Sidewinder article
From Scramble - The Aviation Magazine
Contents
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Infrared target acquisition
All objects above absolute zero temperature emit infrared radiation. This radiation can be used to observe or track military targets. "Heat-seeking missiles" use infrared radiation from the hot exhaust (or other hot surfaces) of a target to guide themselves to their prey.
The target
Infrared energy emitted by aircraft are used by infrared guided missiles to acquire, track and navigate the missile to the target aircraft. The energy primarily originates from the engine(s). All objects or surfaces emit infrared energy in levels and band associated with its absolute temperature and the surface emissivity. Emissivity is the characteristic of the viewed surface to emit energy referenced against that of a perfect emitter (called a black body). As temperature rises, the peak wavelength of the infrared emission becomes shorter. The wavelength is usually expressed in a millionth of a meter, or microns (um). The surface of the sun has a absolute temperature of 6,000K. Its peak emission wavelength coincides with a peak sensivity of the human eye at 0,5 um or yellow light. The hot metal surrounding the exhaust of a jet engine and the afterburning plume of a fighter aircraft may have a peak energy emission in the 2-4 um range, while the exhaust plume of an non-afterburning engine may emit peak energy in the 4-5 um range. The hot metal on the engine may present a surface area of less than a square meter to the attacking missile, but will normally have a high emissivity. The hot exhaust plume is also an infrared source that may extend 20-50 ft behind the engine. It presents a large area from the side but, compared with hot metal, has low emissivity. The energy spectrum of the plume is also altered by the molecular emission from hot gases entrained in the hot air. Most targets are complex emitters with multiple infrared emitters operating at different temperatures and emission levels.
Background/clutter
The missile seeker perceives the target as a hot point source of energy in contrast to the background until it closes to a range where the target expands across seeker field of view, becoming an extended source. Reticle seekers inherently track point source targets more effectively as extended sources, giving them a margin of protection against clutter and background sources. The missile seeker prefers to view the target against a benign, uniform cool background, such a a clear blue sky, which yields a maximum signal-to-noise level. Unfortunately, for the missile seeker designer, the missile seeker must deal with the real world of clouds and extended ground infrared sources. Clouds reflect the light of the sun and can present very warm, extended background sources that can compete with the target aircraft signatures. The ground can be uniform warm or cold, or present hot point source targets such as buildings, vehicles, sun glint from window, water or hot sources, such as open fires and chimneys. Detecting and tracking an aircraft in the presence of background can be very challenging. A simple reticle seeker looking for a point source hot spot may find a cloud illuminated by the sun much more attractive than a jet aircraft. Much of the development efforts on reticle-based seekers have been conducted to improve performance in a clutter environment.
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Early development of infrared detection systems
Lead sulphide
In the 1920s, it was discovered that exposing lead sulphide (PbS) to thermal radiation (infrared rays) reduces the compound's electrical resistance. This is an example of a property called photoconductivity, also seen with illumination by other wavelengths of light. 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). The invention triggered a wide array of military applications, ranging from infrared missile guidance to detection and night vision systems. This section described the early development of the AIM-9 Sidewinder infrared guided air-to-air missile.
Spanner sighting tube
Prior to World War II, most of the major forces attempted to produce night-vision systems using PbS detectors and image intensifiers as displays, mostly for long-distance aircraft detection. None of these proved very successful, and only the German Spanner system entered production. Spanner used a long sighting tube projecting through the aircraft windscreen to give the pilot a view of the air directly ahead of their aircraft, but had limited range. The Spanner was used in specials modified Do-17 Z-10 Kauz II. All of these projects ended with the introduction of useful airborne radar sets, which offered much greater detection range.
Enzian missile
German scientists also experimented with an automatic missile guidance system intended to home in on the heat of aircraft engines and guide the Messerschmidt Enzian missile. A new design known as Madrid, used an PbS photocell mounted on the front of a steerable telescope mirror, with small metal vanes placed in front of the cell in the shape of a cross. By moving the mirror side to side (or up and down), the vanes would block off more or less of the image of the target, and the system continually moved the mirror in both directions while attempting to find the direction that maximized the signal. This kept the mirror pointed at the target. The missile's control system then had the task of attempting to point the missile in the same direction as the mirror. However the system was never actually developed beyond a test-bench mock-up. Years later, the US Navy adopted this system and perfected it in the development of the Sidewinder air-to-air missile.
Local Fuze Project 602
In the late 1940s, Bill McLean worked on an ‘heat homing rocket’. Initially he called his effort "Local Fuze Project 602" using laboratory funding, volunteer help and fuze funding. 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 was born.
