what, pray tell? Show me a single engine of more than 50,000 lbs thrust with an Isp that high that has gotten anywhere near being tested. Sorry but you are just out to lunch here.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Warren Platts on 06/07/2010 03:42 am
This has absolutely zero to do with thorium.

Show me these "real numbers" you used and the results you got from them.  If you used 850 sec Isp and a T/W of 4 then there's no way you could claim it was worth the effort.

My "emotional problem" is the sorrow I would feel about seeing billions of dollars of my and others taxpayer funds wasted on this costly and non-improvement approach to space propulsion.

NON IMPROVEMENT??? Over what, pray tell? Show me a single engine of more than 50,000 lbs thrust with an Isp that high that has gotten anywhere near being tested. Sorry but you are just out to lunch here.
The problem is the thrust/weight ratio.  At T/W of 4, your engine weighs 6.25 tons, compared to a few hundred pounds for an RL-10. Plugging the numbers into the spreadsheet, and the RL-10 actually has better payload mass fractions than the NTR for initial thrust/weights of 1.0.  (RL-10: Isp=450; Mix ratio=5; T/W=59.5) For a delta v of 2.5 km/s, the mass frac is 0.53 for RL-10, and 0.48 for NTR).

Even where the NTR has a better mass fraction, the difference is marginal, and so it's not worth billions in development costs to obtain an increase in performance that theory says can only be marginal.

(I have a question: what is the significance of this "initial thrust/weight" factor? The NTR is much more sensitive to it than is the RL-10 if you run a spread from 0.2 to 1.0)?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 06/07/2010 04:32 am
This has absolutely zero to do with thorium.

Show me these "real numbers" you used and the results you got from them.  If you used 850 sec Isp and a T/W of 4 then there's no way you could claim it was worth the effort.

My "emotional problem" is the sorrow I would feel about seeing billions of dollars of my and others taxpayer funds wasted on this costly and non-improvement approach to space propulsion.

NON IMPROVEMENT??? Over what, pray tell? Show me a single engine of more than 50,000 lbs thrust with an Isp that high that has gotten anywhere near being tested. Sorry but you are just out to lunch here.
The problem is the thrust/weight ratio.  At T/W of 4, your engine weighs 6.25 tons, compared to a few hundred pounds for an RL-10. Plugging the numbers into the spreadsheet, and the RL-10 actually has better payload mass fractions than the NTR for initial thrust/weights of 1.0.  (RL-10: Isp=450; Mix ratio=5; T/W=59.5) For a delta v of 2.5 km/s, the mass frac is 0.53 for RL-10, and 0.48 for NTR).

Even where the NTR has a better mass fraction, the difference is marginal, and so it's not worth billions in development costs to obtain an increase in performance that theory says can only be marginal.

(I have a question: what is the significance of this "initial thrust/weight" factor? The NTR is much more sensitive to it than is the RL-10 if you run a spread from 0.2 to 1.0)?


So what? Payload mass fraction isn't the be all end all statistic to base everything on.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/07/2010 04:41 am
Folks:

If we can't build a NTR with a thrust to weight ratio of better than 25-to-1, I have to agree with KFS.  However from the early work on the DUMBO NTRs, the particle bed reactors performed by the USAF back in the late 1980s and early 90s under the Timberwind program, and the current tungsten ceremt NTR work espeically as applied to the high power Dumbo reactors, a viable NTR with T/W ratios of greater than 25-to-1 should be buildable.  Especially if we pursue thrust levels of 50 kLb-f or greater.  Of course there is the negative politics surrounding anything "nuclear" in this country to contend with, and at the rate the USA is going broke under the current administration, this discussion may be moot to begin with.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Warren Platts on 06/07/2010 07:16 am
So what? Payload mass fraction isn't the be all end all statistic to base everything on.
Neither is Isp.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 06/07/2010 08:03 am
So what? Payload mass fraction isn't the be all end all statistic to base everything on.
Neither is Isp.

When it comes to in-space operations, Isp is far more important than, say, launching into orbit.

Rather than a straight T/W, I would rate different propulsion systems by T*Isp/W along with a coefficient that involves the density of the propellant and resulting tank mass requirements along with a factor regarding the ease of refuelling (i.e. cracking water is a lot easier than finding xenon or kerosene out in space), as well as factoring trip time. Any propulsion using LH2 alone will have serious payload mass fraction performance issues, but punishing it for that when an interplanetary vessel using LH2 can rather conveniently be refuelled is another matter entirely. Likewise punishing a poor mass fraction on a vehicle that can get somewhere twice as fast as a vehicle with a better mass frac is not smart.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Warren Platts on 06/07/2010 08:25 am
I think Kirk's point is that the be all and end all statistic to base everything on is the bang/bucks ratio. The marginal improvement isn't worth the many factors of $$$ required to obtain that.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/07/2010 11:54 am
I think Kirk's point is that the be all and end all statistic to base everything on is the bang/bucks ratio. The marginal improvement isn't worth the many factors of $$$ required to obtain that.


Where a high thrust NTR with an Isp = ~950 seconds really excels in the bang per buck arena is in the reusable single stage to orbit (SSTO) applications.  Too bad no one has the intestinal fortitude to pursue it.  I guess that is a tribute to the fact that humanity has yet to decide that human space flight (HSF) is really that important, yet.  In the meanwhile we will have to wait for the aneutronic fusion reactors/rockets to come on line, if they ever do...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/07/2010 01:24 pm
Where a high thrust NTR with an Isp = ~950 seconds really excels in the bang per buck arena is in the reusable single stage to orbit (SSTO) applications.

Not even close.  This was one of the first things we looked at when I came to NASA ten years ago.  The engine had nowhere near the thrust-to-weight for the earth-to-orbit application.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/07/2010 01:39 pm
Is it possible to build an NTR with a >900 s Isp and >25 T/W?

Perhaps--I don't know how, but I can't exclude the possibility.

What I can exclude is that nothing that was tested back in the 1960s has performance anywhere close to that, and the stuff that was tested doesn't have sufficient performance to "buy" its way onto any deep space mission.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/07/2010 02:14 pm
There are a lot of highly qualified people remaining, several of them AAAS Fellows like Harry Finger, who feel the opposite.  They're not stupid people, Kirk.

I never said that they were.  Good engineers bring their calculations to the table so that they can see each others' work and assess whether they started from the same initial numbers or if there were flaws in the calculation strategy that led to different answers.  I've showed my work and invite you and anyone else to show yours.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 06/07/2010 03:39 pm
More tested? SEP is already operational! Has been for years.
http://en.wikipedia.org/wiki/Dawn_(spacecraft) (http://en.wikipedia.org/wiki/Dawn_(spacecraft))

Let's see, that's 0.02 pounds of thrust...bring sunblock.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: daveklingler on 06/07/2010 03:41 pm
Folks:

If we can't build a NTR with a thrust to weight ratio of better than 25-to-1, I have to agree with KFS.

Eh?  Why?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/07/2010 07:32 pm
More tested? SEP is already operational! Has been for years.
http://en.wikipedia.org/wiki/Dawn_(spacecraft) (http://en.wikipedia.org/wiki/Dawn_(spacecraft))

Let's see, that's 0.02 pounds of thrust...bring sunblock.

Who cares if the thrust is lower? You can afford greater delta-v. Sure, you pay a penalty for having low thrust, but at higher specific powers (like are available today and even better tomorrow) and the incredibly high Isp of electric thrusters (HiPeP has 10 times the Isp of NTR), that penalty is easily compensated for.

Besides, let's compare that to nuclear thermal rockets which have flown in orbit. Those have exactly zero thrust.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: A_M_Swallow on 06/07/2010 07:54 pm
More tested? SEP is already operational! Has been for years.
http://en.wikipedia.org/wiki/Dawn_(spacecraft) (http://en.wikipedia.org/wiki/Dawn_(spacecraft))

Let's see, that's 0.02 pounds of thrust...bring sunblock.

Who cares if the thrust is lower? You can afford greater delta-v. Sure, you pay a penalty for having low thrust, but at higher specific powers (like are available today and even better tomorrow) and the incredibly high Isp of electric thrusters (HiPeP has 10 times the Isp of NTR), that penalty is easily compensated for.

Besides, let's compare that to nuclear thermal rockets which have flown in orbit. Those have exactly zero thrust.

There is a major difference between spacecraft and launch vehicles.

Spacecraft - need high Isp and can live with low thrust.
Launch vehicles - need high thrust and can live with low Isp.

The last few posts have been mixing up the two types of machines.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Warren Platts on 06/07/2010 09:12 pm
More tested? SEP is already operational! Has been for years.
http://en.wikipedia.org/wiki/Dawn_(spacecraft) (http://en.wikipedia.org/wiki/Dawn_(spacecraft))

Let's see, that's 0.02 pounds of thrust...bring sunblock.

Who cares if the thrust is lower? You can afford greater delta-v. Sure, you pay a penalty for having low thrust, but at higher specific powers (like are available today and even better tomorrow) and the incredibly high Isp of electric thrusters (HiPeP has 10 times the Isp of NTR), that penalty is easily compensated for.
There's also a penalty in weight, $$$, transit times, and necessary delta v. Eg that VASIMERTM Earth-Moon space tug has to do 8 km/sec delta v--for a 6 month trip that normally takes 4 km/s. Meanwhile, a single reusable ACES-71 tanker could make the same round trip 10 times over, and deliver much more payload for much less cost and much less delay, thus allowing just in time inventory management.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/07/2010 09:18 pm
Folks:

If we can't build a NTR with a thrust to weight ratio of better than 25-to-1, I have to agree with KFS.

Eh?  Why?

IMO to be useful, a manned inter-planetary propulsion system based on rockets only needs moderate thrust levels of 1,000-to-10,000 lb-f, but a much higher Isp as compared to NTRs that are in the range of 3,000-to-15,000 seconds, dependent on the desired destination and trip times.  An NTR can provide much higher thrust levels, but it can only provide an Isp of ~1,000 seconds max, so it’s out of the running from the get-go using these criteria.  Your requirements may vary…
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/07/2010 09:24 pm
Where a high thrust NTR with an Isp = ~950 seconds really excels in the bang per buck arena is in the reusable single stage to orbit (SSTO) applications.

Not even close.  This was one of the first things we looked at when I came to NASA ten years ago.  The engine had nowhere near the thrust-to-weight for the earth-to-orbit application.


Kirk:

What NTR engines did you examine?  Just the 1960s NERVA family??  If so, Los Alamos, DARPA and the USAF have looked into higher power versions of NTRs with much higher potential thrust to weight ratios.  And if we ever do revist developing NTRs seriously, we will be starting from scratch, so why not go for the gold?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/07/2010 09:39 pm
More tested? SEP is already operational! Has been for years.
http://en.wikipedia.org/wiki/Dawn_(spacecraft) (http://en.wikipedia.org/wiki/Dawn_(spacecraft))

Let's see, that's 0.02 pounds of thrust...bring sunblock.

