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.

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

In 1943, the German Luftwaffe began the use of anti-shipping missiles and guided bombs in the Mediterranean Sea during Allied operations against Italy.

These weapons were released outside of anti-aircraft gun range, which meant that naval operations lacking complete air superiority would be open to attack with no effective response from the ships.

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.

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] 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.

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).

[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.

[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.

[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.

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".

[15] 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.

[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.

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.

[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.

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 defence for task-force sized groups became the primary concern.

In October 1954, a new design emerged that demanded the speed to keep up with a fleet in combat, have guns limited to self-defence, and carrying a single twin-missile launcher.

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 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).

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.

On 21 May 1982 in Falkland Sound, the Antrim which had already had an unexploded 1,000 lb bomb pass through the Seaslug magazine, fired a single missile (some sources say two[28]) at one of a second wave of attacking IAI Dagger fighters.

It was unguided because the aircraft was too low to be acquired; the launch was intended to deter the pilot and to remove the exposed missile from the ship because it posed a fire hazard.

[29] The first combat use in the surface-to-surface role was during a shore bombardment on 26 May, when HMS Glamorgan fired Seaslugs at Port Stanley Airport claiming the destruction of a number of helicopters and a radar installation.

[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.

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.

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) The capabilities of the new Sea Slug Mk 2, an almost 2.5 ton missile, were much improved compared to the previous Mk 1.

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.