Sea Slug missile

Sea Slug missile

Infobox Weapon

caption=Sea Slug Mk. II missile
name=Sea Slug
type=surface to air missile
era=Cold War
manufacturer=Armstrong Whitworth
design_date=Mark 1; 1961
Mark 2; 1965
service=1961 - 1991
used_by=UK (Royal Navy), Chile
wars=Falklands War
diameter=Mk.1; 0.42 m
Mk.2; 0.41 m
wingspan=1.44 m
length=Mk.1; 6.0 m
Mk.2; 6.1 m
weight=Mk.1; 2,080 kg
Mk.2; 2,384 kg
speed=Mk.1; convert|685|mi/h|km/h|abbr=on
Mk.2; convert|1370|mi/h|km/h|abbr=on
vehicle_range=Mk.1; convert|30000|yd|m
Mk.2; convert|35000|yd|m
ceiling=Mk.1; convert|55000|ft|m
Mk.2; convert|65000|ft|m
filling=Mk.1; convert|200|lb|abbr=on blast
Mk.2; Continuous Rod
engine=4 solid fuel jettisoned boosters & liquid fuel sustainer
steering=control surface
guidance=Beam riding
variants= Mark 1, Mark 2
fuze=Mk.1; Radio proximity & impact
Mk.2; Infra-red proximity

Sea Slug was a first generation surface-to-air missile designed by Armstrong Whitworth (later part of the Hawker Siddeley group) for use by the Royal Navy. It came into operational service in the 1960s and was still in use at the time of the Falklands War.

Sea Slug was intended to engage high flying targets such as reconnaissance aircraft or bombers before they could launch stand-off weapons. Later improvements meant that it could also be used against ships.


Work on what became Sea Slug began in 1949 under 'Stage 1' of the Royal Navy's post-war missile program. The weapon was intended to counter high altitude, nuclear armed bombers before they could release their weapons. Work was based on an earlier programme known as "LOPGAP" (Liquid Oxygen / Petrol Guided Anti-aircraft Projectile) and a Liberty Ship specially converted into prototype escort ship, HMS "Girdle Ness", was procured for developmental work. The original system differed in having a triple launcher. Sea Slug Mark 1 finally entered service in 1961 on the County class destroyers fitted with a single, twin missile launcher.


The missile had four wrap-around booster motors which separated after launch, the main motor then ignited, powering the missile to the target. The booster motors were positioned at the front of the missile, but this unusual arrangement gave acceleration and, with the motor nozzles angled outwards at 45°, the missile entered a gentle roll at launch evening out differences in the thrusts of the boosters. This meant that large stabilising fins as used on contemporary missiles in service with the Royal Air Force (Bristol Bloodhound) and the British Army (English Electric Thunderbird) were not required. Once the boosters were clear the control surfaces took over.

Guidance was by radar beam-riding, the beam was provided by the Type 901 fire-control radar. There were 3 flight modes;
*LOSBR (Line Of Site, Beam Riding) where the missile flew up a beam that tracks the target.
*CASWTD (Constant Angle of Sight With Terminal Dive) with the missile climbing at a low angle and then diving onto a low-altitude target
*MICAWBER (Missile In Constant Altitude While BEam Riding), similar to CASWTD, but with a terminal low-level glide phase to, so the Mark 2 variant could be used against ships. This mode suffered from the problems associated with the surface of the water reflecting the guidance beam.

ervice Performance

Sea Slug was a high-performance weapon in its day, with a single-shot kill probability of 92%. It was, however, limited by the complicated handling arrangements and since each County class ship carried only a single fire-control radar only one target could be engaged at once - though two missiles could be fired against it.


There were two main variants of Sea Slug:

Mark 1 (GWS.1)

The Sea Slug Mark 1 was powered by the "NK.1" liquid sustainer rocket motor and "Gosling" booster motor. It had a radio proximity fuze and convert|200|lb|abbr=on blast warhead.
**Attack Velocity: convert|685|mi/h|km/h|abbr=on
**Range: convert|30000|yd|m
**Ceiling: convert|55000|ft|m

Mark 2 (GWS.2)

Sea Slug Mark 2 was based on the aborted Blue Slug programme for a nuclear-armed anti-ship missile using the Sea Slug missile and guidance system. In the event the project was cancelled in favour of "Green Cheese missile" but other project developments were incorporated into what became the Mark 2. It had improved low altitude performance and a limited anti-ship capability and entered service in 1965. It was initially powered by the "Foxhound" sustainer motor, later replaced by the "Deerhound", with "Retriever" boosters. Control was by a modified Type 901M radar and it had an improved infra-red proximity fuze and a continuous-rod warhead with a smaller, convert|56|lb|abbr=on, explosive charge and large steel penetrator.
**Attack Velocity: convert|1370|mi/h|km/h|abbr=on
**Range: convert|35000|yd|m
**Ceiling: convert|65000|ft|m

Nuclear Variant (not built)

In addition, a nuclear-armed variant was planned using a low-yield fission warhead code named "Winkle". "Winkle" was never built as it was quickly supplanted by "Pixie", a very small unboosted warhead with an all-plutonium fissile core tested at Maralinga, which was in-turn, replaced by "Gwen" - an Anglicised version of the US W54 "Gnat" unboosted warhead of approx 1/2 - 2 Kiloton (kt). The final choice of warhead was "Tony" - a UK version of the W44 "Tsetse" boosted warhead, but in the event all nuclear options for Sea Slug were subsequently abandoned, and no nuclear-armed variant of Sea Slug was ever deployed.


;Royal NavyThe County class destroyers were specifically built to carry Sea Slug and its associated control equipment. The magazine was positioned amidships and missiles were assembled in a central gallery forward of the magazine before being passed to the launcher on the quarterdeck. The handling arrangements were designed with a nuclear-war environment in mind and were therefore entirely under cover.

During the Falklands War Sea Slug was only launched once against an aircraft target, by HMS "Antrim", and did not hit. This is hardly surprising as the Royal Navy considered the system to be obsolete and the low-level attacks experienced in the Falklands War were outside the missile's operational capacity. However it was fired again in anger, this time against the Argentine radar at Stanley airfield that the Royal Air Force had been unable to destroy. The impressive fireworks display associated with the launch sequence was something of a morale booster to the troops ashore. Requests from ashore resulted in a further 3 Sea Slugs being fired.

Sea Slug was withdrawn as the Counties were decommissioned. HMS "Fife" was converted to a training ship, and had her Sea Slug systems removed, freeing up large spaces for classrooms.

;Chilean NavyA number of the County Class were sold to Chile for the Chilean Navy and the last of their Sea Slug systems was not decommissioned until 2001.



ee also


* "Naval Armament", Doug Richardson, Jane's Publishing, 1981, ISBN 0-531-03738-X
* [ United Kingdom Aerospace and Weapon Projects]
* [ The Falkland Islands Conflict, 1982: Air Defense Of The Fleet]
* [ "Winkle" - warhead for the planned nuclear variant]

External links

[ Sea Slug Video]

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