In 1947, the designation XF-98 was assigned to an "unmanned fighter" project that was eventually to result in the Hughes Falcon, the world's first operational air-to-air guided missile.
In 1947, the USAAF asked for bids on a new type of aircraft armament system. Two separate projects were to be undertaken--the first was to explore the use of a radar-based airborne fire-control system for manned interceptors and the second was to develop a guided air-to-air missile to be carried by later generations of manned interceptors. Both contracts were won by the Hughes Aircraft Corporation of Culver City, California.
The air-to-air missile component of the system was given the designation XF-98, a fighter designation. It was assigned the name *Falcon*. The F-98 was initially known as the AAM-A-2 under a joint Army/Navy/Air Force guided missile designation from 1947, but in 1951, the Air Force decided to classify air-to-air and surface-to-air guided missiles as fighters (leading to AAM-A-2 becoming F-98). However, The USAF had second thoughts about giving an unmanned guided missile a fighter designation, and in 1955, the Hughes Falcon air-to-air missile was redesignated GAR-1, the GAR standing for *Guided Aircraft Rocket*.
The Falcon initially operated on the principle of semi-active radar homing, that is, it tracked and homed in on a radar echo reflected back from a target illuminated by a radar set carried in the aircraft that launched the missile. The missile did not carry its own radar transmitter.
The Falcon entered production at a new Hughes plant in Tucson in 1954. The airframe was about the size of a person, and the missile was powered by a Thiokol solid-fuel rocket motor. It had a hemispherical nose radome flanked by receiving aerials that looked like small nose fins. These antennae gave proportional navigation and steering signals to the elevons mounted on the trailing edges of the four delta wings. The length was 1.97 meters (6.48 feet), the diameter was 16.3 cm (6.4 inches), and the wingspan was 50.8 cm (20 inches). It carried a 29-pound high-explosive warhead that was detonated by a proximity fuse. The launch weight was about 110 pounds. Maximum speed at engine burnout was Mach 2.8, and effective range was about five miles.
The first Falcon installations were on the wingtip pods of F-89H and J Scorpion interceptors. At about the same time, the Convair F-102A Delta Dagger interceptor entered service with six Falcons housed inside the internal weapons bay. Both aircraft entered service with the Air Defense Command in 1956.
The F-104 designation was originally used for the variant of the Falcon designed to be launched from supersonic fighters (the initial XF-98 was dropped because the burn characteristics of its rocket motor made it incapable of launch from supersonic aircraft), but the supersonic-launch variant that would later become the GAR-1 was redesignated F-98A, and F-104 was re-used for the Lockheed Starfighter.
In late 1956, the first infrared Falcon appeared, the GAR-2. The radar receiver of the GAR-1 was replaced by a passive infrared detector housed behind a glass nose. This missile homed in on the infrared radiation emitted by the hot engine exhaust of the enemy aircraft. The infrared Falcon had the advantage in that it was "fire and forget", enabling the launching aircraft to break away as soon as the missile was fired. However, interceptor aircraft typically carried a mix of both radar and infrared Falcons, and it was common doctrine to launch one missile of each type at an enemy target in order to ensure a kill.
The GAR-1D was an improved version of the radar Falcon, with the control surfaces being made larger and moved to a position well behind the delta wings.
The GAR-2A was an improved version of the infrared Falcon with a better infrared detector. The GAR-2A was built for export under the designation HM-58, and was built under license by SAAB-Scania of Sweden as the RB-28.
The GAR-3 Falcon of 1958 was a vastly-improved version of the radar Falcon with a longer-burning rocket motor, an improved radar receiver mounted behind a more pointed nose, long wing-root fillets, and a larger warhead. Maximum speed was about Mach 4, and effective range was about 7 miles.
The GAR-3A radar homer of 1959 had a new motor having boost/sustainer charges, improved radar homing guidance, provision for electronic countermeasures, and some changes in the airframe.
The GAR-4A was the infrared homing version of the GAR-3. It contained a new infrared seeker capable of locking on to smaller targets at greater ranges.
