The Falcon air-to-air missile that did not enter service – The Hughes AIM-47 Falcon

The Hughes AIM-47 Falcon, which up to 1962 was designated as the GAR-9, was a very long-range air-to-air missile that shared the basic design of the earlier AIM-4 Falcon but offered significantly higher overall performance as well as greater range. The missile was developed from 1958 alongside the Hughes ASG-18 radar fire-control system to provide the armament of the North American F-108 Rapier Mach 3.2 interceptor that was being planned as the US Air Force’s latest weapon in the fight against Soviet bomber incursions into North American airspace. Neither the AIM-47 nor the F-108 entered service, the latter not even being built, but the missile did reach the hardware stage and paved the way to the AIM-54 Phoenix used by the Grumman F-14 Tomcat carrierborne air-defence fighter, and a considerable portion of the F-108 was incorporated in the North American A-5 Vigilante carrierborne strike/attack and reconnaissance warplane.

In the early 1950s the USAF created a requirement for a phenomenally advanced interceptor as what was, at the time, called the LRI-X (Long-Range Interceptor – Experimental). This ‘Zip-Fuel’ interceptor was to carry a fire-control system whose radar would provide mid-course guidance and terminal illumination of targets more than 100 miles (160 km) distant; the first stage of the missile’s flight was entrusted to an autopilot using target position, speed and course data loaded from the fighter’s systems just before missile launch. During 1957 Hughes won the contract to design, develop and manufacture the weapons system that was the core of the F-108’s operational capability. This weapons system comprised the YX-1 radar and fire-control system and the GAR-X missile. The original missile designed by the company for this application had a range of 15 to 25 miles (25 to 40 km) and was to be armed with either a conventional blast/fragmentation HE warhead or a 0.25 kiloton version of the W42 nuclear warhead. When North American’s NSA-246 design was declared winner of the LRI-X contest in April 1958 over designs by Lockheed and Northrop, and North American was contracted to develop an XF-108 prototype, the parallel Hughes fire-control and missile entries became the ASG-18 and GAR-9 respectively. The F-108 programme was cancelled in September 1959, but the USAF decided to continue development of the missile system with both its warhead options.

Advanced development
During the early stages of its development, the GAR-9 grew enormously in capability. The length was increased considerably to allow the incorporation of more fuel for the Aerojet General XM-59 solid-propellant rocket motor and thus increase the missile’s speed to Mach 6 and its range to 100 miles (160 km). As the range now available was beyond the range of effective semi-active radar homing, a new active radar terminal seeker was added to the missile. This seeker was a powerful system in its own right, with no effective maximum range and the ability to secure a lock on a target with a cross section of 100 sq ft (9.29 m²) at a range of 63 nm (72.5 miles; 117 km). The terminal seeker was also changed at one time through the addition of a passive infra-red seeker to improve last-phase performance. This would have demanded a 180 lb (82 kg) increase in missile weight and a 2 in (51 mm) increase in missile diameter, however, making it too large to fit the F-108’s weapons bay. The W42 nuclear warhead was dropped in 1958 in favour of a 100 lb (45 kg) HE type.

Problems with the motor during this development period resulted in brief consideration of using a storable liquid-fuel rocket design, but the Aerojet General rocket motor was finally replaced by the Lockheed XSR13-LP-1 solid-propellant rocket motor, even though this trimmed the missile’s maximum speed from Mach 6 to Mach 4.

It was in this form that the GAR-9 began ground firings in August 1961. For air-launch testing at supersonic speeds the Republic XF-103 had at first been suggested as the test platform, but this fighter was also cancelled before reaching the prototype stage. Thus it was a YB-58A pre-production prototype of the Convair B-58 Hustler that was taken in hand for modification with the ASG-18 radar in a large protruding radome. In-flight launches of the XGAR-9 (from 1962 XAIM-47A) began in May 1962.

Another platform
In 1960 Lockheed had embarked on the development of the F-12 Mach 3 interceptor as a cheaper replacement for the F-108, and the ASG-18/GAR-9 combination was shifted to the F-12. This interceptor had four internal weapons bays, with flip-open doors, on the chines behind the cockpit, one of them filled with electronics and the other three each outfitted for the carriage of one missile. The weapon bays were too small to accommodate the GAR-9, so the GAR-9B (from 1962 AIM-47B) was developed with the missile’s body diameter reduced by 0.5 in (12.7 mm) and the fins modified into flip-out surfaces to reduce the stowed diameter. The length remained unaltered at 12 ft 6.5 in (3.82 m), but the weight was reduced from 818 lb (371 kg) to 800 lb (363 kg).

These were a number of unguided test firings and then guided test firings of the GAR-9A from the YF-12A prototypes. The seven guided tests resulted in six ‘kills’, the single miss being attributable to a power failure in the missile. It was in the autumn of 1962 that the missile was renamed as the AIM-47 within the context of the Department of Defense’s rationalisation of the US forces’ separate designation systems into a single unified arrangement. The last missile launch was from a YF-12A flying at Mach 3.2 at an altitude of 74,400 ft (22675 m) at a Boeing QB-47 Stratojet target drone (an obsolescent bomber adapted for remote control) flying at a height of just 500 ft (150 m).

In 1966 the F-12 project was cancelled. Another project for which the missile was proposed was the North American B-70 Valkyrie Mach 3 strategic bomber, which could have been equipped with the AIM-47 for self-defence. Two XB-70A prototypes were built, but the Valkyrie programme was cancelled after it had become clear to the USAF that the Soviets had developed and deployed effective high-altitude surface-to-air missiles, thereby rendering impractical the current concept of high-altitude bomber attacks on the USSR. Another use for the missile was also considered briefly as the AGM-76A weapon for air-launch against ground-based radars. This would have had the radar seeker of the AGM-45 Shrike and the 250 lb (113 kg) warhead of the Mk 81 free-fall bomb.

In all, Hughes had built some 80 pre-production AIM-47 missiles.

The ASG-18 fire-control system and AIM-47 missile were used as the starting points for the AWG-9 and AIM-54 Phoenix intended for the General Dynamics F-111B ‘swing-wing’ carrierborne interceptor. This project was also cancelled in 1968, but the weapon system finally found a home on the F-14 Tomcat.

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