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Wikipedia dicit:

The RL10 is a liquid-fuel cryogenic rocket engine used on the Centaur, S-IV, and Delta Cryogenic Second Stage upper stages. Built in the United States by Aerojet Rocketdyne (formerly by Pratt & Whitney Rocketdyne), the RL10 burns cryogenic liquid hydrogen and liquid oxygen propellants, with each engine producing 64.7 to 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 three, the RL10A-4-2, the RL10B-2, and the RL10C-1 still being produced and flown on the Atlas V and Delta IV.

The engine produces a specific impulse (Isp) of 373 to 470 s (3.66–4.61 km/s) in a vacuum and has a mass ranging from 131 to 317 kg (289–699 lb) (depending on version). Six RL10A-3 engines were used in the S-IV second stage of the Saturn I rocket, one or two RL10 engines are used in the Centaur upper stages of Atlas and Titan rockets, and one RL10B-2 is used in the upper stage of Delta IV rockets.

Video Credit: Aerojet Rocketdyne