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Anti-aircraft warfare is the counter to aerial warfare[1] and it includes "all measures designed to nullify or reduce the effectiveness of hostile air action" (NATO's definition).[2] It includes surface based, subsurface (submarine launched), and air-based weapon systems, associated sensor systems, command and control arrangements, and passive measures (e.g. barrage balloons). It may be used to protect naval, ground, and air forces in any location. However, for most countries, the main effort has tended to be homeland defence. Missile defence is an extension of air defence, as are initiatives to adapt air defence to the task of intercepting any projectile in flight.
Most modern anti-aircraft (AA) weapons systems are optimized for short-, medium-, or long-range air defence, although some systems may incorporate multiple weapons (such as both autocannons and surface-to-air missiles). ‘Layered air defence’ usually refers to multiple ‘tiers’ of air defence systems which, when combined, an airborne threat must penetrate in order to reach its target; This defence is usually accomplished via the combined use of systems optimized for either short-, medium-, or long-range air defence.
In some countries, such as Britain and Germany during the Second World War, the Soviet Union, and modern NATO and the United States, ground-based air defence and air defence aircraft have been under integrated command and control. However, while overall air defence may be for homeland defence (including military facilities), forces in the field, wherever they are, provide their own defences against airborne threats.
Until the 1950s, guns firing ballistic munitions ranging from 7.62 mm (.30 in) to 152.4 mm (6 in) were the standard weapons; guided missiles then became dominant, except at the very shortest ranges (as with close-in weapon systems, which typically use rotary autocannons or, in very modern systems, surface-to-air adaptations of short-range air-to-air missiles, often combined in one system with rotary cannons).
Terminology
It may also be called counter-air, anti-air, AA, flak, layered air defence or air defence forces.
The term air defence was probably first used by the UK when Air Defence of Great Britain (ADGB) was created as a Royal Air Force command in 1925. However, arrangements in the UK were also called "anti-aircraft", abbreviated as AA, a term that remained in general use into the 1950s. After the First World War it was sometimes prefixed by "light" or "heavy" (LAA or HAA) to classify a type of gun or unit. Nicknames for anti-aircraft guns include "AA", "AAA" or "triple-A" (abbreviations of "anti-aircraft artillery"), "flak" (from the German Flugzeugabwehrkanone), "ack-ack" (from the spelling alphabet used by the British for voice transmission of "AA");[3] and "archie" (a World War I British term probably coined by Amyas Borton, and believed to derive via the Royal Flying Corps, from the music-hall comedian George Robey's line "Archibald, certainly not!"[4]).
NATO defines anti-aircraft warfare (AAW) as "measures taken to defend a maritime force against attacks by airborne weapons launched from aircraft, ships, submarines and land-based sites".[2] In some armies the term all-arms air defence (AAAD) is used for air defence by nonspecialist troops. Other terms from the late 20th century include "ground based air defence" (GBAD) with related terms "short range air defense" (SHORAD) and man-portable air-defense system (MANPADS). Anti-aircraft missiles are variously called surface-to-air missiles, ("SAMs") and surface-to-air guided weapons (SAGWs). Examples are the RIM-66 Standard, Raytheon Standard Missile 6, or the MBDA Aster missile.
Non-English terms for air defence include the German Flak or FlaK (Fliegerabwehrkanone, 'aircraft defence cannon',[5] also cited as Flugabwehrkanone), whence English flak, and the Russian term Protivovozdushnaya oborona (Cyrillic: Противовозду́шная оборо́на), a literal translation of 'anti-air defence', abbreviated as PVO.[6] In Russian, the AA systems are called zenitnye (i.e., 'pointing to zenith') systems. In French, air defence is called Défense contre les aéronefs (DCA) , aéronef meaning 'aircraft'.[7]
The maximum distance at which a gun or missile can engage an aircraft is an important figure. However, many different definitions are used and unless the same definition is used, performance of different guns or missiles cannot be compared. For AA guns only the ascending part of the trajectory can be usefully used. One term is "ceiling", the maximum ceiling being the height a projectile would reach if fired vertically, not practically useful in itself as few AA guns are able to fire vertically, and the maximum fuse duration may be too short, but potentially useful as a standard to compare different weapons.
