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 five decades. When a Sidewinder missile is being launched, NATO pilots use the brevity code Fox two in radio communication, as with all "heat-seeking" missiles.
About 110,000 Sidewinders have been built, of which perhaps one percent have been used in combat, resulting in some 250-300 kills world-wide to date. The missile was designed to be simple to upgrade.
Prior to World War II, most of the major forces attempted to produce night-vision systems using lead sulfide 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. All of these projects ended with the introduction of useful airborne radar sets.
IR detectors found more widespread use for land-based systems. These included everything from sighting systems for tanks and even snipers, to a variety of night driving aids. However the Germans also experimented with an automatic missile guidance system intended to home in on the heat of aircraft engines and guide their Enzian missile. It used a single detector located at the focus of a small telescope, with four vanes positioned between the detector and telescope. The telescope "nodded", causing the signal falling on the detector to increase and decrease depending on how much of the signal was being blocked by the vanes. This signal would then be used as an input to a simple autopilot, by continually turning toward the telescope optical axis, the missile was guided toward the target using what is known as a pure pursuit. Development was not complete when the war ended.
The Sidewinder took several design tips from the Enzian, but made a number of improvements that dramatically improved its performance. 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 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, he 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 center. The off-axis angle could then be estimated by the duration of the reflected pulse of infrared.
The Sidewinder also included a dramatically improved guidance algorithm. The Enzian attempted to fly directly at its target, feeding the direction of the telescope into the control system as it if were a joystick. This meant the missile always flew directly at its target, and under most conditions would end up behind it, "chasing" it down. This meant that the missile had to have enough of a speed advantage over its target that it didn't run out of fuel during the interception.
The Sidewinder guided not on the actual position recorded by the detector, but 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 traveling 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.
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.
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 Sidewinder subsequently evolved through a series of upgraded versions with newer, more sensitive seekers with various types of cooling and various propulsion, fuze, 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 U.S. 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.
The Navy Sidewinder design progression went from the early production B model to the D model that was used extensively in Vietnam. The G and H models followed with new forward canard design improving ACM performance and expanded acquisition modes and improved envelopes. The "Hotel" model followed shortly after the "Golf" and featured a solid state design that improved reliability in the carrier environment where shock from catapult launches and arrested landings had a deteriorating effect on the earlier vacuum tube designs. The Ault report had a strong impact on Sidewinder design, manufacture, and handling.
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 directions, including head-on, which had a dramatic effect on close in combat tactics. In its first combat use by Israel over Lebanon and by the United Kingdom during the Falklands War, the "Lima" reportedly achieved a kill ratio of around 80%, a dramatic improvement over the 10-15% levels of earlier versions. In both initial combat uses of AIM-9L, the 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 AIM-9L was also the first Sidewinder that was a joint variant used by both the US Navy and Air Force. The "Lima" was distinguished from earlier Sidewinder variants by its double delta forward canard configuration and natural metal finish of the guidance and control section. The Lima was also built under license in Europe by a team headed by Diehl BGT Defence. There are a number of "Lima" variants in operational service at present. First developed was the 9L Tactical, which is an upgraded version of the basic 9L missile. Next was the 9L Genetic, which has increased infra-red counter counter measures (IRCCM); this upgrade consisted of a removable module in the Guidance Control Section (GCS) which provided flare-rejection capability. Next came the 9L(I), which had its IRCCM module hardwired into the GCS, providing improved countermeasures as well as an upgraded seeker system. Diehl BGT also markets the AIM-9L(I)-1 which again upgrades the 9L(I)GCS and is considered an operational equivalent to the initially "US only" AIM-9M.
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 capability 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 of the initial AIM-9M-1 began in 1982. The only changes from the AIM-9L to the AIM-9M were related to the Guidance Control Section (GCS). Several models were introduced in pairs with even numbers designating Navy versions and odd for USAF: AIM-9M-2/3, AIM-9M-4/5, and AIM-9M-6/7 which was rushed to the Persian Gulf area during Desert Shield to address specific threats expected to be present. The AIM-9M-8/9 incorporated replacement of five circuit cards and the related parentboard to update infrared counter counter measures (IRCCM) capability to improve 9M capability against the latest threat IRCM. The first AIM-9M-8/9 modifications, fielded in 1995, involved deskinning the guidance section and substitution of circuit cards at the depot level, which is labor intensive and expensive -- as well as removing missiles from inventory during the upgrade period. The AIM-9X concept is to use reprogrammable software to permit upgrades without disassembly.
