This is the Lunar Module Ascent Engine used to lift up the ascent portion of the Lunar Excursion Module (LEM) after the completion of a lunar landing mission for Project Apollo. The ascent portion of the LEM, carrying the Apollo two astronauts, then docked with the Command Module (CM) orbiting the Moon and flown by the CM Commander.
The Ascent Engine engine produced 3,500 pounds of thrust and was built by the Bell Aerosystems Company. The engine was used successfully in all operational Apollo lunar missions between 1969 and 1972, except for Apollo 13 which did not reach the Moon. This engine was donated to the Smithsonian in 1972 by the Grumman Aerospace Co.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.