One of the problems of early jet fighters was their relatively limited range and endurance as compared to their piston-engined predecessors. The USAAF wanted to acquire jet powered escort fighters capable of defeating enemy interceptors, but most of the early jet fighter designs lacked sufficient range to escort bombers all the way to their targets. Since the USAAF had found by painful experience in World War II that fighter escort was absolutely vital for the survival of bombers in enemy airspace, they considered all sorts of proposals for markedly increasing the range of jet fighter escorts, some of which bordered on the bizarre. Some thought that the range problem could be solved by having the bombers tow their escorting fighters into the combat zone, and several experiments were made with B-29s or B-36s towing P-80 or P-84 jet fighters. Other proposals involving having jet fighters operate in parasite fashion from the bellies of large bombers, the best known example of this idea being the McDonnell XF-85 Goblin. Other ideas included the use of mixed power concepts such as that which produced the Convair XP-81. Others involved the construction of large, bulky fighters that were virtually flying fuel tanks, e.g., the Bell XP-83.
Initial attempts to produce jet-powered fighters with the endurance of piston-engined aircraft (e.g. the Bell XP-83 and the Convair XP-81) were disappointing, and in early 1946, the USAAF informally requested proposals for a "penetration fighter" with a combat radius of at least 900 miles and a performance capable of meeting all opposing fighters on more than equal terms. In addition, the USAAF wanted to keep the gross weight of the aircraft below 15,000 pounds. They didn't ask for much, did they? :-)
Lockheed submitted the XF-90 and McDonnell entered the XF-88 in response to this proposal, and the USAAF ordered prototypes of both designs in the spring of 1946. In late 1947, North American entered the penetration fighter fray with a proposal for an extensively revised version of the F-86A.
North American's penetration fighter proposal began life as company project NA-157 on December 17, 1947. In order to keep costs down, the NA-157 retained the swept-wing and the tail assembly of the F-86A, but almost everything else was different. For one, it had an entirely new engine. The engine was to have been the Pratt & Whitney J48-P-1 centrifugal-flow turbojet rated at 8000 lb.st. with afterburning. The J48 was an American-built version of the Rolls Royce Tay. Since the J48 was significantly larger than the J47 of the F-86A, the fuselage had to be increased both in width and in length. In order to meet the range requirement, additional fuel tankage had to be added, bring the total fuel capacity to 1580 gallons. Since the NA-157 was a larger and heavier aircraft than the F-86A, the landing gear was significantly more robust, with twin wheels being used on the main landing gear.
In December of 1947, the USAF ordered two examples of the NA-157 under the designation F-86C, reflecting its Sabre ancestry. Serials were 48-317 and 48-318.
Armament of the F-86C was to have been six 20-mm cannon (with 225 rpg), and an SCR-720 search radar was to have been mounted in the nose. Since the radar set now took up the nose, the air intakes for the turbojet had to be relocated to the sides of the fuselage. The first prototype was fitted with special NACA-designed flush-mounted air intakes in the hopes of reducing aerodynamic drag. However, the second prototype was fitted with more conventional air scoops. The air brakes on the sides of the F-86A fuselage were replaced by a pair of slab-type brakes mounted on the fuselage belly.
In the penetration fighter competition, the USAF initially favored the North American design because of its commonality with other Sabre variants, and in June of 1948 they supplemented the contract for the two F-86Cs with a contract for 118 production aircraft. At that time, it was decided that there were so many differences between the F-86C and the production Sabre that the F-86C should be assigned a new F-number--it was redesignated YF-93A.
It would seem that the F-93 would be assured of a long and fruitful career with the USAF. However, the F-93A production contract was suddenly cancelled in February of 1949. Several reasons were given. One reason was that the projected performance of the B-47 Stratojet was such that it probably would not need a fighter escort. Perhaps the most important reason was a severe reduction in the military budget for FY 1949. With limited funds available, it was decided to give priority to interceptors and to strategic bombers. In addition, a Senior Officers' Board felt that no production order for any penetration fighters should be awarded until a competitive flyoff between the three contenders could be carried out.
Even though the production contract had been cancelled, work on the two YF-93A prototypes (48-317 and 318) continued so that they could be entered in the penetration fighter contest. The YF-93A was actually the last of the three penetration fighter competitors to take to the air. The McDonnell XF-88 competitor had flown on October 20, 1948, and the Lockheed XF-90 had made its first flight on June 3, 1949. The YF-93A did not roll out of the factory until late 1949, and was trucked out to Muroc Dry Lake for flight testing. George Welch took the YF-93A (48-317) on its maiden flight on January 24, 1950.
The flyoff between the Lockheed XF-90, the McDonnell XF-88, and the North American YF-93A took place in the summer of 1950. On August 15, 1950, the Evaluation Board declared the McDonnell XF-88 to be the winner of the contest. However, McDonnell's victory was rather hollow, since no penetration fighters were ever actually manufactured or placed in service because the development of long-range, high-speed jet bombers such as the B-47 and the B-52 eliminated any real need for penetration fighters. In addition, wartime pressures mandated that higher priority be given to the procurement of existing types for use in Korea.
The two YF-93As were eventually handed over to NACA's Ames Laboratory at Moffett Field, California for comparison tests of the flush air intakes. The test results indicated that the standard scoop intakes were actually a better design for high-speed flight than the flush intakes. At one point in their service lives, both planes had their rear fuselages modified to accept a production F-86D tailpipe and stabilizer housing. They were used by NACA as flight test and chase aircraft well into the mid 1950s, and played an important role in testing for most of the "Century Series" of fighter aircraft ranging from F-101 to F-106. They were both retired and scrapped in the late 1950s.
Specification of North American YF-93A:
Engine: One Pratt & Whitney J48-P-6 turbojet rated at 6000 lb.st. dry and 8750 lb.st. with afterburner. Performance: Maximum speed 708 mph at sea level, 622 mph at 35,000 feet. Initial climb rate 11,960 feet per minute. Maximum range on internal fuel 2000 miles. Service ceiling 46,800 feet. Dimensions: Wingspan 38 feet 9 inches , length 44 feet 1 inch, height 15 feet 8 inches, wing area 306 square feet. Weights: 14,035 pounds empty, 21,610 pounds gross, 26,516 pounds combat. Armament: Six 20-mm cannon in the nose.
Sources:
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F-86 Sabre in Action, Larry Davis, Squadron/Signal Publications, 1992.
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The North American Sabre, Ray Wagner, MacDonald, 1963.
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The American Fighter, Enzo Angelucci and Peter Bowers, Orion, 1987.
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Flash of the Sabre, Jack Dean, Wings Vol 22, No 5, 1992.
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North American F-86 Sabre, Larry Davis, Wings of Fame, Volume 10, 1998
- E-mail from Johan Visschedijk on first prototype having recessed intakes, second prototype having scoop intakes, and the speed brakes being two slabs rather than one slab.
