AIM-9 Sidewinder

Sidewinder MissileThe AIM-9 Sidewinder is a heat-seeking, short-range, air-to-air missile carried by fighter aircraft. It is named after the Sidewinder snake, which also detects its prey via body heat. The Sidewinder was the first really effective air-to-air missile, widely imitated and copied. Its latest variants remain in active service with many air forces.

Background Information
Developed by the US Navy starting in the late 1940s, the Sidewinder introduced several new technologies that made it simpler and much more reliable than its Air Force counterpart, the AIM-4 Falcon. After terrible experiences with the Falcon in the Vietnam War, the Air Force replaced its Falcons with Sidewinders.

The primary advantage to the Sidewinder is its sophisticated, yet simple detection and guidance system. During WWII the Germans had experimented with infrared guidance systems in a large missile known as the Enzian, but were unable to get it to work reliably. The Enzian was guided by an IR detector mounted in a small, steerable telescope. A vane in front of the mirror shaded the detector, so the system could locate the target. By continually turning toward the telescope, the missile was guided toward the target using what is known as a pure pursuit.

The Sidewinder improved on this. The first was to replace the "steering" mirror with a mirror rotating around a shaft pointed out the front of the missile. The detector was mounted in front of the mirror. Instead of attempting to track the target in the mirror, the IR sensor would see the target as brief flashes as the mirror lined up with the target. By knowing where the flash was as the mirror spun, the direction (radially) to the target was also known. This system could also track the radial angle to the target by timing the flashes. If the target was further to the side, the flash seen in the detector would be shorter due to the mirror's higher rate of motion at the outside.

This signal makes the tracking system both simpler and better. Instead of simply pointing the missile at the target (which is inefficient), the Sidewinder "remembered" each flash's direction and time. By attempting to zero out the changes, instead of the difference between the detector and missile angles, the Sidewinder flies a course known as proportional pursuit, which is much more efficient and makes the missile "lead" the target.

However this system also requires the missile to have a fixed roll axis orientation. If the missile spins at all, the timing based on the speed of rotation of the mirror is no longer accurate. Correcting for this spin would normally require some sort of sensor to tell which way is "down" and then adding controls to correct it. Instead, small control surfaces were placed at the rear of the missile with spinning disks on their outer surface. Airflow over the disk spins them to a high speed. If the missile starts to roll, the gyroscopic force of the disk drives the control surface into the airflow, cancelling the motion. Thus the Sidewinder team replaced a potentially complex control system with some simple mechanical bits.

First Operation

Sidewider missile

A prototype Sidewinder, the XAAM-N-7 (later AIM-9A), was first fired successfully in September 1953. The initial production version, designated AAM-N-7 (later AIM-9B), entered operational use in 1956, and has been improved upon steadily since. The first combat use of the Sidewinder was in 1958 with the air force of the Republic of China on Taiwan. During that period of time, the ROC was engaged in air battles with the People's Republic of China over the Taiwan Strait. The United States provided a few dozen Sidewinders to ROC forces, which used them to great effect against PRC MiG-15s, adding a new element to an air war which had formerly been fought only with guns.

The Taiwan Strait battles inadvertently produced a new derivative of Sidewinder: shortly after that conflict the Soviet Union began the manufacture of the K-13/R-3S missile (NATO reporting name AA-2 'Atoll'), a reverse-engineered copy of the Sidewinder. It was reportedly made possible after a Taiwanese AIM-9B hit a Chinese MiG-15 without exploding, and served as a "university course" in missile design for Soviet engineers. The K-13 and its derivatives remained in production for nearly 30 years.

Although originally developed for the USN, the Sidewinder was subsequently adopted by the USAF as the GAR-8 (later AIM-9E). During the 1960s the USN and USAF pursued their own separate versions of the Sidewinder, but cost considerations later forced the development of common variants.

The Sidewinder subsequently evolved through a series of upgraded versions with newer, more sensitive seekers with various types of cooling and various propulsion, fuse, and warhead improvements.

Although each of those versions had various seeker, cooling, and fusing differences, all but one shared infrared homing. The exception was the US Navy AAM-N-7 Sidewinder IB (later AIM-9C), a Sidewinder with a semi-active radar homing seeker head developed for the F-8 Crusader. Only about 1,000 of these weapons were produced, many of which were later rebuilt as the AGM-122 Sidearm anti-radiation missile.