The British adopted "effective ceiling", meaning the altitude at which a gun could deliver a series of shells against a moving target; this could be constrained by maximum fuse running time as well as the gun's capability. By the late 1930s the British definition was "that height at which a directly approaching target at 400 mph can be engaged for 20 seconds before the gun reaches 70 degrees elevation".[8]
General description
The essence of air defence is to detect hostile aircraft and destroy them. The critical issue is to hit a target moving in three-dimensional space; an attack must not only match these three coordinates, but must do so at the time the target is at that position. This means that projectiles either have to be guided to hit the target, or aimed at the predicted position of the target at the time the projectile reaches it, taking into account the speed and direction of both the target and the projectile.
Throughout the 20th century, air defence was one of the fastest-evolving areas of military technology, responding to the evolution of aircraft and exploiting technology such as radar, guided missiles and computing (initially electromechanical analogue computing from the 1930s on, as with equipment described below). Improvements were made to sensors, technical fire control, weapons, and command and control. At the start of the 20th century these were either very primitive or non-existent.
Initially sensors were optical and acoustic devices developed during World War I and continued into the 1930s,[9] but were quickly superseded by radar, which in turn was supplemented by optoelectronics in the 1980s. Command and control remained primitive until the late 1930s, when Britain created an integrated system[10] for ADGB that linked the ground-based air defence of the British Army's Anti-Aircraft Command, although field-deployed air defence relied on less sophisticated arrangements. NATO later called these arrangements an "air defence ground environment", defined as "the network of ground radar sites and command and control centres within a specific theatre of operations which are used for the tactical control of air defence operations".[2]
Rules of engagement are critical to prevent air defences engaging friendly or neutral aircraft. Their use is assisted but not governed by identification friend or foe (IFF) electronic devices originally introduced during the Second World War. While these rules originate at the highest authority, different rules can apply to different types of air defence covering the same area at the same time. AAAD usually operates under the tightest rules.
NATO calls these rules "weapon control orders" (WCO), they are:
- Weapons free: weapons may be fired at any target not positively recognised as friendly.
- Weapons tight: weapons may be fired only at targets recognised as hostile.
- Weapons hold: weapons may only be fired in self-defence or in response to a formal order.[2]
Until the 1950s, guns firing ballistic munitions were the standard weapon; guided missiles then became dominant, except at the very shortest ranges. However, the type of shell or warhead and its fuzing and, with missiles, the guidance arrangement were and are varied. Targets are not always easy to destroy; nonetheless, damaged aircraft may be forced to abort their mission and, even if they manage to return and land in friendly territory, may be out of action for days or permanently. Ignoring small arms and smaller machine-guns, ground-based air defence guns have varied in calibre from 20 mm to at least 152 mm.[11]
Ground-based air defence is deployed in several ways:
- Self-defence by ground forces using their organic weapons, AAAD.
- Accompanying defence, specialist aid defence elements accompanying armoured or infantry units.
- Point defence around a key target, such as a bridge, critical government building or ship.
- Area air defence, typically "belts" of air defence to provide a barrier, but sometimes an umbrella covering an area. Areas can vary widely in size. They may extend along a nation's border, e.g. the Cold War MIM-23 Hawk and Nike belts that ran north–south across Germany, across a military formation's manoeuvre area, or above a city or port. In ground operations air defence areas may be used offensively by rapid redeployment across current aircraft transit routes.
Air defence has included other elements, although after the Second World War most fell into disuse:
- Tethered barrage balloons to deter and threaten aircraft flying below the height of the balloons, where they are susceptible to damaging collisions with steel tethers.
- Cables strung across valleys, sometimes forming a "curtain" with vertical cables hanging from them.[12]
- Searchlights to illuminate aircraft at night for both gun-layers and optical instrument operators. During World War II searchlights became radar controlled.
- Large smoke screens created by large smoke canisters on the ground to screen targets and prevent accurate weapon aiming by aircraft.
