RIM-7 Sea Sparrow
The RIM-7 Sea Sparrow is a U.S. ship-borne short-range anti-aircraft and anti-missile weapon system, primarily intended for defense against anti-ship missiles. The system was developed in the early 1960s from the AIM-7 Sparrow air-to-air missile as a lightweight "point-defense" weapon that could be retrofitted to existing ships as quickly as possible, often in place of existing gun-based anti-aircraft weapons. In this incarnation, it was a very simple system guided by a manually aimed radar illuminator.
After its introduction, the system underwent significant development into an automated system similar to other US Navy missiles like the RIM-2 Terrier. Contemporary improvements being made to the Sparrow for the air-to-air role led to similar improvements in the Sea Sparrow through the 1970s and 80s. After that point the air-to-air role passed to the AIM-120 AMRAAM and the Sea Sparrow underwent a series of upgrades strictly for the naval role. It now resembles the AIM-7 only in general form; it is larger, faster and includes a new seeker and a launch system suitable for vertical launch from modern warships.
Fifty years after its development, the Sea Sparrow remains an important part of a layered air defense system, providing a short/medium-range component especially useful against sea-skimming missiles.
History
Background
High-speed jet aircraft flying at low altitudes presented a serious threat to naval forces in the late 1950s. Approaching under the local horizon of the ships, the aircraft would suddenly appear at relatively close ranges, giving the ships only seconds to respond before the aircraft dropped their payloads and withdrew. This gave the aircraft an enormous advantage over earlier weapons such as dive bombers or torpedo bombers, whose low speed allowed them to be attacked with some effectiveness by anti-aircraft guns. The advantage was so great that when the Royal Navy was faced by the threat of the new Soviet, they responded in a non-linear fashion by introducing the Blackburn Buccaneer aircraft to attack them.Further improving the capabilities of aircraft against ships were a variety of precision-guided weapons. Early designs were first used in World War II with manually controlled weapons such as the Fritz X, and evolving into semi-autonomous cruise missiles, such as the Raduga KS-1 Komet, that relied on a combination of initial guidance from the launching aircraft and terminal guidance on the missile itself. These systems allowed the aircraft to launch their attacks from outside the range of shipboard anti-aircraft weapons, in relative safety. Only the presence of defensive fighters operating at long ranges from the ships could provide cover against these attacks, by attacking the launch aircraft before they could close on the ships.
US Navy doctrine stressed long-range air cover to counter both high-speed aircraft and missiles, and development of newer short range defenses had been largely ignored. While the Navy was developing expensive long-range fighters like the Douglas F6D Missileer, most ships were left with older weapons, typically Bofors 40 mm guns or Oerlikon 20 mm cannons. By the early 1960s their capability against modern aircraft and missiles was limited; a lack of fast-reacting mounts, gunsight radars of limited accuracy, and long settling times for the fire control systems all meant that the guns were unlikely to be able to respond effectively against high-speed aircraft.
The introduction of sea-skimming missiles dramatically increased the threat against these ships. Unlike the earlier generation of anti-ship missiles, sea-skimmers approached at low level, like an attack aircraft, hiding themselves until the last moment. The missiles were relatively small and much harder to hit than an attacking aircraft. While the older defenses might be considered a credible threat to a large aircraft at low altitude or a missile approaching at higher altitudes like the KS-1, against a sea-skimming missile they were useless. To successfully counter this threat, ships needed new weapons able to attack these targets as soon as they appeared, accurately enough to give them a high first-attempt kill probability - there would be little time for a second attempt.
Point defense missile system (PDMS)
The US Army faced a similar problem defending against attacks by high-speed jet-powered attack aircraft. In this case the local horizon was generally even more limited, blocked by trees and hills, and engagement times could be measured in seconds. They concluded that a gun-based system was simply unusable in this role; by the time the radar had locked-on and the gunsight calculated proper "lead" there would be no time to shoot at the target while it was within a gun's relatively short range. Missiles, on the other hand, could progressively tune their approach while they were flying toward the target, and their proximity fuses meant they only needed to get "close enough".In 1959 the Army started development of the MIM-46 Mauler, which mounted a new high-speed missile on top of the ubiquitous M113 Armored Personnel Carrier chassis, along with a medium-range search radar and a separate tracking and illumination radar. In order to deal with the quick response times needed, the fire control system was semi-automatic; operators would view targets on the search radar and prioritize them, the fire control system would select ones within attack range and automatically slew the missiles toward them and launch. Since the missile would be operating close to the ground in highly cluttered environments, it used a combination of beam riding along the illumination radar and an infrared seeker in the nose, which allowed tracking as long as either the path in front or in rear of the missile remained free of obstructions.
