In early 2016 the U.S. Navy revealed that it had successfully modified its new SM-6 (Standard Missile 6) anti-aircraft missile so that it could hit surface ships. At first many wondered how effective this could be given that the 1.3 ton SM-6 warhead (what actually hits the target) weighs less than a hundred kilograms (220 pounds). Actually more than the warhead is involved but the amount of explosives is small. What makes a big difference is that the warhead and a large part of the missile hits the ship from above moving faster than a rifle bullet. In early 2014 the U.S. Navy tested the SM-6 against a recently retired American frigate. This is a 4,000 ton ship and while the exact results of the “SINKEX” (sinking exercise where a retired warships is used as the target) are classified it was not surprise (to anyone who could do the math) that the warhead travelling at high speed did a lot of damage to the frigate. The damage was so great that the frigate was judged to be out of action had the SM-6 hit occurred in wartime.
This is consistent with results from earlier SINKEX tests where the older SM-2 ER, which entered service in 1980 and was also capable of being used against ships. In 2015 SM-6 went into full production. SM-6 has longer range and more effective guidance (and resistance to countermeasures like jamming) that the SM-2 and is meant to deal with aircraft, cruise missile and ballistic missiles. All this was demonstrated during live tests in 2014 and 2015, which had SM-6 shooting down aircraft, anti-ship missiles and cruise missiles under a variety of different conditions. This included the longest surface-to-air engagement (missiles shooting down target) in naval history and the use of SM-6 against a surface ship. Max range of the SM-6 is given as 240 kilometers. The longer range and higher speed of the SM-6 make it particularly effective against other ships.
The SM-6 is basically the existing SM-2 anti-aircraft missile with the more capable guidance system of the AMRAAM air-to-air missile, as well as general improvements in the electronics and other components. The SM-6 is a 1.5 ton, 6.55 meter (21.5 foot) long, 533mm (21 inch) diameter missile. It has a max altitude of 33 kilometers (110,000 feet).
The older SM-2 is 1.35 ton, 8 meter (26.2 foot) long missile with a max range of 190 kilometers and max altitude of 24.4 kilometers (80,200 feet). The main change for the SM-6 is the guidance system which is self-contained and will seek out any target it comes within range of. The SM-2 uses a "semi-active" guidance system, which requires that a special targeting radar "light up" the target with a radar beam, which the SM-2 guidance system detects and homes in on. The "active" guidance system of the SM-6 is thus harder to jam and can home in on targets beyond the range of targeting radars. The SM-6 can attack anti-ship missiles as well.
The SM-6 took nine years to develop and was in limited production since 2011, with plans to obtain up to 1,800 missiles at a cost of $4.3 million each. SM-6 will replace many of the SM-2 missiles currently carried by American and Australian warships. Note that anti-aircraft missiles (including the SM series) have long been used as anti-ship weapons.
Meanwhile, the navy has been continuing years of improvements in the Aegis radar and fire control system that controls SM-2, SM-6, and the smaller SM-3 anti-missile version. The SM-3 can destroy ballistic missiles and low orbit satellites. Aegis equipped ships began getting version 4.0 of the Aegis anti-missile software in 2013 and the latest major upgrade (5.0) makes the anti-missile capabilities a standard feature of Aegis software. New destroyers are having anti-missile Aegis software installed as standard equipment. Much of the anti-missile capability of the original Aegis anti-aircraft system came from upgrades to the Aegis software.
The Aegis anti-missile system has had a success rate of over 80 percent in knocking down incoming ballistic missile warheads during test firings. To achieve this, two similar models of the U.S. Navy Standard anti-aircraft missile are in service, in addition to a modified (to track incoming ballistic missiles version) version of the Aegis radar system.
The RIM-161A, also known as the Standard Missile 3 (or SM-3), has a range of over 500 kilometers and max altitude of over 160 kilometers. The Standard 3 is based on the anti-missile version of the Standard 2 (SM-2 Block IV). This SM-3 missile has a shorter range than the SM-2, which can destroy a warhead that is more than 200 kilometers up. The SM-3 is optimized for anti-missile work, while the SM-2 Block IV was designed to be used against both ballistic missiles and aircraft. The SM-2 Block IV also costs less than half of what an SM-3 costs. So going after aircraft with SM-3s is discouraged unless absolutely necessary.