Initial concept 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. A solution for long-range anti-aircraft was required. On 16 March 1944 the first meeting of the "Guided Anti-Aircraft Projectile Committee", was held. 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. 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.
LOPGAP anti-aircraft weapon In December 1944, GAP put out a Naval Staff Target for a new anti-aircraft weapon, capable of attacking targets at altitudes up to and speeds of up to . This project was briefly known as LOPGAP, short for "Liquid Oxygen and Petrol Guided Anti-aircraft Projectile", but soon moved from petrol to
methanol which made the "LOP" inaccurate. 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. Accordingly, Fairey was ordered to stop work on Stooge in favour of LOPGAP. Development was slowed by the
Air Ministry who were opposed to the project as it might take resources away from jet fighter production and a lack of urgency on the part of both the Admiralty and
Ministry of Supply. 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 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 at altitudes up to with a maximum weight of .
Missile projects moved to the Royal Aircraft Establishment 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. 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. 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 and an all-up weight of . Development continued as before but was significantly hampered by the post-war exodus of engineering talent. 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. 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. 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. 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 maximum range. In September 1948 they agreed to develop Seaslug "as a matter of insurance", before further upgrading it in 1949 to "top priority". 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 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 and able to attack supersonic aircraft.
Experimental systems 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. 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. 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. As RTV.1 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.
Project 502 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. The 29 July 1949 update of the Staff Target called for a maximum range of and a minimum of . Maximum altitude should be 55,000 ft, but 45,000 would be considered acceptable. A further update pushed the range to against a , later , 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. For this role, maximum storage density was essential, so the initial design featured a single booster rocket mounted at the missile’s base. This resulted in a very long missile that caused handling issues on small ships. Like most contemporary designs, the single inline booster was eventually replaced by four smaller boosters wrapped around the fuselage, reducing the overall length to about 20 ft (6.1 m). These boosters were positioned within the diameter defined by the missile’s wings, so they did not increase its storage diameter. However, if one booster failed to fire, the thrust would become significantly off-axis. This problem was later mitigated by moving the boosters forward, placing their exhaust near the missile’s centre of gravity, which allowed the small control surfaces to maintain effectiveness despite asymmetric thrust. By comparison, the American Terrier missile was shorter at 13 ft 6 in (4.11 m), but employed an additional tandem booster, bringing its total length to 28 ft 6 in (8.69 m). The designs were continually modified in order to find a suitable arrangement. They started as early as 1953 with a mid-sized cruiser of 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. After continual comparison and revision, these plans finally gelled around what became the
County-class destroyer.
Test firings . This version is based on the RAE's early GPV, and retains the rear-mounted boosters before they moved forward on the "long round". 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 at the
Summerfield Research Station which provided the desired range. 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. 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. ==Description==