Seaslug (missile)

Seaslug 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.

Seaslug Mk. II missile
Typesurface-to-air missile
Place of originUK
Service history
In service1961 - 1991
Used byUK (Royal Navy), Chile
WarsFalklands War
Production history
DesignedMark 1; 1961
Mark 2; 1965
ManufacturerArmstrong Whitworth
VariantsMark 1, Mark 2
MassMk.1; 2,080 kg
Mk.2; 2,384 kg
LengthMk.1; 6.0 m
Mk.2; 6.1 m
DiameterMk.1; 0.42 m
Mk.2; 0.41 m
WarheadMk.1; 200 lb (91 kg) blast
Mk.2; continuous-rod warhead

Engine4 solid fuel jettisoned boosters & solid fuel sustainer
Wingspan1.44 m
Mk.1; 30,000 yards (27,000 m)
Mk.2; 35,000 yards (32,000 m)
Flight ceilingMk.1; 55,000 feet (17,000 m)
Mk.2; 65,000 feet (20,000 m)
SpeedMk.1; 685 mph (1,102 km/h)
Mk.2; 1,370 mph (2,200 km/h)
Beam riding
control surface

Seaslug 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. Seaslug was only fired as an anti-aircraft missile in anger once, from HMS Antrim during the Falklands War, but missed its target. It was also used for surface bombardment.


Seaslug began through wartime work on 'GAP' and 'LOPGAP', the liquid oxygen / petrol guided anti-aircraft projectile. A 1947 navy review put forward a first draft design for Seaslug, with an all-up weight of 1,800 pounds (820 kg), and single-stage vertical launch. However it was recognised that development by the existing GAP participants would take too long and so a larger industry team was required. In 1949 this gave rise to the 'Project 502' group from industry, with Armstrong Whitworth Aircraft and Sperry in March and GEC in September.[1]

Work on what became Seaslug 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. Development made use of on an earlier programme by the Fairey Aviation Company known as "LOPGAP" (Liquid oxygen / petrol guided anti-aircraft projectile), the Clausen Rolling Platform at RAE Aberporth and a Victory ship specially converted into a prototype escort ship, HMS Girdle Ness, was procured. The original system differed in having a triple launcher.[2] The Seaslug was delayed by several technical problems, one of them being the solid fuel rocket that was meant to be used instead of dangerous liquid rocket fuel. The Royal Navy initially favoured a liquid fuel sustainer, but the program delays allowed IMI Summerfield to develop a reliable solid fuel rocket, the Foxhound, available from 1956.

After more than 250 launches, the Seaslug Mark 1 finally entered service in 1962 on County-class destroyers, each fitted with a single twin missile launcher and a complete weapon system with one fire control set and 30 missiles.

In the mid-1950s, Seaslug was planned to arm both cruisers and destroyers. The Seaslug-armed cruisers were cancelled in 1957.[3]

Seaslug needed height, range and bearing information for targets. By 1955 the Royal Navy considered using the Type 984 radar on Seaslug-armed cruisers and destroyers to provide this. Though this was potentially feasible with cruisers, it was rejected with the destroyers because it would have meant sacrificing their 4.5 in gun armament. The gun armament was regarded as essential for the navy's wider role. The solution adopted with the first batch of the County-class destroyers was to network them with ships carrying Type 984. The destroyers were given a reduced version of the Comprehensive Display System (CDS), which was fed by a CDS-link receiver called DPD (Digital Picture Transmission or Translation).[3][4]

The final set for the County ships, actually more a cruiser type than a simple 'destroyer', was quite complex: a Type 965 radar for early warning (P-band, 450 kW peak power, range over 175 km), in the County Batch 2 the double antenna AKE-2 had two different frequency settings; a Type 992Q target indicator radar (3 GHz, 1.75 MW peak power, 90 km range); a Type 278 height finding set (80–90 km); a Type 901 missile guidance radar (X band, 70 km range), that in the Sea Slug Mk 2 had a continuous wave signal (but it was still a beam riding designation radar); a Type 904 fire control radar (used in the MRS-3 system, X-band, 50 kW, 35 km range) for surface targeting.[5]


The missile had four wrap-around booster motors which separated after launch. After separation the main motor ignited to power the missile to the target. The booster motors were positioned at the side of the missile, but this unusual arrangement with the motor nozzles both angled outwards at 22.5° and 22.5° to the left, 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, affected by special rings all around the missile (for increasing drag) were clear the control surfaces became active.

Guidance was by radar beam-riding, the beam to be provided by Type 901 fire-control radar. There were 4 flight modes:

  • LOSBR (Line Of Sight, Beam Riding), in which the missile flew up a beam that tracked 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 at 45°, used against low flying targets at over 12,000 yards away
  • MICAWBER (Missile In Constant Altitude While BEam Riding), used against low level target approaching at 500–800 feet, it allows to switch from CASWTD to LOSBR when the target is closing at the ship
  • Up and over: the standard surface mode, using the Type 901 radar slaved to the Type 903 (from the MRS3 FCS, that in the County class had a 40,000 yds scale range instead of 30,000 and was known also as Type 904) in bearing; the missile is fired ad high elevation and then depressed in order to strike the vessel with a steep dive, without arming the fuse; this cannot assure that the warhead would not detonate in the high speed impact[6]
  • Sea Skimming: the industry suggested a mode to use the missile as a real sea skimmer, but the alleged capability was apparently only a paper project, as no altimeter (needed for this mode) was fitted to the Sea Slug.

