Cold gas thrusters (DIY by inflating a balloon and then letting go) operate on the same concept: the energy stored in the compressed gas is released through the nozzle, providing thrust.<p>All other types of thermal rockets simply provide a way for this energy to be replenished, either by external (laser, solar, microwave) or internal (nuclear) sources.<p>The US built nuclear thermal rockets from 1955 through the 1970s. The Soviets experimented with a few for their manned moon missions. Nuclear thermal rockets heat their reaction mass with a nuclear reactor. No nuclear thermal rocket has flown, but NASA keeps tinkering with derivatives of the Rover&#x2F;NERVA programs.