Passive air defence is defined by NATO as "Passive measures taken for the physical defence and protection of personnel, essential installations and equipment in order to minimise the effectiveness of air and/or missile attack".[2] It remains a vital activity by ground forces and includes camouflage and concealment to avoid detection by reconnaissance and attacking aircraft. Measures such as camouflaging important buildings were common in the Second World War. During the Cold War the runways and taxiways of some airfields were painted green.
Organization
While navies are usually responsible for their own air defence—at least for ships at sea—organisational arrangements for land-based air defence vary between nations and over time.
The most extreme case was the Soviet Union and this model may still be followed in some countries: it was a separate service, on a par with the army, navy, or air force. In the Soviet Union, this was called Voyska PVO, and had both fighter aircraft, separate from the air force, and ground-based systems. This was divided into two arms, PVO Strany, the Strategic Air defence Service responsible for Air Defence of the Homeland, created in 1941 and becoming an independent service in 1954, and PVO SV, Air Defence of the Ground Forces. Subsequently, these became part of the air force and ground forces respectively.[13][14]
At the other extreme, the United States Army has an Air Defense Artillery Branch that provides ground-based air defence for both homeland and the army in the field; however, it is operationally under the Joint Force Air Component Commander. Many other nations also deploy an air-defence branch in the army. Some, such as Japan or Israel, choose to integrate their ground based air defence systems into their air force.
In Britain and some other armies, the single artillery branch has been responsible for both home and overseas ground-based air defence, although there was divided responsibility with the Royal Navy for air defence of the British Isles in World War I. However, during the Second World War, the RAF Regiment was formed to protect airfields everywhere, and this included light air defences. In the later decades of the Cold War this included the United States Air Force's operating bases in the UK. All ground-based air defence was removed from Royal Air Force (RAF) jurisdiction in 2004. The British Army's Anti-Aircraft Command was disbanded in March 1955,[15] but during the 1960s and 1970s the RAF's Fighter Command operated long-range air-defence missiles to protect key areas in the UK. During World War II, the Royal Marines also provided air defence units; formally part of the mobile naval base defence organisation, they were handled as an integral part of the army-commanded ground based air defences.
The basic air defence unit is typically a battery with 2 to 12 guns or missile launchers and fire control elements.[citation needed] These batteries, particularly with guns, usually deploy in a small area, although batteries may be split; this is usual for some missile systems. SHORAD missile batteries often deploy across an area with individual launchers several kilometres apart. When MANPADS is operated by specialists, batteries may have several dozen teams deploying separately in small sections; self-propelled air defence guns may deploy in pairs.
Batteries are usually grouped into battalions or equivalent. In the field army, a light gun or SHORAD battalion is often assigned to a manoeuvre division. Heavier guns and long-range missiles may be in air-defence brigades and come under corps or higher command. Homeland air defence may have a full military structure. For example, the UK's Anti-Aircraft Command, commanded by a full British Army general was part of ADGB. At its peak in 1941–42 it comprised three AA corps with 12 AA divisions between them.[16]
History
Earliest use
The use of balloons by the U.S. Army during the American Civil War compelled the Confederates to develop methods of combating them. These included the use of artillery, small arms, and saboteurs. They were unsuccessful, and internal politics led the United States Army's Balloon Corps to be disbanded mid-war. The Confederates experimented with balloons as well.[17]
Turks carried out the first ever anti-airplane operation in history during the Italo-Turkish war. Although lacking anti-aircraft weapons, they were the first to shoot down an aeroplane by rifle fire. The first aircraft to crash in a war was the one of Lieutenant Piero Manzini, shot down on August 25, 1912.[18][19]
The earliest known use of weapons specifically made for the anti-aircraft role occurred during the Franco-Prussian War of 1870. After the disaster at Sedan, Paris was besieged and French troops outside the city started an attempt at communication via balloon. Gustav Krupp mounted a modified 1-pounder (37 mm) gun – the Ballonabwehrkanone (Balloon defence cannon) or BaK — on top of a horse-drawn carriage for the purpose of shooting down these balloons.[20][page needed]
