The AIM-9 Sidewinder is a heat-seeking, short-range, air-to-air missile carried by fighter aircraft.
The Sidewinder has a high-explosive warhead and an infrared heat-seeking guidance system. Its main components are an infrared homing guidance section, an active optical target detector, a high-explosive warhead and a rocket motor. The guidance section enables the missile to home in on the engine exhaust of target aircraft. An infrared unit costs less than other types of guidance systems and can be used day or night in all weather conditions. The infrared seeker also permits the pilot to launch the missile then leave the area or take evasive action while the missile guides itself to the target.
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 Viet Nam War, the Air Force replaced its Falcons with Sidewinders.
The primary advantage to the Sidewinder was its use of a simple yet very sophisticated detection/guidance system. In WWII the Germans had experiments with infrared guidance systems in a large missile known as the Enzian but were unable to get it to work reliably. The system used an IR detector mounted in a small steerable telescope, and used a vane in front of the mirror to tell on which side of center the target was in order to guide. By continually trying to turn the missile body toward the current direction of the telescope, the missile would guide toward the target using what is known as a pure pursuit.
The Sidewinder improved on this in a number of ways. The first was to replace the "staring" mirror with a system using a mirror that was rotating around a shaft pointed out the front of the missile, with the detector mounted in a fixed location in front of it (not to 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. In addition this system could track the angle-off to the target in a clever manner. If the target was further to the side of the field of view, the flash seen in the detector would be shorter due to the mirror's higher rate of motion at the outside.
Finally this sort of signal made the tracking system both simpler and much better. Instead of simply pointing the missile at the target (which is inefficient) the guidance system, 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 flew 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 angle of flight. 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 control inputs to correct it. Instead the Sidewinder engineers came up with a very clever solution. Small control surfaces were placed at the rear of the missile with spinning disks on their outer surface. Airflow over the disk would spin them to a high speed, and if the missile started to roll, the gyroscopic force of the disk would drive the control surface into the airflow and produce the opposite control input. Thus the Sidewinder team replaced a potentially complex control system with a small bit of metal.
A prototype of the Sidewinder, the AIM-9A, was first fired successfully in September 1953. The initial production version, designated AIM-9B, entered operational use in 1956 and has been improved upon steadily since. The L model was the first Sidewinder with the ability to attack from all angles, including head-on. The AIM-9M, currently the only one operational, 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. The AIM-9M-9 and AIM-9X are future variants presently under development.
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. The AIM-9 is one of the oldest, least expensive and most successful missiles in the entire U.S. weapons inventory.
General Charateristics for AIM-9B