The XLR-11 rocket engine, developed by Reaction Motors, Inc. (RMI), was initially used in the Bell X-1 rocket research aircraft, the first plane to reach the speed of sound, in 1947. Each of the four chambers produced 1,500 pounds of thrust. They were fired separately or in groups for a maximum thrust of 6,000 pounds.
Besides the X-1, the engine was used in the X-1A, X-1B, X1D, X-1E, Douglas D-558-2 Skyrocket, and XF-91. Two XLR-11s also served as the "Interim Engine" for the X-15 research aircraft, while its larger XLR-99 engine was completing its development. This XLR-11 is one of two used in the first test flight of the X-15 on 17 September 1959. The engine was transferred to the NASM in 1963 by the U.S. Air Force.