The Navy began development of AIM-9R, a Sidewinder seeker upgrade in 1987 that featured a Focal Plane Array (FPA) seeker using video-camera type charge-coupled device (CCD) detectors and featuring increased off-boresight capability. The technology at the time was restricted to visual (daylight) use only and the USAF did not agree on this requirement, preferring in another technology path. AIM-9R reached flight test stage before it was cancelled and subsequently both services agreed to joint development of the AIM-9X variant.
China Lake developed an improved compressed carriage control configuration titled BOA. Data from the testing was used for subsequent AIM-9X development. The BOA design reduced size of control surfaces eliminating the rollerons and returned to simple forward canard design. Although the Navy and Air Force had jointly developed and procured AIM-9L/M, BOA was a Navy only effort supported by internal China Lake Independent Research & Development (IR&D) funding. Meanwhile, the Air Force was pursuing a parallel effort to develop a compressed carriage version of Sidewinder for the F-22 called Boxoffice. The Joint Chiefs of Staff directed that the services collaborate on AIM-9X, which effectively put an end to the disparate efforts. The results of BOA and Boxoffice were provided to the industry teams competing for AIM-9X and elements of both can be found in the AIM-9X design.
After looking at advanced short range missile designs during the AIM portion of the ACEVAL/AIMVAL Joint Test and Evaluation at Nellis AFB in the 1974-78 timeframe, the Air Force and Navy agreed on the need for the Advanced Medium Range Air-to-Air Missile AMRAAM. But agreement over development of an Advanced Short Range Air-to-Air Missile ASRAAM was problematic and disagreement between the Air Force and Navy over design concepts (Air Force had developed AIM-82 and Navy had flight-tested Agile and flown it in AIMVAL). Congress eventually insisted the services work on a Joint effort and AIM-9M became the result thereby compromising without exploring the improved off boresight and kinematic capability potential offered by Agile. In 1985, the Soviet Union did field a SRM (AA-11 Archer/R-73) that was very similar to Agile. At that point, the Soviet Union took the lead in SRM technology and correspondingly fielded improved IRCM to defeat or reduce the effectiveness of the latest Sidewinders. As relations improved in the aftermath of the Soviet Union, the West became aware of how potent both the AA-11 and IRCM were and SRM requirements were readdressed.
For a brief period in the late '80s, an ASRAAM effort led by a European consortium was in play under a MOA with the United States in which AMRAAM development would be led by the US and ASRAAM by the Europeans. The UK working with the aft end of the ASRAAM and Germany developing the seeker (Germany had first hand experience improving the Sidewinder seeker of the AIM-9J/AIM-9F). By 1990, technical and funding issues had stymied ASRAAM and the problem appeared stalled so in light of the threat of AA-11 and improved IRCM, the US embarked on determining requirements for AIM-9X as a counter to both the AA-11 and improving the IRCCM features. The first draft of the requirement was ready by 1991 and the primary competitors were Raytheon and Hughes. Later, the UK resolved to revive the ASRAAM development and selected Hughes to provide the seeker technology in the form of a high off-boresight capable Focal Plane Array. However, the UK did not choose to improve the turning kinematic capability of ASRAAM to compete with AA-11. As part of the AIM-9X program the US conducted a Foreign Cooperative Test (FCT) of the ASRAAM seeker to evaluate its potential and an advanced version featuring improved kinematics was proposed as part of the AIM-9X competition. In the end, the Hughes evolved Sidewinder design featuring virtually the same seeker as used by ASRAAM was selected as the winner.
