First conceived of in 1945, this is a rocket that replaces burning chemical fuel with a nuclear reactor that is used to heat up a propellant. This propellant is most likely hydrogen, though ...

NTP was first theorized in the 1940s. Instead of burning chemical propellant, a rocket would use a nuclear reactor to heat propellant – most likely hydrogen.

No hydrogen-oxygen rocket of appreciable size has flown so far, but for a year Aerojet-General Corp. has been ground-testing hydrogen rocket motors at its Sacramento plant.

But oxygen is typically the heavier component of rocket fuel combinations—typically about 80 percent of the mass—and so, is the bigger challenge to get to a fuel depot.) ...

But as Universe Today also notes, the big problem is guaranteeing that only hydrogen escapes the propulsion nozzle, and not uranium fuel. If uranium does escape, the nuclear reaction slows down ...

During the tests, the fuel was subjected to the maximum heat of a reactor for 20 minutes. That’s 4,220 degree Fahrenheit (2,326°C), roughly equivalent to the heat a nuclear rocket engine would ...