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Ballonabwehrkanone by Krupp
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Ballonabwehrkanone by Krupp
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Ballonabwehrkanone on the Prussian corvette Nymphe 1872
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20 mm Becker-Oerlikon Model 1917 AA-gun
By the early 20th century balloon, or airship, guns, for land and naval use were attracting attention. Various types of ammunition were proposed, high explosive, incendiary, bullet-chains, rod bullets and shrapnel. The need for some form of tracer or smoke trail was articulated. Fuzing options were also examined, both impact and time types. Mountings were generally pedestal type but could be on field platforms. Trials were underway in most countries in Europe but only Krupp, Erhardt, Vickers Maxim, and Schneider had published any information by 1910. Krupp's designs included adaptations of their 65 mm 9-pounder, a 75 mm 12-pounder, and even a 105 mm gun. Erhardt also had a 12-pounder, while Vickers Maxim offered a 3-pounder and Schneider a 47 mm. The French balloon gun appeared in 1910, it was an 11-pounder but mounted on a vehicle, with a total uncrewed weight of two tons. However, since balloons were slow moving, sights were simple. But the challenges of faster moving aeroplanes were recognised.[21]
By 1913 only France and Germany had developed field guns suitable for engaging balloons and aircraft and addressed issues of military organisation. Britain's Royal Navy would soon introduce the QF 3-inch and QF 4-inch AA guns and also had Vickers 1-pounder quick firing "pom-poms" that could be used in various mountings.[22][23]
The first US anti-aircraft cannon was a 1-pounder concept design by Admiral Twining in 1911 to meet the perceived threat of airships, that eventually was used as the basis for the US Navy's first operational anti-aircraft cannon: the 3-inch/23 caliber gun.[24]
First World War
![](http://upload.wikimedia.org/wikipedia/commons/thumb/3/3f/Krupps_9_pounder_1909.jpg/220px-Krupps_9_pounder_1909.jpg)
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On the 30th of September, 1915, troops of the Serbian Army observed three enemy aircraft approaching Kragujevac. Soldiers fired at them with shotguns and machine-guns but failed to prevent them from dropping 45 bombs over the city, hitting military installations, the railway station and many other, mostly civilian, targets in the city. During the bombing raid, private Radoje Ljutovac fired his cannon at the enemy aircraft and successfully shot one down. It crashed in the city and both pilots died from their injuries. The cannon Ljutovac used was not designed as an anti-aircraft gun; it was a slightly modified Turkish cannon captured during the First Balkan War in 1912. This was the first occasion in military history that a military aircraft was shot down with ground-to-air artillery fire.[25][26][27]
The British recognised the need for anti-aircraft capability a few weeks before World War I broke out; on 8 July 1914, the New York Times reported that the British government had decided to "dot the coasts of the British Isles with a series of towers, each armed with two quick-firing guns of special design," while "a complete circle of towers" was to be built around "naval installations" and "at other especially vulnerable points". By December 1914 the Royal Naval Volunteer Reserve (RNVR) was manning AA guns and searchlights assembled from various sources at some nine ports. The Royal Garrison Artillery (RGA) was given responsibility for AA defence in the field, using motorised two-gun sections. The first were formally formed in November 1914. Initially they used QF 1-pounder "pom-pom"s (37 mm versions of the Maxim Gun).[23][28]
All armies soon deployed AA guns often based on their smaller field pieces, notably the French 75 mm and Russian 76.2 mm, typically simply propped up on some sort of embankment to get the muzzle pointed skyward. The British Army adopted the 13-pounder quickly producing new mountings suitable for AA use, the 13-pdr QF 6 cwt Mk III was issued in 1915. It remained in service throughout the war but 18-pdr guns were lined down to take the 13-pdr shell with a larger cartridge producing the 13-pr QF 9 cwt and these proved much more satisfactory.[29] However, in general, these ad hoc solutions proved largely useless. With little experience in the role, no means of measuring target, range, height or speed the difficulty of observing their shell bursts relative to the target gunners proved unable to get their fuse setting correct and most rounds burst well below their targets. The exception to this rule was the guns protecting spotting balloons, in which case the altitude could be accurately measured from the length of the cable holding the balloon.
