One offshoot of the nuclear bomber program VelociraptorBlade mentions below was a nuclear powered rocket, which was successfully tested several times in the 60s. A coolant (liquid hydrogen) was flushed through a
compact fission reactor and expelled as a propellant for thrust. It was very efficient, yielding specific velocities about twice as high as liquid hydrogen-oxygen combustion. Except for the weight of the reactor, it was our most promising form of rocket propulsion at the time.

Fortunately, the inherent contamination dangers of this technology eventually led to its abandonment. Still, I can see this power plant design being used as a second stage for sending big payloads on manned planetary missions, where potential radioactive exhaust or accident debris would pose no threat to Earth's biosphere. The engine could be assembled and fueled in low earth orbit and the effects of an accident would be minimized. (Except for the crew!).