Nuclear Thermal Rocket (NTR) is a rocket engine that superheats a propellant with the energy of nuclear fission and ejects it to create thrust. Not limited by the energy of chemical reactions, NTRs can use a variety of gaseous propellants, and although heavier gases (like nitrogen, neon, argon) can be used for more thrust, the most common propellant hydrogen as enables the highest exhaust velocity (
efficiency) by being super light.
The
efficiency of NTRs is also directly proportional to core temperature, however, there is a limit on how much heat a solid core can endure before it melts. Which is a bad thing, the molten remains of the reactor will shoot out the exhaust bell like a radioactive bat from hell, killing anybody nearby and leaving the spacecraft without an engine.
Big-brained rocket engineers thought, "What if the core was molten in the first place?" This led to the design of liquid core NTRs, and even open-cycle gas cores (GCNTR). A gas core NTR provides outrageous temperatures and the
ultimate efficiency (reaching up to 34 km/s of exhaust velocity and runs over 20,000K), but radioactive fission products will escape with the exhaust, producing a radioactive
stream of death. There are solutions to this problem, but the efficiency will be cut in half.
Undoubtedly, NTRs have the potential to make
interplanetary travel a lot less miserable, but stigmatization against nuclear power had held it back. For Sauce, it's the second engine list on Atomic Rockets.
Bill: "Have you seen the latest work of that sci-fi artist?"
Jeb: "
Of course! His NTR series looks so cool and realistic, I really wish to be one of the crews on board. Space,
the final frontier..."
Bill: "Same!
On the bright side, at least we still have the designs for NERVA."