The first issue was ammunition. Before the war it was recognised that ammunition needed to explode in the air. Both high explosive (HE) and shrapnel were used, mostly the former. Airburst fuses were either igniferious (based on a burning fuse) or mechanical (clockwork). Igniferious fuses were not well suited for anti-aircraft use. The fuse length was determined by time of flight, but the burning rate of the gunpowder was affected by altitude. The British pom-poms had only contact-fused ammunition. Zeppelins, being hydrogen-filled balloons, were targets for incendiary shells and the British introduced these with airburst fuses, both shrapnel type-forward projection of incendiary "pot" and base ejection of an incendiary stream. The British also fitted tracers to their shells for use at night. Smoke shells were also available for some AA guns, these bursts were used as targets during training.[30]
German air attacks on the British Isles increased in 1915 and the AA efforts were deemed somewhat ineffective, so a Royal Navy gunnery expert, Admiral Sir Percy Scott, was appointed to make improvements, particularly an integrated AA defence for London. The air defences were expanded with more RNVR AA guns, 75 mm and 3-inch, the pom-poms being ineffective. The naval 3-inch was also adopted by the army, the QF 3-inch 20 cwt (76 mm), a new field mounting was introduced in 1916. Since most attacks were at night, searchlights were soon used, and acoustic methods of detection and locating were developed. By December 1916 there were 183 AA sections defending Britain (most with the 3-inch), 74 with the BEF in France and 10 in the Middle East.[31]
AA gunnery was a difficult business. The problem was of successfully aiming a shell to burst close to its target's future position, with various factors affecting the shells' predicted trajectory. This was called deflection gun-laying, where "off-set" angles for range and elevation were set on the gunsight and updated as their target moved. In this method, when the sights were on the target, the barrel was pointed at the target's future position. Range and height of the target determined fuse length. The difficulties increased as aircraft performance improved.
The British dealt with range measurement first, when it was realised that range was the key to producing a better fuse setting. This led to the height/range finder (HRF), the first model being the Barr & Stroud UB2, a two-metre optical coincident rangefinder mounted on a tripod. It measured the distance to the target and the elevation angle, which together gave the height of the aircraft. These were complex instruments and various other methods were also used. The HRF was soon joined by the height/fuse indicator (HFI), this was marked with elevation angles and height lines overlaid with fuse length curves, using the height reported by the HRF operator, the necessary fuse length could be read off.[32]
However, the problem of deflection settings — "aim-off" — required knowing the rate of change in the target's position. Both France and the UK introduced tachymetric devices to track targets and produce vertical and horizontal deflection angles. The French Brocq system was electrical; the operator entered the target range and had displays at guns; it was used with their 75 mm. The British Wilson-Dalby gun director used a pair of trackers and mechanical tachymetry; the operator entered the fuse length, and deflection angles were read from the instruments.[33][34]
By the start of World War I, the 77 mm had become the standard German weapon, and came mounted on a large traverse that could be easily transported on a wagon. Krupp 75 mm guns were supplied with an optical sighting system that improved their capabilities. The German Army also adapted a revolving cannon that came to be known to Allied fliers as the "flaming onion" from the shells in flight. This gun had five barrels that quickly launched a series of 37 mm artillery shells.[citation needed]
As aircraft started to be used against ground targets on the battlefield, the AA guns could not be traversed quickly enough at close targets and, being relatively few, were not always in the right place (and were often unpopular with other troops), so changed positions frequently. Soon the forces were adding various machine-gun based weapons mounted on poles. These short-range weapons proved more deadly, and the "Red Baron" is believed to have been shot down by an anti-aircraft Vickers machine gun. When the war ended, it was clear that the increasing capabilities of aircraft would require better means of acquiring targets and aiming at them. Nevertheless, a pattern had been set: anti-aircraft warfare would employ heavy weapons to attack high-altitude targets and lighter weapons for use when aircraft came to lower altitudes.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/5/5f/AA-Predictor-Nr1MarkIII-001.jpg/220px-AA-Predictor-Nr1MarkIII-001.jpg)
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Interwar years
World War I demonstrated that aircraft could be an important part of the battlefield, but in some nations it was the prospect of strategic air attack that was the main issue, presenting both a threat and an opportunity. The experience of four years of air attacks on London by Zeppelins and Gotha G.V bombers had particularly influenced the British and was one of if not the main driver for forming an independent air force. As the capabilities of aircraft and their engines improved it was clear that their role in future war would be even more critical as their range and weapon load grew. However, in the years immediately after World War I, the prospect of another major war seemed remote, particularly in Europe, where the most militarily capable nations were, and little financing was available.
