AIM-9 SIDEWINDER

The AIM-9 Sidewinder is a heat-seeking, short-range, air-to-air missile carried mostly by fighter aircraft and recently, certain gunship helicopters. The missile entered service with the United States Navy in the mid-1950s, and variants and upgrades remain in active service with many air forces after five decades. The United States Air Force purchased the Sidewinder after the missile was developed by the United States Navy at China Lake, California. 

The Sidewinder is the most widely used missile in the West, with more than 110,000 missiles produced for the U.S. and 27 other nations, of which perhaps one percent have been used in combat. It has been built under license by some other nations including Sweden. The AIM-9 is one of the oldest, least expensive, and most successful air-to-air missiles, with an estimated 270 aircraft kills in its history of use.

                                            HISTORY  OF  SIDEWINDER

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 B. 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 William "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, under development during the same period. After disappointing experiences with the Falcon in the Vietnam War, the Air Force replaced its Falcons with Sidewinder

                                                  THE SIDEWINDER  DESIGN

Basically, the Sidewinder systems consists of following components :

1. Nose dome
2. Reticle
3. Detector (or “seeker”)
4. Cooling system
5. Guidance Control System
6. Control actuation and flight fins
7. Stabilizing wings
8. Rocket motor
9. Fuzing system
10. Warhead
11. Battery

                                                       NOSE DOME

To allow infrared light to fall on the mirror, and to be passed to the detector, the nose dome has to be transparent to (infrared) light. The initial AIM-9B field model used a (special) glass nose dome window. In all later models, the glass nose dome was replaced by a much smaller polycrystalline Magnesium Fluoride (MgF2) dome, which provides better transparency to longer wavelength (cooler) infrared emissions, thus aldo allowing more faint infrared emissions to be passed to the detector. The forth generation models use a glass dome again, to provide unobstructed (off bore-sight) view for the Focal Plane Array seeker.

                                                     STEERING MIRROR

The Wiorld War II Enzian missile used an infrared detector mounted in front of a steering 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 the instant of 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 reaching the detector would be reflected from the outer edge of the mirror. If the target was closer on axis, the rays would be reflected from closer to the centre of the mirror.

                                                           RETILE

A reticle is essentially a modulator that chops the scene, using sequentially arranged transparent and opaque spokes on a spinning disk in front of the detector. The detector sees the scene chopped by the reticle at the spin rate times the number of reticle spokes. The reticle design allows the sensor to detect when it is spinning past the zero-point, allowing the angle of arrival of target sources to be determined. A single detector can then be used to perceive angular information to the target. The reticle also improves the signal-to-noise ratio by limiting the instantaneous field-of-view (FOV) of the detector. Small, point source targets are emphasized because they transmit their energy through a single reticle spoke. Large, extended source targets are minimized because their energy is spread between transparent and opaque spokes.

                                                   DETECTOR ELEMENT  

LEAD SULPHIDE(PbS)

Early Sidewinder models used lead sulfide (PbS) as photoconductive compound. PbS is relatively cheap and easy to manufacture. The legacy AIM-9B model has an uncooled PbS detector had a peak sensitivity in the 2 um region which limits the missile to stern engagements because the missile seeker has to look at the hot turbine in the in the engine tail pipe to see enough infrared energy to be able to track the target.

                           

INDIUM ANTIMONIDE(InSb)

Third generation (all-aspect) models (from the AIM-9L on), use Indium Antimonide as photo conductive compound, which is much more sensitive and thus offers target acquisition from any aspect at substantially greater ranges. Indium Antimonide seekers cooled to the temperature of liquid nitrogen (77K) have peak sensitivity in the 3-4 um region. Non-afterburning engines have theor peak emission in this region from both the hot metal and the exhaust plume.

MERCURY CADMIUM TELLURIDE(HgCdTe)

HgCdTe is a well established material with excellent sensitivity extending down to the 8-12 micron band (cool targets, ie FLIR applications, satellite tracking, detecting stealth vehicles ) but it is difficult to fabricate arrays from because of a very large variation in sensitivity from detector to detector. Such an array will introduce clutter (noise) into the image it views and this will understandably make it more difficult for the image processing algorithm to sift targets from the background.

PLATINUM SILICIDE

Another alternative is the use of Platinum Silicide which is unfortunately about fifty times less sensitive than HgCdTe and is spectrally limited to the 2.5-4 micron band(ie hot targets such as aircraft/airframes, vehicles, missile exhaust plumes); its strength lies in high uniformity of array sensitivity and ease of fabrication and thus low cost, commercial Silicon fabrication techniques are used as for eg MOS memory chips. This infers another major advantage, the ability to embed within the same slab of Silicon all the necessary electronics to scan and read out the image from the array, thus radically cutting the cost and complexity of the optical system as a whole.

