The AIM-7 Sparrow is a medium-range semi-active radar homing air-to-air missile operated by the United States Air Force, United States Navy, and USMC as well as various allied air forces and navies. Sparrow and its derivatives were the West's principal beyond visual range (BVR) air-to-air missile from the late 1950s until the 1990s. It remains in service, although it is being phased out in aviation applications in favor of the more advanced AIM-120 AMRAAM. The armed forces of Japan employ the Sparrow missile, though it is being phased out and replaced by the Mitsubishi AAM-4. NATO pilots use the brevity code Fox One in radio communication to signal launch of a Semi-Active Radar Homing Missile such as the Sparrow.
The Sparrow emerged from a late-1940s United States Navy program to develop a guided rocket weapon for air-to-air use. In 1947 the Navy contracted Sperry to build a beam riding version of a standard 5-inch (127 mm) HVAR, the standard unguided aerial rocket, under Project Hotshot. The weapon was initially dubbed KAS-1, then AAM-2, and, from 1948 on, AAM-N-2. The airframe was developed by Douglas Aircraft Company. The diameter of the HVAR proved to be inadequate for the electronics, leading Douglas to expand the missile's airframe to 8 in (203 mm) diameter. The prototype weapon made its first aerial interception in 1952.
After a protracted development cycle the initial AAM-N-2 Sparrow entered service in 1956, carried by the F3H-2M Demon and F7U Cutlass fighter aircraft. Compared to the modern versions, the Sparrow I was more streamlined and featured a bullet-shaped airframe with a long pointed nose.
Sparrow I was a limited and rather primitive weapon. The limitations of beam-riding guidance (which was slaved to an optical sight, requiring visual identification of the target) restricted the missile to visual-range attacks and made it essentially useless against a maneuvering target. Only about 2,000 rounds were produced to this standard.
By 1955 Douglas proposed going ahead with development, intending it to be the primary weapon for the F5D Skylancer interceptor, and ten years later an advanced active radar similar to the modern AMRAAM for the Avro Arrow were to be built under license by Canadair this is discussed later. However the small size of the missile forebody and the K-band AN/APQ-64 radar limited performance, and it was never able to work in testing.
The program was cancelled in 1958, and although there was some discussion of Canadair taking over the work, when the Arrow was cancelled all work ended.
Concurrently, in 1951, Raytheon began work on the semi-active radar homing version of Sparrow family of missiles, the AAM-N-6 Sparrow III. The first of these weapons entered United States Navy service in 1958.
The AAM-N-6a was similar to the -6, but used a new Thiokol liquid-fuel rocket engine for improved performance. It also included changes to the guidance electronics to make it effective at higher closing speeds. The -6a was also selected to arm the Air Force's F-110A Spectre (F-4 Phantom) fighters in 1962, known to them as the AIM-101. It entered production in 1959, eventually being built to about 7500 examples.
Another upgrade switched back to a Rocketdyne solid-fuel motor for the AAM-N-6b, which started production in 1963. The new motor significantly increased range, which was up to 35 km for head-on attacks.
During this year the Navy and Air Force agreed on a standardized naming for their missiles, the Sparrows becoming the AIM-7 series. The original Sparrow I and aborted Sparrow II became the AIM-7A and AIM-7B, even though both were long gone from the inventory. The -6, -6a and -6B became the AIM-7C, AIM-7D and AIM-7E respectively.
25,000 AIM-7E's were produced, and saw extensive use during the Vietnam War, where its performance was generally considered disappointing. The mixed results were a combination of reliability problems (exacerbated by the tropical climate), limited pilot training in fighter-to-fighter combat, and restrictive rules of engagement that generally prohibited BVR (beyond visual range) engagements. The Pk (kill probability) of the AIM-7E was less than 10%; US fighter pilots shot down a grand total of 55 aircraft using the Sparrow.
In 1969 an improved version, the E-2, was introduced with clipped wings and various changes to the fusing. Considered a "dogfight Sparrow", the AIM-7E-2 was intended to be used at shorter ranges where the missile was still travelling at high speeds, and in the head-on aspect, making it much more useful in the visual limitations imposed on the engagements. Even so, its kill rate was only 13% in actual combat in 1972, leading to a practice of ripple-firing all four at once in hopes of increasing kill probability. Its worst tendency was that of detonating prematurely, approximately a thousand feet in front of the launching aircraft, but it also had many motor failures, erratic flights, and fusing problems. An E-3 version included additional changes to the fusing, and an E-4 featured a modified seeker for use with the F-14 Tomcat.
Improved versions of the AIM-7 were developed in the 1970s in an attempt to address the weapon's limitations. The AIM-7F, which entered service in 1976, had a dual-stage rocket motor for longer range, solid-state electronics for greatly improved reliability, and a larger warhead. Even this version had room for improvement, leading British Aerospace and the Italian firm Selenia to develop advanced versions of Sparrow with better performance and improved electronics as the Skyflash and Selenia Aspide, respectively.
The most common version of the Sparrow today, the AIM-7M, entered service in 1982 and featured a new inverse monopulse seeker (matching the capabilities of Skyflash), active radar fuse, digital controls, improved ECM resistance, and better low-altitude performance. It was used to good advantage in the 1991 Gulf War, where it scored many USAF air-to-air kills; however its kill probability, overall, is still less than 40%.
The AIM-7P is similar in most ways to the M versions, and was primarily an upgrade program for existing M-series missiles. The main changes were to the software, improving low-level performance. A follow-on Block II upgrade added a new rear receiver allowing the missile to receive mid-course correction from the launching aircraft. Plans initially called for all M versions to be upgraded, but currently P's are being issued as required to replace M's lost or removed from the inventory.
The final version of the missile was to have been the AIM-7R, which added an infrared seeker to an otherwise unchanged AIM-7P Block II. A general wind-down of the budget led to it being cancelled in 1997.
Sparrow is now being phased out with the availability of the active-radar AIM-120 AMRAAM, but is likely to remain in service for a number of years.
An upgraded version with active radar seeker, called Active Sky Flash was proposed by BAe and Thomson-CSF, but did not receive funding because the RAF opted for other missiles.
As with other semi-active radar guided missiles, the missile does not generate radar signals, but instead homes in on reflected continuous-wave signals from the launch platform's radar. The receiver also senses the guidance radar to enable comparisons that enhance the missile's resistance to passive jamming.
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