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Introduction to infrared missile guidance
Direct pursuit
The easiest navigation method is to point directly at the target throughout the engagement. If you continue to point at the target for the duration of the flyout, the missile will hit the target eventually. This method is called direct persuit or pure pursuit. The Enzian used this navigation method. As long as the missile had enough speed advantage over its target and it didn't run out of fuel during the interception, the missile would hit the target.
Proportional pursuit
The Sidewinder also included a dramatically improved guidance algorithm. The Sidewinder isn't guided on the actual position recorded by the detector, but on the change in position since the last sighting. So if the target remained at 5 degrees left between two rotations of the mirror, the electronics would not output any signal to the control system. Consider a missile fired a right angles to its target; if the missile is flying at the same speed as the target it should "lead" it by 45 degrees, flying to an impact point far in front of where the target was when it was fired. If the missile is travelling four times the speed of the target, it should follow an angle about 11 degrees in front. In either case, the missile should keep that angle all the way to interception, which means that the angle that the target makes against the detector is constant. It was this constant angle that the Sidewinder attempted to maintain. This "proportional pursuit" system is very easy to implement, yet it offers high-performance lead calculation almost for free and can respond to changes in the target's flight path, which is much more efficient and makes the missile "lead" the target.
Fixed roll axis orientation
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, these are known as rollerons. 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, the rolleron.
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Counter (counter) measures
Infrared counter measures (IRCM)
Another challenge are infrared counter measures, such as flares and jammers. In order to protect aircraft from infrared tracked missiles, infrared decoys widely used. In general, decoys fall into two categories: simple flares and active infrared decoys.
Flares
Flares are hot bodies radiating energy considerably greater than the aircraft defended and in the same wavelength spectrum, luring the missile to track the flare instead of the aircraft. As the flare energy is greater than that of the target, the missile seeker transfers lock to the flare as long as the flare achieves greater emissivity, while the target and the flare are in the seeker's FOV. Typically, the centroid of the energy emitted energy by the combined sources. Thus, the seeker's FOV is typically limited to a small value. When the sources separate sufficiently in space so that both cannot be contained in the FOV, the seeker-tracking point will shift to the stronger of the two sources.
Infrared jammers
A first generation infrared counter measure was the AN/ALQ-144 Hot Brick. This system consists of a heated Silicon Carbide block that radiates a large amount of infra-red energy, it is surrounded by a large cylindrical mechanical shutter, that modulates the infra-red output, producing a pulsing pattern. Early infrared guided missiles used a rotating reticle, when a target was not on the sensor's centreline, it would produce a pulse as the reticle swept over the target. When the target was on the sensor's centerline, the sensor would produce a constant signal. This constant signal was required by the early missiles to produce a "lock on" that would allow a launch. The AN/ALQ-144 Hot Brick IRCM produced a pattern of pulses that was approximately synchronized with the rotation rate of these reticles. Before launch this would prevent the missile actually locking onto the target, preventing the operator from firing the missile. After launch this would cause the missile to think that the target was off to one side, and cause the missile to steer away from the aircraft carrying the IRCM. The introduction of rosette and "staring" scanning techniques in second generation missiles reduced the effectiveness of the AN/ALQ-144 Hot Brick and AN/ALQ-147, later upgrades restored the effectiveness of the jammers. More modern equipment includes BAE Systems AN/ALQ-204 Matador, typically fitted above the exhaust of non afterburning engines.
Directional Infrared Counter Measures
Active infra-red jamming causes the missile to miss the intended target by disturbing the seeker-tracking process. The active IRCM acts in such a way as either to cause a complete loss of target tracking or to degrade target tracking. An advanced active infrared jamming system is the Northrop Grumman AN/AAQ-24 Directional Infrared Counter Measures (DIRCM), which uses pulsing flashes of IR energy confuse the missile guidance system. The effectiveness of a DIRM system is demonstrated in this video clip.
Infrared counter counter measures (IRCCM)
Infrared counter counter measures (IRCCM) are built in most third generation Sidewinders. IRCCM has two parts: the switch (which detects the flare in the seeker FOV) and activates the response circuitry. The seeker's reaction the the switch is to reject the flare or to limit its effect on target tracking. A simple response is to order the missile to continue the flight as it was performing just prior to the switch. However, this response cannot take account of the target manoeuvre after switching. Switch and response techniques are highly classified and include using correlations (of previous tracking errors of previous signals), adaptive two-colour (using the fact that a flare is substantially brighter at the shorter wavelenghts, by processing the signature in two infrared bands, the missile can discriminate the true target from the flare).
