The F-108 Rapier’s main purpose was to climb to 81,000 ft, reach Mach 3, and fly for up to 1,000 nautical miles to intercept incoming bombers using an automated fire-control system
In many respects, North American Aviation’s (NAA) F-108 project was closely tied to the development of the XB-70 Valkyrie. In 1955, as the USAF began to lose confidence in Republic Aviation’s overly ambitious XF-103 long-range interceptor, it issued a somewhat less demanding requirement, known as GOR 114, for a two-seat long-range interceptor. Although the requirement was revised frequently throughout 1956, NAA was asked in June 1957 to build two F-108A prototypes based on its NA-236 proposal. According to Peter E. Davies in his book North American XB-70 Valkyrie, the aircraft was intended to climb to 81,000 feet, reach Mach 3, and operate over distances of up to 1,000 nautical miles in order to intercept incoming bombers using an automated fire-control system.
As another option, it could cruise at subsonic speed to a patrol area roughly 280 miles from its base and remain there for as long as six hours. From that position, the F-108A could then accelerate to Mach 3 and cover more than 750 miles before launching its missiles at hostile aircraft. Its Hughes AN/ASG-18 fire-control system was designed to direct three large AIM-47/GAR-9 Super Falcon missiles, capable of Mach 6 speeds, fitted with either nuclear or conventional warheads, and offering a range of 110 miles. The aircraft also carried AIM-9 Sidewinders mounted on retractable racks.
In theory, a Rapier would have been expected to destroy at least three bombers within five to ten minutes of combat. Meeting such a demanding mission required space for large amounts of fuel, engines, missiles, and sophisticated avionics, which in turn meant that the F-108A needed a very large airframe measuring about 89 feet in length, a broad 57-foot-span cranked-delta wing, and an all-moving vertical stabilizer.
The aircraft’s estimated maximum takeoff weight was no less than 102,000 pounds. It was to be powered by two General Electric J93 turbojets, each capable of producing 29,300 pounds of afterburning thrust using ethyl-borane “zip” fuel and equipped with thrust reversers. These were the same engines NAA had chosen for the XB-70A, which was being developed alongside the F-108A and its weapons systems. In 1958, the company also considered the Rapier as a possible escort fighter for its own XB-70A, though their range figures did not fully align.
Nevertheless, the company’s position appeared secure as the holder of the USAF’s two most valuable and technologically advanced aircraft contracts for both SAC and Air Defense Command. Using common systems and components across the two programs would have helped lower their immense costs, and this contributed to NAA receiving the WS-110A/XB-70 contract in December 1957.
The Rapier’s design underwent major changes during 1958. Twin vertical stabilizers were introduced in place of the original single fin, after which the single fin returned, now accompanied by two fins positioned above the midpoint of the wing. In the final stage, designers added a large canard foreplane, ventral stabilizers, and downward-angled outer wing panels to improve stability.
Development carried on into 1959, but the program was canceled in September of that year because of its high cost. One immediate result of the cancellation was a significant rise in XB-70 program expenses, since the savings gained from using shared components were lost.
North American XB-70 Valkyrie is published by Osprey Publishing and is available to order here.
Photo by Bzuk and Anynobody via Wikipedia and U.S. Air Force