The AIM-9X Sidewinder, developed by Raytheon engineers, entered service in November 2003 with the USAF (lead platform is the F-15C; the USN lead platform is the F/A-18C) and is a substantial upgrade to the Sidewinder family featuring an imaging infrared focal plane array (FPA) seeker with claimed 90° off-boresight capability, compatibility with helmet-mounted displays such as the new U.S. Joint Helmet Mounted Cueing System, and a totally new three-dimensional thrust-vectoring control (TVC) system providing increased turn capability over traditional control surfaces. Utilizing the JHMCS, a pilot can control the AIM-9X missile by simply looking at a target, thereby increasing air combat effectiveness. It retains the same rocket motor, fuze and warhead of the "Mike," but its lower drag gives it improved range and speed. AIM-9X also includes an internal cooling system eliminating the need for use of nitrogen bottles (U.S. Navy and Marines) in the launch rail or argon internal bottle (USAF). It also features an electronic safe and arm device (ESAD) similar to the AMRAAM allowing reduction in minimum range and reprogrammable InfraRed Counter Counter Measures (IRCCM) capability that coupled with the FPA provide improved look down into clutter and performance against the latest IRCM. Though not part of the original requirement, AIM-9X has demonstrated a Lock on After Launch (LOAL) capability, allowing for possible internal use for the F-35, and even in a submarine launched configuration for use against ASW platforms.
As of September 2008, Raytheon has delivered 3,000 AIM-9X missiles to the armed services and has begun testing the Block II version of the missile.In the Fall of 2006 the AIM-9X had 2 reported failures in their processors. In a flight on September 15, 2006, two AIM-9X missiles were fired and failed to lock on to their target. The missiles were then brought out of service until they fixed the problem. The missiles re-entered active operation by January 2008.
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 reticle, mirror, and five CdS cells or “pan and scan” Focal Plane Array (FPA) (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 on the side of the GCU attaches to the launcher, which detaches from the missile at launch. To cool the seeker head, a 5,000 psi (35 MPa) argon bottle (TMU-72/B or A/B) is carried internally in USAF AIM-9L/M variants while USN uses a rail mounted nitrogen bottle. AIM-9X contains a Sterling cryoengine to cool the seeker elements. Two electric servos power the canards to steer the missile (except AIM-9X). At the back of the GCU is a gas grain generator or thermal battery (AIM-9X) to provide electrical power. The AIM-9X features High-Off-Boresight capability; together with JHMCS (Joint Helmet Mounted Cueing System), this missile is capable of locking on to a target that is in its field of regard said to be up to 90 degrees off boresight. AIM-9X has several unique design features including Built-In-Test (BIT) to aid in maintenance and reliability, an Electronic Safe and Arm Device (ESAD), an additional digital umbilical similar to AMRAAM and Jet Vane Control (JVC).
Next is a target detector with four IR emitters and detectors that detect if the target is moving farther away. When it detects this action taking place, it sends a signal to the Warhead Safe and Arm device to detonate the warhead. Versions older than the AIM-9L featured an influence fuze that relied on the target's magnetic field as input. Current trends in shielded wires and non-magnetic metals in aircraft construction rendered this obsolete.
The AIM-9H model contained a 25-pound expanding rod-blast fragmentary warhead. All other models up to the AIM-9M contained a 22-pound blast fragmentary warhead The missile's warhead rods can break rotor blades (an immediately fatal event for any helicopter).
Recent models of the AIM-9 are configured with an annular blast fragmentation warhead, the WDU-17B by Argotech Corporation. The case is made of spirally wound spring steel filled with 8 pounds of PBXN-3 explosive. The warhead features a safe/arm device requiring five seconds at 20 g acceleration before the fuze is armed, giving a minimum range of approximately 2.5 kilometers.
The Mk36 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 activate the motor igniter. The fins provide stability from an aerodynamic point of view, but it is the "rollerons" at the end of the wings providing gyroscopic precession that prevents the serpentine motion that gave the Sidewinder its name in the early days. The wings and fins of the AIM-9X are much smaller to accommodate two each per side bay in the F-22 Raptor as originally planned, AIM-9X control surfaces are reversed from earlier Sidewinders with the control section located in the rear, while the wings up front provide stability. The AIM-9X also features vectored thrust or Jet Vane Control (JVC) to increase maneuverability and accuracy, with four vanes 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 TC-1 is a Taiwanese development of the AIM 9L which is fitted to and launched from a tracked vehicle. This missile is used as an Air Defense Weapon. Hardigg Industries has designed, qualified and now manufactures a single missile AUR (All Up Round) Container for this missile, which enables the loaded container to be physically handled by 6 men.
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. The United States Navy hosted a 50th anniversary celebration of its existence in 2002.
Please note that this list is not exhaustive.