Who cares if the thrust is lower? You can afford greater delta-v. Sure, you pay a penalty for having low thrust, but at higher specific powers (like are available today and even better tomorrow) and the incredibly high Isp of electric thrusters (HiPeP has 10 times the Isp of NTR), that penalty is easily compensated for.
There's also a penalty in weight, $$$, transit times, and necessary delta v. Eg that VASIMERTM Earth-Moon space tug has to do 8 km/sec delta v--for a 6 month trip that normally takes 4 km/s. Meanwhile, a single reusable ACES-71 tanker could make the same round trip 10 times over, and deliver much more payload for much less cost and much less delay, thus allowing just in time inventory management.
I guess I was talking about in the context of a Mars mission. The low-thrust penalty is worse for Earth-Moon (plus the significant fact that you have longer time to develop that thrust on a Mars mission). For bulk cargo that isn't time-sensitive, high Isp propulsion is still definitely worth it (imagine a huge LEO depot being tugged slowly to EML1/2 or LLO... The longer time you allow for the rad-hard solar arrays to collect power, the higher Isp you can use, thus the less propellant required).
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: pathfinder_01 on 06/07/2010 09:57 pm
More tested? SEP is already operational! Has been for years.
http://en.wikipedia.org/wiki/Dawn_(spacecraft) (http://en.wikipedia.org/wiki/Dawn_(spacecraft))

Let's see, that's 0.02 pounds of thrust...bring sunblock.

Who cares if the thrust is lower? You can afford greater delta-v. Sure, you pay a penalty for having low thrust, but at higher specific powers (like are available today and even better tomorrow) and the incredibly high Isp of electric thrusters (HiPeP has 10 times the Isp of NTR), that penalty is easily compensated for.
There's also a penalty in weight, $$$, transit times, and necessary delta v. Eg that VASIMERTM Earth-Moon space tug has to do 8 km/sec delta v--for a 6 month trip that normally takes 4 km/s. Meanwhile, a single reusable ACES-71 tanker could make the same round trip 10 times over, and deliver much more payload for much less cost and much less delay, thus allowing just in time inventory management.

The vasmir system would take much less propellant mass to do the same work whicn in turn would allow you to use smaller cheaper rockets to do the same work. Imagine being able to supply a lunar base using any 20ton or less to orbit stage.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 06/08/2010 11:03 am
More tested? SEP is already operational! Has been for years.
http://en.wikipedia.org/wiki/Dawn_(spacecraft) (http://en.wikipedia.org/wiki/Dawn_(spacecraft))

Let's see, that's 0.02 pounds of thrust...bring sunblock.

Who cares if the thrust is lower? You can afford greater delta-v. Sure, you pay a penalty for having low thrust, but at higher specific powers (like are available today and even better tomorrow) and the incredibly high Isp of electric thrusters (HiPeP has 10 times the Isp of NTR), that penalty is easily compensated for.
There's also a penalty in weight, $$$, transit times, and necessary delta v. Eg that VASIMERTM Earth-Moon space tug has to do 8 km/sec delta v--for a 6 month trip that normally takes 4 km/s. Meanwhile, a single reusable ACES-71 tanker could make the same round trip 10 times over, and deliver much more payload for much less cost and much less delay, thus allowing just in time inventory management.

Nobody here is suggesting using VASIMR (try spelling it right) for Earth-Moon missions, so you are making a straw man here.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Hop_David on 06/08/2010 03:58 pm
Nobody here is suggesting using VASIMR (try spelling it right) for Earth-Moon missions, so you are making a straw man here.

I suspect longer trip times and increased delta V might also apply to interplanetary trips.

To a lesser extent, I suppose. Dwell time in a perihelion neighborhood is weeks vs minutes in a perigee neighborhood. So a low thrust engine might have a more Hohmann like trip through interplanetary space than earth's neighborhood.

I don't regard the 29 day trips to Mars as plausible as they rely on a power source that makes a kilowatt of electricity for every half kilogram.

The increased delta V of slow spirals vs Hohmann ellipses are less of a concern if you have really high ISP.

But the trip time is a factor. Given a commodity like propellent, more frequent deliveries are preferable. Given probes to Mars, an 8 month trip is preferable to two years.

Star-Drive, have the programmers in your group run orbital simulations assuming a 20 kg/kWe power source? I am guessing plausible electric propulsion will take longer than chemical for trips in the inner solar system. But I admit that's only speculation on my part.

Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/08/2010 05:15 pm
"I don't regard the 29 day trips to Mars as plausible as they rely on a power source that makes a kilowatt of electricity for every half kilogram."

It's 39 days.

"Star-Drive, have the programmers in your group run orbital simulations assuming a 20 kg/kWe power source? I am guessing plausible electric propulsion will take longer than chemical for trips in the inner solar system. But I admit that's only speculation on my part."

Why use 20 kg/kWe nuclear power sources when the makers of the Stretched Lens Array (SLA) solar power supplies, (Entech, et al), using linear triple-junction Spectrolab solar cells that are now over 40% efficient, are already building ~5.0 kg/kWe installed arrays with promises of 2.0-to-3.0 kg/kWe in the next five years?  That is provided there is a market willing to pay for their development.  So we've already run VASIMR based Earth-to-Mars cases using 2.0 kg/kWe solar power supplies using 32% efficient solar cells and a 1.0 kg/kWe SLA system case using 40% efficiency solar cells.
 
See Spectrolab at: http://www.spectrolab.com/DataSheets/PV/CPV/CDO-100-C3MJ.pdf   

Both of these solar power supplies were sized for the Earth-to-Mars mid-way point solar insolation value, and were paired with the 1.0 kg/kWe, 200 MWe VASIMR propulsion subsystem for this analysis.  The 200 MWe VASIMR, 2.0 kg/kWe SLA case provided a one-way trip time to Mars in ~90 days for a manned (52 mT payload) conjunction class mission, but it required refueling in Mars orbit to get back to Earth GEO.  The 1.0 kg/kWe SLA / 200 MWe VASIMR case, which used the 40% eff. solar cells and an assumed lighter SLA deployment structure, allowed a conjunction class Mars round trip with NO refueling at Mars, in a 90-day going /90-day stay/90 day-return class mission.   Both of these examples were based on liquid hydrogen as the VASIMR propellant.   On the down side, both of these solar powered cases required a pair of very large solar arrays measuring 600 to 700 meters on a side dependent on the assumed array solar cell efficiency.  And deployment of that large of a solar array will have to be man tended in high LEO or GEO for there are sure to be hang-ups in the array deployment mechanisms, at least for the first few tries at it.  Lastly, dynamic stability control of these solar array structures will also have to be used to maintain overall structural integrity of the vehicle during thrusting maneuvers.

BTW, IMO if you want to use fission based nuclear power for space propulsion, with all its political and safety issues that it entails, it had better provide a powerplant specific mass of less than 1.0 kg/kWe, or it's not worth the pain and costs involved in its development.  Otherwise solar power eats it lunch, at least for the inner solar system, which includes the inner portion of the asteroid belt...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/08/2010 05:57 pm

For a trip to Mars, electric propulsion trajectories between Earth-escape and Mars escape would look like Hohmann transfers, not like the spirals for LEO to LLO.

And for high-power solar-electric, the transfer would be faster than Hohmann (because you have more delta-v available).
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/08/2010 06:08 pm
"I don't regard the 29 day trips to Mars as plausible as they rely on a power source that makes a kilowatt of electricity for every half kilogram."

It's 39 days.

I don't think that makes any appreciable difference.  It's impossible either way due to lack of suitable power supply.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/08/2010 07:29 pm
"I don't regard the 29 day trips to Mars as plausible as they rely on a power source that makes a kilowatt of electricity for every half kilogram."

It's 39 days.

I don't think that makes any appreciable difference.  It's impossible either way due to lack of suitable power supply.

Kirk:

At the moment your above statement rings true, but like the words  “never” and “always”, "impossible" is a very hard thing to prove, especially when one is given sufficient time and resources to change the current situation. 

For instance, what happens if the Bussard polywell aneutronic fusion reactor concept, currently being pursued in a series of proof of principle tests sponsored by the US Navy, actually works as advertised?  I’ve talked with a couple of senior scientists about this topic who are very familiar with Bussard’s and the Navy’s work on same, and they tell me that there is a one in three chance that the current Wiffle-Ball (WB)-8 prototype reactor will perform as currently predicted based on the WB-7 results.  A one in three chance that in two years time that we will have the practical makings for a 100+ Mega-Watt (MW) electric output reactor that will start out in the 0.5-to-1.0 kg/kWe specific mass range inlcuding power conversion equipment.  And these WB reactors’ specific mass only get smaller as their power outputs get larger.  To me that is well worth pursuing this three in one chance of success just because of the huge potential payoff it promises a space fairing nation or civilization.

BTW, I’ll repeat my previous question from yesterday.   What NTR engines did you examine ten years ago when you first came to NASA?  Just the 1960s NERVA family, or were you allowed to look at the DARPA/DoD fair as well??

All the best.

Paul March
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/08/2010 08:09 pm
BTW, I’ll repeat my previous question from yesterday.   What NTR engines did you examine ten years ago when you first came to NASA?  Just the 1960s NERVA family, or were you allowed to look at the DARPA/DoD fair as well??

We looked at stuff even beyond the starry-eyed world of Timberwind and its buddies.  I'll post more on this in the future.  It was absolutely crazy.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/08/2010 08:12 pm

For a trip to Mars, electric propulsion trajectories between Earth-escape and Mars escape would look like Hohmann transfers, not like the spirals for LEO to LLO.

And for high-power solar-electric, the transfer would be faster than Hohmann (because you have more delta-v available).


Chris:

Quite true on both counts, but your point is?  All our trajectory analyses assumed a spiral out from or into Earth GEO or Mars Phobus orbits to/from interplanetary/heliocentric space.  We then used your second excess delta-V Hohmann case and/or hyperbolic conics using high-power electric propulsion trajectories based on the 1.0 kg/kWe VASIMR engine, combined with various nuclear power sources or the SLA solar arrays I talked about earlier today.  An example of one of these VASIMR based trajectories is appended, along with the solar array powered concept vehicle that comes out with a combined power supply & engine specific mass of 2.0-to-3.0 kg/kWe. 

Correction: I just re-read my solar powered SLA Vehicle slide and noted that I had assumed a specific mass of only 0.5 kg/kWe for the VASIMR subsystem back in March of this year, instead of my above 1.0 kg/kWe used for later analyses.  If we used this more realistic 1.0 kg/kWe figure for VASIMR, the transit times will probably bump up from 90 days to more like ~110 days each way.  Or we have to use the much lighter but less flexible MPD thrusters that already have specific mass figures in this 0.5 kg/kWe range.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/08/2010 08:12 pm
At the moment your above statement rings true, but like the words  “never” and “always”, "impossible" is a very hard thing to prove, especially when one is given sufficient time and resources to change the current situation. 

