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. JATO Units [Illustration: 53. JATO-boosted Martin Mariner aircraft.] JATO (Jet Assisted Take-Off) rockets boost heavy aircraft from short runways or from high-altitude airports where long take-off runs are required. The development of more powerful airplane engines has reduced the use of JATOs in recent years. The first American JATO units were tested at March Field, California, on August 12, 1941. Six solid-propellant engines, each developing 12.8 kilograms (28 pounds) of thrust, boosted a light plane piloted by Capt. Homer Boushey into the air on this occasion. These motors were designed and built by staff members of the Air Corps Jet Propulsion Research Project of the Guggenheim Aeronautical Laboratory of the California Institute of Technology. During World War II, work continued on JATO prototypes: the M17G was developed by Reaction Motors, Inc., to provide 590 kilograms (1300 pounds) of thrust to assist the take-off of PBM flying boats; the M19G, also built by Reaction Motors, Inc., was fueled by gasoline with liquid oxygen as an oxidizer. The liquid-propellant 25ALD1000, developed during World War II. produced 453 kilograms (1000 pounds) of thrust and burned red-fuming nitric acid as an oxidizer and aniline as a fuel. It was successfully used on a variety of aircraft, including the B-24, B-25, C-40, and P-38. After the war ended, JATO engines were used on military aircraft such as the B-47 and F-84 in the United States, while in Britain the JATO Super Sprite became the first rocket engine to receive official type approval for quantity production. The first U.S. JATO unit and the 25ALD1000 are gifts of the Aerojet General Division of the General Tire and Rubber Company. The M17G and M19G JATOs are from the Thiokol Chemical Corporation, and Rolls Royce, Ltd., provided the Super Sprite. LR-87 Engine [Illustration: 54. LR-87 engine in Titan on launch stand.]