Republic XF-91 Thunderceptor

The Republic XF-91 Thunderceptor (originally designated XP-91) is a mixed-propulsion prototype interceptor aircraft, developed by Republic Aviation. The aircraft would use a jet engine for most flight, and a cluster of four small rocket engines for added thrust during climb and interception. The design was largely obsolete by the time it was completed due to the rapidly increasing performance of contemporary jet engines, and only two prototypes were built. One of these was the first American fighter to exceed Mach 1 in level flight.

A unique feature of the Thunderceptor was its unusual inverse tapered wing, in which the chord length increased along the wing span from the root to the tip, the opposite of conventional swept wing designs. This was an attempt to address the problem of pitch-up, a potentially deadly phenomenon that plagued early high-speed models. The Thunderceptor's design meant the entire wing stalled smoothly, more like a straight-wing design.