The RL-10 is an upper stage propulsion system that can be stopped and restarted in space. It is a regeneratively cooled engine which burns liquid hydrogen and liquid oxygen to produce 6800 kilograms (15,000 pounds) of thrust. RL-10s pioneered the use of liquid hydrogen as a rocket fuel. They powered the Centaur launch vehicles that boosted such craft as Surveyor and Viking into space. A six-engine cluster of RL-10s was also used to propel the S4 stage of the Saturn 1.

The RL-10 was developed by Pratt & Whitney Aircraft division of the United Aircraft Corporation.

The RL-10 engine is from the National Aeronautics and Space Administration.