Page "Nuclear thermal rocket" Paragraph 2

In a nuclear thermal rocket a working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor, and then expands through a rocket nozzle to create thrust.

In this kind of thermal rocket, the nuclear reactor's energy replaces the chemical energy of the propellant's reactive chemicals in a chemical rocket.

Due to the higher energy density of the nuclear fuel compared to chemical fuels, about 10 < small >< sup > 7 </ sup ></ small > times, the resulting propellant efficiency ( effective exhaust velocity ) of the engine is at least twice as good as chemical engines.

The overall gross lift-off mass of a nuclear rocket is about half that of a chemical rocket, and hence when used as an upper stage it roughly doubles or triples the payload carried to orbit.