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In 2003 James Dewar published "To the End of the Solar System" a history of the US nuclear rocket program. In a recent interview on the Space Show he talks in detail about the subject. He claims that a NTR with a B4 core is ready "off the shelf" for more testing. Furthermore a nuclear engine of this type could be upgraded and used by Ares V as an upper stage capable of putting 700,000 lbs (317 MT) into LEO. Maybe it's time to look seriously at this in light of the ESAS DRM 5.0 architecture, perhaps this is the type of NTR that NASA intend to use?
For background also see this interview with Dewar
If you build a thermal rocket based on a Rover/NERVA. You would build it according to what people thought they could do in 1971-'72. That is, it would have 825 seconds of Isp it would have power densities of 1500 megawatts for the NERVA (Nuclear Engine for Rocket Vehicle Application), 400 MW for the small engine (Pewee) of 1972. Both would have 10 hours of fuel lifetime with 60 stop & starts had an Isp of 825 and 875 for the small engine. What I'm arguing in the text is if you had kept on working that core you would now have a 4th generation system. And it would have Isp's over 1000, power densities 3000 MW for a NERVA type of core or 800 MW for a Pewee type of core. And maybe 30 hours of engine lifetime with 180 stop & starts.
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No way,
NTR engines will never be allowed for upper-stage applications, because the core will not be in a stable orbit before it fires (and becomes intensely radioactive).
And as a pure orbital engine, we are supposed to put it in a higher 800km+ orbit to ensure it doesn't come down again for a while if it fails.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Never is a long time.
With that amount of lift capacity a lot of shielding can go along with the core together with a mechanism to ensure it lands in one piece. Alternatively it could have an Abort To Orbit capability as the core would only weigh about 5 tons, some numbers are here: NTR Propulsion (PDF)
Today, RTG technology is considered safe enough to survive launch failures of all types. Given ten years of development perhaps NTRs can be engineered to the same safety standards.
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Hmm, I never realy thought about the point you address. When we think of NTR we do tend to think of the NERVA stuff from the 70's and assume that it is the limit for these types of engines. Probably because conventional chemical engines pretty much have reached the limits of performance.
But as the author points out this may not be the case for NTR. There is probably a good bit of additional preformance that can be wrung out of NTRs beyond what the NERVA guys achived. Especially in the area of specific power increases. The improvements he mentions for a 4th generation NTR rocket seem very reasonable to me, and would make the already good NTR even better.
As for using a NTR as an upper stage, its a marginal proposition I think. Even the improved 4th generation NTR rockets are only marginal in this role. Even at twice the power density they are still only okay. Another problem is that an upper stage engine would have to be much larger, in some cases as powerful as a ground based nuclear reactor (2GW or more). Which has obvious implications both in terms of shielding and safety.
I'm also not to sure that abort to orbit is a viable alternative. Mainly because implementing a seperate assent system for the reactor would be expensive and complicated, and the system would ultimatly not be very reliable. There are a large number of possible malfunctions (any of a destructive sort) in which that sort of abort system would probably be useless. I might be willing to slide on enviromental consiquences of a largish NTR disaster, but realisticly that won't be an option.
He who refuses to do arithmetic is doomed to talk nonsense.
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As for using a NTR as an upper stage, its a marginal proposition I think. Even the improved 4th generation NTR rockets are only marginal in this role.
Dewar is saying it can double the lift capacity to LEO, that's quite a margin. It's beyond what chemical rockets can be expected to do for a long time, if ever. Ares V will be the heavy lifter for lunar and Mars exploration, doubling its capacity will change the economics. ISP rules.
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I think Nuclear propulsion could open up a whole new ear of exploration but here are the problems I see and that is Nuclear Thermal Propulsion is too costly, NEP too complicated and too fat, GCNRs too dirty. Don't think any civilian population will be too keen on living near one of these.
Gas Core's radioactive exhaust is a huge PR turn off.
So what happens when the rocket blows up during launch, of course they may not be enough fissile material but you can be sure there will be a media frenzy about a dirty bomb.
