A
nuclear thermal rocket is a proposed
spacecraft propulsion technology. 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. The thermal heater / inert propellant paradigm as opposed to the reactive propellants of chemical rockets turns out to produce a superior
effective exhaust velocity, and therefore a superior propulsive efficiency, with specific impulses on the order of twice that of 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.