Seaslug (missile)

  (Redirected from Sea Slug 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. Tracing its history as far back as 1943, it came into operational service in 1961 and was still in use at the time of the Falklands War.

Sea Slug missile.png
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 on display at Wickenby Aerodrome, Lincolnshire, UK

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

Seaslug's medium-range role was to be supplanted by a very long-range missile known as Blue Envoy, but this was passed over in favour of a new medium-range system, Sea Dart. Sea Dart entered service in 1973 and replaced Seaslug through the 1980s as the County-class destroyers were removed from service.


Initial conceptEdit

In 1943, the German Luftwaffe began the use of anti-shipping missiles and guided bombs in the Mediterranean Ocean during Allied operations against Italy. These weapons were released outside of anti-aircraft gun range, which meant that any operations lacking complete air superiority would be open to attack with little response by the ships.[1]

The Admiralty Signals Establishment (ASE), in charge of the Navy's radar development, was working on new radars featuring radar lock-on that allowed them to accurately track aircraft at long range. This was part of the LRS.1 fire-control system that allowed large dual-purpose guns to attack bombers at long range.[2]

A contemporary project at Cossors, Brakemine, was working on a system to allow a missile to keep itself centered within a radar beam, a concept known today as beam riding. The Navy decided to combine the two concepts, using the LRS.1's Type 909 radar with a new missile that differed from Brakemine primarily in requiring longer range and being more robust for shipborne use.[1]


Fairey Aviation Company was at this time working on another Navy missile project, Stooge. Stooge was more like an armed drone aircraft than a missile, and its low speed and manual guidance meant it was not useful for interceptions outside the immediate area of the ship. Accordingly, Fairey was ordered to stop work on Stooge in favour of a much higher-performance radar-guided weapon,[a] LOPGAP, short for "Liquid Oxygen and Petrol Guided Anti-aircraft Projectile", with the initial requirements released in December 1944.[3] LOPGAP soon moved from petrol to methanol which made the "LOP" inaccurate, and the name accordingly changed simply to GAP.[4]

A March 1945 report called for the first test launches of GAP from converted 3.7-inch air-aircraft gun mounts (the same were used for Stooge and Brakemine) within two months. They predicted the final system would be about 19 feet (5.8 m) long and a twin-launcher would take up about the same room as a twin 5.25" gun turret. An April Staff Target called for the system to be able to engage an aircraft flying at 500 miles per hour (800 km/h) at altitudes up to 40,000 ft with a maximum weight of 500 pounds (230 kg).[5]

In a January 1947 Navy review, the program was given the name Seaslug. This called for a significantly larger weapon than initially envisioned in GAP, capable of single-stage vertical launch, a warhead (and guidance) of 200 lb (91 kg) and an all-up weight of 1,800 lb (820 kg).[6] Development continued as before but was significantly hampered by the post-war exodus of engineering talent.[2]


In 1946, the Royal Aircraft Establishment (RAE) formed its Controlled Weapons Department, soon to become the Guided Weapons Department.[7] They began considering the beam riding concept in partnership with the Telecommunications Research Establishment (TRE), the deliberately oddly-named department of the Air Ministry responsible for radar development. Over the next year, first Brakemine and then Stooge were moved to the RAE. Seaslug followed in 1947.[7]

Two systems emerged from this centralization. The CTV.1 was a small unpowered Brakemine-like system devoted to the development of the guidance systems, launched using three RP-3 rocket motors and controlled through the coast phase. A series of CTV designs followed, providing ever-increasing amounts of telemetry for the guidance and control systems work.[8] GAP became a purely research-oriented system, as opposed to a prototype missile design, and was used primarily as a platform for testing the rocket motors.[9][10]

Noticing that the performance requirements outlined by the Navy, Army and Air Force were roughly the same, they proposed a two-stage development program, with Stage 1 delivering missiles in the mid-1950s with roughly 20 miles (32 km) range with capability mostly against subsonic targets, and a Stage 2 of the early 1960s with greatly extended range on the order of 150 miles (240 km) and able to attack supersonic aircraft. The GAP/RTV.1 efforts would be directed at the Stage 1 design, which would essentially be the Seaslug requirement.

