Certainly any power that possesses a mature nuclear technology infrastructure and a reasonable level of sophistication in space technology could initiate research into combining the two.

Certainly China has publicly expressed interest and intent to eventually go to the moon and beyond. Certainly their scientists are probably exploring the notions of creating nuclear engines--the concept is deceptively simple.

The complexity lies in the fabrication of the fuel elements, choosing a geometry for the core which operates efficiently as nuclear reactor and as a heat exchanger with a gas. Enough heat must be extracted from the reactor core so that it does not melt or buckle under the mechanical loadings of high pressure gas streams and vibration induced by flow oscillations in the nozzle throat and high-frequency vibrations from the turbo pumps.

I suspect that China has little experience with this. Furthermore, almost all of China's Long March boosters utilize RFNA (Red Fuming Nitric Acid) and Kerosene (or more probably the Chinese version of American JP-7 jet fuel.) These are storable propellants, which is fine, but the Chinese have limited experience with the cryogenic technologies associated with utilizing liquid hydrogen as a propellant (which is an almost ideal propellant for a nuclear thermal rocket.) This isn't to rule out the Chinese--far from it, my only intent is to demonstrate that their technology will have to be expanded to encompass cryogenics which I feel certain is something they must be working on. Still, it would be foolish to think that they can't do it. They most certainly can--it's just a matter of engineering and testing, testing, testing!

Russia, I believe has also tested nuclear thermal rockets, although to what extent I have little idea. I remember seeing a photograph of a prototype engine, but the engine's designation escapes me. As I recall, it was to have had similar performance to the United States' Nerva engine, something in the 50,000 lb thrust class. If it weren't for their money woes, the Russians might be in the best overall position to develop and exploit NTR engines. Overall, their technological maturity involving integration of cryogenics, flight hardware, turbo pumps and rocket engines is similar to the US level. It would also be foolish to underestimate them. Should they develop the national 'will' they certainly are capable of developing NTR engines.

India will have to greatly expand its launch capability--which it has publicly stated it is doing. It too has stated that it has plans to send people into space--I am sure that they will progress more or less naturally along the lines that the US and Soviets did in the 1960's. Given time, they too could develop NTR, but their present technological level (rocket wise) suggests about 1961-US, but with the Internet and a lot of public domain data to pull from, India could very well close that gap very fast. I'd give them about 10 to 15 years before they attempt an manned orbital flight. Perhaps 20. If they developed the desire to pursue nuclear propulsion, say a flight test in fifteen years to twenty. Its hard to tell, but they must develop a booster that is capable of launching more than 300 kg into low earth orbit. A manned rated booster will have to launch something like 3000 to 5000kg to LEO. And a Nerva test flight would require something like 25000 to 30000 kg to LEO. So they have a ways to go.

I think Israel has orbited some small satellites, but I haven't heard anything like an ambition to develop indigenous manned launch capability--so I'd have to say they are NTR capable--but their a little 'distracted' politically to muster the resources needed for nuclear propulsion technology.

Japan has also expressed its desire to orbit people and go to the moon. I don't believe they have expressed publicly an intent to develop nuclear thermal propulsion, but surely Japan's aerospace engineers are aware of the benefits of such technologies.

Hmmm, I'd have to say I doubt it.

Anything as technologically advanced as a Triton engine will almost certainly fall under the DOD's 'Controlled Technologies List" for export.

But again, do not underestimate the power of the Internet. Public domain, non restricted information sources can, in the right hands, be almost as useful as restricted documents. A lot of information can be extrapolated, and with a nation that has the will to do it, testing can obviously close the gaps. This is precisely how the USSR managed to duplicate many American systems during the cold war--and in a few cases, to actually improve on the American product.

Nuclear technology is virtually public domain knowledge--the specific details may be restricted but any country capable of indigenously manufacture its own nuclear weapons would not have too many obsticals, I would think.

Combining nuclear technology with rockets will take finesse--but the technological maturity needed to do it can be bought by experimentation--provided that there exists a core team of very competent engineers and scientists. I'm quite sure that such teams (containing all the required knowledge) must exist in any country that possesses both an active nuclear weapons/power program, and a ballistic missile program.

Yes, I'd have to agree. I would imagine that the main 'limiting factor' for pebble bed reactors for power plants would just about have to be economics--thus going with fewer but larger fuel 'pebbles' which incidently will also decrease the back pressure of the coolant gas through the reactor core because of the increased void space between the individual pebbles. This should reduce the 'recirculating power' of the power plant (the mechanical power needed to compress the gas and blow it through the core.

