History
Introduction and Background
On August 28, 1945, the United States Army Air Forces (later separated as the United States Air Force) announced a new requirement for a twin-engine penetration fighter with swept wings capable of escorting jet-powered bombers over enemy territory and back. It was required to have a combat radius of at least 900 miles.
By this time, World War II was over in Europe and the Pacific. The air doctrine was reshaped and advocated that large formations of bombers, escorted by long-range fighters, would cause mass destruction on enemy targets below. Fighter escort was essential for the survival of bombers in enemy airspace. With the Soviet Union being the chief Cold War contender, it was undoubtedly attempting to outpace the West with its numerous jet-powered fighters and bombers.
The requirement sought a new fighter to fulfill the long-range penetration and bomber escort roles. Although both terms had similar concepts, they differed in tactical movements. The penetration fighter’s role would not operate in close formation with bombers. Instead, it would penetrate deeper into enemy airspace and neutralize any threats ahead of the bombers. These fighters could return to the bombers and continue the escort duty after the incursion. The North American P-51 Mustangs had demonstrated this part of the air doctrine during the turning points of the air war in World War II. In the post-war period, the development of jet-powered strategic bombers made propeller-driven fighters and earlier non-afterburning jet fighters unable to keep up. Hence, this requirement was created to bring about more modern aircraft designs powered by afterburning jet engines.
Competitors for the contract included McDonnell’s XF-88, North American’s YF-93, and Lockheed’s XF-90. North American received a contract to construct two prototype YF-93s on December 17, 1947.
YF-93A Development
The YF-93A retained the swept-wing and tail assembly of the F-86A, but everything else was different. Unlike the F-86 and its nose-mounted intake, the YF-93A incorporated a nose cone housing the radar system, and flush intakes were fitted to the sides of the larger fuselage for a more refined design approach. It was planned to incorporate six 20 mm cannons. These guns would reside along the sides of the nose, similar to the placement of the F-86A’s machine guns.
The YF-93A was powered by a new Pratt & Whitney J48-P-1 centrifugal-flow turbojet engine. The J48 was an American-licensed version of the Rolls-Royce Tay. Being larger than the J47 used in the F-86A, the J48 required the YF-93A’s fuselage to be increased both in width and in length. Additional tankage was installed to bring the total fuel capacity to 1,581 gallons, per the range requirement. A pair of slab-type air brakes was mounted on the fuselage belly rather than on the fuselage sides, as seen on the F-86A.
These changes increased the YF-93A’s total weight, making it much larger and heavier than the F-86A Sabre. As a result, it featured stronger landing gear with twin wheels to compensate for the added weight.
Two unarmed prototypes were built with NACA inlet ducts; the first (48-317) and the second (48-318) were later retrofitted with more conventional intakes. Six months later, in June 1948, the USAF placed a production order for 118 F-93As. The USAF, already familiar with other North American Sabre variants, seemed confident in the aircraft’s operational prospects.
However, in February 1949, the USAF abruptly canceled the production order after dropping the requirement for a penetration fighter. This change was due to shifts in USAF priorities and significant reductions to the military budget. Additionally, the introduction of the Boeing B-47 Stratojet, with its impressive performance, was deemed to make fighter escorts unnecessary. The eventual introduction of the “state-of-the-art” B-52 was also anticipated. As a result, the production order for any penetration fighter was put on hold pending a competitive fly-off between the XF-88A, XF-90A, and YF-93A.
Flight Testing
The YF-93A did not roll out of the factory until late 1949 and was transported to Muroc Dry Lake for flight testing. On January 24, 1950, test pilots took the YF-93A (48-317) on its maiden flight. The YF-93A achieved a speed of 708 mph and was able to reach Mach 1.05, though only in a dive. However, it was later found that the use of NACA inlet ducts, although intended to improve performance, was inferior to conventional intakes for air combat at high angles of attack.
With limited time, the USAF conducted a fly-off among the three competing designs in the summer of 1950. The evaluation of the Lockheed XF-90A, the McDonnell XF-88A, and the North American YF-93A took place between June 30 and July 8, 1950. On August 15, 1950, the Evaluation Board selected the McDonnell XF-88A as the winner. McDonnell secured the production contract in September 1950. However, the penetration fighter project was eventually shelved altogether due to the outbreak of the Korean War and evolving USAF priorities.
The two YF-93As were then transferred to NACA’s Ames Laboratory at Moffett Field, California, where they were used in additional testing, primarily to study the performance of flush air intakes. At one point, both aircraft had their rear fuselages modified to accept a production F-86D tailpipe and stabilizer. They were employed as chase and test aircraft during the mid-1950s. Both were retired from NACA service in 1956 and later scrapped.
Had the F-93A entered production, it would likely have served as an escort for B-29 Superfortresses and intercepted MiGs over Korea between 1951 and 1953.