For instance, what happens if the Bussard polywell aneutronic fusion reactor concept, currently being pursued in a series of proof of principle tests sponsored by the US Navy, actually works as advertised?

It's impossible to prove a negative, but I'm an engineer and try not to be too much of a dreamer.  Others can pin their hopes to whatever they want, but the physics has convinced me sufficiently that trip times to Mars of <2 months won't be coming in my lifetime, if ever.

I'm not even sure you should want such a thing, even if you could do it, it would still be constrained by orbital mechanics to particular launch dates, so who cares anyway?

But if you have a few billion of your own $ to invest in whatever tickles your fancy, feel free to do so.  As for taxpayer dollars, I'll argue strenuously to invest them as wisely as possible, and VASIMR-type propulsion systems that rely on impossible power sources and NTR-style engines are a waste of money.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mmeijeri on 06/08/2010 08:25 pm
I'm not even sure you should want such a thing, even if you could do it, it would still be constrained by orbital mechanics to particular launch dates, so who cares anyway?

Dates? Aren't the windows on the order of weeks, or even months if you start from a high energy staging orbit? And do you think that's intrinsically a problem, or just not worth it because there are better solutions? Having Mars missions "only" every couple of years doesn't sound too bad. I'd love to have that problem...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/08/2010 08:33 pm
Yeah, and I'd love to be a billionaire from selling electricity from lightweight power supplies.  You guys are fantasizing about a spacecraft the size of a Star Destroyer that can go and come from Mars in a year.  Keep dreamin' friends.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mmeijeri on 06/08/2010 08:36 pm
Kirk, I was just asking a question. FWIW I'm not in favour of NTR or propulsion that requires magical energy sources.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 06/08/2010 10:55 pm
At the moment your above statement rings true, but like the words  “never” and “always”, "impossible" is a very hard thing to prove, especially when one is given sufficient time and resources to change the current situation. 

For instance, what happens if the Bussard polywell aneutronic fusion reactor concept, currently being pursued in a series of proof of principle tests sponsored by the US Navy, actually works as advertised?

It's impossible to prove a negative, but I'm an engineer and try not to be too much of a dreamer.  Others can pin their hopes to whatever they want, but the physics has convinced me sufficiently that trip times to Mars of <2 months won't be coming in my lifetime, if ever.

I'm not even sure you should want such a thing, even if you could do it, it would still be constrained by orbital mechanics to particular launch dates, so who cares anyway?


Firstly, with a constant thrusting system, there are no constraints to specific launch dates, as the vessel follows a more or less straight line trajectory, which allows for  a launch window of 2-3 months every two years for short term transits.

Secondly, since you are dissing Polywell without apparently having read the background material, with a Polywell reactor, you won't have trip times as long as 2 months. Nowhere near that. Polywell will allow for a single stage vehicle to go from Earth surface to Mars in under a week. The outer solar system will require a few weeks more. This would allow you to depart on a mission to Mars at any time you want for trip times of less than 30 days.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 06/08/2010 10:56 pm
Yeah, and I'd love to be a billionaire from selling electricity from lightweight power supplies.  You guys are fantasizing about a spacecraft the size of a Star Destroyer that can go and come from Mars in a year.  Keep dreamin' friends.

This is the advanced concepts forum. It is named that way for a reason. I think you are looking for the History forum...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: KelvinZero on 06/08/2010 11:16 pm
I'm not even sure you should want such a thing, even if you could do it, it would still be constrained by orbital mechanics to particular launch dates, so who cares anyway?

My two cents (again :) )
If better drives are possible, I would prefer to see them used to allow larger,  longer term, better supplied missions for the same cost. Using these fantastic advances so we could leap to mars and back while holding our breaths instead of attacking and solving these problems of sustainable life support and ISRU would be a great pity to me.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Cinder on 06/08/2010 11:29 pm
But would you inherently want such a slow poke/high mass mission as first pathfinder mission? 
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/09/2010 12:51 am
This is the advanced concepts forum. It is named that way for a reason. I think you are looking for the History forum...

Actually, it seems like you guys are looking for the History forum, with all your talk about NERVA and Rover and so forth...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/09/2010 12:54 am
Firstly, with a constant thrusting system, there are no constraints to specific launch dates, as the vessel follows a more or less straight line trajectory, which allows for  a launch window of 2-3 months every two years for short term transits.

No.  Think of the solar system's gravitational topology as looking like one of those funnels you see at the mall and drops the coins into.  The kind where the coin spins around and around before falling it.

(http://www.gumball.com/images/products/display/coin-vortex-machine.jpg)

We're the coins.

No straight lines.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/09/2010 12:55 am
Secondly, since you are dissing Polywell without apparently having read the background material, with a Polywell reactor, you won't have trip times as long as 2 months. Nowhere near that. Polywell will allow for a single stage vehicle to go from Earth surface to Mars in under a week. The outer solar system will require a few weeks more. This would allow you to depart on a mission to Mars at any time you want for trip times of less than 30 days.

Mmm, hmmm.  You bet.

Whatever.  You're not talking seriously anymore.  Keep fantasizing.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: 93143 on 06/09/2010 02:29 am
Secondly, since you are dissing Polywell without apparently having read the background material, with a Polywell reactor, you won't have trip times as long as 2 months. Nowhere near that. Polywell will allow for a single stage vehicle to go from Earth surface to Mars in under a week. The outer solar system will require a few weeks more. This would allow you to depart on a mission to Mars at any time you want for trip times of less than 30 days.

Mmm, hmmm.  You bet.

Whatever.  You're not talking seriously anymore.  Keep fantasizing.

...actually, I'm going to have to agree here.  mlorrey is probably thinking of M-E thrusters, which are a much longer shot than Polywell but absolutely civilization-changing if they work.  Constant-thrust 1 gee surface-to-surface Mars transit in ~2-5 days (depending on launch date) is the least of it...

Polywell could most likely do a fast Mars transit in a month or so, maybe less (though you'd have to start in LEO; the research would have to go much better than expected to produce a reactor that could power an SSTA capable of fast Mars transit).

Let's see...  5 GW reactor 10 m in diameter with full 360x360° shielding, assuming 4/12/16 MeV gamma emissions from p-¹¹B at the branching probability of 0.0001 as stated on Wikipedia, is about 1500 mT.  Most of that (1200-1300 mT) is shielding.  With 1.67 GW waste heat (75% direct conversion efficiency), radiators at 1200 K, add 300 mT for 16,000 m² at about 20 kg/m², so 1,800,000 kg for 5,000,000 kW - 0.36 kg/kW.

Now do shadow-shielding - you can probably cut the shield mass by a factor of 6 or so, so the total mass goes down to about 700-800 mT.  Squeeze 10 GW into that form factor (hey, it might work; I actually estimated a much smaller reactor for 5 GW, but I'm trying to be conservative here), use more mass-efficient radiators at ~10 kg/m², and you've got maybe 1000-1100 mT for 10 GW, or ~0.1 kg/kW.

Get the reactor radius down to 3 m, the radiator mass down to 1 kg/m², keep the power at 10 GW (this is ironically less of a stretch than doing it at 5 m, but it's still quite high), and you could conceivably get down near 0.02-0.03 kg/kW...  but it's not likely...

Yeah, most of that is armwaving considering the low TRL.  But the basic layout of the reactor is well known enough for order-of-magnitude estimates of the mass...  anyone got a better one?

...I hope the direct conversion system doesn't have to be huge...  the full-shielding scenario gets very heavy very quickly...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/09/2010 03:24 am
Does anyone here want to talk about anything realistic or just fantasize?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: 93143 on 06/09/2010 03:29 am
Does anyone here want to talk about anything realistic or just fantasize?

Look, we're both rocket scientists here.  Are you going to respond intelligently or not?

My post above was a very quick BoE effort based mostly (but not entirely; I'm no good with radiator masses) on numbers I've calculated previously.  If you see something wrong with it, point it out.

If you just want more detail, I can provide that - but not right away...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Cinder on 06/09/2010 03:46 am
His cutoff for realism is electric or thermal nukes. Polywell probably doesn't make it because the concept isn't proven yet.

Either way I don't agree that it doesn't belong in Advanced Concepts, but in this thread I'd certainly like to read more about this:
We looked at stuff even beyond the starry-eyed world of Timberwind and its buddies.  I'll post more on this in the future.  It was absolutely crazy.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/09/2010 04:08 am
93143:

"...I hope the direct conversion system doesn't have to be huge...  the full-shielding scenario gets very heavy very quickly..."

The P-B11's ~2.4 MeV helium ion powered direct energy power conversion systems currently run between a calculated 0.10-to-0.30 kg/kWe specific mass with overall conversion efficiency in the 70-to-80% range, dependent on whether you need moderate voltage dc or low frequency ac power distribution.  If all you need is high frequency RF in the form of a high power HF or VHF such as VASIMR uses, then ~0.10 kg/kWe has all ready been demonstrated at the MWe level by the Japanese. 

As to the shielding mass issue you mention, a lot of these low probability p-B11 neutron and gamma branching reactions are extremely dependent on the actual Maxwellian plasma temperature at the center of the operating polywell reactor.  In other words since we are dealing with a very non-Maxwellian system of colliding ion and electron beams, your quoted branching reactions could be much lower than your source is assuming.  In fact, Rick Nebel has been quoted as saying that the neutron's ~0.1% power branching reactions for p-B11 reaction are several orders of magnitude lower in the non-Maxwellian colliding beam system used in the polywell process.  I can only assume that the gamma branching probabilities may also be much smaller than 0.01% of the total reactor power output as well.  However, only time and experiments will tell us for sure who is right.     
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: stealthyplains on 06/09/2010 04:17 am
please don't sidetrack this thread with dubious fusion contraptions, as there is nothing meaningful to be said unless one is proven to break even.

i too would love to hear about the crazy space reactors

i would also like to know what the highest specific power spacecraft solar power system is
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/09/2010 04:23 am
Does anyone here want to talk about anything realistic or just fantasize?

Look, we're both rocket scientists here.  Are you going to respond intelligently or not?

My post above was a very quick BoE effort based mostly (but not entirely; I'm no good with radiator masses) on numbers I've calculated previously.  If you see something wrong with it, point it out.

If you just want more detail, I can provide that - but not right away...

A NASA 2004 solid core fission based electric propulsion system study in the 10 MWe output range used a 5.0 kg/kWe specific mass for their heat-pipe based radiator system.  I believe it was a GRC related study.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: KelvinZero on 06/09/2010 04:57 am
But would you inherently want such a slow poke/high mass mission as first pathfinder mission? 