Sidewinders Variants

AIM-9L / AIM-9M / AIM-9M-7
The next major advance in IR Sidewinder development was the AIM-9L ("Lima") model, introduced in 1978. This was the first "all-aspect" Sidewinder with the ability to attack from all angles, including head-on. In its first combat uses by Israel over Lebanon and by Britain during the Falklands War, the "Lima" reportedly achieved a kill ratio of around 80%, a dramatic improvement over the 10-15% levels of earlier weapons. In both cases, the users' opponents had not developed any tactics for the evasion of a head-on missile shot of this kind, making them all the more vulnerable.

The subsequent AIM-9M ("Mike") has the all-aspect capability of the L model while providing all-around higher performance. The M model has improved defense against infrared countermeasures, enhanced background discrimination capability, and a reduced-smoke rocket motor. These modifications increase its ability to locate and lock on a target and decrease the missile's chances for detection. Deliveries began in 1983. The AIM-9M-7 was a specific modification to AIM-9M in response to threats expected in the Persian Gulf war zone.

AIM-9X
Now entering service is the AIM-9X, a new variant with an imaging infrared focal plane array seeker with claimed 90° off-boresight capability, compatibility with helmet-mounted sights (the new U.S. JHMCS, Joint Helmet-Mounted Cueing System), and a totally new thrust-vectoring system replacing the traditional control surfaces. It retains the same motor and warhead of the "Mike," but its lower drag gives it improved range and speed.

SIDEARM / AGM-122A
The Sidewinder was also adapted into a new missile, the AGM-122A Sidearm, which is an Anti-radiation missile utilizing an AIM-9C guidance section modified to detect and track a radiating ground-based air defense system radar. The target detecting device is modified for air-to-surface use, employing forward hemisphere acquisition capability. Sidearm stocks have apparently been expended, and the weapon is no longer in the active inventory.

Architecture
The Aim-9 is made up of a number of different components manufactured by different companies including AeroJet and Raytheon. The missile is divided into four main sections: guidance, target detector, warhead, and rocket motor.

The Guidance and Control Unit (GCU) contains most of the electronics and mechanics that enable the missile to function. At the very front is the IR seeker head utilizing the rotating spindle, mirror, and five CDS cells or “pan and scan” CCD (AIM-9X), electric motor, and armature all protruding into a glass dome. Directly behind this are the electronics that gather data, interpret signals, and generate the control signals that steer the missile. An umbilical that attaches to the launcher, witch is pulled from the missile at launch. A 5,000 PSIG argon bottle or Sterling self generating liquid nitrogen engine (AIM-9X) to cool the electronics. Two electric servos power the canards to steer the missile (Except AIM-9X). At the back is a gas grain generator or thermal battery (AIM-9X) to provide electrical power. The AIM-9 X features High-Off-Boresight capability, together with JHeMoCS (Joint Helmet Mounted Cuing System) this missile is capable of locking on to a target that it is behind it. The Aim-9X also features a Built-In-Test to aid in maintenance and reliability.

Next is a target detector with eight IR emitters/detectors that detonate the warhead in the event of a near miss. Versions older than the AIM-9L featured an influence fuse that relied on the targets magnetic field as input. Current trends in shielded wires and non-magnetic metals in aircraft construction rendered this obsolete.

Recent models of the AIM-9 are configured with an annular blast fragmentation warhead. The case is made of spirally wound spring steel filed with 12 pounds of tritonol. The fuse requires five seconds at 20 Gs acceleration to arm and features a safe/arm device.

The solid propellant rocket motor provides propulsion for the missile. A reduced smoke propellant makes it difficult for a target to see and avoid the missile. This section also features the launch lugs used to hold the missile to the rail of the missile launcher. The forward of the three lugs has two contact buttons that electrically pre-arm the warhead and activate the motor igniter. The fins provide stability from an aerodynamic point of view but it is the rolerons at the end of the fins providing gyroscopic precession that prevents the uncanny motion that gave the Sidewinder its name in the early days. The canards and fins of the AIM-9X are smaller to accommodate its use on the F-22 and this time it is the fins that do the steering while the wings up front provide stability. The AIM-9X also features Vectored Thrust to increase maneuverability and accuracy with four veins inside the exhaust that move as the fins move. The last upgrade to the missile motor on the AIM-9X is the addition of a wire harness that allows communication between the guidance section and the control section as well as a new 1760 bus to connect the guidance section with the launcher’s digital umbilical.

The Sidewinder is the most widely used air-to-air missile in the West, with more than 110,000 missiles produced for 27 nations excluding the United States. It has been built under license by other nations (including Sweden, which builds it under the local designation Rb24). The AIM-9 is one of the oldest, least expensive and most successful air-to-air missiles.

It has been said that the design goals for the original Sidewinder were to produce a reliable and effective missile with the "electronic complexity of a table model radio and the mechanical complexity of a washing machine" — goals which were well accomplished in the early missiles.