An unmanned aircraft is equipped with a helium-cooled Brayton cycle nuclear reactor. Heated helium gas drives turbines which in turn rotate propellers to maintain the aircraft aloft for a protracted period of time. After the helium gas expands in the turbines, it is passed through a closed loop including radiator tubes which radiate waste heat from the helium gas to space. The cooled helium gas is returned through the closed loop for repeat of the Brayton cycle.