'first steps are not for cheap, think about it...
did China build a great Wall in a day ?' ( Y L R newmars forum member )
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Comparison between RTGs and NERVA is nonsense, since they are completely different animals with the only commonality between them is that they both make heat and radiation.
RTGs are designed as completely sealed units that can protect the fuel inside from leakage during most any possible failure mode. The American Plutonium fueled units reach relatively low temperatures, and can be shielded by tough ceramics and graphite. They also generate fairly small amounts of radiation (medium-intensity alpha source) and the elements themselves are very strong versus the conditions they have to operate at.
NTR engines on the other hand, are entirely the opposite in nearly every way, with the biggest difference is that propellant (liquid Hydrogen) is intentionally brought into relatively intimate contact with the fuel elements and vented out the nozzle. Thus the failure of the fuel element(s) directly leads to them being lost out the nozzle and into the atmosphere. Also, because of the corrosive nature of hot Hydrogen and the velocity at which it passes through the engine, ablation of the fuel elements and the loss of core material to the atmosphere is probably unavoidable.
The core itself is naturally stressed to the limits of temperature in order to maximize Isp; the operating temperature of NERVA was decided upon by how hot they could get the thing before it melted. This is a recipe for an engine that is going to fail from time to time, and failure short of orbit is not acceptable.
Because, the core produces infinitely more radiation than an RTG does, even when it is shut down! With the high operating power of the reactor versus the amount of fuel aboard (which is minimized), the reactor will produce a very large amount of "screaming hot" radioactive waste very quickly. Dispersion of even a portion of this would yield terrible fallout. Coupled with the close-to-failure operating temperature and the rigors of a Hydrogen rocket engine, failure of the engine will naturally lead to this outcome.
And, added shielding to protect the "hot" core in the event of orbital-speed reentry is a lousy proposition, particularly since the core needs to have holes on both ends for the Hydrogen feed and nozzle. Failure to block the hole on the side with the reentry shielding, likely the top of the core due to its center of mass, would result in the core being destroyed in reentry and its contents dispersed. This is already assuming it reenters shield-first and has a relatively soft landing.
Furthermore what sort of systems could protect the core in the event of failure? You cannot use superhigh temperature-safe graphite as a reactor "liner" like RTGs because it would act as a neutron reflector. Ceramics would be subject to shattering just like fuel elements too. And of course, metals melt.
An out-of-control nuclear reactor of this size is also capable of generating a very large amount of energy very quickly, such as the SL-1 reactor in Idaho of years back, for a brief moment, generated 7000 times its design output. Thats not a typo... I know that it is not very likely, but I just don't think we will ever get to unlikely enough. Materials science is reaching a plateu about what kind of temperatures are practical, and if you just slap a valve on the bottom of the nozzle & Hydrogen intake, its just not going to contain the radioactivity regardless what you make it out of.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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I also think the painting of NERVA's superiority as an upper stage is unfair and somewhat exaggerated. First of all, you will not be able to use the small RL-10 sized megawatt scale reactors, as they just don't generate the thrust. A larger gigawatt-scale engine, likely weighing in the low tens of tonnes, will be required to generate SSME scale thrust.
Then there is the general requirement that the core be placed in a 800km storage orbit, to prevent it from reentering before the radioactive waste inside decays to a safe level. This is not a trivial ding against its payload capacity.
Then there is the mass of the larger Hydrogen tankage, which will need to have considerably higher volume to feed the NTR engine and hence higher mass.
Then there is the radiation shield, reentry heat shield, and safety reactor bypass/cooling/containment systems. Probably some extra armor for the reactor too, which I bet will weigh in the low tens of tonnes if the mass of Orion and the reactor are any indicators.
If you add all these up, we're only talking a 50% increase in payload, not a 100% increase. And just how much is this core going to cost compared to the marginal cost for simply building a second Ares-V?
Edit: oh yeah, and high boiloff ullage too!
