The Mystery Airplane
The Republic XF-91 falls into the category of mixed powered point defense interceptors that followed World War II inspired by German experiments with the rocket powered Messerschmitt Me-163B Komet and Bachem Natter. Designed to a December 1945 Army Air Force requirement for a bomber interceptor that would be supersonic and could climb to 47,500 feet in two and a half minutes, Republic's AP-31 design was ordered on 29 March 1946. It was to join the French Sud-Quest Trident and British Saunders-Roe SR. 53 as mixed turbojet and rocket powered interceptors that became aeronautical dead ends.
Two XF-91s were ordered, serial numbers 46-680 and 46-681. Designed with a unique reverse taper wing that was narrower at the root than at the tip and with the ability to vary the wing's angle of incidence, the first prototype was flown on 9 May 1949 by Carl Bellinger at Muroc. The second aircraft followed on 14 October 1949. Both were flown initially with just the General Electric J47. It was not until 1952 that the four XLR-11 rocket motors were available. Unfortunately, these rockets were not known for reliability, often exploding without warning. On 9 December 1952, at Edwards AFB, Russell Roth made the first supersonic flight by an American combat aircraft reaching Mach 1.18 on combined turbojet and rocket power.
One downfall of the Thunderceptor was its short flight endurance, only 25 minutes. Flight tests also showed its intended top speed of Mach 1.4 was impossible as the airframe would have ben shaken to pieces. Meant to be armed with four 20 mm cannon, development was halted in favor of the program that produced the missile armed Convair F-102 Delta Dagger. The surviving XF-91, 46-680 was delivered to the Air Force Museum in May 1955 for display. Needless to say, the Thunderceptor made Bill Yenne's and Jim Winchester's books of "World's Worst Aircraft."