                                                             COOLING

With an infra-red guided missile such as the Sidewinder, the discriminating ability of the seeker head — i.e. the ability to discriminate between different heat sources and their respective backgrounds — depends on the seeker head's own temperature, relative to the temperature of the ambient air. Therefore, the seeker head of an active missile is cooled up to minus 160 degrees Celsius in order to establish optimal sensitivity. The effective range of a cooled missile is 10-16 km, depending on the weather conditions — clouds tend to "mask" infra-red radiation — and the degree of humidity. The initial AIM-9B was uncooled. As a result, target acquisition and lock-on was extremely difficult, as experienced in combat by he US services. From the AIM-9D model on, the infra red detector was cooled. The US Navy and US Marine Corps used 6 litre nitrogen bottles in the LAU-7 launch rail, providing for 2.5 hours of seeker cool down, reflecting the primary fleet defence requirement. The US Air Force opted for Peltier thermoelectric cooling, allowing unlimited cooling time while the missile was on the launch rail (and – of course – power was applied). Later models use an internal Argon cooling system, eliminating the need for use of nitrogen bottles or internal bottles. The seeker head is cooled with specially treated air (officially the expensive Argon should be used instead).

 

                                        GUIDANCE CONTROL SECTION

The control actuation section adjusts flight fins near the nose of the missile based on instructions from the guidance electronics. A servo assembly includes a gas generator that feeds high-pressure gas to pneumatic pistons. The pistons are connected to rocker arms, which move the flight fins back and forth. The command signal from guidance control activates electric solenoids, which open and close valves leading to these pistons in order to tilt the fins from side to side. The flight fins themselves steer the missiles through the air -just like the flaps on an airplane wing.

                            CONTROL ACTUATION AND FLIGHT FINS

 The control actuation section adjusts flight fins near the nose of the missile based on instructions from the guidance electronics. A servo assembly includes a gas generator that feeds high-pressure gas to pneumatic pistons. The pistons are connected to rocker arms, which move the flight fins back and forth. The command signal from guidance control activates electric solenoids, which open and close valves leading to these pistons in order to tilt the fins from side to side. 

                      REAR STABILIZING WINGS AND ROLLERONS

The rear stabilizing wings provide the necessary lift to keep the missile aloft. Each of the four rear wings is outfitted with a simple stabilizing device called a rolleron. The rolleron is essential to provide a fixed roll axis orientation of the missile. Basically, a rolleron is a metal wheel with notches cut into it. As the missile speeds through the air, the air current spins the rolleron like a pinwheel. A spinning wheel resists lateral forces acting on it. In this case, the gyroscopic motion counteracts the missile's tendency to roll - to rotate about its central axis.

                                               ROCKET MOTOR

 The rocket motor provides the thrust to propel the missile through the air. The Hercules Mk 36 is used mostly (in several versions), although some earlier models used the Thiokol Mk.17. Once the propellant has burned up, the missile glides the rest of the way to its target. The Mk 36 Mod 7 rocket motor uses a single-grain propellant. A non propulsive head closure located on the forward end of the motor tube, blows out if the motor is accidentally ignited without the warhead installed, making the motor non propulsive (a fire hazard vice a missile hazard). The Mk 36 Mod 8 rocket motor is basically identical to the Mod 7 motor except that the Mod 8 motor is equipped with a safe-arm ignition assembly. The purpose of this assembly is to prevent accidental or inadvertent rocket motor ignition. The safe-arm ignition assembly must be manually rotated to the armed position before flight.

                                                 FUZE  SYSTEM

The Sidewinder isn't designed to go off when it actually hits the target; it's designed to go off when it gets very close to the target. The missile control system uses an fuzing system to figure out when it's within range and to detonate the warhead. Early models used a passive infrared fuse, later models have eight laser-emitter gallium-arsenide (GaAs) laser diodes and eight infrared sensors arranged around the outside of the missile airframe, just behind the flight fins. 

                                        

                                   WARHEAD

Early generation Sidewinders carried a blast/fragmentation warhead. The US Navy opted for a continuous rod warhead. Third and fourth generation Sidewinders carry the 20-pound (9-kg) WDU-17/B warhead. The WDU-17/B consists of a case assembly filled with PBXN-3 high explosive, booster plates, an initiator device and nearly 200 titanium fragmentation rods. When the fuzing system detects the target, it triggers the warhead by sending an explosive charge through the initiator (a train of low-explosive material) to the booster plates. The explosive charge from the initiator ignites low-explosive material in the booster plate channels, which ignites explosive pellets surrounding the high-explosive material. The pellets ignite the high explosive, causing it to release a huge amount of hot gas in a short amount of time. The powerful explosive force from this expanding gas blasts the titanium rods outward, breaking them apart to form thousands of metal pieces, all zipping through the air at top speed.

                                   BATTERY

A battery to provide power to the onboard electronics after the missile’s release from the launcher rail. While attached to the launcher rail, the missile is powered by an umbilical cable.