In the late 1950s, the US began to realize the threat Soviet long range anti-ship missiles such as the SS-N-2 Styx and AS-1 Komet presented to the fleet. The primary counter to such missiles was to intercept the launch platforms at long range, either by attack aircraft sinking ships equipped with the Styx, or fighters shooting down bombers carrying Komet.
The backup was the long range Talos, Terrier and Tartar guided missile equipped cruisers and destroyers.
But the cost and weight of the missile ships meant there were barely enough to support the fast carrier task forces. The huge numbers of other warships, amphibious warfare ships, and auxiliaries were left with only the virtually useless twin 3”/50 gun for air defense.
And so, the Navy embarked on a program to field a low cost, lightweight missile system for the self defense of ships such as the Knox class ocean escorts.
For several years, the Navy cooperated with the US Army developing the RIM-46 Mauler. It was eventually cancelled for technical reasons. The Army instead opted to field a ground launched version of the AIM-9 Sidewinder at the MIM-72 Chapparal. The Navy considered adopting it as well, as it was very light, and very cheap. The problem was, the missile seeker, based on the AIM-9D, had no front quarter engagement capability. And almost by definition, any self defense missile system means any target you’re shooting at is pointing right at you, that is, you only see its front quarter. That meant a radar guidance system was needed. And what radar guided missile did the US Navy have handy? The AIM-7E Sparrow III, used on the F-4 Phantom!
With very minimal modifications, the AIM-7E became the RIM-7E* Sea Sparrow. The Mk112 ASROC pepperbox launcher was modified to become the 8 round box launcher for the Sea Sparrow. Guidance was by the Mk115 director. The Mk115 was a crude system, in which an operator optically tracked the target, manually traversing and elevating the director. The director also had two continuous wave illuminator radars.
Mk115 manned director for Basic Point Defense Missile System (Sea Sparrow).
The director operator would be cued to incoming threats by voice commands from the ship’s Combat Information Center and associated search radars.
The Sea Sparrow had Semi-Active Radar Homing. That is, it guided on the reflected radar energy that bounced off the target being illuminated by the Mk115. Because the Mk115 had a fairly wide illumination beam, that allowed for some degree of minor tracking errors on the part of the operator. Not much, but some.
The missile, launcher, and director were collectively known as the Basic Point Defense Missile System, and BPDMS would be installed on many Knox class frigates, amphibious warfare ships, and fleet auxiliaries.
But BPDMS had an obvious shortcoming. The director operator had to be able to see the target. At night, or in inclement weather, the chances of a successful intercept plummeted to just about 0%.
Our NATO allies also faced a similar missile threat. And so, working with several European allies, the Navy began work on an improve Sea Sparrow system, on that came to be known variously as either Improved Point Defense Missile System (IPDMS) or more commonly, NATO Sea Sparrow Missile System (NSSM).
NSSM saw improvements to all three components of the system. The missile used an improved motor, and utilized folding wings so it could fit in a smaller launcher box. The new launcher was lighter, and of course, used smaller launcher boxes. The biggest improvement was in the director system. The manual tracking of the Mk115 was abandoned. Instead, the Mk95 radar director automatically tracked the target via radar, as well as providing illumination for missile guidance.
NSSM formed the basic air defense weapon of the 31 ships of the DD-963 Spruance class destroyers, as well as arming most aircraft carriers, quite a few amphibious ships, and many auxiliaries. On board the Spruance class, the NSSM was controlled by the Mk91 Fire Control System. The ship’s primary air search radar, the SPS-40, wasn’t really that great at detecting missile threats. The Mk 91 consisted primarily of the NSSM with its missile, Mk29 launcher, Mk95 directors, and the addition of the Mk23 TAS. Mk23 TAS, or Target Acquisition System, was a relatively short ranged 2D radar with a rapid rotation rate optimized for detecting low flying anti-ship missiles. After detecting a target, the Mk23 cues the Mk95, which in turn cues the Mk29 in azimuth and elevation, and then illuminates the target at launch.
Our Canadian friends operate a vertical launch version of the Sea Sparrow in the Mk48 VLS system, but otherwise, it works similarly to ours.
The final version of the Sea Sparrow is so radically different, it’s actually a new missile, the RIM-162 Evolved Sea Sparrow Missile. Using the latest guidance section and warhead of the Sparrow family, the ESSM has an entirely new, much larger motor, and instead of using mid-body mounted wings for steering, uses after fins for steering and mid-body strakes for stability. It can be fired either from the Mk29 launcher, from the Canadian Mk48 VLS, or can be “quad packed” with four missile in once cell of a Mk41 VLS, such as those aboard the DDG-51 Burke class guided missile destroyers. When fired from existing Mk29s, it’s guided by the Mk91 Fire Control System. When fired from the Mk41, it is guided by the ships Aegis system and its Mk99 illuminators.
Similar guidance systems are used by allied navies, such as the Dutch.
*Under the tri-service missile designation system, AIM stands for “Air Launched Intercept Missile, while RIM is Ship Launched Intercept Missile.