RL10

The RL10 is a liquid-fuel cryogenic rocket engine built in the United States by Aerojet Rocketdyne that burns cryogenic liquid hydrogen and liquid oxygen propellants. Modern versions produce up to 110 kN (24,700 lbf) of thrust per engine in vacuum. RL10 versions were produced for the Centaur upper stage of the Atlas V and the DCSS of the Delta IV. More versions are in development or in use for the Space Launch System and the Centaur V of the Vulcan rocket. The expander cycle that the engine uses drives the turbopump with waste heat absorbed by the engine combustion chamber, throat, and nozzle. This, combined with the hydrogen fuel, leads to very high efficiency, with in-service variants achieving specific impulses (Isp) of up to 465.5 s (4.565 km/s) in a vacuum. Mass ranges from 131 to 317 kg (289–699 lb) depending on the version of the engine. The RL10 was the first liquid hydrogen rocket engine to be built in the United States, with development of the engine by Marshall Space Flight Center and Pratt & Whitney beginning in the 1950s. The RL10 was originally developed as a throttleable engine for the USAF Lunex lunar lander. The engine was electric spark ignited.