These same basic engagement parameters - high-speed and the associated fleeting sighting times - applied to sea-skimming aircraft and missiles as well. The Navy intended to adapt the Mauler to shipboard use by removing its search radar and wiring it into the existing ship-borne radar systems instead. The 9-box launcher and illuminator radar would be retained in a relatively compact mount. Development started in 1960 under the "Point Defense Missile System", the naval version to be known as the "RIM-46A Sea Mauler". The Navy was so confident in the Sea Mauler that they modified the design of their latest frigates, the, to incorporate a space on the rear deck for the Sea Mauler launcher.
The Navy's confidence in Mauler proved misplaced; by 1963 the program had been downgraded to a pure technology development effort due to continued problems, and was canceled outright in 1965. All three of the stakeholders, the US Army, US Navy and British Army, started looking for a replacement. While the British took a longer-term approach and developed the new Rapier missile, the US Army and Navy scrambled to find a system that could be deployed as quickly as possible. Facing the problem of guidance in a cluttered environment, the Army decided to adapt the infrared AIM-9 Sidewinder missile into the MIM-72 Chaparral. This was based on the AIM-9D, which could only acquire targets from the rear, and would be useless for the Navy where its targets would be approaching head on. They required a radar-guided system, and this naturally led to the AIM-7 Sparrow. They also considered Chaparral for smaller ships due to its much smaller size, but no such fits were ever attempted.
Basic point defense missile system (BPDMS)
Quickly organizing the "Basic Point Defense Missile System", BPDMS, the then-current AIM-7E from the F-4 Phantom was adapted to shipboard use as RIM-7E with surprising speed. The main developments were the new Mark 25 trainable launcher developed from the ASROC launcher, and the Mark 115 manually aimed radar illuminator that looked like two large searchlights. Operation was extremely simple; the operator would be cued to targets via voice commands from the search radar operators, and he then slewed the illuminator onto the target. The relatively wide beam of the radar only needed to be in the general direction of the target, the continuous wave signal being Doppler shifted by the moving target and showing up strongly even if it was not centered in the beam. The launcher would automatically follow the motions of the illuminator, so that when the missile was fired it would immediately see the signal being reflected off the target.In this form the Sea Sparrow was tested on the destroyer escort starting in February 1967, but this installation was removed when Bradley was sent to Vietnam later that year. Testing continued, and between 1971 and 1975 Sea Sparrow was fitted to 31 ships of the Knox class, hulls 1052 to 1069 and 1071 to 1083. The "missing ship" in the series, , was instead used to test an upgraded version.
The Sea Sparrow was far from an ideal weapon. Its rocket engine was designed with the assumption that it would be launched at high speed from an aircraft, and therefore is optimized for a long cruise at relatively low power. In the surface-to-air role one would rather have very high acceleration in order to allow it to intercept sea-skimming targets as soon as possible. The power profile is also suitable for cruising in thin air at high altitudes, but at low altitudes it does not produce enough power to overcome drag and dramatically decreases range; some estimates indicate that the Sea Sparrow may be effective only to, about one quarter of the range of the air-launched Sparrow. An engine of much higher power would greatly improve performance, in spite of a shorter burning time.
Another problem is that the Sparrow is steered with its mid-mounted maneuvering wings. These were used on the Sparrow because they required less energy for basic maneuvers during cruise, but this made the missile less maneuverable overall, which was not well suited to the quick-reaction weapon. Additionally, the powered wings meant that they could not easily be adapted to fold, and therefore the launcher cells were sized to the wings instead of the missile body, taking up much more room than required. Although the Sea Sparrow was meant as a small missile system that could be fit to a wide variety of ships, the launcher was relatively large and was deployed only to larger frigates, destroyers and aircraft carriers. Finally, the manually aimed illuminator was of limited use at night or in bad weather, which was hardly encouraging for a ship-borne weapon where fog was a common occurrence.