The RL-10 was the world's first operational liquid hydrogen/liquid oxygen high energy rocket engine and was restartable in space. Two RL-10 engines, each producing 15,000 pounds of thrust made up the Centaur upper stage used with the Atlas and Titan launch vehicles. A cluster of six RL-10 engines also powered the second stage of the Saturn 1 vehicle.
The first successful flight of the Atlas-Centaur took place on October 26, 1966, and was the first time the RL-10 made a full-thrust re-start in space. The Titan-Centaur made its first successful flight on December 10, 1974, and placed the Helios solar probe into an accurate solar orbit. The first Saturn 1 launch with RL-10s took place on January 29, 1964 and lifted a test payload into space, then on February 16, 1965 placed a Pegasus satellite into orbit.
The NASA - Marshall Space Flight Center transferred this RL-10 to the Smithsonian Institution in 1975.
Transferred from the NASA - Marshall Space Flight Center