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. Lockheed received a contract to construct two prototype XF-90s on June 20, 1946.
XF-90A Development
In 1947, the XF-90 had originally been a delta-winged design, but wind tunnel tests indicated that such a configuration would not be viable. It was later redesigned with 35-degree sweptback wings, a sharply-pointed nose, and two Westinghouse J34-WE-11 axial-flow turbojet engines mounted side-by-side in the rear fuselage and fed by side-mounted air intakes. The wings were featured with the leading-edge slats, Fowler flaps, and ailerons on the trailing edge. The cockpit was pressurized and fitted with an ejector seat and a bubble canopy. The internal fuel could be supplemented by wingtip-mounted tanks, totaling a capacity of 1,665 US gallons. One of the most innovative features of the XF-90 design was its variable incidence vertical tail fin, which could be adjusted to move forward or back to assist in adjusting the horizontal stabilizers. The aircraft was designed to be fitted with six 20-mm cannons horizontally below the engine intakes, with three guns to each intake. It was also capable of carrying two 1,000-lb bombs and eight 5-inch rockets.
Flight Testing
Consequently, the early changes to the XF-90 designs forced a delayed first flight. The first prototype XF-90 (46-687) was finally ready for flight testing in the spring of 1949. The prototype was initially fitted with non-afterburning J-34-WE-11 turbojet engines. It was found to lack the necessary power for take-off, so it was fitted with Rocket-Assisted Take-Off canisters for support. The prototype made its maiden flight on June 3, 1949, out of Edwards Air Force Base. Although the test flight was completed without problems, the performance was disappointing because of its excess weight and the power of its non-afterburning J-34 engines. In search of more power, this prototype was also the first to be retrofitted with afterburning engines and redesignated XF-90A. It flight-tested with the afterburners on September 16, 1949, during 24th flight testing.
Lockheed built the second prototype XF-90 (46-688) and initially fitted it with afterburning J34 turbojets. This prototype was never redesignated XF-90A. The XF-90 completed its first flight on April 12, 1950. It had achieved a maximum speed of 668 mph in level flight at sea level and reached a maximum speed of Mach 1.12 in a dive. It went on to exceed the sound barrier several times without any structural problems. This XF-90 had almost cost the life of an experienced test pilot when it had great difficulty in pulling out of a dive but avoided a total crash towards the earth.
Although the improved performance was better than most early fighter jets, it was still insufficient to be considered acceptable to the USAF. The fly-off between the Lockheed XF-90A, the McDonnell XF-88A, and the North American YF-93A occurred between June 30, 1950, and July 8, 1950, for the final evaluation by the USAF officials. On August 15, 1950, the Evaluation Board declared the McDonnell XF-88A the winner. McDonnell secured the production contract in September 1950, but the penetration fighter project was later canceled and abandoned due to the changes in Air Force priorities, along with a budget deficit.
Aftermath
The XF-90s’ inadequate performance was mainly attributed to the excessive weight and underpowered powerplants. The XF-90s were built with 75ST aluminum rather than the then-standard 24ST aluminum alloy, along with heavy forgings and machined parts. These materials made the airframe very sturdy. However, these materials resulted in an aircraft with an empty weight more than 50% heavier than its competitors. The bottom line of the issue with this overall aircraft design was described as “overbuilt and underpowered.”
After losing the production contract, the two remaining prototypes were subjected to additional testing. The disposition of the first prototype (46-687) was unknown. Despite conflicting sources, It was not handed over to any NACA tests as it did not show up in NACA records. Presumably, it was destroyed during the crash fire tests along with almost 100 other aircraft. The other XF-90A (46-688) had survived three atomic blasts at Frenchman Flat within the Nevada Test Site in 1952. In 2003, the XF-90A (46-688) was recovered from the Nevada Test Site; it was decontaminated and restored as the museum aircraft on display at the National Museum of the United States Air Force in Dayton, Ohio.
