British naval anti-aircraft weaponry – a brief history

With reference to the Explosion Museum of Naval Ordnance, Gosport

All the photographs of preserved naval AA weapons below show items in the extensive collection of the Explosion Museum

The threat that aircraft could pose to naval vessels was recognised almost as soon as fixed-wing flight became a reality, and well before any useful weapons could be carried into the air. At the Harvard Aeronautical Society’s Harvard-Boston aviation meet in September, 1910, the outline of a battleship was marked out on the ground and the aviators present attempted to score ‘bulls eyes’ with plaster of paris ‘bombs’ by hitting the outlines of funnels marked within the ‘ship’. It was indeed possible for the pilots of the Bleriot, Farman and Curtiss machines present to drop their bombs down the funnels of the facsimile ship from a height of around 100 ft.

This prompted the Lieutenant C. A. Blakely of the U. S. Navy to suggest that “some sort of a pyrotechinic bomb set to explode at a great height might be utilized” for the future defence of warships against aircraft.

Despite this demonstration, the Royal Navy paid relatively little attention to arming its ships against aircraft in the years leading up to the First World War. Dreadnought battleships and battle cruisers of the WW1 period were generally armed with two 3in BL quick-firing guns Mk1 with a mounting that could elevate up to 90°. These had a range of 11,200 yards at 45° elevation, and were only of use against fixed-wing aircraft, as zeppelins flew far too high. Indeed, when a reconnaissance zeppelin was seen near the Grand Fleet the morning after the Battle of Jutland, the battleships tried to knock it down with their main armament of 12in-15in guns!

3in QF on HMAS Australia
3in Quick Firer on the battle cruiser HMAS Australia

The potency of aircraft with regard to shipping increased dramatically during the First World War – an attack by carrier-launched torpedo aircraft against the German High Seas Fleet was even planned, though the war ended before the mission could be staged. The RN had commissioned Vickers to scale up its 1pdr belt-fed quick firing gun to fire a 2lb round, as an anti-aircraft gun for cruisers and smaller vessels.

Nevertheless, with the war’s end the Royal Navy lost any sense of urgency in improving its ships’ defences against air attack. HMS Warspite, for example, retained the two 3pdr guns as its sole anti-aircraft defence until 1927, when the weapons were replaced by a mere four 4pdr guns and a small number of rifle-calibre machine-guns. Although a new anti-aircraft gun was ordered in 1923, the QF 2pdr Mark VIII did not begin to enter service until 1930. This weapon would become the iconic naval anti-aircraft gun of the Second World War, known as the ‘pom-pom’ after the distinctive sound it made when firing. The weapon began life, however, with a typical British compromise – it was almost certainly designed to be able to use existing 2lb ammunition of which there was a vast stock left over from the First World War. It was also based on thinking that would later be revealed to be flawed.

2pdr pom-pom
The 2-pounder ‘pom-pom’ as it was rarely seen, as a single barrel. Four, eight or even 16 guns to a mounting were more common

The ‘pom-pom’ was almost always fitted in multiple mounts and this at least allowed the number of anti-aircraft guns on RN ships to be improved dramatically. In her 1937 refit, Warspite was fitted with 32 2pdrs in four eight-barrel mountings, as well as four 4in high-angle guns.

By this time, there were broadly two methods by which a ship could attempt to defend itself from aircraft. The first was by destroying any aircraft which came within range, and the second was to deter aircraft from getting within range. There were also two methods of actually destroying or damaging an aircraft – the first was to physically hit it with a projectile, the second was to detonate a projectile sufficiently close to pepper the aircraft with shrapnel.

The ‘pom-pom’ was intended as much as a deterrent as a weapon in its own right. It was hoped that by pumping a huge volume of shells into the air in a ‘curtain’ around a ship or, more pertinently, a group of ships, no enemy aircraft could hope to survive within the zone of the barrage. This was not dissimilar to the theory espoused for heavily armed day bomber aircraft which were intended to defend each other as well as themselves with defensive machine-gun fire. The theory proved to be equally flawed. In any case, the pom-pom suffered from a lack of a tracer round, which limited both the crew’s ability to aim it and the weapon’s value as a deterrent. The increasing speed of aircraft meant that accurate aiming and direction was more important than ever. Unfortunately, an up-to-date director, the MkIV, was not available for the pom-pom until 1941, and many ships struggled on with the obsolete MkIII.