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Now, I am by no means anti-nuclear! I am anti nuclear launch vehicle, engines like NTR and GCNR are just fine, and in fact will eventually be needed for Mars colonization and missions to the outer planets, but not for launch. The risk is too high versus the benefits. If NASA wants to cut the size of the Ares-V EDS stage to just make it to orbit, and uses a small low-thrust nuclear engine for TLI/TMI, then thats great! ...But the reactor must not be started up before reaching a stable orbit.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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GCNRevenger, thanks for such a detail description of the challenges of using a NTR as an upper stage. Clearly if the cost of containing the significant risks associated is too high, then it would be better to use chemical propulsion to place the NTR into a stable orbit. However that is still an "if", and engineers do like challenges.
So what's the worst case scenario. AFAIK it would be a runway reaction leading to or resulting from a H2 internal explosion inside the core, and then the debris raining down on a populated area. Now such an explosion would have to be contained anyway to ensure crew survival, so a NTR core would be inside a substantial containment vessel. It ought to be possible to design this vessel so any internal explosion would cease the fission reaction and seal both the inlet and outlet. Then of course this vessel must be able to survive reentry and landing. Another solution would be to split the core into a number of smaller units, isolating each element so damage to one doesn't affect the others, then it can abort to orbit or reenter.
NTR could make a useful engine for missions to and from the lunar surface.
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I think that it is quite clear that the large added costs needed to make an NTR-powered launch vehicle "safe enough," both financial and in dead mass along with the other performance and price drawbacks, will simply exceed the marginal cost of launch vehicles. End quote, full stop.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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GCNRevenger, Ciclops, AustinStanley
I was wondering is there any way to make safer an NTR which doesn't reach orbit. I'm thinking inline with context of the US Navy ASAT test, if the NTR gets stuck in a decaying orbit
Then what to do ?
- would you send up a craft with more propellant or an ion drive to push it away from Earth
or could you simply 'blast it' with a kinetic interception like those ASAT tests and make it 'safe' ?
'first steps are not for cheap, think about it...
did China build a great Wall in a day ?' ( Y L R newmars forum member )
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What, are you seriously suggesting launching a missile at an aerial nuclear reactor?????
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I think that it is quite clear that the large added costs needed to make an NTR-powered launch vehicle "safe enough," both financial and in dead mass along with the other performance and price drawbacks, will simply exceed the marginal cost of launch vehicles.
Almost all that cost and "dead" mass will be part of a NTR anyway as the core must be robust enough to withstand reentry or explosion of the chemical stages. A faulty NTR upper stage could reenter after firing, so it will also require a safe and permanent shutdown mechanism. See what happened to Mars 96 after it was placed in a "safe" parking orbit. The additional cost and mass is only that needed to seal the containment vessel.
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There is no such thing as a means to make a reactor stop producing heat and radiation suddenly; it is not that difficult to stop the fission reaction that splits the Uranium, that is not the problem. The problem arises from the radioactive waste produced, which goes right on emitting heat & radiation even if you manage to stop any more Uranium from splitting. Not as much radiation as the running reactor, but enough to create the worst nuclear disaster ever.
This rules out simply blowing the reactor up, as the radioactive waste would be dispersed and massive nuclear fallout would ensue.
Nor will pumping a neutron absorber into the reactor fix anything, as that would also only serve to stop the fission reaction but again does nothing about the radioactive waste.
Because the reactor must operate at such a high power level (likely in the gigawatt range), the reactor itself is going to be quite large and heavy. Building some hypothetical containment vessel will simply make it so heavy that you may as well skip the NTR engine and stick with chemical rockets.
Not that you could build such a containment system anyway.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Because the reactor must operate at such a high power level (likely in the gigawatt range), the reactor itself is going to be quite large and heavy. Building some hypothetical containment vessel will simply make it so heavy that you may as well skip the NTR engine and stick with chemical rockets.
Not that you could build such a containment system anyway.
The core will be inside a pressure vessel and that will have shielding to protect the crew. In turn it will all have to be able to withstand reentry and launch explosion which will help with containment. When hot, even modest shielding should be able to restrict dangerous exposure levels to close to the reactor. In the event of an uncontrolled reentry, the risk would only be in the immediate vicinity of the crash area and manageable. Some exposure numbers would help. 90% of the Earth's surface area is uninhabited.
