Republic XF-91 Thunderceptor

The Republic XF-91 Thunderceptor, first designated XP-91, was a mixed-propulsion prototype interceptor developed by Republic Aviation. A jet engine provided power for most of a flight and a cluster of four small rocket engines added thrust for the climb and interception. Its most unusual feature was an inverse tapered wing, whose chord grew from root to tip, the opposite of a conventional swept wing, which was an attempt to cure pitch-up and let the whole wing stall smoothly. The first of two prototypes flew on 9 May 1949 and broke the speed of sound in December 1951, making it the first American fighter to exceed Mach 1 in level flight, and with jet and rockets together it reached Mach 1.71. The design was largely obsolete by the time it was finished, and only two were built.