This is the RL-10, the world's first operational liquid-hydrogen/liquid oxygen high energy rocket engine and was re-startable in space. Two RL-10s, each of 15,000 pounds of thrust, made up the Centaur upper stage of Atlas and Titan launch vehicles. A cluster of six RL-10s also powered the second stage of the Saturn 1, a precursor to the Saturn V manned Project Apollo lunar launch vehicle.
The Atlas-Centaur first successfully flew in 1966 and was the first full-thrust re-start in space. Atlas-Centaur missions included Surveyor lunar probes, Mariner planetry probes, and Pioneers 10-11 to Jupiter and Saturn. This RL-10 was transffered in 1966 by NASA to the NASM.
Transferred from NASA, Marshall Space Flight Center