Davros - 30/12/2005 6:07 PMNo, I'm afraid there is no cure for chronic ideologues short of brain transplant. - sigh! -
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?
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.
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?
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
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 ;)
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?
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.
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.
vanilla - 1/1/2006 5:30 PMQuoteRocket 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?
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.
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
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.
vanilla - 1/1/2006 8:44 PMQuoteRocket 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.
SimonShuttle - 2/1/2006 5:12 AMQuoteRocket 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.
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.
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.
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.
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.
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?
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?
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?
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!
Colby - 2/1/2006 5:12 PMVanilla, 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!
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!
Chris Bergin - 2/1/2006 12:12 PMhttp://www.nasaspaceflight.com/_docs/
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?
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.
Justin Space - 3/1/2006 2:35 PMWhat 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.
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! :)
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!
Avron - 3/1/2006 10:34 PMNo, 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.
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?
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
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?
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.
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.
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.
Tap-Sa - 4/1/2006 11:33 AMYes, 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.
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?
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.
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?
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).
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.
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.
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?
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.
vanilla - 5/1/2006 4:14 AM
I put two gifs on the website
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.
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?
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?
JonClarke - 6/1/2006 3:31 PMProtecting 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.
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.
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.
vanilla - 7/1/2006 7:27 AMQuoteJonClarke - 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?
Avron - 7/1/2006 9:03 AMThanks 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).Quotebraddock - 7/1/2006 9:00 AMThis has been one hell of a fantastic thread, have learned a lot...
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.
Many thanks to vanilla for the 'tour' and providing so much easy to follow feedback to all the questions.
Tap-Sa - 7/1/2006 10:07 AMWow, 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.
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.
JonClarke - 8/1/2006 4:01 AM
What do you see as the application of moltern salt reactors in space? Surface power sources? NEP?
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.
braddock - 9/1/2006 9:02 PMThe 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.
How could excess fission products be removed? Would you require an on-board centrifuge or some-such?
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?
JonClarke - 10/1/2006 2:31 PMBoy, 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.
What's the half life of the gasesous fisson products, xenon and krypton? How much are actually produced?
vanilla - 13/1/2006 3:32 PM
Is there interest in continuing this discussion and getting into the topic of power conversion systems?
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?
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.
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?!).
vanilla - 15/1/2006 1:08 PMHydrogen 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.
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.
realtime - 16/1/2006 12:48 AMHydrogen 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.
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.
vanilla - 16/1/2006 9:02 AMQuoterealtime - 16/1/2006 12:48 AMHydrogen 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.
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.
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.
vanilla - 17/1/2006 2:01 AMIf 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.
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.
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.
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
vanilla - 16/1/2006 8:02 AMHere 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.
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.
simonbp - 28/1/2006 2:28 PMHere 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.
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 ;)
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.
simonbp - 28/1/2006 8:46 PMI 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.
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 ;)
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.
Tap-Sa - 29/1/2006 8:08 AM
does anyone have any more details about this?
Tap-Sa - 29/1/2006 8:08 AMWow! 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.
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.
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?
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.
Tap-Sa - 29/1/2006 8:08 AMMore 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.
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?
Stowbridge - 29/1/2006 12:10 PMI 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.
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?
PlanetStorm - 1/2/2006 6:16 PMWell, 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.
>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.
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.
simonbp - 2/2/2006 11:57 PMAh, a man with whom I can discuss the beauties and barriers of thermodynamics... :)
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 ;)
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...
vanilla - 31/1/2006 3:51 PMNote 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.
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.
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.
Mark Max Q - 25/3/2006 11:42 AMTo 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.
Sorry if this has been answered, but how do you 'start' a nuclear propulsion engine?
Avron - 23/3/2006 10:04 PMQuotevanilla - 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...
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
Avron - 26/3/2006 11:54 PMAs 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).
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?
vanilla - 27/3/2006 5:24 PMQuoteAvron - 26/3/2006 11:54 PMAs 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).
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?
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.
Avron - 27/3/2006 11:01 PMI'm sorry, I must be going senile and forgetting which threads I said which things on...
Somehow I missed that answer, thanks..
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?
Star-Drive - 11/5/2006 11:12 PMTimberwind 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.
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.
publiusr - 12/4/2006 2:05 PMIt 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..."
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.
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
mauk2 - 30/8/2006 1:12 AM
Potassium metal, on the other hand? Nosirree bob. Not nice. Tin would be much better.
Rob in KC - 5/9/2006 5:30 PM
Is there an alternative to both chemical and nuclear?
mauk2 - 6/9/2006 4:15 PMTethers 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.
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. :(
vanilla - 30/8/2006 7:35 AMQuotemauk2 - 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.