The GAR-11 Nuclear Falcon of 1960 was designed to give a better PSSK (Probability of Single-Shot Kill) in head-on attacks. Infrared homing was considered to be ineffective in head-on encounters, and since semi-active radar homing was less precise, it was decided that the best option was to increase the yield of the warhead. A nuclear warhead similar to that of the Genie was carried, triggered by four active-radar proximity fuse aerials mounted flush with the body ahead of the wings. The diameter was 11 inches, and the launch weight was 203 pounds.
The GAR-11A was similar to the GAR-11, but it carried a conventional high-explosive warhead. This version was exported as HM-55 and was built under license by SAAB-Scania of Sweden as RB-27.
The GAR-9 was a fourth-generation Falcon designed for the North American F-108 Rapier interceptor. Following launch, the ASG-18 radar of the F-108 was to be used for mid-course guidance and target illumination, and an infrared sensor was to be used for terminal homing. Propulsion was to be by a Lockheed-developed storable liquid-fueled rocket. Mach 6 performance was envisaged, and ranges of up to 115 miles were anticipated. Following the cancellation of the F-108 project in 1959, work on the GAR-9 missile project was transferred to the Lockheed F-12 project. The F-12 project was itself terminated after only four aircraft were built, leaving the GAR-9 with no launching aircraft.
In 1962, the GAR-1, 2, 3, and 4 Falcon missiles were redesignated AIM-4,
where the AIM stood for "Airborne Interception Missile". The GAR-11 was redesignated AIM-26
and the GAR-9 was redesignated AIM-47. The various Falcon redesignations are summarized as follows:
The AIM-4D was the final production version of the Falcon, It was specifically designed for air-to-air combat, all previous Falcons having been designed for the interception and destruction of enemy bombers. It combined the small airframe of the earlier Falcons with the more powerful rocket motor and more sophisticated infrared seeker of the AIM-4G. The AIM-4D had a launch weight of 134 pounds and had a maximum effective range of about 6 miles. Numerous AIM-4As and Cs were remanufactured to the AIM-4D standard.
The AIM-4D did see some combat in Vietnam. The F-4D Phantom retained the AIM-7 Sparrow capability of the F-4C, but it deleted the Sidewinder capability on the inboard underwing pylon in favor of the Hughes AIM-4D Falcon infrared-homing missile.
However, the infrared Falcon proved relatively unsuccessful in air-to-air combat in Vietnam, shooting down only four MiG-17s and one MiG-21 between October 26, 1967 and February 5, 1968. The Falcon was definitely not a good dogfighting missile, having been originally designed back in the 1950s for bomber interceptions. One of the basic problems in using the Falcon for dogfighting was that its aerodynamic design made for relatively limited manoeuvrability. The moveable surfaces at the end of the four delta wings of the Falcon did not provide sufficient aerodynamic force for the rapid changes of direction that were required to be effective against highly manoeuvrable fighters.
The Falcon proved to be somewhat temperamental in service, requiring a lot of careful setting up and tweaking. In addition, the Falcon had a tendency to cause engine flameouts on the F-4D when fired. Perhaps the most significant problem with the AIM-4D was that its fire control system required 6-7 seconds to actually launch the missile after the firing button was pushed, which is an eternity in a dogfight. The internal systems and aerodynamic surfaces of the Falcon were powered by an internal turbo-alternator and hydraulic power unit which was driven by a gas generator. This system took a few seconds to spin up and take over control from the aircraft fire control system. Also, the analog computers in the fire control system had to calculate several pre-launch attack parameters and pass them along to the missiles' guidance system, which also took a second or two.
Thirdly, the Falcon required a direct hit to explode, since there was no proximity fuse. The leading edges of the four delta wings were made of fibreboard, and the intent was that upon impact the missile would bury itself in the fuselage of the target up to the midpoint of the missile's wing. The fibreboard would then crush, completing a circuit and detonating the warhead. In addition, the explosive warhead was quite small, only about 4 pounds.
Consequently, combat pilots in Vietnam were very uncomplimentary about the Falcon. As a result of the barrage of complaints from the field, the Sidewinder capability on the inboard underwing pylons was hastily restored. However, in fairness to the Falcon, virtually all air-to-air missiles would prove to be troublesome in Vietnam and less deadly to enemy aircraft than anticipated.