3” AA Equipments
These equipments were introduced during the Great War and were the standard HAA equipment untl the issue of mobile 3.7” equipments. The equipments had a high rate of fire and quick traverse but due to the improvement of aircraft performance by the Second War, were no longer adequate in terms of effective ceiling and muzzle velocity. At the start of the Second War, many were still employed in both HAA and LAA roles (including AA batteries H9, H10 and H12 in Suffolk). They were finally withdrawn from service in 1943.
Effective ceiling against a bomber travelling at 400 mph directly approaching which could be engaged for a period of 20 seconds, using Predictors Nos 1, 2 or 5, was 15,700 ft.
Lethal radius of burst:
Category A* damage: 45 ft
Category B** damage: 30 ft
Rate of fire: 20 to 25 rounds per minute
Gun detachment: normally 9 men
Right: Static 3" HAA Gun
3.7” AA Equipments
This equipment was introduced in 1937 and originally planned for a mobile employment in GDA’s and also with Field Forces. A static mounting was introduced in 1939 due to a bottleneck in the production of mobile platforms. Many improvements were made during the war, most notably automatic fuse setting and loading gear which doubled the rate of fire. When deployed in a HAA role, four guns were normally sited round a command post, containing the fire control instruments.
The guns had slow rates of traverse and elevation which made it difficult to bring fire on close aircraft. The mobile equipment could also be effectively employed in an anti-tank role – arrangements were made to do this for Home Defence in 1941 (Bargain Scheme).
Effective ceiling against a bomber travelling at 400 mph directly approaching which could be engaged for a period of 20 seconds depended upon the type of fuse, type of projector or the method of fire control:
Lethal radius of burst:
Category A damage: 55ft
Category B damage: 35 ft
Rate of fire:
With M.F.S No.9 and hand loading – 8 to 10 rounds per minute
With M.F.S No.11 and automatic gear – 23 to 25 rounds per minute
With M.F.S No. 11A and automatic gear – 20 rounds per minute
Gun detachment: 9 men with each mobile equipment or between 7 to 11 men with each static equipment (depended upon type of mounting).
Above: Top left: Static 3.7" HAA Gun. Bottom left: Mobile 3.7" Gun.
Right: Static 3.7" on Metrovick mounting.
5.25” AA Equipments
Naval twin equipments were issued for land service in 1942. Subsequently two types of single gun equipments were designed for land service – one design for an AA role only and one for twin Coastal Artillery / AA role. The gun designed for AA role only was fitted with an open back mild steel shield. Four guns were sited together adjacent to a command post. The guns featured electric firing, air blast gear, hydraulic power operated traversing, elevating, loading tray and ramming gears. Only 31 AA sites were started or completed by November 1945 which included H2 in the Harwich GDA.
The effective ceiling was limited by the type of fuse and predictor or method of fire control employed. The maximum heights at which a 400 mph directly approaching aircraft could be engaged for a period of 20 seconds was:
Lethal radius of burst:
Category A damage: 100ft
Category B damage: 70 ft
Rate of fire – 7 to 10 rounds per minute
Gun detachment: 13 men
Right: 5.25" HAA Gun on Metrovick mounting.
Z AA Equipments
A 3” rocket projector (known as Z Anti-aircraft equipments) were first developed operationally during 1940 for the protection of vulnerable points on the South coast against dive bombing attacks. A single projector was produced for this purpose. As more light AA guns became available, Z AA equipments role changed from LAA to HAA defence. In a HAA role a large number of 3” rocket projectors, usually 64 twin equipments, were sited close together with their fire controlled by telephone form a plotting and control post.
Each rocket consisted of a tail unit with four fins, H.E shell and a time fuze. The rocket was launched from the rails of the projector at a velocity of 200 ft per second and accelerated whilst propellant was still in the tail to a maximum velocity of 1,500 ft per second. Fins fitted to the tail ensured stability in flight. Surface wind and air currents caused a variable dispersion from the normal trajectory and it was for this reason that high density fire was essential. They could obtain a range of 19,000 ft or a ground range of 7,000 yards.
Right: Mobile nine rocket Z projector
Other guns in use in an AA role were 4.5” statics based on a Naval design and 3.7” High Velocity Equipments (basically a 4.5” gun which has a special 65-calibre 3.7 inch barrel fitted in its jacket) but documentary evidence suggest neither of these equipments were employed in Suffolk.
* Category A damage = prevention of accurate bombing
** Category B damage = failure of aircraft to return to base
Notes on Anti-Aircraft Artillery, Military College of Science Publication, 1944