Nuclear Thermal Rockets have been studied extensively over the last 65 years, with hard engineering experiments from about 1955-1970 in the USA and Russian work a bit later. A joint NASA/DoD/DoE Workshop in 1990 summarised a lot of know-how to that date. High-performance reactor cores have improved somewhat since then, but no terrestrial application proposes running them as hard as an NTR between 2500-3500 K.
Here’s some tabulated comparative data for further thought.
Space-Tug application using Chemical Rockets, Fission Rockets and futuristic Fusion Rockets – the latter having essentially a Technology Readiness Level (TRL) of 0.
Notice the mass ratio comparisons of the “tankage” (which includes pressurisation systems and tank meteoroid shielding) and the bare tank-to-propellant. Just liquid hydrogen, seen for the nuclear systems, requires heavier tankage due to insulation etc.
Another set of comparisons for similar systems. Notice that the tankage fractions imply a maximum mass-ratio for a single stage system. Getting more than 8 is virtually impossible given present engineering limitations. Throw-away tanks are probably needed to even attain that. Staging gets higher delta-vees of course, but implies throw-away reactor cores.
The Pebble Bed NTR is one of the relatively new reactor concepts. A technical issue, discussed at the workshop, is the migration of fission fuel through the protective cladding at high operating temperatures. In the 30 years since that issue might’ve been solved. Would be interested in hearing from anyone in the know.
Finally a comparison of the different NERVA derivatives that were feasible with 1990 know-how. There were several advanced concepts discussed by the Workshop – Liquid Core, Vapour Core and Gas/Plasma Core reactors – but they’re all low TRL systems. The other advanced Solid-Core concepts discussed were DUMBO (which has material challenges of its own) and the Low Pressure NTR, which promises a higher Isp by dissociating molecular hydrogen into atomic hydrogen.
Workshop is available on the NASA NTRS:
Nuclear Thermal Propulsion: A Joint NASA/DOE/DOD Workshop