For more than 40 years, the AIM-7 Sparrow was one of the U.S. Navy’s two most widely used air-to-air missiles (the AIM-9 Sidewinder was the other). It was also adopted by the U.S. Air Force and many foreign militaries. Though conceived in 1946, it is still in service more than 70 years later, albeit in several substantially different forms. The missile’s history exemplifies the Navy’s very successful spiral approach to developing weapons, which improves established systems step by step, rather than by inventing whole new weapons.
In 1946, the Navy realized that fighters encountering jet bombers needed something more than the guns of the past. At typical jet speeds, a fighter approaching a bomber head on would have very little time to react, hence too little time for accurate gunfire. Two separate naval organizations claimed responsibility for missile development: the Bureau of Ordnance (BuOrd), which considered missiles to be guided projectiles; and the Bureau of Aeronautics (BuAer), which considered them pilotless aircraft. The two bureaus’ entries into the new field of air-to-air missiles were BuOrd’s semiactive radar Meteor and active radar Oriole, and BuAer’s Sparrow. By the early 1950s, BuAer had won.
The bureau understood that the most difficult part of missile development was the guidance system. Instead of establishing a primary development contract with an aircraft company for the airframe and motor, it contracted with Sperry Gyroscope, a company that made navigation instruments for aircraft and ships, in December 1946.
Sparrow I (AAM-N-2), like most post-war missile development programs, used a beam-riding guidance system, in which an aircraft’s fire-control radar would lock on to a target and the missile would fly along the radar beam. The missile’s rocket motor accelerated it to supersonic speed, after which it coasted. During initial testing, a radar operator would manually guide the beam to hold it on the target. For the operational version, the fire-control radar would indicate to the pilot the course to steer before and after firing. In 1952, a Sparrow I successfully intercepted an F6F Hellcat fighter at a range of 4,000 yards. The missile joined the fleet on F3H-2M Demon fighters beginning in March 1956, about when the first U.S. surface-to-air missile, Terrier, became available.
In 1951, before Sparrow I was even operational, BuAer had begun to see the Sparrow as a modular system, and explored alternative guidance systems. By 1956, the bureau recognized that beam-riding was not as promising as it once had seemed, because radar beams widen with distance from their emitters, decreasing accuracy and increasing chances of a miss. Sparrow II (AAM-N-8), developed by Douglas Aircraft with the Bendix Corporation handling the guidance components, would have an active-radar seeker. Sparrow III (AAM-N-6), under Raytheon’s development, would use a semiactive homing system. Sparrow IV never advanced to the point of receiving a missile designation, but it was to have used infrared (IR) guidance.
An active or IR seeker placed the least burden on the fighter firing the missile but required a lock before the missile could be launched. Semiactive guidance was much more attractive, because the signal on which the missile homed became stronger as the missile approached the target, but the missile itself did not require bulky radar emitters. In 1957, BuAer dropped the active-guidance Sparrow II, selecting Sparrow III for the next-generation F-4H-1 (later redesignated F-4) Phantom II fighter. The Canadian military took over development of the Sparrow II to equip the new CF-105 Avro Arrow interceptor, but the missile program died with the airplane’s cancellation in 1959.
Unfortunately, the Sparrow III earned a very poor reputation in Vietnam, with a ratio of less than one kill per ten missiles fired. The reason was simple. Sparrow III had been conceived as a beyond-visual-range missile to counter large bombers attacking a fleet. Radar guidance always entailed problems of target identification, but these were limited when fighters were under tight control from their carriers and most aircraft approaching the fleet were hostile. In Vietnam, however, most airplanes were friendly; identification errors would put friendly aircraft at risk. To solve that problem, pilots sometimes had to close with targets, identify them visually, and then fall back to fire from beyond the missile’s minimum range. This gave targets both warning and time to evade. Violently maneuvering targets were difficult for the semi-active missile guidance system to track. In addition, the motor of the AIM-7E version used in Vietnam burned for only part of the flight, after which the missile coasted, losing energy and maneuverability. There were also reliability problems.
In 1963 Raytheon proposed a new version, Advanced Sparrow, with a new motor, which would burn continuously, and airframe modifications to make it more maneuverable. This eventually became the AIM-7F, but it was not available until the end of the Vietnam War. Postwar, it became the standard U.S. long-range air-to-air missile for both the Navy and the Air Force.
The Sparrow had enough growth potential that it remains in Naval service. The system was so important that radars on board the F-14 Tomcat and F/A-18 Hornet were both modified to provide illumination for Sparrow missile seekers. The most common version after AIM-7F was AIM-7M (in this case, “M” stands for monopulse, the radar format for a better type of seeker). AIM-7P is an upgraded AIM-7M incorporating a data link, so that it can be fired without first locking on. It was Sparrow missiles that shot down enemy aircraft during the Gulf of Sidra incident (1981) and the First Gulf War (1991), not longer-range AIM-54 Phoenix missiles. In both cases, the system confronted exactly the kind of small maneuverable targets that had frustrated Sparrow users during the Vietnam War.
Active-radar seekers returned in the 1990s with the current AIM-120 advanced medium-range air-to-air missile (AMRAAM), but with the important improvement that the missile locks on to its target well after launch, based on settings and commands from the launching fighter. That is far beyond what could have been done in the 1950s and 1960s. Nevertheless, the Navy retained the Sparrow in service rather than adopting the actively guided AMRAAM, because the former was considered good enough and AMRAAM was too expensive.
Both Italy and Great Britain developed modified versions (respectively the Aspide and the Skyflash). China derived its LY-60/PL-10 family of missiles from Italian Aspides licensed in the 1980s. The Soviet K-25 air-to-air missile, copied from the Sparrow, did not enter production.
During the Sparrow’s original development, BuAer thought the missile might work as a surface-to-air weapon and conducted tests in 1955. This seems to have been a BuAer initiative; the larger Navy showed no immediate interest. In 1964, the U.S. Navy was interested in a simple surface-to-air missile to arm smaller ships and joined the U.S. Army in a program to develop the sophisticated Mauler missile. The Mauler proved far too expensive and was canceled, but those involved in the Sparrow surface-to-air tests remembered how well it had worked. The missile became the Sea Sparrow, which remains in service in the navies of the United States and its allies as both the RIM-7 Sea Sparrow and the RIM-162 evolved Sea Sparrow missile, but whose development is another story.
Note: When Navy and Air Force designations were unified in 1962, all air-to-air missiles received air intercept missile (AIM) designators. For clarity, I have used only the AIM designators in the text. Previous designations had been: Sparrow I, AAM-N-2; Sparrow II, AAM-N-3; and Sparrow III, AAM-N-6.