The SM-2 Block IV (RIM-156A) is an extended-range surface-to-air missile interceptor modified for terminal phase ballistic missile defense. The Block IV was the first extended-range SM-2 interceptor to fit inside the MK41 VLS. The Navy deemed the differences between the Block IV and previous SM-2 ER (RIM-67) variants as significant enough to warrant its own designation, the RIM-156A. Because of programmatic instability, the Navy only procured 100 units. The Navy is phasing out SM-2 Block IV in favor of the SM-6. Although limited production has minimized the role the SM-2 Block IV has played in naval air defense, the interceptor provided a base for an interim sea-based terminal BMD capability. Knowledge gained in its development has furthermore contributed to the creation of the SM-6, with the two sharing numerous components.  

Development

The concept for the SM-2 Block IV came from the 1983 Outer Air Battle study as part of a Navy effort to define its role in a potential conflict with the Soviet Union around the same time that the Army developed its AirLand Battle doctrine.¹ Navy planners saw the extended range of the Block IV as necessary to intercept Soviet bombers before they were able to release multiple cruise missiles, effectively thinning the raid sizes air defenses would encounter.

The Navy initiated development of the Block IV in July 1987. The requirements for the missile included the ability to operate at both high and low altitudes against low radar cross-section, high speed, and maneuvering missiles with resiliency against electronic countermeasures. The first test of an SM-2 Block IV took place at White Sands Missile Range in 1990, and it had its first test at sea in 1994. In May 1995, the Navy approved the SM-2 Block IV for LRIP, but shortly abandoned its procurement in favor of the development of a new variant of the missile, the SM-2 Block IVA (RIM-156B).²

Part of the Navy Area Theater Ballistic Missile Defense (NATBMD) program the Navy and BMDO intended the SM-2 Block IVA to serve as a terminal phase BMD interceptor. Its design included an enhanced radiofrequency seeker to provide more precise measurements for the warhead fuse. It also included a strap-on infrared seeker as well, like the Block IIIB missile. Due to cost overruns, however, the DOD canceled the NATBMD program, along with the IVB, in 2001.

The cancellation of NATBMD did not, however, remove the requirement to develop a sea-based terminal phase ballistic missile defense. In 2006, the Navy and Missile Defense Agency (MDA) tested a modified SM-2 Block IV against a ballistic missile target. According to the MDA press release after the test, the modified SM-2 used a warhead “similar in concept to that used in the deployed Israeli Arrow system” and successfully intercepted the target.³ According to contemporary sources, the test was conducted in response to a 2003 Deputy Secretary of Defense directive and was financed by the Navy.⁴ The Navy ultimately funded the modification of 75 of the 100 SM-2 Blk IVs produced. Five of the modified missiles have been expended in testing, leaving about 70 modified Block IV missiles available to the fleet as an interim defensive layer against antiship ballistic missiles.⁵

Deployment Configurations

The SM-2 Block IV’s compatibility with the Mk 41 VLS permits deployment onboard Ticonderoga-class cruisers and the Arleigh Burke-class destroyers. This flexibility was a major improvement over previous extended-range SM-2s. The size of the older RIM-67 missiles required them to be fired from above deck launchers. The size and weight of these launch apparatuses meant that only large-cruiser size ships could carry them, limiting the number of ships that could contribute to the air defense mission.

The Navy only built 100 SM-2 Block IV interceptors, modifying 75 of them for terminal, endoatmospheric ballistic missile defense while preserving their capability against aircraft and cruise missiles.⁶ These modifications include changes to the fuze and autopilot systems. Unlike the SM-2 Block IVA developed under the Navy Area Theater BMD program, the 75 Block IVs MDA modified for SBT missions did not include an infrared seeker.

Guidance and Homing

Compared to earlier SM-2 ER (RIM-67) variants, the SM-2 Block IV included upgrades to the guidance and steering control of the missile enabled by advancing technology. It includes a new digital autopilot system, improving the accuracy of the missile. Designers also moved the dorsal strakes on the missile body to increase maneuverability and improve the missile’s stability in flight.⁷ This configuration was also applied to SM-6. The SM-2 Block IV also includes seeker upgrades allowing it to see low radar cross-section missiles better. The Block IV design also included microprocessors inside its seeker similar to those in Patriot interceptors and the AMRAAM, giving it the necessary sensitivity to detect smaller targets.⁸

To destroy incoming missiles, the Block IV uses a blast fragmentation warhead with the MK45 mod 10 target detecting device (TDD), which includes software upgrades to previous variants.

Propulsion

One of the most significant differences between the Block IV and its SM-2 ER forebears was the replacement of the much longer MK70 booster with the much shorter MK72. This change enabled the interceptor to fit inside the MK72 VLS tube. The MK72 also differs from the MK70 in that it uses four thrust motors for control of the missile during its launch. The extra booster gives the Block IV a reported range of 370 km.⁹