The F-108 Rapier main purpose was to climb to 81,000ft, reach Mach 3, and fly for up to 1,000 nautical miles to intercept incoming bombers using an automated fire-control system.
North American Aviation (NAA) F-108 project was in many ways an Integral part of the XB-70 Valkyrie’s development. As the USAF began to lose faith in Republic Aviation’s too ambitious XF-103 long-range interceptor in 1955, it issued a somewhat less demanding requirement (GOR 114) for a two-seat, long-range Interceptor. Although the USAF’s requirement underwent frequent changes throughout 1956, in June 1957 NAA was asked for two prototypes of the F-108A, based on its NA-236 proposal. As told by Peter E. Davies in his book North American XB-70 Valkyrie, its main purpose was to climb to 81,000ft, reach Mach 3, and fly for up to 1,000 nautical miles to intercept incoming bombers using an automated fire-control system.
Alternatively, it would cruise sub-sonically to a patrol circuit about 280 miles from its base and then loiter for up to six hours. The F-108A could then dash at Mach 3 for more than 750 miles to release its missiles at intruders. The fighter’s Hughes AN/ASG-18 fire-control system would have guided three bulky AIM-47/GAR-9 Super Falcon Mach 6 missiles with nuclear or conventional warheads and a 110-mile range. AIM-9 Sidewinders were also carried on extending racks.
Theoretically, a Rapier would have to shoot down at least three bombers in five to ten minutes of combat.
The volume of fuel, engine and missile accommodation and advanced avionics required to fulfill this demanding role meant that the F-108A would have to have a very large airframe some 89ft in length, a big “cranked delta” wing spanning 57ft and an all-moving vertical stabilizer.
Estimated maximum takeoff weight was at least 102,000lb. Power was to come from a pair of General Electric J93 turbojets, each developing 29,300lb of thrust in afterburner with ethyl-borane “zip” fuel and fitted with thrust reversers. These were the same engines selected for the XB-70A in parallel development with the two aircraft weapons systems by NAA. In 1958 the company even envisaged the Rapier as an escort fighter for its own XB-70A, although the range statistics were not entirely compatible.
However, the company’s future as the holder of the USAF’s two most lucrative and advanced aircraft contracts for both SAC and Air Defense Command seemed assured. Sharing some systems and parts between the two projects would have reduced their enormous costs, and this was a factor in the awarding of the WS-110A/XB-70 contract to NAA in December 1957.
The Rapier design evolved considerably in 1958. Twin vertical stabilizers replaced the single fin, and then the single unit was reinstated with two fins above the mid-point of the wing surface. Finally, a large canard foreplane and ventral stabilizers were added, together with downward-canted outer wing panels for improved stability.
Development continued through 1959, but the cost of the program caused its cancelation in September 1959. An immediate consequence of termination was a considerable increase in costs for the XB-70 program due to the removal of shared component economies.
North American XB-70 Valkyrie is published by Osprey Publishing and is available to order here.