Yes, in one sense at least. I really like the way these long trip times force missions to focus on technology that allow long term stays. That is what really interests me. Of course I would not use that argument to kill research into faster trip times that could allow more opportunities to fix things if something goes wrong. I would hate for all the money to go into an apollo style mission with short stays and then the entire architecture thrown away because it just wasnt sustainable.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: 93143 on 06/09/2010 05:04 am
The P-B11's ~2.4 MeV helium ion powered direct energy power conversion systems currently run between a calculated 0.10-to-0.30 kg/kWe specific mass with overall conversion efficiency in the 70-to-80% range, dependent on whether you need moderate voltage dc or low frequency ac power distribution.  If all you need is high frequency RF in the form of a high power HF or VHF such as VASIMR uses, then ~0.10 kg/kWe has all ready been demonstrated at the MWe level by the Japanese.

I believe the QED drives are supposed to mostly use the power at full voltage DC.  VASIMR doesn't, because it wasn't designed with Polywell in mind.  Surely it's not impossible to design a high-Isp space drive that uses mostly MV-range DC?  The VASIMR sinks most of its power into ionizing and heating the propellant, right?  Sounds like a good job for a relativistic electron beam to me...

Any idea what the collector system mass would look like without any conversion or stepdown equipment?

Given the 0.1 kg/kWe figure for the collectors and power conversion to support a VASIMR, my estimate for the 5 GW reactor mass goes up to about 700 mT.  With shadow shielding and 20 kg/m² radiators at 1200 K, that's ~0.25 kg/kW system mass.  Still not too bad...

Quote
As to the shielding mass issue you mention, a lot of these low probability p-B11 neutron and gamma branching reactions are extremely dependent on the actual Maxwellian plasma temperature at the center of the operating polywell reactor.  In other words since we are dealing with a very non-Maxwellian system of colliding ion and electron beams, your quoted branching reactions could be much lower than your source is assuming.  In fact, Rick Nebel has been quoted as saying that the neutron's ~0.1% power branching reactions for p-B11 reaction are several orders of magnitude lower in the non-Maxwellian colliding beam system used in the polywell process.  I can only assume that the gamma branching probabilities may also be much smaller than 0.01% of the total reactor power output as well.  However, only time and experiments will tell us for sure who is right.

I know about the neutron thing - neutronicity is supposedly ~1e-8 of what you'd get from a neutronic core of comparable power.  The gammas, on the other hand...  I'm hoping.  But the only data we have is that one paper referenced on Wikipedia, which is quite difficult to access, and Art Carlson's speculation that the branching probability has to do with the fine structure constant and won't vary with collision energy.

The trouble is that the gammas dominate the shielding requirements, to the point that you can essentially ignore the neutrons and bremsstrahlung.  Assuming Wikipedia and Art Carlson are right about the branching probability, it takes more than a foot of lead to attenuate the gammas down to a safe level of long-term exposure...

A NASA 2004 solid core fission based electric propulsion system study in the 10 MWe output range used a 5.0 kg/kWe specific mass for their heat-pipe based radiator system.  I believe it was a GRC related study.

Temperature?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 06/09/2010 06:29 am

The trouble is that the gammas dominate the shielding requirements, to the point that you can essentially ignore the neutrons and bremsstrahlung.  Assuming Wikipedia and Art Carlson are right about the branching probability, it takes more than a foot of lead to attenuate the gammas down to a safe level of long-term exposure...


Why are you depending on lead as the shielding and not depleted uranium? The halving thickness of depleted uranium is less than 1/5th that of lead, so if you need a foot of lead, you should only need about 2.4 inches of depleted uranium. This reduces the shielding mass significantly, by more than 2/3. So total mass should be around 250 mT, which is exactly what Bussard originally estimated...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: 93143 on 06/09/2010 08:39 am
Why are you depending on lead as the shielding and not depleted uranium? The halving thickness of depleted uranium is less than 1/5th that of lead, so if you need a foot of lead, you should only need about 2.4 inches of depleted uranium. This reduces the shielding mass significantly, by more than 2/3. So total mass should be around 250 mT, which is exactly what Bussard originally estimated...

Reference?

That seems unrealistically good - unlike neutrons, gammas are essentially shielded by mass.  Lead is a little better than most other stuff, but generally there aren't large differences in shield masses from substance to substance.

The gammas come off at 4, 12, and 16 MeV, which is in the worst part of the curve for lead (Compton declining, photoelectric out of the picture, pair production just starting to pick up) - but I compared with tungsten once and if my fancy inverse extrapolation means anything, lead is slightly better.  Bigger, since it's less dense, but lighter.

I've found a couple of tables (http://onlineshowcase.tafensw.edu.au/ndt/content/rad_safety/task4/accessible.htm#topic5) that show DU vs. lead for a cobalt-60 source (1.17 and 1.33 MeV gammas) and an iridium-192 source (beta and gamma, energies unknown).  The DU is 1.68 times as dense as lead and is 1.9 times as effective length-wise at cobalt energies (10-15% better mass-wise), and roughly 3 times as effective (so 1.5-2 times better mass-wise) at iridium energies...  but the lengths for iridium are lower, indicating that the energies are lower.  So it looks like it might still be better than lead at the high energies a p-¹¹B Polywell puts off, but probably not three times as good mass-wise unless something really dramatic happens to the curve between 1 and 4 MeV...  and if the advantage is too small the shielding mass required to protect personnel from the depleted uranium's radioactivity could wipe it out...

I would be extremely happy to see evidence that the total mass of a reactor with 360x360° shielding could be half what I've been assuming.  It would make SSTO a lot easier, especially in terms of ground operations...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/09/2010 11:25 am
Guys, bring the fusion talk to the appropriate threads. Hopefully you haven't already scared off sorenson.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: isa_guy on 06/09/2010 11:41 am
Which of the current state of the art reactors types  ( such as liquid metal or molten-salt reactors ) are the best candidates to power a manned nuclear electric Mars Transfer Vehicle? Also when i was looking for  some information from NASA's sites on the subject of space nuclear reactors  i found  these to reports http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020066157_2002110044.pdf , http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040010797_2004001506.pdf , and the one thing that amaze me in kirk's and the second reports is the chart that compares various kinds of reactor types and i noticed that they shows that molten salt reactors could have a potential power density of 2500 (MW/m3). Is it really possible to have this amount of power density and  what could be the mass of such reactor ?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/09/2010 03:15 pm

A NASA 2004 solid core fission based electric propulsion system study in the 10 MWe output range used a 5.0 kg/kWe specific mass for their heat-pipe based radiator system.  I believe it was a GRC related study.

Temperature?


See attached NASA 2002 study slide, but its a potassium liquid metal Rankine system running at 1,500K.  Also the radiator mass used in this study was 5.0 kg/m^2, not 5.0 kg/kWe.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: 93143 on 06/09/2010 06:38 pm
Guys, bring the fusion talk to the appropriate threads. Hopefully you haven't already scared off sorenson.

Hey, technically it's nuclear electric...  anyway, we're probably about done here...

See attached NASA 2002 study slide, but its a potassium liquid metal Rankine system running at 1,500K.  Also the radiator mass used in this study was 5.0 kg/m^2, not 5.0 kg/kWe.

Hmm... ~1000 K radiators at 5 kg/m².  Not bad at all...  at 1200 K the power per unit area doubles; I wonder what that does to the mass and feasibility...

Oh, and I believe I forgot to divide the surface area of my radiators by 2 to account for the two-sided action...  clumsy...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/10/2010 02:51 pm
BTW, I’ll repeat my previous question from yesterday.   What NTR engines did you examine ten years ago when you first came to NASA?  Just the 1960s NERVA family, or were you allowed to look at the DARPA/DoD fair as well??

We looked at stuff even beyond the starry-eyed world of Timberwind and its buddies.  I'll post more on this in the future.  It was absolutely crazy.

Here's my calculations on NTR-SSTO:

http://selenianboondocks.com/2010/06/ssto-ntr-bad/
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/10/2010 04:42 pm
BTW, I’ll repeat my previous question from yesterday.   What NTR engines did you examine ten years ago when you first came to NASA?  Just the 1960s NERVA family, or were you allowed to look at the DARPA/DoD fair as well??

We looked at stuff even beyond the starry-eyed world of Timberwind and its buddies.  I'll post more on this in the future.  It was absolutely crazy.

Here's my calculations on NTR-SSTO:

http://selenianboondocks.com/2010/06/ssto-ntr-bad/

Kirk:

If you go into a problem looking for ways to make it fail, it will fail by definition.  Putting aside for the moment the political and safety issues surrounding the use of nuclear fission based engines for launch or deep space operations, picking Stan Borowski’s 15,000 lb-f bimodal NTR as the engine candidate for an SSTO is setting yourself up for a negative results, and you should have known that to begin with unless that was your intent from the start.   

Now Stan B. never said that his current version of an NTR was for SSTO applications, but it is instead for “low-thrust”, deep space operations only.  If you want to build a NTR powered SSTO vehicle that will work, you had best find a way to develop an NTR that has a thrust to weight (T/W) ratio of NOT 3-to-1, but more like 30-to-1 or higher, so even the NERVA Phoebus-2A NTR that demonstrated 10-to-1 T/W ratios back in the 1960s would be found wanting.  Such high power/thrust to weight ratio NTR designs were investigated though in the 1950s & 60s in the Los Alamos Dumbo NTR work, and later in the 1980s with the USAF Timberwind NTRs based on the particle bed reactor designs.   (Yes they had problems, but what rocket system doesn’t?).  We also now have Glenn Research Center (GRC)’s LOX Augmented NTR (LANTR) concept where LOX is injected just downstream of the rocket nozzle that could double or even triple the 100% thrust levels available from the base NTR engine in question when the LOX is being injected into the engine’s expansion bell.  So given a Dumbo NTR with a T/W of 30-to-1 and a “first stage” 3-to-1 LANTR booster for the first two minutes of the flight, we could get an NTR SYSTEM that would produce a liftoff T/W of ~90-to-1 with an effective Isp of ~650 seconds, rising to ~950 seconds when the LOX is cut off two minutes into the flight.  If you can’t make that work as a SSTO candidate, you didn’t try very hard to do so.

As to the hydrogen’s low density problem, have you ever thought about using liquid deuterium instead as the reaction mass for the NTR?  It has a density of 162.4 kg/m^3 verses liquid hydrogen’s density of 67.8 kg/m^3 or ~2.4 times denser.  That will cut the required propellant tank volume in half for a given propellant load with not much decrease in the NTR’s Isp performance as compared to straight hydrogen.  Yes there is an issue with deuterium’s cost of $10-to-$20/Lb , but since deuterium makes up 0.0154% of all the hydrogen in sea water, and there are already commercial processes that can separate deuterium from sea water, deuterium ‘s price can come down if the demand for same is there.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/10/2010 05:15 pm
You really think that Kirk Sorensen, the man who writes the "Energy from Thorium" blog and is chief nuclear technologist (or will be?) at Teledyne Brown Engineering, approaches analyzing nuclear thermal rockets with an eye towards having them fail?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/10/2010 05:29 pm
As to the hydrogen’s low density problem, have you ever thought about using liquid deuterium instead as the reaction mass for the NTR?

