| A liquid hydrogen/oxygen rocket engine used in the Centaur upper stage and the Saturn I S-IV upper stage.
The RL-10 has the distinction of being part of the first production model liquid hydrogen fueled booster.
The ancestry of the RL-10 goes back to 1946, when the U.S. Navy began investigating using liquid hydrogen as a rocket propellant. Much of the early research studies were conducted by the Jet Propulsion Laboraory under the supervision of Theodore von Karman. Although no flight hardware was developed under this program, it provided useful information to others working on cryogenic engines such as as NACA and the U.S. Air Force, not least with regard to techniques for the large scale production and storage of liquid hydrogen.
After the Atlas rocket entered service, thought was given to adding an upper stage that could boost larger payloads into orbit. In November of 1958, the General Dynamics/Astronautics company recieved the contract to begin building what would be known as the Centaur. The rocket engines were subcontracted to Pratt & Whitney, who began developing a engine known as the LR-115.
In August of 1960, NASA signed a contract with Pratt & Whitney to produce an uprated version of the LR-115, the LR-119, for its Saturn launchers. The orginal intent was to use a cluster of four LR-119s, but development problems forced NASA to rethink. In the end, to compensate for the lower thrust of the LR-115, a cluster of six modified LR-115s, now known as RL-10s, were used. The first flight qualified RL-10 arrived at NASA in June 1962. The RL-10 was used to power
the Saturn I's S-IV stage (The S-IVB stage used in the Saturn IB and Saturn V used one J-2 engine instead of the RL-10 cluster).
Above and beyond its use of liquid hydrogen, the RL-10 featured a number of innovations in its design and construction. Extensive mathematical modelling was used to design the nozzle and bypass a lengthy trial and error testing program. The injector used a new porous mesh design which improved efficiency. It also used liquid hydrogen as a lubricant in the fuel turbopump rather than conventional lubricants.
The RL-10, in a two engine configuration, also became standard on the Centaur. It is still used, albeit with some modifications over time. The current model is the RL-10-A4.
The RL-10 is 1.7 meters high, 1 meter across and the RL-10-A4 can generate 185,000 newtons of thrust. The orginal RL-10 generated 67,000 newtons of thrust. | 
