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Nuclear thermal rockets (NTRs) are notable as the most mature technology (the second-most-mature is Project Orion) to provide both high thrust and higher specific impulse than chemical rockets.

But "high thrust" doesn't mean the same as "high enough thrust for planetary takeoff", and most NTR designs have been intended for use as upper stages, orbit-only engines, or sometimes for planetary takeoff and landing on low-gravity bodies.

For those of us who are perpetually frustrated by the failure of the world's space programs to take advantage of nuclear power, there is a tantalizing promise of much higher thrust-to-weight ratio nuclear thermal rockets such as the DUMBO design that was developed alongside NERVA, and the military-funded TIMBERWIND project during the "Star Wars" era. While these still may not be enough to make the dream of a nuclear SSTO launcher or shuttle practical, they seem worthy of attention... if it is actually probable for them to develop the levels of performance claimed.

Various optimists have suggested that it should not be technologically difficult to build nuclear thermal rockets with T/W well over 20, which would make a variety of planetary takeoff options viable, and perhaps even make the nuclear SSTO feasible. This optimism has sometimes even been extended to closed-cycle gas-core NTRs. Other trustworthy figures have looked with intense skepticism on these hopes and suggested that in the absence of an actual demonstrator, things like the claimed T/W of over 30 for DUMBO should be considered mere fantasy.

Is it technologically realistic that, if there was a real will to produce high-thrust NTRs, that they could be built? Are there fundamental problems limiting how fast one can transfer heat out of a nuclear reactor?

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    $\begingroup$ I think your "without massive further development" clause kinda answers your own question for you. The former has never been built, let alone tested, and the latter seems like little more than a nice idea at this point. I don't see anyone building and testing either on Earth any time soon... even nuclear reactors and unpopular, so the political will for more expensive and dangerous projects filled with unknowns is probably never going to materialise. $\endgroup$ – Starfish Prime Dec 6 '19 at 11:42
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    $\begingroup$ Nuclear thermal rockets ironically have a limitation that chemical rockets don't have. In a chemical rocket, the energy to push the reaction mass is already part of it. In a NTR, the energy comes from nuclear fuel and the reaction mass is separate. We want to contain the fuel and expel the propellant. To get the energy into the propellant it has to get through the reactor wall by conduction. So the temperature of the propellent can only be as high as the reactor wall. We have no material that can stand enough heat to exploit the vast potential of nuclear fuel's power density. $\endgroup$ – Johnny Robinson Dec 8 '19 at 22:54
  • $\begingroup$ @Johnny Robinson yes, but that already achieves typical NTR performance, and you have things like particle bed reactors. And then there are concepts like the Nuclear Lightbulb. $\endgroup$ – ikrase Dec 8 '19 at 23:02
  • $\begingroup$ I'm aware of these exotic ideas like the gas core nuclear thermal rocket with quartz reactor walls. The idea to get around conduction as a means to heat the propellant. (I ran out of my character count to go into any of that.) Ideas for fusion rockets that stay in orbit, these go ahead & mix the fuel with the propellent. This includes magnetic bottle plasma reactors and laser micro implosion pulse drive. But all of that is so far away. I'm betting on nuclear electric rockets in my lifetime. If Space-X can lift 100 tons for $2 million, NTR lift vehicles may be moot. $\endgroup$ – Johnny Robinson Dec 8 '19 at 23:28

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