No, I have never thought of that, nor would I, because the key to the high Isp of the NTR is the low molecular weight of the exhaust products.  Doubling the molecular weight doesn't help.

The fact that liquid deuterium is staggeringly, staggeringly expensive doesn't factor into this at all.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/10/2010 05:32 pm
If you go into a problem looking for ways to make it fail, it will fail by definition.

No, that is not the case at all.  I have gone into many analyses thinking that something wouldn't work, only to have my mind changed by the numbers.  My opinion or bias doesn't change numerical values or outputs.  That is why I carefully show my work, so that you can follow what I've done and identify deficiencies if there are any.

I used numbers Borowski has put out there.  Frankly, I think they're no good anyway (reality would be much worse) but I used them to show that if you used his avowed performance the idea still fails miserably.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/10/2010 05:33 pm
If you can’t make that work as a SSTO candidate, you didn’t try very hard to do so.

I show my work.  If you think you have a different answer, feel free to calculate and attempt to defend it.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: KelvinZero on 06/10/2010 10:42 pm
Im not at all a fan of nuclear launch vehicles, but I have a question,

Why do nuclear and beamed power designs not exploit the atmosphere as propellant?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/10/2010 10:44 pm
Im not at all a fan of nuclear launch vehicles, but I have a question,

Why do nuclear and beamed power designs not exploit the atmosphere as propellant?
They have.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/10/2010 11:28 pm
Im not at all a fan of nuclear launch vehicles, but I have a question,

Why do nuclear and beamed power designs not exploit the atmosphere as propellant?

For a launch vehicle, the first thing you want to do is "get clear" of the atmosphere in your ascent trajectory, so using atmospheric constituents as propellant is counter-productive.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: KelvinZero on 06/11/2010 12:13 am
Got Examples, RobotBeat?

Hi kfsorensen,
Im aware of this issue. You would need a launcher to operate in two modes, including a smallish propellant tank for the second mode. This is difficult Im sure, but still often discussed for other SSTO methods. (Im not counting that any SSTO has to function efficiently in two atmospheric pressures of course. Im referring to Skylon etc.)

I would have thought the motivation for a two-mode operation would be even greater when you have the possibility of omitting all your propellent and the mass of its tanks for the first mode.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mmeijeri on 06/11/2010 12:26 am
For Mars, with its lower gravity, rarified atmosphere and need for ISRU, an NTR SSTO is more plausible and Zubrin has proposed this. You might call it airbreathing, but it would suck up and liquefy CO2 before taking off, not during flight.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/11/2010 12:41 am
For Mars, with its lower gravity, rarified atmosphere and need for ISRU, an NTR SSTO is more plausible and Zubrin has proposed this. You might call it airbreathing, but it would suck up and liquefy CO2 before taking off, not during flight.
Or make a chemical air-breathing SSTO rocket for Mars by sucking up the CO2, electrolyzing it into CO and O2. That way you could use just mundane rocket motors with their far superior Thrust/Weight. Isp wouldn't actually be that different, if they are both limited by material properties of their combustion chamber and throat and nozzle. But dry-weight would be far less for the chemical one versus the NTR version.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mmeijeri on 06/11/2010 12:43 am
I think Zubrin's argument was that that still needs ground infrastructure, including nukes. By putting the nuke inside the hopper it would be more mobile. A niche application to be sure, but that by itself doesn't invalidate the concept.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/11/2010 03:50 am
If you can’t make that work as a SSTO candidate, you didn’t try very hard to do so.

I show my work.  If you think you have a different answer, feel free to calculate and attempt to defend it.

Kirk:

Well, we appear to have a different approach on how to optimize a system.  I try to prefilter out the myrid cases that I know won't work from other sources before going to the time and trouble of doing a detailed analysis on a system that doesn't have a prayer of working as you did with your 15k Lb-f NTR analysis.  However, if you want to see one of my rough back of the envelope type of nuclear SSTO design presentations I did for a brown bag group of Advanced Deep Space Transports (ADST) engineers here at JSC a few years back, find it attached.  I consider it more of a design sketch book than a detailed engineering analysis, but it provides some design avenues to explore for the more well versed in the rocket arts than I am.  My main expertise is in electrical power generation, distribution and control along with electric propulsion systems such as VASIMR and the more interesting to me field propulsion techniques I've been exploring with Jim Woodward and Sonny White.

All the best.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 06/11/2010 04:13 am
Why are you depending on lead as the shielding and not depleted uranium? The halving thickness of depleted uranium is less than 1/5th that of lead, so if you need a foot of lead, you should only need about 2.4 inches of depleted uranium. This reduces the shielding mass significantly, by more than 2/3. So total mass should be around 250 mT, which is exactly what Bussard originally estimated...

Reference?
http://en.wikipedia.org/wiki/Uranium-238#Radiation_shielding

While gammas striking depleted uranium produces alphas, and there is natural beta at a very low level from the uranium itself, these both are blockable with polymer casing. Depleted uranium is widely used in medical radiation equipment for shielding purposes, and is also used in various fission reactor designs as well. Lead is, well, a poor substitute for applications where excess mass doesn't really matter.

Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: 93143 on 06/11/2010 07:38 am
http://en.wikipedia.org/wiki/Uranium-238#Radiation_shielding

I'd say the link in my previous post (http://forum.nasaspaceflight.com/index.php?topic=1139.msg603455#msg603455) trumps that source, unfortunately.

It demonstrates that while U-238 may be much better at stopping hard X-rays and low-energy gamma rays, it is nowhere near five times as good once you get near 1 MeV.

Remember, the gamma spectrum from p-¹¹B is very high-energy, essentially consisting of three Dirac deltas at 4 MeV, 12 MeV, and 16 MeV.  At those energies, the advantage of U-238 is most likely minimal to nonexistent, unless the curve does something really funky between 1 and 4 MeV...

Also, there is apparently a bit of EM (X/gamma) from the DU:  http://en.wikipedia.org/wiki/Depleted_Uranium#Shielding_in_industrial_radiography_cameras

It could be useful if the gamma thing turns out to be a red herring, and the hottest stuff we have to stop is bremsstrahlung...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 06/11/2010 08:45 am
http://en.wikipedia.org/wiki/Uranium-238#Radiation_shielding

I'd say the link in my previous post (http://forum.nasaspaceflight.com/index.php?topic=1139.msg603455#msg603455) trumps that source, unfortunately.

It demonstrates that while U-238 may be much better at stopping hard X-rays and low-energy gamma rays, it is nowhere near five times as good once you get near 1 MeV.

Remember, the gamma spectrum from p-¹¹B is very high-energy, essentially consisting of three Dirac deltas at 4 MeV, 12 MeV, and 16 MeV.  At those energies, the advantage of U-238 is most likely minimal to nonexistent, unless the curve does something really funky between 1 and 4 MeV...

Also, there is apparently a bit of EM (X/gamma) from the DU:  http://en.wikipedia.org/wiki/Depleted_Uranium#Shielding_in_industrial_radiography_cameras

It could be useful if the gamma thing turns out to be a red herring, and the hottest stuff we have to stop is bremsstrahlung...

Actually that reference says the DU produces beta electron radiation, as I previously stated. This is easily neutralized with polyeurethane steel cladding and can potentially be used as an additional source of power generation.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kkattula on 06/11/2010 09:10 am
If you can’t make that work as a SSTO candidate, you didn’t try very hard to do so.

I show my work.  If you think you have a different answer, feel free to calculate and attempt to defend it.

Kirk, I've read your articles on Selenian Boondocks and have no problem with your calculation methods. Your propellant and gross mass sensitive terms seem to be a very handy shortcut for evaluating rocket performance.

However, I have to agree with Paul that the parameters you chose for your NTR-SSTO analysis were inappropriate:

  -  Borowski's Triton NTR is a tri-mode design for in-space applications, NOT a launch vehicle engine. It includes a Brayton cycle electrical power  generator and radiators. Also a backup turbo pump. Neither of these would be required on a SSTO engine.

  -  I thought your LH2 tank factor and ullage were a little conservative too, but the very low engine T/W was clearly the show stopper.

  -  On the other hand your required delta v of 9200 m/s seemed very low.  Low density propellant & high Isp would incur high gravity losses. Something over 9600 m/s seems reasonable.

What you have done, is clearly demonstrate that engine 'package' (including thrust structure & shielding) T/W well in excess of 10 is required for solid core NTR SSTO.

Paul suggests there are NTR designs that easily exceed that FOM, so I can't accept your claim that NTR SSTO is not practical.

That said, I think it is more realistically applicable to deep-space missions. Particularly, moderate thrust missions departing from depots at L1/L2, where gravity losses are much less a factor, so quite low T/W is acceptable.

Yes, development costs are considerable for modest performance gains over e.g. LOX/LH2. Buit those costs only apply to the first mission.  Each subsequent mission gains a significant cost reduction at current per kg launch costs.

Oddly enough, a chemical SSTO RLV that substantially reduced per kg launch costs, would make NTR and other deep-space propulsion technologies less economical by greatly extending the payback period.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: 93143 on 06/11/2010 10:40 pm
Quote
Also, there is apparently a bit of EM (X/gamma) from the DU:  http://en.wikipedia.org/wiki/Depleted_Uranium#Shielding_in_industrial_radiography_cameras

Actually that reference says the DU produces beta electron radiation, as I previously stated. This is easily neutralized with polyeurethane steel cladding and can potentially be used as an additional source of power generation.

You must think I'm some kind of idiot.

Quote
a semi-infinite  slab of depleted uranium has a contact dose rate of about 2.1 mSv per hour of which ~1.95 mSv per hour is attributable to beta radiation and the remaining 0.15 mSv per hour attributable to gamma/x-ray/bremsstrahlung radiation from the uranium

Anyway, it's beside the point...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: clongton on 06/12/2010 10:32 pm
Is it possible to build an NTR with a >900 s Isp and >25 T/W?

Perhaps--I don't know how, but I can't exclude the possibility.

What I can exclude is that nothing that was tested back in the 1960s has performance anywhere close to that, and the stuff that was tested doesn't have sufficient performance to "buy" its way onto any deep space mission.

Gas Core. Solid Core designs all suffer from the same limiting liability, material thermal limits. Gas Core designs are admittedly further afield than any Solid Core designs, but if the goal is SSTO, Gas Core is the only reasonable near term approach. Solid cores just don't offer enough performance to make them worth the expense for ground to orbit applications (in-space is different). I agree with Kirk that there are better ways to spend our money.

