Nuclear thermal rockets are indeed a great idea. However, keep this in mind: each pound of thrust requires about 20 kilowatts (thermal) of power. To equal the SSME, that works out to 9400 megawatts. That is a BIG reactor. SSME has a thrust-to-weight of about 70:1. The best nuclear thermal rocket has a thrust-to-weight of possibly 30 (assuming values given in this article).
Nuclear thermal rockets are thus not useful (or politically possible) as boosters launching from the ground.
A space engine, with a thrust of 20000 to 50000 pounds, is a better match.
I once read a wonderful article in 1969, entitled, To Mars and Back in 30 Days by Gas-Core Nuclear Rocket. Never forgot it. But gas-core nuclear rockets are way in the future--if ever.
--Boris