Republic XF-91 Thunderceptor

History of the Republic XF-91 Thunderceptor

After World War II and early in the Cold War, the U.S. government now faces the threat of Soviet and nuclear-armed high-altitude bombers attacking the United States. In response, in August 1945, the United States Army Air Forces (USAAF) sent an interceptor to meet the bomber fleet armed with 4 x .50 caliber heavy machine guns at supersonic speed and an impressive rate of climb at high altitude.

With turbojets still in their infancy, rocket propulsion was the mainstream at the time. No fewer than six competing companies submitted their designs to the competition, including Bell Aircraft, United Vultee, Douglas, North America, Northrop and Republic.

In the end, no one was selected, and the Republic's project -- the XF-91 Thunder -- came to an end in the history of military aviation.

Republic Aviation was best known for its P-47 Thunderbolt fighter jets during World War II and developed the powerful F-84 Thunderjet of the early jet era. The F-84 was the basis for the Thunder fighter, although it was modified so much that there were few interchangeable parts between the twothe XF-91 itself became an entirely new fighter design.

The design of the aircraft was led by Alexander Kartveli, who also lent his talents to the design of the famous P-47.

In its early form, a swept-wing model was shown with a V-tail, although this was not used in the full-scale, flyable prototype seen later. Curtiss-Wright XLR27 series rocket engines were selected to power the airframe, and work began shortly after the USAF requested it. This airworthy prototype with a conventional tail fin made its first flight from Edwards Air Force Base on May 9, 1949. By this time, however, the Reaction Motors rocket had replaced the expected Curtis-Wright propulsion system. The original 4 x machine gun weapon requirement has also been changed to a 4 x 20mm artillery weapon.

The second prototype joins the first, except it uses the original Vee-Tail unit.

The XF-91 proved to be an interesting solution for interceptor requirements, as it combined 4 Reaction Motors XLR-11 RM-9 rocket engines and 1 General Electric J47-GE-3 turbojet. Both are mounted on the rear. With this arrangement, the aircraft is propelled by four rocket engines from takeoff until it reaches the desired intercept altitude. After achieving the goal and completing the mission, the plane lands using turbojet power, as the rocket runs out of fuel in a short period of time, leaving the plane essentially unpowered, with the turbojet on hand.

With the backup power system, the pilot can steer his aircraft back with the necessary thrust and control. Another interesting design quality is the use of an inverted tapered variable incidence wing design that rotates from +6 degrees to -2 degrees as needed during flight.

Despite promising proposals from the Republic, the USAF decided early on not to use the XF-91 as a serial interceptor. Instead, thanks to its other engineering initiatives, it was allowed to live in the name of research aircraft. As such, the two prototypes were used as flight research test beds to further advance other Air Force programs and some future designs.

The first prototype achieved supersonic flight in December 1952 and proved to be very robust in several aspects of its overall assembly, eventually being used to test the radar, codenamed XF-91A, whose nose is now covered with radome ports (in Air port) ) below and behind the connection to the tip of the nose). The second continued to be tested with its Vee-Tail unit until both prototypes were eventually abandoned.

The first prototype (s/n 46-0680) eventually became a museum exhibit 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 Edwards Air Force Base.

Once the prototype is complete, it will reach a top speed of 126 mph, with service capped at 50,000 feet. Seating is under a largely unobstructed canopy and forward amidships. The turbojet is sucked in from a small nose air intake, and the rocket motor is buried in the tail itself. The main wing assembly is mounted in the middle of the fuselage side, with a 35-degree sweep and a taper from tip to root rather than root to tip. This gives the aircraft a unique floor plan when viewed from top to bottom.

The tail has a vertical tail with a mid-level horizontal plane. Overall, the XF-91 was a very clean design for its time. As expected, the tricycle landing gear is retractable - although the main gear legs have a pair of in-line wheels instead of the more traditional side-by-side pairing.

Weapons should consist of 4 x 20mm cannons - most likely in the bow area.

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