Now, whether or not the huge funding needed to take this from concept to flight hardware would ever be available is another matter entirely. IMO there would need to be lots of DoD money in the mix.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mlorrey on 06/13/2010 07:00 am
Is it possible to build an NTR with a >900 s Isp and >25 T/W?

Perhaps--I don't know how, but I can't exclude the possibility.

What I can exclude is that nothing that was tested back in the 1960s has performance anywhere close to that, and the stuff that was tested doesn't have sufficient performance to "buy" its way onto any deep space mission.

Gas Core. Solid Core designs all suffer from the same limiting liability, material thermal limits. Gas Core designs are admittedly further afield than any Solid Core designs, but if the goal is SSTO, Gas Core is the only reasonable near term approach. Solid cores just don't offer enough performance to make them worth the expense for ground to orbit applications (in-space is different). I agree with Kirk that there are better ways to spend our money.

Now, whether or not the huge funding needed to take this from concept to flight hardware would ever be available is another matter entirely. IMO there would need to be lots of DoD money in the mix.

Kirk very well knows that Timberwind prototypes were tested in the early 1990's with T/W of 35.

The point of the Timberwind reactor program was as propulsion to launch an ICBM sized rocket with a SDI beam weapon or x-ray laser payload that would be powered by the Timberwind reactor once in orbit. This would avoid any treaty restrictions on basing SDI weapons in space and allow the US to basically keep any such capability highly classified and disguised as ICBMs until/unless it reached a point of sufficient international destabilization that an ability to neutralize enemy offensive nuclear capability was required. It would have been a single use SSTO design.

As I recall, the primary reason it was shelved was that Teller's X-Ray Laser system, powered by a nuke detonation, failed.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: clongton on 06/13/2010 01:08 pm
Is it possible to build an NTR with a >900 s Isp and >25 T/W?

Perhaps--I don't know how, but I can't exclude the possibility.

What I can exclude is that nothing that was tested back in the 1960s has performance anywhere close to that, and the stuff that was tested doesn't have sufficient performance to "buy" its way onto any deep space mission.

Gas Core. Solid Core designs all suffer from the same limiting liability, material thermal limits. Gas Core designs are admittedly further afield than any Solid Core designs, but if the goal is SSTO, Gas Core is the only reasonable near term approach. Solid cores just don't offer enough performance to make them worth the expense for ground to orbit applications (in-space is different). I agree with Kirk that there are better ways to spend our money.

Now, whether or not the huge funding needed to take this from concept to flight hardware would ever be available is another matter entirely. IMO there would need to be lots of DoD money in the mix.

Kirk very well knows that Timberwind prototypes were tested in the early 1990's with T/W of 35.

The point of the Timberwind reactor program was as propulsion to launch an ICBM sized rocket with a SDI beam weapon or x-ray laser payload that would be powered by the Timberwind reactor once in orbit. This would avoid any treaty restrictions on basing SDI weapons in space and allow the US to basically keep any such capability highly classified and disguised as ICBMs until/unless it reached a point of sufficient international destabilization that an ability to neutralize enemy offensive nuclear capability was required. It would have been a single use SSTO design.

As I recall, the primary reason it was shelved was that Teller's X-Ray Laser system, powered by a nuke detonation, failed.

I am aware of the Timberwind reactor, and what you say is true. However, in terms of an ICBM-sized launch vehicle, what is HSF going to gain from that? Timberwind does not scale up nicely so it is difficult to see how it benefits HSF. That's why I said the performance is too limited. *If* we are to spend huge resources to develop a nuclear-powered SSTO LV, I believe it would be better spent on gas-core, which has much higher potential. I have seen studies of gas core concepts capable of 1,000mT IMLEO in a RLV, using powered EDL return. I have yet to see anything similar in a solid core. I hesitate to put too much credence in those, but they are interesting and the math does work, but would take a lot of funding to put credibility to those studies. The potential gains are, imo, worth the expense of serious investigation. And if we're going to spend that kind of money, then spend it on something that can really make a difference, not just something that *is* different.

It comes down to economics in the end, getting the best bang for the buck (no pun intended).

Should we do this? My opinion is yes, but then I am not afraid of nuclear power; I respect it but I do not fear it. I do not know the feelings of the rest of the American public after the decades-long hugely successful FUD campaigns waged against nuclear power by the Gas, Oil and Petroleum industry, as well as by many well-meaning but misguided environmentalists, among whom I generally count myself.

Edit: spelling and spending clarification
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/13/2010 02:32 pm
And you "very well know" that Timberwind is dynamically unstable and melts down which is why I excluded it from consideration.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: 93143 on 06/13/2010 07:26 pm
I know that; that's why I've been steering clear of using it as an example.

You still haven't explained why you think Dumbo was unworkable...
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Archibald on 06/13/2010 07:43 pm
The big question is: what does Kirk Sorensen think about gas core nukes ?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/13/2010 09:55 pm
It's hard for me to imagine that gas-core nuclear thermal rockets should offer any comfort to anyone hoping for an NTR SSTO.  Their T/W is likely to be far WORSE than a solid-core NTR, and the prospect for holding the gaseous material in the core is not good enough to stand up to the environment required for launch.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: clongton on 06/14/2010 12:01 am
It's hard for me to imagine that gas-core nuclear thermal rockets should offer any comfort to anyone hoping for an NTR SSTO.  Their T/W is likely to be far WORSE than a solid-core NTR, and the prospect for holding the gaseous material in the core is not good enough to stand up to the environment required for launch.

Everything I've read points to the opposite Kirk. But let me add the caveat that I'm speaking mostly in terms of the nuclear lightbulb. There are several different approaches to the gas core and for most of them you are correct. The nuclear lightbulb however eliminates the thermal limits altogether, getting so hot that the energy moves into the far ultraviolet range. That's because we can let the reaction get as hot as we want to because the fused silica containment is almost completely transparent to energy in that spectra.

I hesitate to rely on Wikipedia for anything these days but the general description it offers for this type of engine is pretty good. I'll paste it here.

A nuclear lightbulb is a hypothetical type of spacecraft engine using a fission reactor to achieve nuclear propulsion. Specifically it would be a type of gas core reactor rocket that separates the nuclear fuel from the coolant/propellant  with a quartz  wall. It would be operated at such high temperature (approx. 25,000°C) that the vast majority of the electromagnetic emissions would be in the hard ultraviolet range. Fused silica is almost completely transparent to this light, so it would be used to 'bottle' the uranium hexafluoride and allow the light to escape and be used to heat hydrogen gas for a rocket propellant or to generate electricity using photovoltaics.

This type of reactor shows great promise in both of these roles, as a rocket engine it has the enormous advantage of being completely reusable, greatly reducing costs by amortizing the cost of the rocket over many launches. As a method to generate electricity, nuclear lightbulbs can be combined with photovoltaics. This method also does not involve the release of radioactive material from the rocket, unlike other designs which would cause nuclear contamination if used in a planetary atmosphere.


The math says that the isp and t/w would be massively better than any chemical engine and would also best any solid core concept known to date by a considerable amount.

Most of this is still theory, although there was a successfully fired proof of concept device constructed in the early 1960's, iirc.

Here's a few references:

1: Thode, L., Cline, M., Howe, S. (July-August, 1998). Vortex  formation and stability in a scaled gas-core nuclear rocket  configuration. Journal of Propulsion and Power.
 2: Poston, D., Kammash, T. (January, 1996). A computational model for  an open-cycle gas core nuclear rocket. Nuclear Science and Engineering.

3: Sforza, P. M., Cresci, R.J. (May 31, 1997). Fuel Efficient  Hydrodynamic Containment for Gas Core Fission Reactor Rocket Propulsion.  DOE/75786-3.
4: Innovative Nuclear Space Power and Propulsion Institute. Gas Core Reactors.
5: Unknown author. Nuclear Rocket  Technologies.
6: Sahu, J., Nietubicz, C. (September, 1985). Navier-stokes  computations of projectile base flow with and without mass injection.  AIAA Journal.

7: Koroteev, A.S., Son, E.E. Development (of) Nuclear Gas Core  Reactor in Russia [Online] Summary available: http://pdf.aiaa.org/preview/CDReadyMASM07_1064/PV2007_35.pdf (http://pdf.aiaa.org/preview/CDReadyMASM07_1064/PV2007_35.pdf)
8: Bussard, R.W., DeLauer, R. D. (1965), Fundamentals of Nuclear  Flight, McGraw-Hill , ISBN: 0070093008

edit: removed broken links
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 06/14/2010 01:20 am
The fused silica is fragile and any darkening would lead to a hot spot and failure of the "lightbulb" and its containment.  This is no system that would pass muster for a frequently-used earth-to-orbit launch vehicle, even if the T/W values were there, which for a system reliant on a single-surface heat transfer is hard to imagine.

No, I'll just say it.  There's no way this has any sort of thrust-to-weight ratio for Earth to orbit.  Sorry.

There's nothing in any of the Wikipedia articles to indicate a T/W for a gas-core engine of any significance.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/14/2010 05:18 am
It should be noted that darkening of the fused silica is unavoidable through any chemical treatment, since there will be neutrons flying everywhere, transmuting the once-pure silica into stuff contaminated with phosphorus compounds (for instance).
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: isa_guy on 06/14/2010 10:39 am
What about using gas core NTR ( which has significantly higher isp than solid core NTR ) to power a manned missions to mars beyond .
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: clongton on 06/14/2010 11:34 am
The fused silica is fragile and any darkening would lead to a hot spot and failure of the "lightbulb" and its containment.  This is no system that would pass muster for a frequently-used earth-to-orbit launch vehicle, even if the T/W values were there, which for a system reliant on a single-surface heat transfer is hard to imagine.

No, I'll just say it.  There's no way this has any sort of thrust-to-weight ratio for Earth to orbit.  Sorry.

There's nothing in any of the Wikipedia articles to indicate a T/W for a gas-core engine of any significance.

I'm not indicating that we can do this now. The state of the art does not support that. If you will look at my previous posts I indicated that its potential warrented funding investigation to determine if it could become feasable. That's all I was advocating. What we know today indicates that the silica will start to darken. Is there a way to prevent that? Maybe - we don't know, but it's worth finding out because if it can be prevented, then this approach is very much worth pursuing.