Whereas in a nuclear rocket reactor utilizing a much more highly enriched fuel, the limiting factor would almost have to be thermomechanical--limiting the thermal gradient from the interior of the pebble to its exterior so that the inside doesn't melt. This thermal gradient would be a complex function of the thermal conductivity of the ceramic matrix used to make the pebble, the pebble's size, and the volumetric power generated. If the fuel was actually contained with smaller 'fuel grains' then the specific fuel loading (the amount of fissionable material per unit volume) could be tailored so that fuel pebbles near the center of the reactor will have a slightly less enriched fuel than the ones near the boundry of the core. The exact tailoring of the fuel loading in the core would depend on computer modeling, but the desired result would likely be a more or less even tempurature profile across the diameter of the core. This results in less thermal stress which can be a big problem for components, especially ones involved with rockets.

I'd have to agree with Jaro, though: the Chinese could do it. It's just a matter of R&D, and lots of experimentation. Just about every rocket engine development program must go through a process where new engines that are tested will 'blow up.'
High performance rocket engines operate very close to the ultimate strength of materials that a few are bound to be blown to bits during initial testing. Nuclear rocket engines typically don't operate at the extremes of, say a Space Shuttle Main Engine with a chamber pressure of 3500 psi, so mechanical failure of a test engine might not be a problem. It would require the use of a secure ground test facility. With a test facility equipped with scrubbers and electrostatic precipitators, almost no radioactivity would be emitted during a full scale test.

I have been reading up on the gas core nuclear engines, but I have to ask some basic questions.

How are these different then using the pebble base reactors?

I liked the gas core engines because they didn't have any radiation in the exiting thrust - from what I understand. Of course from what I could see no one has built one - yet.

Which one would be easier to build or better to build?

Finally, is there any international law stopping anyone from building and flying a nuclear rocket?

thanks

 

To this, I would just add that the gas core concept, wherein the fissioning fuel is so hot it is actually gasseous, almost hast to require the presence of seperators to prevent fuel leakage. Various schemes for this have been proposed, however it would seem that a gas core or vapor core reactor concept is probably best served in a Nuclear Electric Propulsion mode. This is a closed cycle where fuel and coolant are constantly recirculated, and the heat from the reactor is utilized to produce electricity, either directly through the use of an MHD (magneto hydrodynamic generator) or in a Rankine or Brayton Cycle turbogenerator setup, or some combination of these. This also require the use of a radiator to exhaust waste heat from these processes. The electricity produced would then be utilized in an array of highly efficient ion or VASIMR thrusters. Because of the additional complexity (and weight) of such a propulsion system, this would surely only be used for long duration, deep space missions. These NEP (Nuclear Electric Propulsion) systems are typically low thrust, high specific impulse thrusters, versus an NTR (Nuclear Thermal Rocket) which typically has about twice the specific impulse of high performance chemical propulsion systems with a similarly large thrust.

It has even been proposed that future spacecraft may actually have a combination of different propulsion systems to take advantage of features of both NEP and NTR: NTR with its higher thrust is ideal for planetary escape and capture (to quickly cross planetary Van Allen radiation belts,) while NEP is much better for the slow build of speed necessary for fast hyperbolic interplanetary transfers. Judicious application of both systems on future manned spacecraft could provide more advantages that either could alone.

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Originally posted by: Ashley
"If I remember right, the NSWR rocket used that concept.  Subcritical amounts of uranium salts mixed with water for storage and fuel.  The stuff was then brought to a critical state in the reaction chamber, (or just outside it to lower thermal stress) and allowed to react.  (This method is only theoretical, having had no testing at all) "

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I disagree. Reactors comprizing aquous solutions of uranium salts have been tested numerous times. Also, several accidents involving supercritical quantities of aquous solutions of uranium salts have occured in the past, the last one having been the Tokaimura fuel fabrication plant accident in Japan, about six years ago.

It is true however that the specific concept called the NSWR hasn't been tested. It proposes what is essentially a continuous supercritical operation, which can best be described as a contradiction in terms -- you either have continuous critical operation, or a supercritical power burst. But you can't have it both ways. The concept is a dud.

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Originally posted by: larry
"How small could you build a NTR?"

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If we're talking only about the reactor part of an NTR, the answer is "very small."

The main limiting factor is adequate volume for propellant passageways through the reactor core, in order to effectively transfer heat from the solid fuel (be it pebbles or prismatic fuel blocks, rods, or what have you). Beyond that, its simply the minimum critical mass & volume. This is pretty small, for both fast neutron and moderated reactors, on the order of a few dozen pounds. A working NTR reactor can thus fit into a volume about the size of a large water mellon. Obviously, you wouldn't be able to extract as much heat out of something that small, as you would from a reactor the size of, say, a domestic hot water tank, without destroying it in short order. But it might be adequate for a low-thrust lunar cargo tug, or perhaps a small unmanned lunar cargo shuttle :