This NF-3 AO will solicit only missions that do not require nuclear sources for power generation or propulsion.
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?
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?
Nuclear thermal propulsion is a waste of time if you're trying to improve payload performance. See my analysis here: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!!!).
http://selenianboondocks.com/2010/02/pf-expressions-example/
and here
http://selenianboondocks.com/2010/02/payload-fraction-example-proof/
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.
I mean, I'm not afraid of nukes at all. But WHAT is the huge draw for nuclear-electric for a MTV vs solar?
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?
Good to know!
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.
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:
...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.
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.It was a really, really bad experience.
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.
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.
Luckily, there are quite a few other approaches to high specific power solar arrays.Do not underestimate the trouble in qualifying any of them.
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.
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.
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.
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.
Solar Thermal.
I still marvel that it is routinely left out of the discussion
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.
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.
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.
The plastic substrate may degrade. Using aluminum foil as its own substrate would add a lot of weight.Also, the concentrator itself may degrade, which would effect solar thermal greatly.
Aluminium foil is resistant to degrading as a mirror.
Why do you make the assumption that solar thermal us limited to hydrogen propellent?
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.
The plastic substrate may degrade. Using aluminum foil as its own substrate would add a lot of weight.Also, the concentrator itself may degrade, which would effect solar thermal greatly.
Aluminium foil is resistant to degrading as a mirror.
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).
NTR is unobtanium
NTR is unobtanium
No argument there.
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:
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.
"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.
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!"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.
...
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.
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.
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!"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.
...
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.
QuoteEven 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.
QuoteEven 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?
What do you do when you run out of coolant (hydrogen used for propulsion) on a nuclear lightbulb?
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.
NASA's current reference designs sit at 925 seconds.
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 .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.
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.
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.
The Sun is even better than nuclear fission: it's nuclear fusion. :P ;D
I agree, but I was being somewhat facetious.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.
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.
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.
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. ;)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. ;)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?
...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. ;)
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.
...
QuoteEven 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.
QuoteEven 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?
Perhaps you should devote a little bit of time toward fixing what you perceive as the problems with the NTR.
Yes for in-space. No for launch (not enough thrust).
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
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?
What is the "specific power" listed above as SPPWR, then?
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/
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...
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.
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.
Really? Who has done this?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.
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.
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.
Really? Who has done this?
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.
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.
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.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.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. ;)
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.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.QuoteThe 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. ;)
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.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.
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.QuoteThe 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.
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.
Solar system ? You probably meant up to the martian orbit because beyond that solar power becomes useless.
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.
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.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.
Solar system ? You probably meant up to the martian orbit because beyond that solar power becomes useless.
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.
?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.
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.
And, PS, the VASIMR runs at least at 60% efficiency, and higher efficiency (like 80%) is being developed.
I'm not married to it. I think VASIMR has certainly gotten more than its share of hype.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.
?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.
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.
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.
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.
The problem with inflatable structures is that for thin, lightweight inflatables, gas gradually escapes, I believe. Inflatables can work good for deployment, though.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.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.
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).
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.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?
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).
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.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.
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.
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?
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 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.
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?
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).
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?
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?
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.
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 :) .
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.
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."
Has the viability of a 4 month trip to Mars possibly using 12MW solar arrays VASIMR been checked?The basic concept of an opposition-class mission to Mars with 10MW worth of SEP is viable, according to this:
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."
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.
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.
Solar cell massSolar arrays 12,000 kW / 7 kW/kg = 1,715 kg or 1.8 mT
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."
12,000 kW * 7 kg/kW = 84,000 kg or 84 mT
with the powerplant and VASIMR dry specific mass alpha = 0.50 kg/kWe to see if this example could close.
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 :) .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).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.
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.
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 :) .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.
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.
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.
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.
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
Hey, just another throwaway idea..I wouldn't classify that as a throw-away idea. It sounds like a great idea to me!
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
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.
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.
... but his 0.5 kg/kWe number is a fantasy.
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.
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.
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: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.
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.
Interesting, I wasn't really aware of the STCAEM study. I wonder what the transfer times for a conjunction class mission would be.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.
Could you do a conjunction class mission with the long spiral out and spiral down times associated with SEP?
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:QuoteThe 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
It doesn't use aerobraking. Anyways, so what if it does?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:QuoteThe 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:
True, but if you used a lower Isp for the thrusters, you could lower that time (at the expense of more IMLEO).[/quote]It doesn't use aerobraking. Anyways, so what if it does?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:QuoteThe 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
True, but if you used a lower Isp for the thrusters, you could lower that time (at the expense of more IMLEO).It doesn't use aerobraking. Anyways, so what if it does?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:QuoteThe 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 NTRQuoteSurface time 20 days compared to 30 for NTRI 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.
