AIM-120 AMRAAM

The AIM-120 Advanced Medium-Range Air-to-Air Missile, or AMRAAM (pronounced am-ram), commonly known to air crews as the "Slammer," is a modern air-to-air missile (AAM).

Origins

The AIM-7 Sparrow medium range missile was developed by the US Navy in the 1950s. With an effective range of about a dozen miles, it was a semi-active radar guided missile which would home in on reflections from a target illuminated by the radar of the launching aircaft. It was effective at visual to beyond visual range. Up to 4 were carried in special recesses designed for that missile on the gun-less F-4 Phantom and later fighters, and they were effective in dogfights when they worked. Together with the short range infrared guided AIM-9 Sidewinder, they replaced the AIM-4 Falcon IR and radar guided series for use in air combat by the USAF as well. A disadvantage to this system is that only one target can be painted at a time; also, the firing aircraft must remain pointed in the direction of the target, which can make it difficult or dangerous to keep the target illuminated in combat.

The US Navy later developed the AIM-54 Phoenix for fleet air defence. It was an impressive 1000 lb Mach 5 missile designed to counter cruise missiles and their bomber launchers. 8 of its first incarnation was proposed for the straight wing F6D Missileer, and then the F-111B. When Grumman built a dogfighter, they left in enough weight and volume for the Phoenix in the F-14 Tomcat. It was the first US fire-and-forget radar-guided missile: one which used its own active guidance system to guide itself without help from the launch aircraft when it closed on its target. This gave the Phoenix the still unprecedented theoretical capability of tracking and destroying up to six targets as far as 100 miles away.

But the Phoenix could only be carried under the belly of the huge 60,000 lb F-14, making the Tomcat the only US fighter with a fire-and-forget radar missile. A full load of 6,000 lbs was so heavy it exceeded a typical Vietam era bomb load. Only two or four missiles of a full load could be brought back onboard for landing. It has been said that Grumman first added an "ACM" button for dogfighting which simply would release all six missiles. Although highly lauded in the press, the Phoenix was of no use at close ranges, and was rarely used in combat before its retirement in 2005.

By the 1990s, the reliability of the Sparrow had improved so much from the dismal days of Vietnam that it accounted for the largest number of aerial targets destroyed in Desert Storm, and proved effective against the Mach 3 MiG-25 Foxbat. But while the USAF had passed on the Phoenix and their own similar AIM-47/YF-12 to optimize dogfight performance, they still desired the Navy's fire and forget capability. They asked for a missile that could be fitted on fighters as small as the F-16, fit in the same spaces that were designed to fit the Sparrow since the Phantom. They would have to fit the new F-22 Raptor which needed to fit all its missiles internally in weapons bays like the old F-106 Delta Darts in order to maintain a stealthy radar cross-section. The Navy also desired to add this capability to F/A-18E/F Super Hornet which was emerging as the only available replacement for the aging Tomcat.

Development

AMRAAM was developed as the result of an agreement, no longer in effect, among the United States and several other NATO nations to develop air-to-air missiles and to share production technology. Under this agreement the U.S. was to develop the next generation medium range missile (AMRAAM) and Europe would develop the next generation short range missile (ASRAAM). The breakdown in this agreement led to Europe developing the MBDA Meteor, a competitor to AMRAAM and the U.S. pursuing upgrades of the AIM-9 Sidewinder. After protracted development, deployment of AMRAAM (AIM-120A) began in September 1991.

The eastern counterpart of AMRAAM is the very similar Russian R-77 AA-12 Adder, commonly known in the west as "Amraamski."

Operational features summary

AMRAAM has an all-weather, beyond-visual-range (BVR) capability. It improves the aerial combat capabilities of U.S. and allied aircraft to meet the future threat of enemy air-to-air weapons. AMRAAM serves as a follow-on to the AIM-7 Sparrow missile series. The new missile is faster, smaller, and lighter, and has improved capabilities against low-altitude targets. It also incorporates an active radar in conjunction with an inertial reference unit and micro-computer system, which makes the missile less dependent upon the fire-control system of the aircraft.

Once the missile closes in on the target, its active radar guides it to intercept. This feature, called "fire and forget," frees the pilot from the need to continuously illuminate the missile's target with a radar lock, enabling the pilot to aim and fire several missiles simultaneously at multiple targets and perform evasive maneuvers while the missiles guide themselves to the targets.