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The Sidewinder system
Basically, the Sidewinder systems consists of following components:
- Nose dome
- Reticle
- Detector (or “seeker”)
- Cooling system
- Guidance Control System
- Control actuation and flight fins
- Stabilizing wings
- Rocket motor
- Fuzing system
- Warhead
- Battery
- Hanger and umbilical cable
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.
Infrared trackers
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. Rotating on a fixed shaft, the mirror's linear speed was higher at the outer edge. Therefore if a target was further off-axis its "flash" in the detector occurred for a briefer time, or longer if it was closer to the centre. The off-axis angle could then be estimated by the duration of the reflected pulse of infrared.
Reticle
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.
Spin rate
Reticles are basically spinning disks which have alternating bands of opaque and transmissive material. The energy passing the reticle is focussed (by mirrors) on the detector element which outputs pulses whose amplitude corresponds to the received target energy. The pulse freqency, known as the carrier frequency, is a product of the spin rate of the reticle and the number transmissive/opaque spoke pairs in the reticle. The spin rate of the reticle may or may not be held constant throughout the missile time of flight. On some missiles the weight and cost of a spin governor is eliminated and the spin rate is akkiwed to gradually decline as the missile approaches the target. Missiles with relative short fly-out times, normally 15s for SAMs, will see the spin rate degrade 10-15 Hz during the flyout. Most missiles however maintain a relatively constant spin rate. When the reticle is first spun up, a phase reference is established to encode the angular position of the target. The missile seeker uses the phase reference to determine the angle of the target to jeep the seeker telescope pointed at the target and to correct the missile flyout to intercept the target. The seeker always acts to center the target within its FOV but in doing so, it creates the tracking information that allows guidance laws to be applied to guide the missile to the target. Essentially, two principles were used to feed target information to the GCS: Frequency Modulated reticles and Amplitude Modulated reticles.
Amplitude Modulated tracking
Most older Sidewinder seeker systems employ amplitude modulation (AM) tracking. The detector amplifier unit is fed into a band-pas filter which strips the DC signal from extended sources. The band-pass filter output is an interrupted sine wave usually called missile audio. The signal is normally amplified and used by the gunner to determine when the missile has established a target track. If a circular target was located at the centre of the rotating reticle, the amount of light transmitting the reticle does not change with the spinning motion of the reticle. Hence, the spatial integral of infrared light through the reticle would be a Direct Current value and the modulation (Alternating Current (AC) response) would be zero. As the target in the Field of View moves towards the reticle periphery, the modulation grows. The construction of AM reticles are relatively simple and they have adequate discrimination against low-radiance backgrounds. For small targets or point sources, the AM configuration is useless in generating an error signal for target location as modulation peaks at a very small distance from the reticle centre. Moreover, they have low discrimination against high-radiation backgrounds, hence the development of FM reticles.
Frequency Modulated tracking
Introduced in the AIM-9L, the Frequency Modulated reticle necessitating some fundamental changes to the guidance electronics. The FM reticle provides superior performance by reducing the effect of a target's increasing size with decreasing range on the seeker error signal output, a factor which can affect the behaviour of AM seekers, while providing the potential for better countermeasures rejection. In addition, design changes were adopted to bridge the dead zone about the missile's immediate boresight, a characteristic of conventional reticle seekers. To widen the manoeuvre envelope, lambda compensation is employed, a technique which prevents the seeker from reaching its angular limit during the early phase of its flight. If a target can force the seeker past its angular limit, lock is broken and the missile lost.
Spinning rate
Reticles of initial AIM-9B model spun at 70Hz, later models spun at a higher frequency, typically 100Hz or 125Hz, improving the target track rate. This was improved from 11 degs to more than 16.5 deg/s in later models such as the AIM-9J and AIM-9P-4/-5. In the forth generation Sidewinder model, the infrared sensor array is coupled with a conical scanning system.
Rosette scanning
A newer and mechanically/optically simpler arrangement is that of the rosette scanning tracker. A rosette scanning tracker will usually employ a single fixed detector and moving optics which have a small instantaneous field of view. The optics then scan a much larger field of view in a rosette pattern, which has the appearance of a flower with multiple petals. The strength of this technique lies in the simplicity of the scanning mechanism and a minimum of hardware in the optical path; the penalty is in the additional complexity of the electronics required cf. a rotating reticle tracker. Jam resistance of these trackers has not been discussed at length in the open literature, no doubt for fear of compromising weapons such as the FIM-92 Stinger SAM. Aparently, rosette scan has not been employed in the Sidewinder.