Nevertheless, in 1939 huge faith was placed in the ability of the ‘pom-pom’ to ward off any enemy aircraft. This was demonstrated in the month the war began when aircraft from Ark Royal shot down a fleet shadower, but were too late to prevent the task-force’s position being reported. Instead of being rearmed and launched to meet an enemy bomber force, the carrier’s aircraft were struck down in the hangars and their tanks drained of fuel to guard against fire. One of the four Junkers Ju88s that found the ships pressed its attack despite the AA barrage, and narrowly missed Ark Royal with a 1,000kg bomb. This incident led to a rethink, and in future, anti-aircraft fire was used in conjunction with aircraft whenever they were available.

Other anti-aircraft weapons had been developed or pressed into service in the immediate pre-war period. These included at one end of the spectrum, the obsolete Hotchkiss 3pdr quick firing gun dating from the 1890s, which was hastily attached to high-angle mountings and fitted to merchantmen and warships where no better weapons were available. At the other end of the spectrum, the first tentative steps with rocket projectiles were being made in the form of the Unrotated Projector MkI, which fired a battery of ten 3in rockets firing parachute mines.

Hotchkiss 3pdr QF
An emergency AA weapon, the 3-pounder Hotchkiss QF gun pressed into service at the start of WW2

In January 1941, when HMS Illustrious and the Western Mediterranean fleet was attacked by a massive force of German and Italian aircraft in the Mediterranean, a combined defence of aircraft and guns helped prevent the ship’s loss, although it was badly damaged. The pom-poms worked well, and it was estimated that over 30,000 rounds were fired with few problems. The combined anti-aircraft defences of the fleet destroyed four Junkers Ju87 Stukas.

2pdr pom-pom
Although flaws were apparent in the 2-pounder pom-pom early in the war, the weapon continued to be produced throughout the war as with this 1943 example

Unfortunately, aircraft were not available to the HMS Prince of Wales and HMS Repulse when the two capital ships were attacked by a massive force of bombers and torpedo aircraft. The ships pom-poms were also hampered by ammunition which had degraded in the heat and humidity of the tropics. HMS Repulse’s pom-poms shot down two Japanese aircraft, but the newer 40mm Bofors autocannon made a better account of itself with its greater reliability, tracer ammunition and greater range.

Bofors 40mm
The highly effective and widely used 40mm Bofors autocannon, Mk N1 variant

Bofors 40mm breech
The breech of the Bofors 40mm showing the 4-round ammunition clip

Autocannon like the Swedish Bofors and the smaller 20mm Swiss-designed Oerlikon were under production in Britain but were not available at the beginning of the war. The Bofors had begun to become available from 1942, and quickly showed itself to be an improvement on the pom-pom. British naval versions of WW2 fired rounds from clips of four, and could theoretically maintain a firing rate of 120 rounds per minute – although this required a great deal of dexterity in the gun crew to replace the clip every two seconds.

The Oerlikon was an effective short-range weapon, and suitable for mounting on light and coastal vessels such as air-sea rescue launches and submarines – in fact the weight of the basic gun compared favourably with that of the .50in Browning machine gun. It could fire up to 500 rounds per minute, from magazines that could carry up to 60 rounds and was available to the RN in single and twin mountings (German derived weapons commonly used quadruple mountings). The 20mm shell was able to penetrate aircraft armour, unlike rifle-calibre machine gun bullets.

Oerlikon 20mm double mount
Oerlikon 20mm cannon in double mount

Oerlikon 20mm single mount
Oerlikon 20mm cannon in a single mount

The Oerlikon was a successful weapon, and in greatly developed versions is still in service, though on larger vessels it tended to be supplanted by the Bofors due to the latter’s greater stopping power. This feature was of great use against Kamikaze attacks in the Pacific.

From before the Second World War, another way of strengthening the anti-aircraft armament of warships was to make the secondary armament or even primary armament dual purpose, with high-angle mountings and different types of ammunition available for different roles. These were heavier weapons which were intended more to destroy aircraft through detonating a shell at a specified altitude, throwing shrapnel across a wide area.

The introduction of dual-use weapons helped to minimise the amount of specialist anti-aircraft weaponry that ships were required to carry, and was more efficient in terms of the crew as well. British 4in, 4.5in and 5.25in guns were mounted to warships with air defence in mind as well as anti-ship and naval gunnery support functions. The BL 4.5in gun was developed as a dual-use weapon for aircraft carriers, to defend against destroyer or torpedo-boat attack and air strikes, and was later developed with a different mounting for destroyers that could elevate up to 55°.

