Seaslug (missile)

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.^[3] A contemporary British Army project at Cossors, Brakemine, was working on a system to allow a missile to keep itself centred 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 901 radar with a new missile that differed from Brakemine primarily in requiring longer range and being more robust for shipborne use.^[1]

In December 1944, GAP put out a Naval Staff Target for a new anti-aircraft weapon,^[2] capable of attacking targets at altitudes up to 50,000 ft (15,000 m) and speeds of up to 700 mph (1,100 km/h).^[4] This project was briefly known as LOPGAP, short for "Liquid Oxygen and Petrol Guided Anti-aircraft Projectile",^[5] but soon moved from petrol to methanol which made the "LOP" inaccurate.^[6]

The Fairey Aviation Company at this time was working on a missile project for the Ministry of Supply, Stooge. Stooge was more like an armed drone aircraft than a missile. It was flown to a location in front of the target and then cruised toward it until its warhead was triggered by the operator. It was designed primarily to defeat kamikaze attacks at short range. Its low speed and manual guidance meant it was not useful for interceptions outside the immediate area of the ship, and thus did not meet the need for a longer-ranged missile capable of dealing with stand-off weapons.^[4]

Accordingly, Fairey was ordered to stop work on Stooge in favour of LOPGAP.^[i] Development was slowed by the Air Ministry who were opposed to the project as it might take resources away from jet fighter production^[ii] and a lack of urgency on the part of both the Admiralty and Ministry of Supply.^[4]

A March 1945 report called for the first test launches of LOPGAP from converted QF 3.7-inch air-aircraft gun mounts within two months. The same mounts had also been used, with different modifications, for Stooge and Brakemine. They predicted the final system would be about 19 ft (5.8 m) long and a twin-launcher would take up about the same room as a twin 5.25-inch gun turret. An April Staff Target called for the system to be able to engage an aircraft flying at 500 mph (800 km/h) at altitudes up to 40,000 ft (12,000 m) with a maximum weight of 500 lb (230 kg).^[7]

In 1945 a new Guided Projectiles Establishment was set up under the Controller of Supplies (Air) and in 1946 development of all ongoing missile projects moved to the Royal Aircraft Establishment's (RAE) new Controlled Weapons Department, soon to become the Guided Weapons Department.^[8] 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.^[8]

In a January 1947 Navy review, the program was given the name Seaslug. This called for a significantly larger weapon than initially envisioned, 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).^[9] Development continued as before but was significantly hampered by the post-war exodus of engineering talent.^[3] Shortly after the new definition was produced, this project also moved to the RAE. Efforts by the Navy to change the name from Seaslug to the more ominous-sounding "Triumph" failed.^[4]

Development slowed, and in July 1947 the Admiralty approached Henry Tizard to argue for a more "virile leadership" of the program. Tizard called a meeting of the Defence Research Policy Committee (DRPC) and started a process of pushing through four key missile programs that were intended to enter service in 1957, Seaslug, a longer ranged Army/Air Force surface-to-air missile known as Red Heathen, the Blue Boar television guided glide bomb, and the Red Hawk air-to-air missile.^[4]

In March 1948 a new report from the DRPC noted there was not enough manpower for all four projects, and put Seaslug at the bottom of the priority list, claiming air attack would be less likely than submarine in the event of war. They suggested the much longer ranged Red Heathen was more important in the short term. The Admiralty was of another opinion on the matter and argued against the change in priority.^[4]

The Navy found an unlikely ally in the Army, who were concerned that Red Heathen was too difficult to move to in a single step and suggested that Seaslug might be the basis for a more immediate medium-range weapon that could be used both on land and sea. The DPRC also began to have concerns about accurately guiding Red Heathen at its desired 100,000 yd (91 km) maximum range. In September 1948 they agreed to develop Seaslug "as a matter of insurance", before further upgrading it in 1949 to "top priority".^[10] As a result of these changes, the program was seen as having two stages, Stage 1 would deliver 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 would have a greatly extended range on the order of 150 miles (240 km) and able to attack supersonic aircraft.^[10]

