The background to Seaslug.

Seaslug's background can be traced back to the British wartime 'Brakemine' project which started in 1943, when two Cossor engineers (LH Bedford and L Jofeh) worked out a practical radar guidance system during an overnight train journey. Their resulting report reached the GOC of Anti-Aircraft Command (General FA Pile) at about the same time as a paper written by Captain HB Sedgefield of REME. General Pile convened a meeting to discuss this on 27th April 1943, and approval was given for Cossor to run a project to investigate this -at their own expense! This was an experimental solid-fuelled missile about 2 metres long with an 840mm wingspan, fired from a modified 3.7" AA gun mounting.  By the contract end in 1947 Cossor had solved the guidance problems of beam riding and perfected the necessary techniques of triggering high-speed photography etc., which was of value to all future missile projects; the project appeared to have been cancelled just as it appeared to be showing results. The truth, however, was different. The project was merged with Ben to become LOP/GAP -RTV1 (Liquid Oxygen Petrol/Guided Anti-aircraft Projectile -Rocket Test Vehicle 1) which was used to prove concepts.

A Staff requirement was launched by the Director of Gunnery Division (DGD) in November 1943 as anti-aircraft gunnery would be useless in the face of 600mph aircraft at 30-40,000 feet. "No projectile of which control is lost when it leaves the ship can be of any use to us in this matter."

In 1944 the Director of Naval Ordnance (DNO) was asked to develop an up-to-date Fire Control System for anti-aircraft guns of up to 5·25" calibre for HA & LA use. It was to have a fully stabilised but it was recognised that anti-aircraft gunnery was only efficient out to 7,000 yards range; beyond this it only had a deterrent effect. It was expected that for greater ranges a controlled projectile would be used.
Since there was a similar General Staff Requirement the Guided Air Projectile (GAP) project began. On the 16th March 1944 the first GAP committee met, and it set up four sub-committees: Propulsion (Dr Crow), Controlled Projectile Development Aerodynamics (Mr B Lockspeir), Radio and Radar (Dr Paris), and Stabilisation and Servos (Colonel Kerrison). It was tasked with a target date for the first prototype of June 1947! The GAP committee made its first report on 2nd May 1944 with the warning that the minimum period for development under the most favourable circumstances would be two years.

For the control of the projectile in the air it was readily agreed that self control to maintain its course was to use beam riding. The newly proposed X-band set (known as the Type 901) soon came to be regarded as the natural control set for the missile. On 12th January 1945 DGD wrote "It is now reasonably certain that the radar for LRS1 [the Type 901] will also do as the radar for GAP. Guided Weapon System 1 (GWS1), as the missile project became, took precedence over LRS1 and was given an over-riding piority, but work on LRS1 did not stop until 1949.

Armstrong Whitworth Aircraft (AWA), Sperry Gyroscope and the General Electric Co (GEC) collaborated to work on the Navy's Project 502 to establish if a guided missile was a practical/workable proposition. A great deal of work was done using Shorts' General Purpose Test Vehicle (GPV) at Woomera to assess the guidance system. A GPV can be seen behind the Seaslug missile here, it is worth noting it also has the forward mounted boost motors. The Project 502 team went on to design Seaslug and work on the system started in 1949 with the missile airframe being designed by AWA (which became Whitworth Gloster, then Hawker Siddeley Dynamics and finally part of British Aerospace Dynamics), the guidance package by Sperry and the guidance radar by GEC. There are some reports of the name Seaslug appearing as early as 1946.

RTV1 was used to prove both guidance and fuzing,with the first successful interception occuring in 1954. At this stage it was believed that it would take too long to design a reliable solid-fuel motor that could provide thrust for long enough (35-40 seconds) so the team concentrated on liquid fuelled engines; Liquid oxygen & petrol, HTP & kerosene and Nitric Acid (RFNA) & kerosene. The Navy opted for the last combination as it was regarded as the least unwelcome fuel combination on board a ship. As it happened the rest of the programme suffered delays which gave IMI Summerfield enough time to develop an adequate motor -Foxhound- which became available in 1956.
HMS Girdle Ness' trials started in 1956, so apart from the first few they would have used Foxhound solid-fuel motors. After a prolonged test period over test 250 missiles were fired, most at the British test range at Aberporth from fixed launchers as well as HMS Girdleness; others were fired in the Mediterranean and at Woomera.
Seaslug finally entered service in November 1962 when the system was accepted on board HMS Devonshire. The Mk1 missile was only really capable of engaging trans-sonic targets and its electronics were comprised of thermionic valves (or tubes).
With Seaslug's entry into service the Project 502 team continued to work on improving the missile with the Mk2 version -featuring solid-state electronics, a higher ceiling and longer range- entering service in 1971. The Mk2 missile was supersonic and used discrete solid-state components.

The story does not end here, however. In the late 1960s the Project 502 team proposed an 'improved Seaslug' for the Navy's Small Ship Integrated System (SIGS) in competion with Bristol Aeroplane GW Department. Bristol's submission had too short a range to be acceptable so they looked to Bristol Engines for a better power source which led to a proposal called RP25. The Navy approved of the design but wanted the Project 502 team to have design authority. This design turned into a relatively small, sleek missile to which the Ministry of Aviation allocated the designation CF299 -SeaDart!

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Last updated: 17th December 2022.>
Copyright SR Jenkins, February 2012.
If you have any photographs, information or stories about Seaslug, I'd be delighted to hear from you.