Article updated 6/15/2017 to add information and improve readability. Also replaced image chart with a table and added a section on the Rolling Airframe Missile.
The US Navy uses a variety of air defense missiles, each of which fills a specific niche. A table below, ordered by unit price, contains important specifications. The rest of the article offers specific details about each missile.
The RIM-66/RIM-156 Standard Missile-2
The RIM-66 Standard Missile 2 (SM-2) is the primary air defense missile of the US Navy and is carried aboard all destroyers and cruisers. Like the other missiles in the Standard family, it is fired from the Mk 41 Vertical Launching System (VLS). It was designed for fleet air defense against airplanes and cruise missiles, which it destroys using a blast fragmentation warhead. The US Navy classifies the SM-2 as a medium-range missile, but its 90 nmi range would be considered long by many other navies. Since the SM-2 has been in service for decades, its capabilities are proven. The SM-2 costs less than the SM-6 and has a longer range than the ESSM, so it occupies a middle ground in terms of capabilities. The SM-2 is ideal for engaging threats at moderate range; the more-expensive SM-6 is used when a target is outside the SM-2’s envelope. The vast majority of SM-2 missiles in US service are Block III+ (Block III+ denotes Block III or one of its variants.) All Block III+ missiles are being upgraded to Block IIIB. The US also created a RIM-156 Block IV variant which boasts extended range and is capable of ballistic missile defense. The Block IV was produced for a while but then canceled in favor of the SM-6 — there are about 72 SM-2 Block IVs left in service.
The SM-2 is directed during most of its flight by command guidance, where the launching ship wirelessly controls the missile’s path. Because command guidance is not particularly accurate, the missile switches to semi-active radar homing (SARH) when it gets closer to the target. SARH utilizes radars aboard the launch ship to bathe the target in a beam of radio energy (essentially a radar spotlight), which the seeker follows. Newer Block IIIB SM-2 missiles also have infrared seekers in addition to the SARH receivers, enabling Block IIIB missiles to find targets without radar illumination.
The RIM-174 Standard Missile-6
The Standard Missile 6 (SM-6) is a long-range missile based on the SM-2 Block IV. The main difference between the SM-2 Block IV and SM-6 is that the latter features extended range and an active radar homing seeker based on the AIM-120 AMRAAM air to air missile’s seeker. This allows the SM-6 to find and attack targets without illumination from the launching ship, greatly extending its effective range, as illumination radars are limited by the earth’s curvature. The SM-6 Dual I variant is also capable of operating in the anti-ballistic missile role against endo-atmospheric (in atmosphere) targets (exo-atmospheric targets are handled by the SM-3). Anti-ship capability has also been added to the SM-6, which has a small warhead relative to a bona fide anti-ship missile but compensates, to an extent, with its high speed. Currently, the SM-6 is not slated to fully replace the SM-2, as it is significantly more expensive. Furthermore, many low-flying, stealthy targets cannot be detected until within the range of the SM-2 or ESSM — the SM-6 would be overkill in such circumstances.
The RIM-161 Standard Missile-3
The RIM-161 Standard Missile 3 (SM-3) is a heavily modified version of the SM-2. Its speed and range are in a class above the other missiles, enabling it to intercept fast-moving ballistic targets. Instead of an explosive warhead, the SM-3 uses a kinetic kill vehicle which separates from the missile body and slams into the target at immense speed. As a result, the SM-3 can only be used against exo-atmospheric (in space) ballistic targets. Because it is solely a ballistic missile defense weapon, the SM-3 is only carried aboard ships equipped with the Aegis Ballistic Missile Defense system (not all destroyers and cruisers have been upgraded to this standard). The SM-3 is guided during midcourse by command link and GPS — terminal guidance is provided by an infrared seeker. A new variant, the SM-3 Block IIA, features an improved propulsion system, allowing for significantly higher speeds and greatly expanding the missile’s range. In ballistic missile interception tests, the SM-3 has a record of 29 successes in 36 attempts.
The RIM-162 Evolved Sea Sparrow Missile
The RIM-162 Evolved Sea Sparrow Missile (ESSM) features a compact airframe; four of the missile can fit in a single Mk 41 VLS cell, as opposed to only one SM-2, SM-3, or SM-6. This means that warships can carry a large number of ESSMs while still having cells left over for Standard missiles, Tomahawks, etc. The ESSM’s small airframe also significantly reduces unit price. The tradeoff is reduced range and a smaller warhead relative to the SM-2. As a result of its small size and relatively low cost, the ESSM is ideal for defending against a massed cruise missile attack. The ESSM utilizes midcourse command link guidance and terminal SARH, similarly to SM-2 variants IIIA and below. In addition to the Mk 41 VLS, ESSMs can be fired from a trainable Mk 29 launcher, which holds eight missiles, or the Mk 48 and Mk 56 VLS systems, which house two ESSMs per cell. The Mk 29 launcher is typically found aboard aircraft carriers, while the Mk 48 and 56 are used by small warships.
The RIM-116 Rolling Airframe Missile
The RIM-116 Rolling Airframe Missile (RAM) is a small missile used to defend ships against cruise missiles at very close range. Unlike the larger air defense missiles described above, which are generally used by frigates, destroyers, and cruisers designed for intensive combat operations, the RAM is installed aboard a wide variety of classes, including aircraft carriers, amphibious warfare ships, and both Littoral Combat Ship variants. Rather than using fins for stabilization, the whole missile spins (like a bullet from a rifle), hence the name. Thanks to its tiny airframe, RAM launchers are very small and can be easily installed to the deck of most vessels. There are two different launchers used with the RAM; the Mk 49 is a 21-round trainable launcher which requires targeting input from the host ship. The SeaRAM is a self-contained unit which combines an 11-round RAM launcher with the sensors of the Phalanx Close-in Weapons System (CIWS) to create a fully independent ship defense solution which requires no input other than electrical power. Because the RAM uses infrared and passive radar homing, it is a “fire and forget” weapon and requires no targeting input after launch.