High Quality Content by WIKIPEDIA articles! The RL10 is a liquid-fuel cryogenic rocket engine used on the Centaur, S-IV and DCSS upper stages. Built in the United States of America by Pratt & Whitney Rocketdyne, the RL10 burns cryogenic liquid hydrogen & liquid oxygen propellants, with each engine producing 64.7–110 kN (14,545–24,729 lbf) of thrust in vacuum depending on the version in use. The RL10 was the first liquid hydrogen rocket engine to be built in the United States, and development of the engine by Marshall Space Flight Center and Pratt & Whitney began in the 1950s, with the first flight occurring in 1961. Several versions of the engine have been flown, with two, the RL10A-4-2 and the RL10B-2, still being produced and flown on the Atlas V and Delta IV.