Sea Slug missile

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Sea Slug

Sea Slug Mk. II missile

Type

surface to air

Nationality

Era

Cold War

Launch platform

Ship

Target

Aircraft

History

Builder

Armstrong Whitworth

Date of design

Mark 1; 1961
Mark 2; 1965

Production period

Service duration

1961 - 1991

Operators

UK (Royal Navy), Chile

Variants

Mark 1, Mark 2

Number built

Specifications

Type

Anti-aircraft

Diameter

Mk.1; 0.42 m
Mk.2; 0.41 m

Wing span

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

Propulsion

drop-off booster & sustainer

Steering

control surface

Guidance

Beam riding

Speed

Mk.1; 685 mph
Mk.2; 1,370 mph

Range

Mk.1; 30,000 yards
Mk.2; 35,000 yards

Ceiling

Mk.1; 55,000 feet
Mk.2; 65,000 feet

Payload

Warhead

Mk.1; 200 lb blast
Mk.2; Continuous Rod

Trigger

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.

1 Development
2 Description
3 Service Performance
4 Variants
- 4.1 Mark 1 (GWS.1)
- 4.2 Mark 2 (GWS.2)
5 Operators
- 5.1 Royal Navy
- 5.2 Chilean Navy
6 See also
7 References

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 destroyer 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 (Bloodhound) and the British Army (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.

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 destroyer 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:

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 200 lb blast warhead.

Particulars
- Attack Velocity: 685 mph
- Range: 30,000 yards
- Ceiling: 55,000 feet

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, 56 lb, explosive charge and large steel penetrator.

Particulars
- Attack Velocity: 1,370mph
- Range: 35,000 yards
- Ceiling: 65,000 feet

The 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 the 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 (D20) was converted to a training ship, and had her Sea Slug systems removed, freeing up large spaces for classrooms.

A 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.