Rocket Engine, Liquid Fuel, RL-10

Shown here is the RL-10 rocket engine. Designed and built by the Pratt and Whitney Aircraft Division of the United Aircraft Corporation, the RL-10 was the world's first operational liquid hydrogen/liquid oxygen, restartable rocket engine. Two RL-10 engines, each capable of producing 15,000 pounds of thrust, powered the Centaur upper stage carried atop the Atlas and Titan launch vehicles. The National Aeronautics and Space Administration (NASA) launched the first Atlas-Centaur rocket in 1966, during which the RL-10 successfully demonstrated a full-thrust re-start in space.

A cluster of six RL-10 engines also powered the second stage of the Saturn I launch vehicle, predecessor of the giant Saturn V rocket that carried astronauts to the Moon between 1969 and 1972.