As for the missiles the XF-108 was intended to carry, the GAR-9 was a member of the Hughes Falcon family, but was considerably larger than its cousins, with a length of over 12 feet. By comparison, the GAR-1D and GAR-3A Falcons then in service at the time were around 6 and 7 feet long, respectively.
The GAR-9 had been specifically developed for the Long-Range Interceptor Experimental (LRI-X) program, for which the XF-108 was selected in 1957 as the winning design, being chosen ahead of the rival Republic XF-103.
Originally intended to carry a conventional or nuclear warhead, the GAR-9 would have had a range of 100 miles or more, flying to its target using semi-active radar homing, with the engagement process being managed by the AN/ASG-18 fire-control system on the Rapier. Design changes to the GAR-9 increased its weight and diameter, leading to modifications to the XF-108’s weapons bay, although it’s unclear if the rotary mechanism was changed in the process.
Ultimately, however, despite good progress in the development effort, XF-108 was canceled in 1959, with the Air Force failing to find the funds needed to keep the program alive, despite its promised performance. Moreover, the concern about Soviet bombers was beginning to diminish, now that intercontinental ballistic missiles (ICBMs) appeared to pose a much graver threat. It would be left, instead, to Convair F-106 Delta Darts to defend North American skies against bombers, a job that they did well into the 1980s.
A similar fate befell the XB-70, too, this being canceled in 1961 — before any examples had been flown. The reasons included developments in Soviet surface-to-air missiles, which were expected to put it at risk, as well as the appearance of cheaper, nuclear-armed ICBMs.
Despite the demise of the XF-108 program, work continued on the GAR-9, which was redesignated as the AIM-47 in 1963. It was now intended to arm the Lockheed YF-12A, the interceptor version of the A-12 spy plane, which performed several successful test launches of the AIM-47 before it, too, was canceled.
Thereafter, the AIM-47 and ASG-18 helped inform the design of the AIM-54 Phoenix and AN/AWG-9 radar that, as part of the F-14 Tomcat, ensured air defense of the U.S. Navy carrier battle groups for much of the Cold War.
While the developmental DNA of the XF-108 was destined to endure, the U.S. Air Force was destined never to field a Mach-3 fighter, let alone one armed with a revolving missile launcher.
Contact the author: [email protected]