After World War 2 and during the early Cold War years, the American government now faced the Soviet Union and threat of nuclear-armed, high-altitude bombers attacking the United States. In response, the United States Army Air Forces (USAAF) commissioned for an interceptor design in August of 1945 with supersonic speeds at high-altitude and an impressive rate-of-climb to meet the incoming hordes of bombers with 4 x 0.50 caliber heavy machine guns. As turbojets were still in their infancy, rocket propulsion was the call of the day. No fewer than six competing concerns entered their designs into the fray including Bell Aircraft, Consolidated Vultee, Douglas, North American, Northrop, and Republic. In the end, none were chosen with Republic's venture - the XF-91 "Thunderceptor" - slipping into the pages of military aviation history among the others.
The Republic Aviation concern was well recognized for their P-47 Thunderbolt fighter effort of World War 2 and came to develop the capable F-84 Thunderjet of the early jet age. It was the F-84 that served as the basis for the Thunderceptor though modified so heavily that the two shared little in the way of interchangeable components - the XF-91 becoming its own, all-new fighter design. Design work on the aircraft was headed by Alexander Kartveli who also lent his talents to the design of the famous P-47.
In its early form, a swept-wing mockup was presented with a Vee-style tail unit though this was not utilized in the full-scale flyable prototype seen later. A Curtiss-Wright XLR27 series rocket motor was selected to power the airframe and work began shortly after the USAAF requirement was put forth. The flyable prototype, seen with a conventional tail unit, achieved first flight on May 9th, 1949 from Edwards Air Force Base. By this time, however, Reaction Motors rockets had replaced the intended Curtiss-Wright propulsion system. The original 4 x machine gun armament requirement was also changed to a 4 x 20mm cannon fitting. A second prototype joined the first and this one differed in that it utilized the original Vee-tail unit.
The XF-91 proved something of an interesting take on the interceptor requirement for it coupled 4 x Reaction Motors XLR-11-RM-9 rocket motors to 1 x General Electric J47-GE-3 series turbojet. Both were installed at the tail section. In this arrangement, the aircraft was initially powered by the four rocket motors from take-off to reaching the intended interception altitude. After meeting its targets and completing its mission, the aircraft then landed under the power of its turbojet engine for the rockets used up their fuel in short order, leaving the aircraft essentially unpowered with its turbojet installation on hand. With the reserve propulsion system, the pilot could then guide his aircraft back down with the thrust and control necessary. Another interesting design quality was use of a variable-incidence, inverse tapered-wing design which pivoted from +6 to -2 degrees during flight as required.
Despite the promising nature of the Republic offering, the USAAF decided early on not to pursue the XF-91 as a production interceptor. Instead, it was allowed to live under the guise of research plane thanks in large part to its other engineering initiatives. The two prototypes, therefore, served as flight research testbeds furthering other Air Force programs and some future designs. The first prototype went on to achieve supersonic flight in December of 1952 and proved several facets of its overall arrangement quite sound and was eventually used to test radar under the designation of XF-91A, its nose now covered in a radome fitting (the air intake fitted further under and aft of the nosecone). The second continued tests with its Vee-tail unit in place until both prototypes were given up for good. The first prototype (s/n 46-0680) ended as a museum showpiece at the National Museum of the United States Air Force (Dayton, Ohio) while the second prototype (s/n 46-0681) was lost during testing in 1951 at Edwards AFB.
As completed, the prototypes allowed for a maximum speed reaching ,126 miles per hour with a service ceiling in the 50,000 feet range. Seating was for one under a largely unobstructed canopy and set well-ahead of midships. The turbojet engine was aspirated by a small nose intake with the rocket motors buried within the tail section itself. The main wing assemblies were mid-mounted at the fuselage sides and featured 35-degree sweepback as well as tapering from wingtip-to-wingroot as opposed to wingroot-to-wingtip. This gave the aircraft a unique planform when viewed from the top-down profile. The tail unit showcased a single vertical tail-fin with mid-mounted horizontal planes. In all, the XF-91 was a very clean design of the period. The tricycle undercarriage was expectedly retractable - though its main landing gear legs sported a pair of inline wheels as opposed to a more traditional side-by-side pairing.
Armament was to be 4 x 20mm cannons - mostly likely fitted in the nose section.
(OPERATORS list includes past, present, and future operators when applicable)
Ability to intercept inbound aerial threats by way of high-performance, typically speed and rate-of-climb.
✓X-Plane (Developmental, Prototype, Technology Demonstrator)
Aircraft developed for the role of prototyping, technology demonstration, or research / data collection.
31.2 ft (9.52 m)
43.2 ft (13.18 m)
18.1 ft (5.51 m)
14,132 lb (6,410 kg)
28,307 lb (12,840 kg)
+14,176 lb (+6,430 kg)
(Showcased structural values pertain to the base Republic XF-91 Thunderceptor production variant)
4 x Reaction Motors XLR11-RM-9 rocket motors developing 1,500 lb of thrust each; 1 x General Electric J47-GE-7/-17 turbojet engine developing 6,900 lb of thrust with afterburner.
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