This is the General Electric X-405 rocket engine for the first stage of the Vanguard launch vehicle. Vanguard was the first U.S. rocket specifically designed to launch a satellite. The X-405 produced 27,000 pounds of thrust. The thrust chamber was gimballed to provide thrust vector control.
Oon 17 March 1958, a Vanguard rocket successfully placed Vanguard 1 into orbit, America's second satellite after Explorer 1 that was successfully launched with a modified Restone rocket on 31 January 1958. Vanguard 2 was placed into orbit on 17 February 1959 and Vanguard 3 was launched 18 September 1959. This engine was donated to the Smithsonian in 1976 by the Wright-Malta Corp.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.