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."