Airframe
Produced by Hughes, the AIM-120 is a departure from designs such as the AIM-54 Phoenix, the AIM-4 Falcon, and even the AGM-65 Maverick.  Fat and stubby, these missiles stood apart from the thin, sleek designs that are generally associated with air-intercept missiles. This new design from Hughes was long and slender, looking much more like an AIM-7 Sparrow. Intended to replace the Sparrow, its design was dictated by the need to occupy stations that had housed the AIM-7.
The AIM-120 is smaller and lighter than its predecessor. It is tipped by a ceramic radome, which covers the missile's small radar antenna. Extending from the radome, the fuselage of the missile is covered in a thin layer of steel and titanium. The AIM-120 modular airframe is segmented into assemblies for guidance, warhead, propulsion, and directional control. In addition, a small fairing runs nearly two-thirds the length of the missile, providing protection for wiring connecting the guidance and control sections. The end of the missile features the exit nozzle for the rocket motor.
Stabilizing and Control Fins
Protruding from the airframe are two sets of cruciform wings, with the first set located at the approximate mid-point of the missile and the second set mounted at the tail end. The forward wings are fixed and are designed to provide stability in flight while the aft fins are tasked with directional control. Their movements correspond with signals fed along the external fairing from the guidance section. The directional fins are electronically controlled with power to the control actuators supplied by four lithium-aluminum batteries. The actuators and batteries are arranged around the motor outlet tube. Both sets of fins are detachable for storage and transport. The fasteners are designed for quick attachment or detachment.
The most distinguishing feature of the AIM-120C is the "clipped" fins. Unlike previous versions, the wings do not end in near points but rather, are more noticeably blunted. This reduces the overall wingspan of the fins from 20.7" to approximately 15". This reduction in size allows for the internal carriage of six AIM-120Cs in the F-22 Raptor. During times when the C variant is unavailable, the number of AIM-120s that can be carried within the Raptor is reduced to four.
Propulsion
The Hercules solid-fuel boost-sustain rocket motor encompasses nearly half of the missile. Designated the WPU-6/B (Weapons Propulsion Unit), the solid-fuel propellant and ignition core for the motor is located in the rear half of the missile, running along the fuselage from the mounting-point of the forward wings to just before the rear fins. The propellant consists of a hydroxyl-terminated polybutadiene (HTPB), a substance with a texture similar to that of a pencil eraser. This fuel accounts for almost the entire space within this section of the airframe. However, where the propellant ends it tapers off to a thin blast tube through which the thrust passes to the nozzle. Surrounding the blast tube are the control actuators for the rear fins.
The WPU-6/B is classed as a dual thrust motor. This boost/sustain model is designed to rapidly propel the missile to maximum velocity and then ensure maximum endurance. Initially the fuel burns quickly in the boost phase but then the consumption is throttled back to achieve maximum range. The motor burns for xxxxxx seconds and accelerates the AIM-120 to a speed of approximately Mach 4. The combined burn-time and velocity give the AMRAAM a theoretical range of over 40 miles.
Multiple Launch Modes
The AIM-120 motor features two launch methods, an improvement on the Sparrow. While the AIM-7 is only able to be cold-launched, the AMRAAM is capable of cold-launch or rail-launch. This offers a great deal of flexibility to ordinance planners as the AMRAAM can be mounted to stations previously available only to the rail-launched AIM-9 Sidewinder. During cold-launch firings, the missile is ejected from the aircraft prior to the motor firing. Launchers utilizing this method require a certain degree of clearance for the missile's post-ejection path. As a result, missiles limited to this one type of launch face certain hardpoint restrictions. This factor imposes limitations on the aircraft payload. For example, prior to AMRAAM, an F-15E carrying bombs on its Conformal Fuel Tanks (CFTs) would be incapable of employing a BVR missile for defense as the AIM-7 could only be mounted to the CFT stations. Conversely, rail-launch missiles ignite their motor first, which then propel them along the launch rail. As these missiles fire straight ahead from the aircraft, their clearance requirements are less stringent. In addition, these types of missiles typically feature a much larger launch envelope.
However, this is not so say that an AMRAAM can be mounted to a standard Sidewinder rail. In order to facilitate both the AIM-120 and AIM-9, a new Missile Rail Launcher (MRL) was required. The new system can be installed on all current stations utilized by the Sidewinder with the exception of the wingtip stations on the F/A-18C/D. At this time, these new rails are available only for the F-15, F-16, and F/A-18. The new rails differ slightly for each aircraft and as a result, carry different designations:
| MRL Designations |
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F-15
F-16
F/A-18C/D
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LAU-128A/A
LAU-129A/A
LAU-127A/A
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These new MRLs can replace the previous launchers at the operational level as they require no new fittings or interface cables.
Warhead
Located just forward of the forward fins is the WDU-33/B (Weapons Detonation Unit), a 45-pound blast/fragmentation warhead. In addition to the explosive material, the warhead contains 198 rectangular metal projectiles measuring approximately five inches in length. Upon detonation, the warhead scatters the metal fragments outwards, past the immediate blast radius, thus increasing the effective range and lethality of the warhead.
The WDU-33/B is both impact-fused and proximity-fused. While capable of detonating on impact, the AIM-120 does not need to make contact with the target. Its proximity fuse allows the warhead to ignite when the target is within the blast/fragmentation range of the missile.
Additional Protection on Carriers
The fairing, which protects the interface wiring, serves an additional purpose when the missile is deployed with carrier-borne aircraft. The casing houses the Thermally Initiated Venting System (TIVS), a feature which is designed to release pressure which may build up in the rocket motor due to the heat of a ship-board fire. The crowded flight deck of an aircraft carrier is a particularly harsh environment. Fires are especially deadly at sea when small spaces, aviation fuel, and ordinance are combined.
Beneath the fairing runs a thermal cord which when ignited, trips an Out-Of-Line Device (OOLD) which in turn detonates a shaped charge designed to deplete the rocket motor. The result is a venting of pressure in the rocket-fuel chamber before the detonation of the missile and its warhead. The OOLD prevents the accidental triggering of the shaped charge by accident. In addition, the system prohibits the TIVS from engaging after missile launch, which would drastically affect the missile's performance.
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