Electrical power when the missile was in flight was provided by a flux switching alternator with a six tooth rotor. "The 1.5 kVA Seaslug generator ran at 24,000 rev/min with a frequency of 2,400 Hz."[7]

Service performance

Seaslug was a high-performance weapon in its day, with a single-shot kill probability of 92% though other sources give lower kill probabilities: 75% for the Mk 1 and 65% for the Mk 2.[8] 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.

The cost of an individual Seaslug missile round in 1961 was approximately £50,000.


There were two main variants of the Seaslug:

Mark 1 (GWS.1)

The Seaslug Mark 1 was powered by the solid-fuel Foxhound (390 kg fuel) sustainer motor[lower-roman 1] and Gosling(145 kg) booster motors. It had a radio proximity fuze and 200 lb (91 kg) blast warhead. The Mark 1 was a beam rider missile, meaning the target had to be continually illuminated by the directing radar, so the system was limited to engaging only the number of targets that there were radars to track and lock on.

  • Particulars
    • Attack Velocity: 685 mph (1,102 km/h)
    • Range: 30,000 yards (27,000 m)
    • Ceiling: 55,000 feet (17,000 m)

Mark 2 (GWS.2)

The Seaslug Mark 2 was based on the aborted Blue Slug programme to develop an anti-ship missile using the Seaslug missile and guidance system. The project was cancelled in favour of the 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 1971. The Mark 2 utilized an improved beam-riding guidance system.[lower-roman 2] and solid-state electronics. It was powered by the Deerhound sustainer motor, 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 (25 kg), explosive charge (RDX-TNT) and an unfold diameter of about 70 feet (10 mm steel rods were used)

  • Particulars
    • Attack Velocity: 1,370 mph (2,200 km/h)
    • Range: 35,000 yards (32,000 m)
    • Ceiling: 65,000 feet (20,000 m)

The capabilities of the new Sea Slug Mk 2, an almost 2.5 ton missile, were much improved compared to the previous Mk 1. The boosters gave a total of about 60 tons-force, with 186 kg fuel for each one (145 in the Mk 1), accelerating it to over mach 2. When they separated because the extreme drag made by the rings all around the missile, the solid fuel sustainer Deerhound started to burn its 440 kg propellant (390 for the Mk 1) and gave about 1,820 kg/s for 38 seconds. The slender missile remained over mach 2-2.5 until the flameout. The missile was made fully controllable about ten second after the firing, followed by a radio-beacon while it was centered in the radar beam; and armed the proxy fuse (infra-red) at about 1 km from the target, if 'hot', while if 'cold' the missile was detonated by command sent from the ship. The range could be even more than 35,000 yards, especially at high altitude, with head-on supersonic targets. One of the longest shots recorded was made by HMS Antrim against a target over 58,000 yards away, with an impact at 34.500 with about 46 seconds flight time.[9] The missile was capable to reach potentially aven higher altitude and longer range than nominally assested: even after the engine flameout (over 40 seconds after launch), it retained very high speeds, and one of them even surpassed 85,000 feet before being self destruct, about one minute after the firing [10]

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 — a British version of the US W54 Gnat unboosted warhead of approximate yield 1/2 - 2 Kiloton (kt). The final warhead choice was Tony - a UK version of the W44 Tsetse boosted warhead, but all nuclear options for Seaslug were subsequently abandoned, and no nuclear-armed variant of Seaslug was ever deployed.


Royal Navy

The County-class destroyers were specifically built to carry Seaslug 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 Seaslug 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. It was fired again in anger, this time against an Argentine radar at Stanley airfield that the Royal Air Force had been unable to destroy. During a shore bombardment HMS Glamorgan triangulated the last known position of the radar with her ESM and fired a Seaslug. She later fired several at the runway to cover it with debris which would have damaged any aircraft attempting to land or take off. Results, if any, are unknown, but the spectacle of the launch sequence was something of a morale booster to the troops ashore.

Seaslug was withdrawn as the County-class ships were decommissioned. HMS Fife was converted to a training ship, and had her Seaslug systems removed, freeing up large spaces for classrooms.

Chilean Navy

Some of the County-class destroyers were sold to Chile for the Chilean Navy. The system was decommissioned after the rebuild of the four ships purchased by Chile in the early 1990s.

Former operators

Media appearance

A Seaslug system appears in the 1970 UFO TV series episode "Destruction".


  1. There is a common error about a liquid-fuel sustainer on this model.
  2. Another common error is that the Mark-2 has semi-active guidance; actually, it was the same beam-rider than Mark-1


  1. Twigge, Stephen Robert (1993). The Early Development of Guided Weapons in the United Kingdom, 1940-1960. Taylor & Francis. p. 28. ISBN 3718652978.
  2. Wise, Jon (2007). John Jordan (ed.). RFA Girdle Ness: Sea Slug Missile Trials Ship. Warship 2007. London: Conway. pp. 9–28. ISBN 1-84486-041-8.
  3. Harding, Richard, ed. (204). The Royal Navy, 1930-2000: Innovation and Defence. Routledge. p. 259. ISBN 0714657107.
  4. Boslaugh, David L (1999). When Computers Went to Sea: The Digitization of the United States Navy. Matt Loeb. p. 66. ISBN 0471472204.
  5. "Seaslug". SR Jenkins.
  6. "Seaslug". SR Jenkins.
  7. Guided Weapons, by Geoffrey Lee, et al, pub Brassey's, 3rd edition, 1998, ISBN 1-85753-152-3, p59.
  8. "Seaslug". SR Jenkins.
  9. "Seaslug". SR Jenkins.
  10. "Seaslug". SR Jenkins.


  • Naval Armament, Doug Richardson, Jane's Publishing, 1981, ISBN 0-531-03738-X
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