The relatively small CTV could safely be launched at the Larkhill Range, part of the Royal School of Artillery. It was equipped with a parachute that allowed it to be recovered. This was not possible for the much longer-ranged RVT, which was fired from RAF Aberporth out over Cardigan Bay in Wales. The desire to reclaim the RTVs as well led to the opening of a parallel launch facility at the RAAF Woomera Range Complex and a program that led development of supersonic parachutes.[11]

As RTV testing continued, the decision was made to build a larger version, RTV.2, which would be more typical of a production missile. During early testing, the design was further modified and renamed GPV, for General Purpose Test Vehicle. Several liquid rocket motors were tested as part of this program. Early tests demonstrated shifts in the center of gravity that required active damping, which in turn led to the lengthening of the overall fuselage to become the "long round". This version used forward-mounted boosters, which were mounted so their exhaust was just in front of the mid-mounted wings.[12]

Project 502Edit

As work continued, it was recognized 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.[6]

The 29 July 1949 update of the Staff Target called for a maximum range of 30,000 yd (27 km) and a minimum of 5,000 yd (4.6 km). Maximum altitude should be 55,000 ft, but 45,000 would be considered acceptable. A later updated pushed the range to 30,000–60,000 yd (27–55 km) against a 600 kn (1,100 km/h), later 650 kn (1,200 km/h), target. It was assumed the targets would "jink" at 1G, so the missile needed to maneuver at 4G at sea level and 2.5G at 40,000 ft. Additional requirements were the ability to switch between targets in 6 seconds.[5]

The designers ultimately selected a maximum range of 30,000 yards, which included 6,000 yd (5.5 km) of coasting after motor burn-out. This was about 50% better than the contemporary US Terrier design. Hit probability was estimated to be 40% at maximum range, so salvos of three missiles would be fired at once, demanding a three-place launcher. This was later reduced back to a twin-launcher when it was realized accessing the missile in the middle launcher was difficult.[5]

Changing requirementsEdit

When the deployment of the Seaslug was first being considered, three classes of ships were considered. The Task Force Ship would be capable of 30 kn (56 km/h) and would be tasked with fleet air defence. The Ocean Convoy Escort was a 17 kn (31 km/h) vessel that would provide direct cover over seagoing convoys, while the 12 kn (22 km/h) Coastal Convoy Escort would do the same closer to shore. At that time it was believed that the aircraft carriers would be able to provide adequate cover over the ocean, so attention turned to the Coastal Convoy Escort. Beginning in May 1953 a Beachy Head-class repair ship was converted into a prototype escort ship, HMS Girdle Ness, to test this fitting.[13]

For this role, the densest possible storage was required, so the initial design of a booster rocket fit to the end of the missile, as in most contemporary designs, was abandoned in favour of four smaller boosters wrapped around the fuselage, giving shorter overall length of about 20 ft (6.1 m) without making the missile larger in diameter as the boosters fit within the area of the control fins. In the case one of the boosters did not fire the thrust would be significantly off-axis, which was addressed by moving the boosters forward so their exhaust was near the center of gravity of the missile, allowing the existing control surfaces to be effective. In contrast, the Terrier missile was somewhat shorter at 13 ft 6 in (4.11 m), but this required an additional tandem booster bringing the overall length to 28 ft 6 in (8.69 m).[5]

In 1954, during another review of the Navy's future operations, consideration turned from a "hot war" against the Soviets to a series of "warm wars" in the third world. Among other changes brought about by this review, including the cancellation of a future all-gun cruiser class and ending further conversion of WWII-era destroyers to Type 15 frigates, the new environment meant that air cover by carriers could not be guaranteed, and the need for air defense for task-force sized units became the primary concern. A cut to carrier construction, capping the fleet at four, released funds for missile ship construction. In October 1954, a new design emerged that demanded the speed to keep up with a fleet in combat, have guns limited to self-defense, and carrying a single twin-missile launcher.[14]

The designs were continually modified in order to find a suitable arrangement. They started as early as 1953 with a mid-sized cruiser of 15,000 long tons (15,000 t) carrying 60 to 90 missiles and a crew of 900. Admiral Ralph Edwards pointed out it would be more useful to have a larger number of small ships with 10 to 20 missiles than one larger one, but attempts to design such a ship resulted in one with room for the weapons but not the crew needed to operate them. In May 1955 a wide variety of plans for designs between the two extremes were compared, ranging from 9,850 tons down to 4,550.[15] After continual comparison and revision, these plans finally gelled around what became the County-class.[16]


