Republic XF-91 Thunderceptor

Last revised November 21, 1999

The Republic XF-91 Thunderceptor was an experimental mixed-power interceptor, being powered by both a jet engine and by a battery of rocket motors. It was Republic's first swept-wing design.

The project began in 1946 when Alexander Kartveli and his team at Republic Aviation began to explore the possibility of using rocket engines to power aircraft. The Messerschmitt Me 163 Komet of World War 2 had demonstrated that truly spectacular performance could be attained with rocket propulsion, although at the expense of an extremely short endurance and a very low combat radius. Republic proposed to overcome this inherent disadvantage of rockets by using a conventional jet engine for cruising flight, with rocket engines being used only for fast takeoff and climb to combat altitude and for extra boost during combat emergencies.

In 1948, Republic submitted its ideas to the USAF. The Republic proposal was to be powered by a single 5200 lb.s.t. General Electric J47-GE-3 turbojet engine, augmented by a Curtiss-Wright XLR27 four-chamber rocket motor rated at a total thrust of 13,000 pounds.Two rocket chambers were to be located above the jet engine exhaust,two below.

The fuselage had some resemblance to the F-84 Thunderjet, with some parts actually being in common. However, an entirely new wing was to be fitted. The cantilever, mid-mounted wing was swept back at an angle of 35 degrees and had a variable incidence. This meant that the wing could be pivoted around its attachment point to the fuselage, allowing the angle of incidence to be adjusted by the pilot for the most effective angle during takeoff, cruise, and landing. The angle of incidence could be varied between -2 and +6 degrees. In contrast to the usual practice, the swept wing was thicker and wider at the tip than it was at the roots. It was hoped that this unusual wing configuration would provide greater lift outboard and would reduce the tendency of the wingtips to stall at low speeds. In addition, it was expected that this innovation would reduce the amount of aerodynamic drag at the critical wing/fuselage junction.

The retractable landing gear consisted of a single nosewheel plus a pair of main landing gear members attached to the wings. The nosewheel was fairly conventional, and retracted forward into the forward part of the nose. The main landing gear was anything but conventional. Each of the main landing gear members consisted of a pair of bogie-type wheels mounted in tandem. Since the wing roots were too thin to accommodate the main landing gear wheels when retracted, the landing gear retracted outwards, the wheels being stowed inside the thickened wingtips. This configuration permitted the XF-91 to carry external stores or drop tanks on pylons inboard of each undercarriage unit.

The design featured a ventral-mounted airbrake, and was to be equipped with a braking parachute to shorten the landing run. A pressurized, air-conditioned cockpit was provided, and the cockpit canopy was similar to that of the F-84.

The USAF was sufficiently impressed that they ordered two examples of this design under the designation XF-91. Serials were 46-680 and 46-681.

Unfortunately, the Curtiss-Wright rocket motor did not perform properly during tests. Rather than delay the entire program, Republic decided to switch to the lower-powered but tried and true Reaction Motors XLR11 four-chambered rocket motor rated at a total thrust of 6000 pounds. This rocket engine was the powerplant of the X-1 and was considered highly reliable. The individual thrust chambers of the Curtiss-Wright engine were originally to have been positioned in vertical pairs in fairings above and below the exhaust, but with the switch to the XLR11, the lower fairing was enlarged so that it could accommodate all four chambers in a diamond pattern.

The first prototype rolled out of the factory on February 24, 1949. The first prototype (46-680) took to the air for the first time at Edwards AFB on May 9, 1949, Carl Bellinger being the pilot. The first flights were powered by the turbojet alone. Later that year, flights were made with an afterburner and then later with the rocket engines installed. In December 1951, the XF-91 became the first American combat aircraft to go supersonic in level flight. When all five powerplants were running, the XF-91 coulld achieve a maximum speed of 1126 mph (Mach 1.71). Not bad for the early 1950s!

Had the more powerful Curtiss-Wright engine been available, theXF-91 could probably have achieved Mach 2 performance.

The XF-91 showed signs of having the pottential of developing into anexcellent warplane. However, the Korean War had demonstrated that Mach 1 could be exceeded by aircraft that were considerably less sophisticated than the XF-91, and this extremely advanced warplane never saw production. Tests continued with the two prototypes. One was flown experimentally with a V-type butterfly tail, and the first XF-91 was refitted with a nose radome housing an APS-6 radar installation in the same manner as the F-86D Sabre.

The XLR11 rocket engine was very reliable and never suffered an inflight failure. On one test flight, it proved its worth when the J47 jet engine flamed out. Unable to restart the jet engine, the pilot fired the rocket motors and was able to reach Edwards AFB and land successfully.

The second prototype was eventually destroyed, but the first XF-91 (46-680) is currently on display at the Wright Patterson Air Force Base Museum.

Specification of the XF-91:

Engines: One General Electric J47-GE-3 axial-flow turbojet, 5200 plus four Reaction Motors XLR11-RM-9 rocket motors rated at 1500 each. Dimensions: wingspan 31 feet 3 inches, length 43 feet 3 inches, height 18 feet 1 inches, wing area 320 square feet. Weights: 14,140 pounds empty, 18,600 pounds combat, 28,300 pounds maximum. Performance: Maximum speed: 1126 mph at altitude (Mach 1.71), 984 mph at 47,500 feet. Climb to 47,500 feet in 2.5 minutes, service ceiling 50,000 feet, normal range 1175 miles. The planned armament of four 20-mm cannon was never installed.


  1. Fighters of the United States Air Force, Robert F. Dorr and David Donald, Temple Press Aerospace, 1990.

  2. The Thunder Factory-An Illustrated History of the Republic Aviation Corporation, Joshua Stoff, Motorbooks International, 1990.

  3. The American Fighter, Enzo Angelucci and Peter Bowers, Orion, 1987.