Author Topic: Role of NTR/BNTR/NEP in future architectures  (Read 141803 times)

Offline kfsorensen

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Role of NTR/BNTR/NEP in future architectures
« 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.

Offline lmike

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #1 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)

Offline FransonUK

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #2 on: 12/30/2005 03:23 PM »
Wow, cool image. How long would that take to construct in orbit?
Don't ya wish your spaceship was hot like me

Offline realtime

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #3 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


Offline kfsorensen

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #4 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.

Offline kfsorensen

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #5 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.

Offline GirlygirlShuttlefan

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #6 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?

Offline kfsorensen

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #7 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.

Offline Davros

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #8 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?

Offline realtime

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #9 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.


Offline Bruce H

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #10 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.

Offline kfsorensen

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #11 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.

Offline To The Stars

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #12 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.

Offline kfsorensen

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #13 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?

Offline To The Stars

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #14 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

Offline simonbp

RE: Role of NTR/BNTR/NEP in future architectures
« Reply #15 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 ;)

Offline JonClarke

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #16 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

Offline braddock

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #17 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... :(

Offline JonClarke

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #18 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

Offline kfsorensen

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RE: Role of NTR/BNTR/NEP in future architectures
« Reply #19 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.

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