This is the Space Shuttle Main Engine (SSME). Three SSME's plus two Solid Rocket Boosters (SRBs) power the reusable Space Shuttle. Each SSME produces 375,000 lbs of thrust or a total of 1,125,000 lbs and uses liquid oxygen and liquid hydrogen as propellants.
This SSME is made of up of components of SSMEs that have flown into space. The flights have included the first four Shuttle missions, the second Hubble Space Telescope repair mission, the missions that launched the Magellan and Galileo space probes, and the John Glenn flight. The engine was donated by Rocketdyne to the Smithsonian in 2004.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.