To say that we should not spend the money to find out, just because there are a lot of unknowns, is the same thing as saying we should not investigate thorium fueled molten salt surface reactors because there's a lot of unknowns. Both propositions are silly. Both technologies offer tremendous potential for the enrichment of mankind if we can make them work efficiently. But if we don't spend the money to find out, then we will never know and in that case we can continue to burn coal in our power plants and petroleum in our rockets. The potential benefits for mankind from both technologies is huge. We need to find out. In terms of the MSRs, we now know that it works. Why? Because we spent the money to find out. All I'm saying is that we should do the same for the nuclear lightbulb.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Star-Drive on 06/14/2010 09:30 pm
The fused silica is fragile and any darkening would lead to a hot spot and failure of the "lightbulb" and its containment.  This is no system that would pass muster for a frequently-used earth-to-orbit launch vehicle, even if the T/W values were there, which for a system reliant on a single-surface heat transfer is hard to imagine.

No, I'll just say it.  There's no way this has any sort of thrust-to-weight ratio for Earth to orbit.  Sorry.

There's nothing in any of the Wikipedia articles to indicate a T/W for a gas-core engine of any significance.

I'm not indicating that we can do this now. The state of the art does not support that. If you will look at my previous posts I indicated that its potential warrented funding investigation to determine if it could become feasable. That's all I was advocating. What we know today indicates that the silica will start to darken. Is there a way to prevent that? Maybe - we don't know, but it's worth finding out because if it can be prevented, then this approach is very much worth pursuing.

To say that we should not spend the money to find out, just because there are a lot of unknowns, is the same thing as saying we should not investigate thorium fueled molten salt surface reactors because there's a lot of unknowns. Both propositions are silly. Both technologies offer tremendous potential for the enrichment of mankind if we can make them work efficiently. But if we don't spend the money to find out, then we will never know and in that case we can continue to burn coal in our power plants and petroleum in our rockets. The potential benefits for mankind from both technologies is huge. We need to find out. In terms of the MSRs, we now know that it works. Why? Because we spent the money to find out. All I'm saying is that we should do the same for the nuclear lightbulb.

Chuck:

An thought provoking read: 

http://www.nuclearspace.com/nucrocketbook_dewarview.aspx

As to stronger nuclear light bulb materials there are several other materials that might be useable such as a-axis sapphire, see attached transmittance chart, but it needs improvements in its neutron damage capabilities to be competitive with fused quartz. 

In regards to enlarging the nuclear light bulb's surface area to maximize heat transfer, a modular light bulb approach could supply a solution such as that shown in the attached sketch.

Bottom line is that if we look for solutions to the gaseous reactor rocket's problems, we might find them.  If we don't look, we will never do so.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: clongton on 06/15/2010 01:24 am
Bottom line is that if we look for solutions to the gaseous reactor rocket's problems, we might find them.  If we don't look, we will never do so.

That's my entire point. Here is a technology that actually has the potential to be a genuine game changer *IF* we can learn how to overcome the few known shortfalls. It is one of the few "out of the box" ideas that has sufficient merit to warrant committing enough funds to pursue investigation. If, after that, it looks promising, then fund development. If not, oh well, at least we will know. But if we don't go find out, then we will never know.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Robotbeat on 06/20/2010 02:49 am
What about fission fragment rockets? Any actual experimentation with that concept? What do the non-involved experts think of the concept's viability?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/12/2012 02:44 pm
Interesting article about nuclear thermal rockets.  Not a particularly promising technology in my opinion, but interesting nonetheless:

Nuclear Space Rockets and the Most Fascinating NASA Man You’ve Never Heard Of (http://www.txchnologist.com/2011/nuclear-space-rockets-and-the-most-fascinating-nasa-man-youve-never-heard-of)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 01/16/2012 03:10 am
In 2000, my interest in lunar and Mars bases led me to investigate liquid-fluoride thorium reactor technology, and I gave a talk on it last year at TEDxYYC in Calgary that is now being featured on the TED.com site:

Talks | TEDx: Kirk Sorensen: Thorium, an alternative nuclear fuel (http://www.ted.com/talks/kirk_sorensen_thorium_an_alternative_nuclear_fuel.html)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Rhyshaelkan on 01/16/2012 04:02 am
Great video there. I would love to see a future with widespread Solar Power Sats as the base load with equally widespread use of backup thorium plants. Power Sats lose power for a around an hour, for three days during the Spring and Fall equinoxes. Plus having backups incase the Sun burps and fries the Power Sats is just wise.

Thorium NEP would be useful for spacecraft from the asteroid belt and beyond. How difficult is it to separate out the protactinium which is lethal to the thorium chain? Much processing?

Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: simonbp on 01/16/2012 04:08 am
Kirk,

What's the external neurton flux from LFTR like, compared to the brayton-cycle Uranium reactors typically assumed for NEP or Mars ISRU? I.E., what would the shielding requirements be like?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Archibald on 04/04/2013 09:35 am
I bring this (excellent) thread back to life - I have tracked down a couple of 1981 paper discussing the use of Molten Salt Reactors for space applications. The authors had a lot of positive things to say ;)


SPACE NUCLEAR POWER—A STRATEGY FOR TOMORROW
David Buden and Ma j . J. Angelo, Jr.
American Institute of Aeronautics and Astronautics
Conference on Large Space Platforms: "Toward
Permanent Manned Occupancy in Space," San Diego,
February 2-4, 1981.

http://www.osti.gov/bridge/product.biblio.jsp?osti_id=7054185

REACTORS FOR NUCLEAR ELECTRIC PROPULSION
David Buden and Major Joseph A. Angelo, Sr.
AlAA/JSAAA/DGLR 15th International Electric Propulsion
Conference, April 21-23, 1981, in Las Vegas, Nevada

http://www.osti.gov/bridge/product.biblio.jsp?osti_id=6634294

As for the authors -

Dr. Joseph A. Angelo, Jr.
Chairman, Space Technology Program
Florida Institute of Technology
Melbourne, FL 32901

David Buden
Advanced Reactors Program Manager
Los Alamos National Laboratory
Los Alamos, NM 87545
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: cordwainer on 04/08/2013 12:48 am
Here are a couple of concepts for NTR's that I thought up.

1. Surround your nuclear reactor with a series of rotating sleeves, fill each sleeve with tantalum or some other thermal seeding material then expose a portion of each rotating sleeve to your reactors neutron radiation. Make your sleeves out of graphene or fused quartz, other portions of the sleeve would be cooled cryogenically when necessary and some sort of moderating material or shield could be used to close the exposed slits to your reactor. This might be one way to use a fast neutron reactor to get higher Isp thrust  without the cooling and moderation issues of a nuclear light bulb.

2. Embed fissionable materials on a lattice of woven nanotube fibers and stuff the fibers into a fan-shaped reaction chamber and place a linear aerospike along the center of the wide section of the fan. Design your reaction chamber in two pieces that fit together like a clamshell or bellows. Fill reaction chamber with cryogenic gas and allow to diffuse through fibers, squeeze clamshell together until fissionable materials undergo critical mass and watch out cause the pressure and heat will probably bust the seals off your clamshell. Still it seems feasible to create a working design, whether it would be useful as a propulsion system I don't know but it might have a use in a nuclear pumped gas-dynamic laser.

Do these ideas seem feasible to anyone?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: cordwainer on 04/11/2013 08:15 pm
I tend to think NEP would be more fuel efficient for all but the fastest Mars transits. Tri-modal NTR like Pratt&Whitney's Triton design would make a great Earth-Orbit to Moon shuttle. Liquid Oxygen harvested from the regolith could be used to give improved LANTR thrust for a quicker and more fuel efficient ride back to Earth.

 Dusty Plasma Fission Rockets would be well suited for manned missions to Mars or for robotic missions to further planets. A five year round trip to Jupiter still seems a little too long to have astronauts in space to me, and the amount of nuclear fuel for such a manned mission with one of these rockets seems exorbitant. Imploding Fusion Liner technology seems likely to give this design a run for it's money though since fusion fuels are more plentiful than fission ones. Darn! I really want those Dusty Plasma Fission reactors to so we can burn all those nasty weaponizable nuclear fuels!
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: kfsorensen on 04/12/2013 04:12 pm
I bring this (excellent) thread back to life - I have tracked down a couple of 1981 paper discussing the use of Molten Salt Reactors for space applications. The authors had a lot of positive things to say ;)

SPACE NUCLEAR POWER—A STRATEGY FOR TOMORROW
http://www.osti.gov/bridge/product.biblio.jsp?osti_id=7054185

REACTORS FOR NUCLEAR ELECTRIC PROPULSION
http://www.osti.gov/bridge/product.biblio.jsp?osti_id=6634294

Those are great finds!  Thanks so much for linking them.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Archibald on 04/13/2013 01:50 pm
I was quite sure you would appreciate them. Interestingly, the authors  are from Los Alamos and not Oak Ridge (kind of outing for the MSR, which was rarely discussed outside ORNL)
There are a lot more papers by the authors Buden and Angelo but unfortunately no trace of MSR in them.  :(
Their conclusions are pretty close from yours - MSR is very superior to solid core for space power.

Oh, and btw - thanks to John Goff   ;D
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: cordwainer on 04/16/2013 05:30 pm
While molten salt reactors and heat pipes are quite effective I would think cermet or pellet bed reactors would be safer for space based power generation?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Archibald on 11/08/2017 02:00 pm
Bringing back this old thread because things have changed a lot since 2013 and even since 2006, when Kirk Sorensen started this thread.

https://www.nanalyze.com/2015/10/6-nuclear-energy-companies-building-molten-salt-reactors/

As of today, no less than 6 companies are studying MSR (including Mr. Sorensen).

Some have brought back the old MSRE design, others are attacking the more difficult (but more promising) thorium breeder.

One of these companies even has funding from Bill Gates. Another has a contract with Indonesia for an experimental MSR.

The sheer soundness of the design is now recognized.

Hans Blix (former director of IAEA, Vienna) publically endorsed thorium as a proliferation-proof nuclear fuel cycle.

hopefully it will find its way into the space program sooner rather than later.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: A_M_Swallow on 11/08/2017 09:10 pm
There are now some good starting blocks for people wanting to build a Nuclear Electric Propulsion (NEP) probe within the next 5 year.

Nuclear power source - Kilopower (1kWe to 10kWe)
https://ntrs.nasa.gov/search.jsp?R=20170002010 (https://ntrs.nasa.gov/search.jsp?R=20170002010)

Ion thrusters - for instance NEXT
https://www.nasa.gov/content/next-provides-lasting-propulsion-and-high-speeds-for-deep-space-missions (https://www.nasa.gov/content/next-provides-lasting-propulsion-and-high-speeds-for-deep-space-missions)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Asteroza on 11/08/2017 10:24 pm
Using an bimodal NTER as possible replacement/enhancement for the helicon in a VASIMR would be an interesting trade path. Ups NTR ISP due to going to conventional nozzle melting temps (which the mag accelerator/nozzle path of the VASIMR can handle), and you have most of the helium infrastructure for bimodal ops in an NTER already (missing generator turbine and radiator though), thus can supply house loads and conventional electric thrusters.