Test firings of the GAP-based examples, now known as Rocket Test Vehicle 1, or RTV.1, demonstrated beam riding in October 1956. The Navy had set a date of 1957 for a broad modernization of the fleet, so they desired Seaslug to be cleared for service in 1956. To this end, they accepted the use of liquid fuels in spite of the Navy's concerns with these fuels on ships. However, by 1956 a new solid fuel rocket had been developed by IMI Summerfield which provided the desired range.[17]

Continual tests took place over the next four years using both the Clausen Rolling Platform at RAE Aberporth and the Girdle Ness. A final series of tests at sea, which culminated in sixteen successful firings, finally cleared the missile for service in 1961.[17] After more than 250 launches, the Seaslug Mark 1, also known as Guided Weapon System 1, or GWS.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. The Seaslug-armed cruisers were cancelled in 1957.[18]

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. During development, the projected weight of the radar doubled, to the point where it could still potentially be mounted on cruisers, but was rejected for 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 outside the hot war mission. 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).[18][19]

The final set for the County ships, actually more a cruiser type than a 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.[20]


The missile had four wrap-around booster motors that 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 were jettisoned the control surfaces became active.

Guidance was by radar beam-riding, the beam to be provided by Type 901 fire-control radar. There were four 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 switching from CASWTD to LOSBR when the target is closing at the ship
  • Up and over: the standard surface attack mode, using the Type 901 radar slaved to the Type 903 in bearing; the missile is fired at high elevation and then depressed in order to strike the vessel with a steep dive, without arming the fuse.[21]

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."[22]

Service performanceEdit

Seaslug was a high-performance weapon in the 1960s, with a single-shot kill probability of 92%, although other sources give lower kill probabilities: 75% for the Mk 1 and 65% for the Mk 2.[23] 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.[citation needed]

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

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.


The firing of the first Seaslug test missile from HMS Girdle Ness (A387). This version is based on the RAE's early GPV, and retains the rear-mounted boosters before they moved forward on the "long round".

There were two main variants of the Seaslug:

Mark 1 (GWS.1)Edit

The Seaslug Mark 1 was powered by the solid-fuel Foxhound (390 kg fuel) sustainer motor[i] 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)Edit

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.[ii] 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.[24] 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 [25]

Nuclear variant (not built)Edit

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.


Map with Seaslug operators in blue

Royal NavyEdit

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.

Chilean NavyEdit

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 operatorsEdit

Media appearanceEdit

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. ^ a b Friedman 2012, p. 197.
  2. ^ a b Harding 2005, p. 254.
  3. ^ Twigge 1993, p. 246.
  4. ^ Morton 1989, p. 209.
  5. ^ a b c d Friedman 2012, p. 179.
  6. ^ a b Twigge 1993, p. 28.
  7. ^ a b Smith 1965, p. 101.
  8. ^ Smith 1965, pp. 104-105.
  9. ^ Smith 1965, p. 105.
  10. ^ Twigge 1993, p. 247.
  11. ^ Smith 1965, p. 106.
  12. ^ Smith 1965, p. 108.
  13. ^ 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.
  14. ^ Friedman 2012, p. 181.
  15. ^ Friedman 2012, p. 182.
  16. ^ Friedman 2012, p. 184.
  17. ^ a b Friedman 2012, p. 180.
  18. ^ a b Harding 2005, p. 259.
  19. ^ Boslaugh, David L (1999). When Computers Went to Sea: The Digitization of the United States Navy. Matt Loeb. p. 66. ISBN 0471472204.
  20. ^ "Seaslug". SR Jenkins.
  21. ^ "Seaslug". SR Jenkins.
  22. ^ Guided Weapons, by Geoffrey Lee, et al, pub Brassey's, 3rd edition, 1998, ISBN 1-85753-152-3, p59.
  23. ^ "Seaslug". SR Jenkins.
  24. ^ "Seaslug". SR Jenkins.
  25. ^ "Seaslug". SR Jenkins.



External linksEdit

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