This is the so-called Interim Engine of the X-15 rocket research aircraft and is made up of two joined XLR-11 rocket engines. The XLR-11 was the standard engine for the earlier Bell X-1 and other rocket research aircraft. The Interim powerplant was used in the X-15 from 1959-1961 due to developmental delays of that aircraft's newer and more powerful XLR-99 Pioneer rocket engine. The Interim engines achieved the X-15's first 29 powered flights. The Interim engine produced a total of 16,400 pounds from its four chambers and was sufficient for the preliminary test flights of the X-15. It achieved some notable records in that phase of the X-15 program. This Interim Engine was transferred to the Smithsonian in 1963 from the U.S. Air Force.