Two test 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.^[11] GAP became a purely research-oriented system, RTV.1 (rocket test vehicle), as opposed to a prototype missile design, and was used primarily as a platform for testing the rocket motors.^[12][13] 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 RTV, 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.^[14]

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

As experimental work progressed, the Ministry of Supply began forming an industry team to build production systems. In 1949 this gave rise to the 'Project 502' group from industry, with Armstrong Whitworth Aircraft and Sperry in March and GEC in September.^[9]

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 further update 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.^[7]

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,^[iii] 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 would make maintenance difficult.^[7]

When the deployment of the Seaslug was first being considered, three classes of custom missile-firing ships were considered. The Task Force Ship would be capable of 30 kn (56 km/h) and would 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 for ships travelling closer to shore. At that time it was believed that aircraft carriers would be able to provide adequate cover over convoys or fleets in the ocean, so attention turned to the Coastal Convoy Escort. Beginning in May 1953 a Beachy Head-class repair ship, HMS Girdle Ness, was converted into a prototype escort ship to test this fitting.^[16]

For this role, the densest possible storage was required, so the initial design of a single booster rocket at the base end of the missile. This led to a very long design that presented handling problems on the small ships. As was the case for most contemporary designs, the single inline booster was abandoned in favour of four smaller boosters wrapped around the fuselage, giving shorter overall length of about 20 ft (6.1 m). The boosters were positioned so they lay within the diameter defined by the missile's wings, so they did not make it any larger in diameter when stored. If one of the boosters did not fire the thrust would be significantly off-axis, a possibility which was later addressed by moving the boosters forward so their exhaust was near the centre of gravity of the missile, allowing the missile's small control surfaces to remain effective enough even against significant asymmetric thrust. In contrast, the American Terrier missile was somewhat shorter at 13 ft 6 in (4.11 m), but used an additional tandem booster which took the overall length to 28 ft 6 in (8.69 m).^[7]

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

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.^[18] After continual comparison and revision, these plans finally gelled around what became the County-class destroyer.^[19]

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

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 (Bloodhound) and the British Army (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.^[24]

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."^[25]

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.^[26] The first four ships of the County-class (Batch 1) operated the Seaslug Mk 1, while the final four (Batch 2) were fitted with the ADAWS command and control system which enabled them to carry the more capable Mk 2 version. A proposal to refit the Batch 1 ships with ADAWS was dropped in 1968.^[27]

During the

The last firing of the Seaslug Mk 1 was in December 1981 by HMS London, the final GWS1 (or Batch 1) ship in active service.^[30] HMS Fife was converted to a training ship, and had her Seaslug systems removed, freeing up large spaces for classrooms and was completed in June 1986.^[31] Fife and the remaining GWS2 ships were sold to Chile between 1982 and 1987. Initially, the British government had hoped that the Chileans would accept a package to upgrade the ships to operate Seadart, but this was not taken up and they were transferred complete with Seaslug.^[32] The Chilean ships were later refitted with an extended flight deck in place of the Seaslug launcher.^[33]

There were two main variants of the Seaslug:

The Seaslug Mark 1 was powered by the solid-fuel Foxhound (390 kg fuel) sustainer motor^[iv] and Gosling (145 kg) booster motors. It had a radio proximity fuze and 200 lb (91 kg) blast warhead.

The Mark 1 was a

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

The Seaslug Mark 2 was based on the aborted

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 yd (33 mi; 53 km) away, with an impact at 34.500 with about 46 seconds flight time.[34] The missile was capable of reaching potentially even higher altitude and longer range than nominally attested: even after the engine flameout (over 40 seconds after launch), it retained very high speeds and one of them even surpassed 85,000 ft (26,000 m) before self-destructing, about one minute after the firing.^[35]

For both Mark 1 and Mark 2 Sea Slug there were

In addition, a