This would allow fast deep oberth burns at high thrust, then electric propulsion during cruise to up mission deltaV.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: mikelepage on 11/11/2017 02:22 am
There was previous discussion of the role of nuclear thermal propulsion/bimodal nuclear thermal propulsion in a previous thread.  Is there interest in continuing this discussion in terms of the roles these technologies, as well as nuclear electric propulsion, might play in future exploration architectures?

Firstly Kirk, a thank you for your work on the promotion of Thorium MSRs.

Secondly, I just saw this, and find it interesting to revive this thread now, since I doubt there was any chance of the various public authorities granting approval to launching such devices until the launch vehicles themselves were very much safer (e.g. as BFR is supposed to be).  If in 5-10 years such a launch vehicle is flying regularly with high safety rates, then I can see Nuclear rockets flying.

Perhaps it's now worth revising/optimising such prototype designs for a 150 ton payload in a 9m fairing?
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: tea monster on 11/12/2017 07:19 am
Don't forget the Nested X3 when considering options for deep space ion drives.

https://www.space.com/38444-mars-thruster-design-breaks-records.html
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Nilof on 11/28/2017 01:59 am
The X3 is a bit overpowered for a kilopower source. The kilopower can only provide 10 kW ish while the X3 needs ~100 kW.

It could be used in a SEP first stage to give an outer solar system probe some extra initial velocity though.

https://youtu.be/vSP1nUnJ9EI
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: A_M_Swallow on 11/28/2017 04:31 am
The NEXT ion thruster can throttle down to 0.5kW. So run off 6.9kW solar power in the inner solar system and then switch to kilopower.
https://en.wikipedia.org/wiki/NEXT_(ion_thruster) (https://en.wikipedia.org/wiki/NEXT_(ion_thruster))

The 2.3kW NSTAR thruster is also available.
https://en.wikipedia.org/wiki/NASA_Solar_Technology_Application_Readiness (https://en.wikipedia.org/wiki/NASA_Solar_Technology_Application_Readiness)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Zed_Noir on 11/28/2017 04:43 pm
The X3 is a bit overpowered for a kilopower source. The kilopower can only provide 10 kW ish while the X3 needs ~100 kW.
....

The flippant response to that is 10 Kilowatt modules. Impractical as that may seem. :P
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: BrightLight on 11/28/2017 05:34 pm
The X3 is a bit overpowered for a kilopower source. The kilopower can only provide 10 kW ish while the X3 needs ~100 kW.
....

The flippant response to that is 10 Kilowatt modules. Impractical as that may seem. :P
10 kW low radiation (4.8 Ci) HEU power systems are now being tested at the Nevada Nuclear Test Site.  Higher power is also in the design phase for future testing.

A paper in NTRS date November 16, 2017
https://ntrs.nasa.gov/search.jsp?N=0&Ntk=All&Ntx=mode+matchallany&Ntt=kilopower
NASA/TM-2017-219467, GRC-E-DAA-TN38737, E-19342
"NASA’s Kilopower Reactor Development and the Path to Higher Power Missions"
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Nilof on 11/29/2017 10:16 pm
The X3 is a bit overpowered for a kilopower source. The kilopower can only provide 10 kW ish while the X3 needs ~100 kW.
....

The flippant response to that is 10 Kilowatt modules. Impractical as that may seem. :P

Nuclear reactors are really heavy for the power they give you though, at ~7 W per kg for kilopower vs 200 W/kg to 400 W/kg (at 1 AU) for ROSA-type solar arrays. Part of the design requirements of the X3 was to have a good enough power to weight ratio that it wouldn't be a bottleneck when combined with good solar arrays in the inner solar system.

With nuclear, some of the advantages of Hall thrusters compared to grid ions (low weight for a given thrust and power) are reduced, since the nuclear reactor will be the bottleneck. With such low requirements on mass efficiency and higher requirements on specific impulse and energy efficiency, grid-ions like NEXT seem to make more sense.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: KelvinZero on 11/29/2017 10:38 pm
Hey, probably a dumb idea, but a while ago I was wondering about going from fission directly to sunlight wavelengths, without all the fiddly turbines and radiators in between. You just want some medium at a certain temperature and the only heat loss you are interested in is radiation, at sunlike temperatures.

(The point of this was to melt down into the ice and create biospheres, but that isnt important)

My new idea is that perhaps the way to combine SEP and NEP would be to have one of these nuclear lightbulbs for when the sun is too weak. Rather than heavy shielding it is a long way from the ship and uses a parabolic mirror to focus the light on the massive disk-shaped solar panel that you used in the inner solar system.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Asteroza on 11/29/2017 11:02 pm
Hey, probably a dumb idea, but a while ago I was wondering about going from fission directly to sunlight wavelengths, without all the fiddly turbines and radiators in between. You just want some medium at a certain temperature and the only heat loss you are interested in is radiation, at sunlike temperatures.

(The point of this was to melt down into the ice and create biospheres, but that isnt important)

My new idea is that perhaps the way to combine SEP and NEP would be to have one of these nuclear lightbulbs for when the sun is too weak. Rather than heavy shielding it is a long way from the ship and uses a parabolic mirror to focus the light on the massive disk-shaped solar panel that you used in the inner solar system.

You want a portable sun?

The closest thing I remember offhand was a continuous nuclear pumped laser, where uranium hexaflouride gas lazes in the reactor cavity and exits via a window, possibly intermediated afterwards with a quantum dot or similar frequency converter laser target to get the sunlight frequencies.

But that seems terribly rube goldberg since you would probably need separate reactor cooling/radiator systems. If you have to have that anyways, might as well go for a thermal power conversion system using turbines and generate power (almost) directly.

If you want a combined light/heat source then you face materials limitations as you would effectively have to deep heat soak the reactor vessel. Nuclear lightbulb systems were using expendable coolant as reaction mass for propulsion to keep the quartz bulb from melting (effectively fully surrounding the reactor vessel with a cooling jacket), so I'm not sure how one would expose the bulb light to the solar panels.


SEP is essentially predicated on not taking the power source with you, only a receiver. If one insists on taking a solar panel system designed for the inner solar system farther out, having an external laser beam power to you strikes me as architecturally more efficient.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: KelvinZero on 11/30/2017 12:38 am
The idea is that it doesn't need cooling. It just incandesces at a certain temperature. Like the filament of an incandescent bulb the material has to be able to withstand that temperature. Overheating would imply a different temperature. You would just turn down your reactor. You are only interested in heat being dissipated through incandescence.

I don't think this would work for pumped lasers because they always need cooling because they will always have high entropy heat that has to be dissipated in addition to the low entropy laser beam.

I also like the idea of beamed laser light, but for this thread the subject is NEP. I can think of reasons people may want this autonomy in the future but that is way off topic. Just treat it as a technical problem.

(edit)
I was just thinking about my incandescent idea again. I googled around and found this:

miniature power generator converts infrared to electricity (http://www.laserfocusworld.com/articles/print/volume-40/issue-7/world-news/world-news/miniature-power-generator-converts-infrared-to-electricity.html)
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: tea monster on 11/30/2017 12:19 pm
Can anyone suggest some graphical material on any studies on a 100kw space reactor? Preferably something more modern that has been properly studied.

I'm interested in making a model of one and I want it to be as realistic/practical as possible.
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Vacuum tube on 12/26/2017 02:40 pm
Can anyone suggest some graphical material on any studies on a 100kw space reactor? Preferably something more modern that has been properly studied.

I'm interested in making a model of one and I want it to be as realistic/practical as possible.

Not exactly 100kW , but there is 1MW RUGK nuclear reactor in TEM (aka index 12AN30 )
(https://pp.vk.me/c624016/v624016809/5ed/6fjM5I4LiR4.jpg)
early version:
(https://ic.pics.livejournal.com/tnenergy/75246910/21994/21994_original.png)
later version
(https://hsto.org/getpro/geektimes/post_images/825/77c/07b/82577c07b410bfa481d82b376b3593e8.png)

same thing in "Resurs" ground stand in NITI (in vacuum chamber, testing for resource, assembly ended in November)
(https://s5.postimg.org/v3pdxaxzr/2017-07-06_19-35-57.png)


 if you don't fear russian language, and really whant to know what it is  there is even scans of initial technical requirements (without pictures obv):
http://zakupki.gov.ru/44fz/filestore/public/1.0/download/priz/file.html?uid=35F1847850940008E053AC110725D86B

with corrections from 2016 :
http://zakupki.gov.ru/44fz/filestore/public/1.0/download/priz/file.html?uid=35F37D275D8100D0E053AC110725C3E2

Currently project suck military money so dont expect to find many illustrations, but there is enough information left about it when it been just space tug , unfortunately all in russian segment of internet

also you can lover your demand and take simle road with Killopower
(https://s5.postimg.org/xu64xsefr/2017-05-14_23-02-33.png)
(https://s5.postimg.org/qfc7ro36f/2017-05-14_22-34-17.png)

there is even video:
https://www.youtube.com/watch?v=KobRfGqlpGc

of course this is only 1-10 kW small simple reactor, direct competitor to RTG, but programme more open

choice is your
Title: Re: Role of NTR/BNTR/NEP in future architectures
Post by: Asteroza on 12/26/2017 10:13 pm
The idea is that it doesn't need cooling. It just incandesces at a certain temperature. Like the filament of an incandescent bulb the material has to be able to withstand that temperature. Overheating would imply a different temperature. You would just turn down your reactor. You are only interested in heat being dissipated through incandescence.

I don't think this would work for pumped lasers because they always need cooling because they will always have high entropy heat that has to be dissipated in addition to the low entropy laser beam.

I also like the idea of beamed laser light, but for this thread the subject is NEP. I can think of reasons people may want this autonomy in the future but that is way off topic. Just treat it as a technical problem.

(edit)
I was just thinking about my incandescent idea again. I googled around and found this:

miniature power generator converts infrared to electricity (http://www.laserfocusworld.com/articles/print/volume-40/issue-7/world-news/world-news/miniature-power-generator-converts-infrared-to-electricity.html)

As a minor followup, Sandia labs were working on the FALCON (Fission Activated Laser CONcept) nuclear pumped laser in the early to mid 90's, for a 10-100MW CW laser, ostensibly as an alternative to electric lasers in support of Dr. Kare's beamed power heat exchanger SSTO work, and other civil uses like fancy pants thick welding. Sandia hasn't done much with it since the SDI program closed, but felt the technology is mostly novel uses of existing mature technologies, with a low pressure reactor design.

https://www.osti.gov/scitech/biblio/12982617 (https://www.osti.gov/scitech/biblio/12982617)
https://www.osti.gov/scitech/biblio/10120505 (https://www.osti.gov/scitech/biblio/10120505)

trolling around the OSTI archives for FALCON brings up a few more details, such as the interesting takeaway that Dr. Kare's beamed power rule of thumb (1MW/1 Kg payload) may actually extend up 1MW/3Kg.