F-16, 1st Launch of AIM-9X Sidewinder Air-to-Air Missile
The AIM-9 Sidewinder is a heat-seeking, short-range, air-to-air missile carried by fighter aircraft and recently, certain gunship helicopters. It is named after the Sidewinder snake, which detects its prey via body heat and also because of the peculiar snake-like path of flight the early versions had when launched.The Sidewinder was the first truly effective air-to-air missile, widely imitated and copied; yet its variants and upgrades remain in active service with many air forces after 5 decades. When a Sidewinder missile is being launched, NATO pilots use the brevity code Fox Two in radio communication, as with all rear-aspect, or “heat seeking” missiles.
Physics of Infrared Detection
In the 1940s, it was discovered that exposing lead sulfide to heat energy reduces the compound’s electrical resistance. This is an example of a property called photoconductivity; photoconductivity is also seen with illumination by other wavelengths of light[1] (see Ron Westrum’s book in reference section). One can measure the resulting current and then link that result to an action - in this case, a seeker head causing the missile to fly toward the heat source (a target aircraft or missile).
History
Early development
The development of the Sidewinder missile began in 1946 at the Naval Ordnance Test Station (NOTS), Inyokern, California, now the Naval Air Weapons Station China Lake, California as an in-house research project conceived by William Burdette McLean. McLean initially called his effort “Local Fuze Project 602″ using laboratory funding, volunteer help and fuze funding to develop what it called a heat-homing rocket. It did not receive official funding until 1951 when the effort was mature enough to show to Admiral “Deak” Parsons, the Deputy Chief of the Bureau of Ordnance (BUORD). It subsequently received designation as a program in 1952. The Sidewinder introduced several new technologies that made it simpler and much more reliable than its United States Air Force (USAF) counterpart, the AIM-4 Falcon that was under development in the same time period. After disappointing 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 concept and made it work.
Geometric arrangement of mirror, IR detector and target.
Geometric arrangement of mirror, IR detector and target.
The first was to replace the “steering” mirror with a forward facing mirror rotating around a shaft pointed out the front of the missile. The detector was mounted in front of the mirror. When the long axis of the mirror, the missile axis and the line of sight to the target all fell in the same plane, the reflected rays from the target reached the detector (provided the target was not very far off axis). Therefore, the angle of the mirror at detection estimated the direction of the target in the roll axis of the missile.
The yaw/pitch direction of the target depended on how far to the outer edge of the mirror the target was. If the target was further off axis, the rays reflected from the outer edge of the mirror. If the target was closer on axis, the rays reflected from closer to the centre of the mirror. The mirror’s linear speed was higher at the outer edge, though its thickness was the same. Therefore if a target was further off-axis its reflection occurred for a briefer time. The off-axis angle could then be estimated by the duration of the reflected pulse of infrared. If the pulse was long, the target was on-axis.
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[2], 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 a simple mechanical solution.
From:Wikipedia.org
