The RL -10 was the first U.S. rocket engine using liquid hydrogen as fuel. Modernized versions of this engine are used in current launchers. The upper stage of the Delta IV and the Centaur upper stage of the Atlas and Titan rockets use the RL -10 engine.
The development of the RL -10 began in 1958 with the appointment of a Centaur upper stage by the military research organization ARPA. The engine manufacturer Pratt & Whitney should it develop a suitable drive. On 1 July 1958, the project was transferred to NASA.
The first tests on the ground began in 1959. On November 7, 1960, a power plant exploded in the test stand in West Palm Beach. The first flight should take place in the second stage of an Atlas - Centaur on May 8, 1962 but the rocket exploded in flight due to an error of the first stage. A successful test flight this configuration took place on 27 November 1963. The engine was also used in the second stage of the Saturn I and completed its maiden flight at the mission SA -5 on 29 January 1964.
The version RL10A -5 was the project Delta Clipper DC-X also called McDonnell Douglas used after the crash and the explosion of the prototype while landing during the fourth test flight, the project was not continued.
The one or two engines of the version RL10A -4-2 are used in the upper stage of the Atlas V Lockheed Martin, and an engine version RL10B -2 in the upper stage of the Delta IV of Boeing.