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The XP/XF-91 Thunderceptor was originally designed in 1946 as a “point defense” interceptor with a high rate of climb to defend high value targets. It was the result of Republic Aviation’s post World War II use of captured German aircraft industry documents – notably the description of the Messerschmitt rocket powered fighter, the Me 163, and its successors.
The XF-91 design was rewarded with an Air Force contract for two prototypes in 1949. Its overall design was based on the swept wing version of the Republic F-84 Thunderjet, the F-84F, which also used German swept wing research.
In fact, the highly innovative XF-91 had very little in common, even in appearance, with its ancestors. The most striking change was in the mid-mounted swept wing; it was much wider and thicker at the tip than at the fuselage join – a modification called inverse taper. This was an attempt to solve a new problem with the swept wing, wing-tip stall, which made swept wing aircraft very hard to control in low speed, high angle of attack situations like landing.
The Thunderceptor’s wide wing tips were intended to solve this problem, and another feature of the wing was that it was variable incidence.
Variable incidence meant the wing leading edge was raised for better angle of attack on take off and landing and lowered it to keep the wing aligned with the fuselage for normal operation. This innovation (patented, incidentally, in 1912) was intended to lower landing and takeoff speed and used successfully on later aircraft, notably the Navy’s F8U Crusader. The final innovation was the aircraft’s power plant.
Using information from the Me 163, the Republic designers used a mixed-propulsion system with a single, afterburning General Electric J47-GE-3 engine augmented by four Reaction Motors XLR11-RM-9 rocket motors of 1,500 pounds of thrust each – two located above the jet engine exhaust and two below. This combination offset the disadvantage of rockets – very short time of fire – by using the J47 for cruising flight and the rocket engines for takeoff, climb and for extra boost during combat emergencies.The four rockets, when fired together, provided 6,000 pounds of thrust – more than the power of the engine for 70 seconds – and gave the XF-91 spectacular performance for short periods.
The first prototype made its first flight May 9, 1949, and became the first U.S. fighter to exceed Mach 1 in level flight in December 1951. It later showed a top speed of Mach 1.17, and only a high frequency vibration kept the test pilots from trying to reach its projected top speed of Mach 1.4.
Additionally, the other two innovations, the inverse taper and variable incidence wing, worked as advertised.
There were other innovations with the prototype XF-91s. One was a “butterfly” or “V” tail replacing the conventional rudder/stabilizer combination, and others were a variety of radar nose configurations to make into a viable all-weather inceptor. But in the end, the Thunderceptor was never ordered into production.
Why did the XF-91 not become the F-91 if its performance was so high and the innovations successful? There were several reasons. The short range, rapidly climbing “point defense” interceptor was no longer a viable concept and the XF-91’s limited endurance, only 25 minutes, was a major handicap.
But overall it seems the reason was over engineering – answering problems with solutions that added extra weight, complexity and increased maintenance costs.
The Air Force found the problems could be solved more easily (and cheaply) by other means, and that high performance was possible from improved variants of more conventional fighters like the F-86.
Today, the XF-91 would be called a “technology demonstrator,” and it is perhaps unfortunate that it was given a “pursuit/fighter” designation.
Considered as a potential Air Force fighter, it was a failure because it never reached production, but as a technology demonstrator it proved very useful, even if none of the technologies was finally adopted.
For questions or comments, contact Dr. Michel at marshall.michel@ramstein.af.mil.