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I am telling you that the NTR engine is not designed to withstand reentry, nor does it have to be protected from explosion as the core is not radioactive until it has been activated. And who said anything about it carrying a crew?
The pressure vessel is just not going to fix the problem: its going to have multiple openings that you cannot assure will close in the event of core failure. The core and Hydrogen inside will be so hot that isn't anything good to build a vessel & valves out of. The high pressure and temperature in the core will cause it to leak and the contents dispersed.
I don't think you really appreciate the difference in radioactivity between "fresh" nuclear waste and other things like Pu-238; it really is so radioactive that it is on a whole different plain or category of deadly. Coming anywhere near even a small amount of fresh waste will give you a lethal dose instantly, and the consequence of a commercial-sized reactor worth of waste being dispersed really is a Chernobyl-size disaster. Remember, radioactivity decreases exponentially with time respective to an isotopes' half life, but conversely the radioactivity immediately following a fission reaction from short-lived isotopes is truly terrible!
I am telling you man, its just not worth the risk! Even a partial core loss will kill hundreds or thousands of people, and it is a gruesome and painful way to die.
There is just no good way to ensure that the cores' contents can be contained in the event of a failure, the temperatures are too high which will make metals melt, the ultrahot Hydrogen is too corrosive, and the energy involved inside a core - even if you shut it down - is just too great to bottle up. And then this same contraption has to be able to survive the fiery plunge of reentry and impact without breaking open. You can't possibly assert that this will be light weight, reliable, and cheap.
For goodness sakes, just build another rocket!
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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NTRs rely upon heat transfer out of the core material into the coolant gas. To operate an NTR efficiently (ie, with high specific impulse and high power) it needs to operate at very high temperatures. This tends to degrade or partially melt the core material. Basically, engineering an NTR for reuse would mean accepting a lower ISP and a lower thrust-weight ratio. NTR cores will also generate significant decay heat following use and will tend to 'melt-down' when the propellant runs out, so it is a good idea to eject the things into space with all due expedience following use.
Spread of contamination from a used NTR cores would be a real headache if there were an accident over a populated area. They could spread contamination over hundreds of square miles of land. An accident would certainly be expensive and politically damaging.
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Why use NTR? Why not have an Orion upper stage? What's the difference?
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Why use NTR? Why not have an Orion upper stage? What's the difference?
NTR has been tested and it works. Only very simple experiments have been done with pulse propulsion, and none with nukes. An Orion upper stage would be an extremely risky engine to build and enormously expensive, testing would be almost impossible on the ground.
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this design looks good. It seems safe, and has high thrust.
http://www.freerepublic.com/focus/f-chat/1490237/posts
but what's a 'nuclear lightbulb'?
-Josh
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A nuclear "light bulb" GCNR engine would be fantastically expensive to build, even more than a regular GCNR engine. Basically, you have to build a capsule made of IR/vis/UV transparent material that has 25000K+ Uranium Hexafluoride plasma on one side and <10K Hydrogen on the other, without it cracking. You are basically running a tiny, slow-motion atomic bomb inside the "light bulb" and capturing the heat/UV given off with liquid Hydrogen outside the light bulb.
The safety of such a system is a non-starter either, that if the "light bulb" failed it would immediately spew a vast amount of radioactive waste out the back of the engine. I am also a little skeptical of the fuel storage system, the failure of which could also make a mess.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Oh, I see, I thought it had something to do with shape.
I guess you can't trust everything on the internet. Would magnetic cconfinement work for a GCNR?
Magnetic confinement would probably help, increasing the Isp and reducing Uranium loss, but a GCNR engine is still inherently pretty dirty since some Uranium will mix with the propellant and be lost. Also, failure of the magnetic field - which I might add must be very powerful yet also be in close proximity to the hot/radioactive core - would increase the fuel loss significantly.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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