Guidance system overview

Interception course stage

AMRAAM uses two-stage guidance when fired at long range. The aircraft passes data to the missile just before launch, giving it information about the location of the target aircraft from the launch point and its direction and speed. The missile uses this information to fly on an interception course to the target using its built in inertial navigation system (INS). This information is generally obtained using the launching aircraft's radar, although it could come from an infra-red search and tracking system (IRST), from a data link from another fighter aircraft, or from an AWACS aircraft.

If the firing aircraft or surrogate continues to track the target, periodic updates are sent to the missile telling it of any changes in the target's direction and speed, allowing it to adjust its course so that it is able to close to self-homing distance while keeping the target aircraft in the basket in which it will be able to find it.

Not all AMRAAM users have elected to purchase the mid-course update option, which limits AMRAAM's effectiveness in some scenarios. The RAF initially opted not to use mid-course update for its Tornado F3 force, only to discover that without it, testing proved the AMRAAM was less effective in BVR engagements than the older semi-active radar homing BAE Skyflash weapon--the AIM-120's own radar is necessarily of limited range and power compared to that of the launch aircraft.

Terminal stage and impact

Once the missile closes to self-homing distance, it turns on its active radar seeker and searches for the target aircraft. If the target is in or near the expected location, the missile will find it and guide itself to the target from this point. If the missile is fired at short range (typically, visual range), it can use its active seeker just after launch, making the missile truly fire-and-forget. At the point where an AMRAAM switches to autonomous self-guidance, the NATO brevity word "PITBULL" would be called out on the radio to inform other pilots, just as "Fox Three" would be called out upon launch.

Kill probability and tactics

General considerations

Once in its terminal mode, the missile's advanced electronic-counter-counter-measure (ECCM) support and good maneuverability mean that the chance of it hitting or exploding close to the target is high (on the order of 90%), as long as it has enough remaining energy to maneuver with the target if it is evasive. The kill probability (PK) is determined by several factors, including aspect (head-on interception, side-on or tail-chase), altitude, the speed of the missile and the target, how hard the target can turn, etc. Typically, if the missile has a sufficient amount of energy during the terminal phase, which comes from being launched close enough to the target from an aircraft flying high and fast enough, it will have an excellent chance of success. This chance drops as the missile is fired at longer ranges as it runs out of overtake speed at long ranges, and if the target can force the missile to turn it might bleed off enough speed that it can no longer chase the target.

Lower-capability targets

This leads to two main engagement scenarios. If the target(s) is/are not armed or not armed with any medium or long-range fire-and-forget weapons, the aircraft firing the AMRAAM need only to get close enough to the target, depending upon whether the target is heading towards or away from the firing aircraft, and launch the missile(s) to have a reasonable chance of hitting. Especially against low-maneuverability targets, in this situation the missiles are unlikely to miss. If the target aircraft are approaching the launching aircraft, especially if they are moving fast, the missile can be launched at long range since the range will be closing fast. In this situation, even if the target(s) turn around, it is unlikely they can speed up and fly away fast enough to avoid being overtaken and hit by the missile(s) (as long as the missiles are not released too early). It is also unlikely they can outmaneuver the missiles since the closure rate will be so great. In a tail-on engagement, the firing aircraft might have to close to between one-half and one-quarter maximum range (or maybe even more for a very fast target) in order to give the missile sufficient energy to overtake the targets.

If the targets are armed with missiles, the fire-and-forget nature of the AMRAAM is invaluable, enabling the launching aircraft to fire missiles at the target and then turn and run away. Even if the targets have longer-range semi-active radar homing (SARH) missiles, they will have to chase the launching aircraft in order for the missiles to track them, effectively flying right into the AMRAAM. If the target aircraft fire missiles and then turn and runs away, their own missiles will not be able to hit. Of course, if the target aircraft have long range missiles, even if they are not fire-and-forget, the fact that they force the launching aircraft to turn and run reduces the kill probability, since it is possible that without the mid-course updates the missiles will not find the target aircraft. However the chance of success is still good and compared to the relative impunity the launching aircraft enjoy, this gives the AMRAAM-equipped aircraft a decisive edge. If one or more missiles fail to hit, the AMRAAM-equipped aircraft can turn and re-engage, although they will be at a disadvantage compared to the chasing aircraft due to the speed they lose in the turn, and would have to be careful that they're not being tracked with SARH missiles.

