"a nuclear-powered spaceship" . "nuclear motor" . "NERVA (Nuclear Engine for Rocket Vehicle Application)" . "NERVA/Rover" . . "nuclear thermal third stage" . . "nuclear-powered" . "Nerva" . "nuclear powered rocket engines" . "two 222.5 KN-thrust engines" . . "NERVA" . . . . "Pewee" . "NERVA" . "nuclear" . . . .