Artificial gravity during transit was relatively easy for NTR, not so for SEP.
It's all about the details, not just the smallest IMLEO.
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.
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.
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.
What about SEP?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.
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.
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.
I totally understand that. However, solar sails can also use thin-film solar cells instead of just metalized mylar. Behold, IKAROS: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 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
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.
865 sec is not nearly enough Isp to justify putting an NTR on any deep space mission.
More tested? SEP is already operational! Has been for years.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.
It's not like we're starting from scratch;
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.
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.
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).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).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.
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.Neither is Isp.
So what? Payload mass fraction isn't the be all end all statistic to base everything on.Neither is Isp.
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 $$$ required to obtain that.
Where a high thrust NTR with an Isp = ~950 seconds really excels in the bang per buck arena is in the reusable single stage to orbit (SSTO) applications.
There are a lot of highly qualified people remaining, several of them AAAS Fellows like Harry Finger, who feel the opposite. They're not stupid people, Kirk.
More tested? SEP is already operational! Has been for years.
http://en.wikipedia.org/wiki/Dawn_(spacecraft) (http://en.wikipedia.org/wiki/Dawn_(spacecraft))
Folks:
If we can't build a NTR with a thrust to weight ratio of better than 25-to-1, I have to agree with KFS.
More tested? SEP is already operational! Has been for years.
http://en.wikipedia.org/wiki/Dawn_(spacecraft) (http://en.wikipedia.org/wiki/Dawn_(spacecraft))
Let's see, that's 0.02 pounds of thrust...bring sunblock.
More tested? SEP is already operational! Has been for years.
http://en.wikipedia.org/wiki/Dawn_(spacecraft) (http://en.wikipedia.org/wiki/Dawn_(spacecraft))
Let's see, that's 0.02 pounds of thrust...bring sunblock.
Who cares if the thrust is lower? You can afford greater delta-v. Sure, you pay a penalty for having low thrust, but at higher specific powers (like are available today and even better tomorrow) and the incredibly high Isp of electric thrusters (HiPeP has 10 times the Isp of NTR), that penalty is easily compensated for.
Besides, let's compare that to nuclear thermal rockets which have flown in orbit. Those have exactly zero thrust.
There's also a penalty in weight, $$$, 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.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.
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?
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.
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).There's also a penalty in weight, $$$, transit times, and necessary delta v. Eg that VASIMERTM Earth-Moon space tug has to do 8 km/sec delta v--for a 6 month trip that normally takes 4 km/s. Meanwhile, a single reusable ACES-71 tanker could make the same round trip 10 times over, and deliver much more payload for much less cost and much less delay, thus allowing just in time inventory management.More tested? SEP is already operational! Has been for years.
http://en.wikipedia.org/wiki/Dawn_(spacecraft) (http://en.wikipedia.org/wiki/Dawn_(spacecraft))
Let's see, that's 0.02 pounds of thrust...bring sunblock.
Who cares if the thrust is lower? You can afford greater delta-v. Sure, you pay a penalty for having low thrust, but at higher specific powers (like are available today and even better tomorrow) and the incredibly high Isp of electric thrusters (HiPeP has 10 times the Isp of NTR), that penalty is easily compensated for.
There's also a penalty in weight, $$$, 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.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.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.
Nobody here is suggesting using VASIMR (try spelling it right) for Earth-Moon missions, so you are making a straw man here.
"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 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.
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??
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).
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'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?
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?
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.
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?
This is the advanced concepts forum. It is named that way for a reason. I think you are looking for the History forum...
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.
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.
Does anyone here want to talk about anything realistic or just fantasize?
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.
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...
But would you inherently want such a slow poke/high mass mission as first pathfinder mission?
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.
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.
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...
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?
Guys, bring the fusion talk to the appropriate threads. Hopefully you haven't already scared off sorenson.
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.
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.
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/
As to the hydrogen’s low density problem, have you ever thought about using liquid deuterium instead as the reaction mass for the NTR?
If you go into a problem looking for ways to make it fail, it will fail by definition.
If you can’t make that work as a SSTO candidate, you didn’t try very hard to do so.
Im not at all a fan of nuclear launch vehicles, but I have a question,They have.
Why do nuclear and beamed power designs not exploit the atmosphere as propellant?
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 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.
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.
http://en.wikipedia.org/wiki/Uranium-238#Radiation_shieldingWhy 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
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...
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.
QuoteAlso, 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.
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
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.
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.
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.
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.
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.
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.
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.
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
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?
The X3 is a bit overpowered for a kilopower source. The kilopower can only provide 10 kW ish while the X3 needs ~100 kW.
....
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.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
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
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.
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.
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)