Similarly armed targets

The other main engagement scenario is against other aircraft with fire-and-forget missiles like the Vympel R-77 (NATO AA-12 "Adder") — perhaps MiG-29s, Su-27s or similar. In this case engagement is very much down to teamwork and could be described as "a game of chicken." Both flights of aircraft can fire their missiles at each other beyond visual range (BVR), but then face the problem that if they continue to track the target aircraft in order to provide mid-course updates for the missile's flight, they are also flying into their opponents' missiles. This is why teamwork is so important and advanced missiles with guidance systems with hand-off capability can help overcome this problem. The other main tactic would be to sneak up behind the enemy aircraft and launch missiles without them noticing, giving the launching aircraft sufficient time to leave the danger zone of the enemy after launching. Even if the enemy detects the launch and turns around, the speed and possibly altitude it loses during the turn puts its missiles at an energy disadvantage which may be sufficient for the other aircraft to defeat it successfully. This typically requires excellent ground-control intercept (GCI) or airborne radar (AWACS — Airborne Warning and Control System) facilities in order to be successful.

Variants and upgrades

Air-to-air missile versions

There are currently three variants of AMRAAM, all in service with the United States Air Force and USN. The AIM-120A is no longer in production and shares the enlarged wings and fins with the successor AIM-120B currently in production. The AIM-120C has smaller "clipped" aerosurfaces to enable internal carriage on the USAF F-22 Raptor. AIM-120B deliveries began in 1994, and AIM-120C deliveries began in 1996.

The AIM-120C has been steadily upgraded since it was introduced. The AIM-120C-6 contained an improved fuze (Target Detection Device) compared to its predecessor. The AIM-120C-7 development began in 1998 and included improvements in homing and greater range (actual amount of improvement unspecified). It was successfully tested in 2003 and is currently being introduced into active service (early 2005). It helps the U.S. Navy replace the F-14 Tomcats which are being retired and replaced with F/A-18E/F Super Hornets—the loss of the F-14's long-range AIM-54 Phoenix missiles (already retired) can be partially offset with a longer-range AMRAAM, but note that the AMRAAM does not have a longer range than the Phoenix.

The AIM-120D is a planned upgraded version of the AMRAAM with improvements in almost all areas, including 50% greater range (than the already long range AIM-120C-7) and better guidance over its entire flight envelope yielding an improved kill probability (PK).

There are also plans for Raytheon to develop a Ramjet-powered derivative of the AMRAAM, the Future Medium Range Air-Air Missile (FMRAAM). It is not known whether the FMRAAM will be produced since the target market, the British Ministry of Defence has chosen the Meteor missile over the FMRAAM for a BVR missile for the Eurofighter Typhoon aircraft.

Ground-launch systems

Raytheon successfully tested launching AMRAAM missiles from a five-missile carrier on an HMMWV (hum-vee). They receive their initial guidance information from a radar not mounted on the vehicle (probably the MPQ-64 Sentinel radar system or possibly a PATRIOT missile battery radar) and help to provide low-level, close-in defence while the PATRIOT system engages targets at higher altitudes and further ranges. The missile's range would be lower when launched from the ground, due to the lack of speed or altitude of the launch vehicle. This system will be known as the SLAMRAAM (Surface Launched (SL) and AMRAAM).

The Norwegian Advanced Surface-to-Air Missile System (NASAMS), developed by Kongsberg Defence & Aerospace, consists of a number of vehicle-pulled launch batteries (containing six AMRAAMs each) along with separate radar trucks and control station vehicles.

Operators

In early 2006, the Bush administration revealed its plan to sell 500 AIM-120C-5 AMRAAMs to Pakistan to be used with F-16s.

External links

Origins

Development

Operational features summary

Guidance system overview

Interception course stage

Terminal stage and impact

Kill probability and tactics

General considerations

Lower-capability targets

Similarly armed targets

Variants and upgrades

Air-to-air missile versions

Ground-launch systems

Operators

See also

In service

Under development

External links

Gourt Home::Gourt :: AIM-120 AMRAAM

Origins

Development

Operational features summary

Guidance system overview

Interception course stage

Terminal stage and impact

Kill probability and tactics

General considerations

Lower-capability targets

Similarly armed targets

Variants and upgrades

Air-to-air missile versions

Ground-launch systems

Operators

See also

In service

Under development

External links