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THE 456th FIGHTER INTERCEPTOR SQUADRON |
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THE PROTECTORS OF S. A. C. |
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The REPUBLIC XF-91 |
Click on Picture to enlarge
The XF-91, a high-speed experimental interceptor, was America's first rocket-powered combat-type fighter to fly faster than the speed of sound. The airplane had a number of unusual design features -- an inverse taper wing (wider at the tips than at the roots), a variable incidence wing that could be varied in flight (high angle of attack for takeoff and landing and low angle of attack for high-speed flight), a main landing gear that retracted outward with the tandem wheels being housed in the wing tips and a rocket engine that augmented the standard jet engine to provide an outstanding rate of climb.
The airplane made its first flight on May 9, 1949. Numerous other test flights were made, providing valuable research data, but the airplane was not put into production because it did not carry sufficient fuel for a flight of longer than 25 minutes and did not incorporate the latest type of fire control system.
Type Number built/converted Remarks XF-91 2 Jet-rocket fighter SPECIFICATIONS:
Span: 31 ft. 3 in.
Length: 43 ft. 3 in.
Height: 18 ft. 1 in.
Weight: 28,300 lbs. loaded
Engines: General Electric J47-GE-3 of 6,700 lbs. thrust with afterburner and Reaction Motors rocket of 6,000 lbs. thrust (four XLR11-RM-9s of 1,500 lbs. thrust each)
Armament: Designed for four 20 mm cannons
Crew: One
PERFORMANCE:
Maximum speed: 984 mph
Cruising speed: 560 mph
Endurance: 25 minutes
Service ceiling: 48,700 ft.
The REPUBLIC XF-91 "Thunderceptor"
Joe Baugher
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| On the left is Republic XF-91 (S/N 46-680) after the nose radome installation. On the right is XF-91 (S/N 46-681) after the Vee-tail modification. |
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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.
Engines: One General Electric J47-GE-3 axial-flow turbojet, 5200 lb.st.
plus four Reaction Motors XLR11-RM-9 rocket motors rated at 1500 lb.st. 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.
Sources:
Joe Baugher
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06/08/2009 |
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