This is the RL-10, the world's first operational liquid-hydrogen/liquid oxygen high energy rocket engine and was re-startable in space. It was developed and built by the Pratt & Whitney Aircxraft Company. 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, Mariners (Mars, Venus, and Mercury probes), and Pioneers 10-11 to Jupiter and Saturn. This engine was transferred to the Smithsonian in 1974 from the NASA Marshall Space Flight Center.
Transferred from NASA, Marshall Space Flight Center