German wartime research and NACA wind tunnel studies indicated advantages from sweptback wings in aircraft. Three aircraft were built by Douglas and named D-558-2 'Skyrocket'. Originally all were designed for ground take-off and used mixed power propulsion systems: a turbojet and a rocket engine. In time, only one D-558-2 used a mixed propulsion system. From experience gained during the X-1 rocket program and from Skyrocket mixed propulsion flights, the Navy and the NACA had all three aircraft modified for air launching from a P2B-1 Superfortres (Navy version of the Air Force B-29). Although not designated an 'X (experimental) vehicle' the D-558-2 was esentially an X-vehicle aircraft in design and function, and contributed much to aeronautics research.
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