BL 4.5in MkIV
A 4.5in Quick Firing MkIV gun in ‘UD’ Upper Deck mounting

BL 4.5in MkIV breech
The breech of the 4.5in Quick Firing gun, which could be loaded with dedicated AA ammunition

By the end of the Second World War, the advent of jet power meant that aircraft were flying higher and faster than had been remotely possible only a few years before. Autocannon and high-angle large-calibre guns continued to play a part, but to be truly effective against fast jets, a far more accurate and destructive weapon was necessary. The switch from guns to missiles as the predominant naval anti-aircraft weapon took place in the 1960s, development work having begun in the late 1940s. Weapons such as the ‘Unrotated Projector’ had been introduced before the technology to guide and propel a rocket projectile was available. By the 1960s, however, lightweight, powerful rocket motors and radio control and radar guidance systems had made shipboard missile defence a practical possibility.

The Sea Slug and Sea Cat missiles both entered service with the Royal Navy in 1962. These were dramatically different weapons. The Sea Slug was a large, long-range projectile with four external booster rockets and radar guidance. It was designed to ride a ‘beam’ emanating from the launching vessel, directed by its fire control radar. It had a range of around 40km and a maximum altitude of 23,000m (around 75,500ft). The launcher was enormous, some nine metres in length, and was consequently only able to be fitted to larger ships. The county-class missile destroyers were the main vessels to operate Sea Slug, and some of the weapons were still in service by the Falklands conflict of 1982. Despite being state-of-the-art on entry into service, the Sea Slug was completely obsolete by the 1980s and was unsuitable for the style of combat experienced in the Falklands. It could only engage aircraft at relatively high altitude and needed considerable warning. It was used only once in the Falklands in the air defence role (though it was also used as a surface-to-surface and anti-radar missile) and it has been suggested that the launch of HMS Antrim’s Sea Slug during an attack was as much to rapidly clear the launcher in case a bomb hit it and detonated the warhead.

Sea Slug
The large Sea Slug long range missile with its four booster rockets clustered round the nose, next to a later short-range Sea Wolf missile

The Sea Cat, by contrast, was a small short range weapon designed to supplant the Bofors 40mm cannon. Initial versions of the Sea Cat were guided by radio control, with an observer steering the missile onto the target, though later versions had radar assistance and even full radar guidance. Its mounting carried four missiles, and was still small enough to fit comfortably on smaller frigates. The missile was steadily improved and in the Falklands conflict, it remained the chief anti-aircraft weapon of many of the older and smaller ships, up to the Type 21 frigate.

Sea Cat launcher
The four-round Sea Cat launcher

Sea Cat round
Inert Sea Cat round

Despite being a much simpler weapon than Sea Slug, it was slightly more successful, with around 80 launches and HMS Plymouth’s Sea Cat battery responsible for a possible ‘kill’ of a Mirage V Dagger. It was, however, too slow and not designed for the sort of low-level, wavetop-height attacks that characterised the Falklands. It has now been replaced by the much more accurate, fully-automatic Sea Wolf short-range missile. Although there were some flaws with the Sea Wolf’s guidance and hardware during the Falklands campaign (it tended to become confused by multiple targets and buildup of salt on the launchers caused failures) it was the most successful missile system during the conflict.

The notion of individual anti-aircraft weapons have come to be replaced over time by weapons systems – combinations of data gathering, target detection and fire control, launchers and the weapons themselves. The emphasis has also shifted from engaging aircraft to any airborne threat, including missiles.

By the 1970s, efforts to replace the clumsy Sea Slug were underway and in 1977, Sea Dart appeared. It was the main long-range air defence for Type 42 destroyers and was a significant advance on the RN’s previous missile systems. The Sea Dart was supersonic, accelerated through the ‘sound barrier’ by a first stage solid rocket motor before the kerosene fuelled Odin cruise motor takes over. Sea Dart can reach speeds of Mach 2.5, and can engage targets at over 30 nautical miles at a variety of altitudes.

HMS Coventry Sea Dart
The tail of a Sea Dart missile launched by HMS Coventry during the Falklands conflict – it was found sticking out of a peat bog on East Falkand, and recovered by a Chinook crew

Sea Dart motor
The first-stage rocket motor of a Sea Dart missile launched by HMS Exeter, which successfully shot down an Argentine aircraft in 1982

Sea Dart
A complete Sea Dart round

Sea Dart and Sea Wolf are themselves due to be replaced by Sea Viper, a combined system with both long and short range missiles. The system promises far better target discrimination and performance than even the sophisticated Sea Dart and Sea Wolf – it is currently the main air defence of the new Type 45 destroyers.

The Explosion Museum is well worth a visit with a fantastic collection, great setting and helpful and knowledgeable staff

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