Focal Plane Array (FPA)
Advancements in infrared countermeasaure (flares and jammers) and the desire to aim for point selection (such as the cockpit) has spurred the development of increasingly capable seekers to process the scene and led to imaging seekers. Imaging seekers do not employ a reticle, but use an array of detector elements that detect energy from the scene and build a spatial map of that scene. In fourth generation models, such a Focal Plane Array (FPA) seeker is employed, providing an actual image of the target instead of a signal only. It has an array of sensors that generate an electrical signal when exposed to the infrared light given off by hot objects.
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. From the AIM-9D on, the PbS detector was cooled, which reduced the self generated noise in the detector material, thus requiring fewer photons to make a detectable output. This resulted in longer lock-on ranges, albeit at the cost of complexity and a higher price tag. In general, cooling PbS elements made them more sensitive in the 2 um region and still limited to stern hemisphere engagements, except in cases were sun glint or perhaps landing lights provided sufficient signature for tracking.
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. As the exhaust plume blossoms out on either side of the engine outside of the nozzle, permits the seeker to see the hot gasses even from the front of the aircraft. Thus, cooled InSb seekers have an all-aspect target capability, meaning they can detect and track a target even from head-on.
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. As a result additional signal processing is usually required to compensate for array non uniformity. At the time of writing Rockwell International were offering 128 x 128 element arrays in HgCdTe.
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. Platinum Silicide arrays of 512 x 512 elements are now offered by Hughes, the highly sought (by the military) 1024 x 1024 array size is expected to become available in the next two years as the technology matures. Iridium Silicide offers like HgCdTe operation in the 8-12 micron band but is very immature as a production technology today.
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). The air is filtered and de-hydrated, then compressed to 345 bar (5,000 psi) and stored in a small stainless-steel bottle, which is placed near the missile foreplanes. The de-hydration process is necessary in order to prevent the head from being frozen. A small amount of compressed air is continuously being expanded, causing a small stream of air, cooling the seeker head. Within two minutes the seeker head temperature is at the required level. The initial cooling is taking up most of the air; maintaining the temperature at the required level is taking up relatively less air. Nevertheless, the time the seeker head temperature can be maintained at the required level is limited. During long missions, the amount of compressed air available must be used wisely. Therefore, in most cases the missiles will only be activated when they actually will be used.
Guidance control section
The Guidance Control Section (GCS)'s main goal is to keep the infrared emission of the enemy aircraft roughly centered so that the missile nose continues to point toward the target. If the infrared image moves off center, the control system sends a signal to the servo assembly. To compensate for the target's own motion, the control system uses proportional navigation, explained earlier. Legacy Sidewinders used vacuum tube electronics, resulting in very bad reliability. Second generation Sidewinders introduced solid state electronics.
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. The flight fins themselves steer the missiles through the air -just like the flaps on an airplane wing.
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. This is accomplished by the use of a hex-head T-handle. The rocket motor burns up solid propellant material to generate a high-pressure gas that streams out the back of the missile (the motor uses special low-smoke propellant material to help hide the missile from the enemy). This provides the initial thrust necessary to get the missile off the launcher and push it through the air at supersonic speeds (the current model flies at about Mach 2.5).
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. When the Sidewinder is in flight, the DSU-15/B target detecting systems lasers are constantly emitting laser beams in a spoke pattern around the missile. If the missile gets close enough to the target, the laser beams will reflect off the aircraft body and bounce back to the infrared sensors. The control system recognizes that the missile is right next to the target and triggers the detonation of the warhead.
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. If the warhead goes off within range of the target, the speeding titanium fragments will break apart the enemy aircraft's fuselage. In some cases, the missile may go right up the target's tailpipe, demolishing the aircraft from the inside. The WDU-17/B is referred to as an annular blast fragmentation warhead because the explosive force carries the metal fragments outward in all directions, in an annular, or ring-shaped, pattern.
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.
Hanger and umbilical cable
Before launching, the missile sits under one of the aircraft's wings, mounted to a launcher on the wing by a center and aft hangers. An "umbilical cable" near the nose of the missile connects the onboard electronic control system to the aircraft's computer system.
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References
- Encyclopedia of optical engineering, Volume 3, Ronald G. Driggers
- Materials for infrared windows and domes: properties and performance, Daniel C. Harris
- Heat Seeking Missile Guidance, Australian Aviation, March, 1982, Carlo Kopp
