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Republic XF-91 Thunderceptor

High-Altitude Interceptor Prototype

Republic XF-91 Thunderceptor

High-Altitude Interceptor Prototype


The Republic XF-91 Thunderceptor mated rocket-propelled take-offs with turbojet-controlled landings.
National Flag Graphic
ORIGIN: United States
YEAR: 1949
MANUFACTURER(S): Republic Aviation Corporation - USA
OPERATORS: United States (cancelled)

Unless otherwise noted the presented statistics below pertain to the Republic XF-91 Thunderceptor model. Common measurements, and their respective conversions, are shown when possible.
LENGTH: 31.23 feet (9.52 meters)
WIDTH: 43.24 feet (13.18 meters)
HEIGHT: 18.08 feet (5.51 meters)
WEIGHT (EMPTY): 14,132 pounds (6,410 kilograms)
WEIGHT (MTOW): 28,307 pounds (12,840 kilograms)
ENGINE: 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.
SPEED (MAX): 984 miles-per-hour (1584 kilometers-per-hour; 855 knots)
RANGE: 1,168 miles (1,880 kilometers; 1,015 nautical miles)
CEILING: 54,134 feet (16,500 meters; 10.25 miles)
RATE-OF-CLIMB: 47,500 feet-per-minute (14,478 meters-per-minute)


4 x 20mm cannons

Series Model Variants
• XF-91 - Base Model Designation; 2 examples completed as s/n 46-0680 and 46-0681, the second destroyed during tests in 1951.


Detailing the development and operational history of the Republic XF-91 Thunderceptor High-Altitude Interceptor Prototype.  Entry last updated on 12/28/2016. Authored by Staff Writer. Content ©
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.

Republic XF-91 Thunderceptor (Cont'd)

High-Altitude Interceptor Prototype

Republic XF-91 Thunderceptor (Cont'd)

High-Altitude Interceptor Prototype

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.


Our Data Modules allow for quick visual reference when comparing a single entry against contemporary designs. Areas covered include general ratings, speed assessments, and relative ranges based on distances between major cities.

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Relative Maximum Speed Rating
Hi: 1000mph
Lo: 500mph
    This entry's maximum listed speed (984mph).

    Graph average of 750 miles-per-hour.
City-to-City Ranges
Graph showcases the Republic XF-91 Thunderceptor's operational range (on internal fuel) when compared to distances between major cities.
Aviation Era
Pie graph section
Pie graph section
Pie graph section
Pie graph section
Pie graph section
Useful in showcasing the era cross-over of particular aircraft/aerospace designs.
Unit Production Comparison
Comm. Market HI*: 44,000 units
Military Market HI**: 36,183 units

  * Commercial Market High belongs to Cessna 172.

  ** Military Market High belongs to Ilyushin Il-2.

Altitude Visualization
Small airplane graphic
Supported Roles
Ground Attack
Aerial Tanker
A2A=Air-to-Air; UAV=Unmanned; CAS=Close Support; ASW=Anti-Submarine; AEW=Airborne Early Warning; MEDEVAC=Medical Evac; EW=Electronic Warfare; SAR=Search-Rescue
Supported Arsenal
Graphical image of an aircrat automatic cannon
Commitments / Honors
Military lapel ribbon for Operation Allied Force
Military lapel ribbon for the Arab-Israeli War
Military lapel ribbon for the Battle of Britain
Military lapel ribbon for the Battle of Midway
Military lapel ribbon for the Berlin Airlift
Military lapel ribbon for the Chaco War
Military lapel ribbon for the Cold War
Military lapel ribbon for the Cuban Missile Crisis
Military lapel ribbon for pioneering aircraft
Military lapel ribbon for the Falklands War
Military lapel ribbon for the French-Indochina War
Military lapel ribbon for the Golden Age of Flight
Military lapel ribbon for the 1991 Gulf War
Military lapel ribbon for the Indo-Pak Wars
Military lapel ribbon for the Iran-Iraq War
Military lapel ribbon for the Korean War
Military lapel ribbon for the 1982 Lebanon War
Military lapel ribbon for the Malayan Emergency
Military lapel ribbon representing modern aircraft
Military lapel ribbon for the attack on Pearl Harbor
Military lapel ribbon for the Six Day War
Military lapel ribbon for the Soviet-Afghan War
Military lapel ribbon for the Spanish Civil War
Military lapel ribbon for the Suez Crisis
Military lapel ribbon for the Vietnam War
Military lapel ribbon for Warsaw Pact of the Cold War-era
Military lapel ribbon for the WASP (WW2)
Military lapel ribbon for the World War 1
Military lapel ribbon for the World War 2
Military lapel ribbon for the Yom Kippur War
Military lapel ribbon for experimental x-plane aircraft
* Ribbons not necessarily indicative of actual historical campaign ribbons. Ribbons are clickable to their respective campaigns/operations.