Four years of war had seen the creation of a new and technically demanding branch of military activity. Air defence had made huge advances, albeit from a very low starting point. However, it was new and often lacked influential 'friends' in the competition for a share of limited defence budgets. Demobilisation meant that most AA guns were taken out of service, leaving only the most modern.
However, there were lessons to be learned. In particular the British, who had had AA guns in most theatres in action in daylight and used them against night attacks at home. Furthermore, they had also formed an Anti-Aircraft Experimental Section during the war and accumulated large amounts of data that was subjected to extensive analysis. As a result, they published the two-volume Textbook of Anti-Aircraft Gunnery in 1924–1925. It included five key recommendations for HAA equipment:
- Shells of improved ballistic shape with HE fillings and mechanical time fuses
- Higher rates of fire assisted by automation
- Height finding by long-base optical instruments
- Centralised control of fire on each gun position, directed by tachymetric instruments incorporating the facility to apply corrections of the moment for meteorological and wear factors
- More accurate sound-location for the direction of searchlights and to provide plots for barrage fire
Two assumptions underpinned the British approach to HAA fire; first, aimed fire was the primary method and this was enabled by predicting gun data from visually tracking the target and having its height. Second, that the target would maintain a steady course, speed and height. This HAA was to engage targets up to 24,000 ft (7.3 km). Mechanical time fuses were required because the speed of powder burning varied with height, so fuse length was not a simple function of time of flight. Automated fire ensured a constant rate of fire that made it easier to predict where each shell should be individually aimed.[35][36]
In 1925 the British adopted a new instrument developed by Vickers. It was a mechanical analogue computer - the Predictor AA No 1. Given the target height, its operators tracked the target and the predictor produced bearing, quadrant elevation and fuse setting. These were passed electrically to the guns, where they were displayed on repeater dials to the layers who "matched pointers" (target data and the gun's actual data) to lay the guns. This system of repeater electrical dials built on the arrangements introduced by British coast artillery in the 1880s, and coast artillery was the background of many AA officers. Similar systems were adopted in other countries and for example the later Sperry M3A3 in the US, was also used by Britain as the Predictor AA No 2. Height finders were also increasing in size; in Britain, the seven-foot optical base World War I Barr & Stroud UB 2 stereoscopic rangefinder was replaced by the nine-foot optical base UB 7 and the eighteen-foot optical base UB 10 (only used on static AA sites). Goertz in Germany and Levallois in France produced five m (16 ft) instruments. However, in most countries the main effort in HAA guns until the mid-1930s was improving existing ones, although various new designs were on drawing boards.[36][37]
From the early 1930s eight countries developed radar; these developments were sufficiently advanced by the late 1930s for development work on sound-locating acoustic devices to be generally halted, although equipment was retained. Furthermore, in Britain the volunteer Observer Corps formed in 1925 provided a network of observation posts to report hostile aircraft flying over Britain. Initially radar was used for airspace surveillance to detect approaching hostile aircraft. However, the German Würzburg radar put into use in 1940 was capable of providing data suitable for controlling AA guns, and the British Radar, Gun Laying, Mark I, was designed to be used on AA gun positions and was in use by 1939.[38]
The Treaty of Versailles prevented Germany having AA weapons, and for example, the Krupps designers joined Bofors in Sweden. Some World War I guns were retained and some covert AA training started in the late 1920s. Germany introduced the 8.8 cm FlaK 18 in 1933, the 36 and 37 models followed with various improvements, but ballistic performance was unchanged. In the late 1930s the 10.5 cm FlaK 38 appeared, soon followed by the 39; this was designed primarily for static sites but had a mobile mounting, and the unit had 220 V 24 kW generators. In 1938 design started on the 12.8 cm FlaK.[39][40]
Britain had successfully tested a new 3.6-inch gun, in 1918. In 1928 a 3.7-inch (94 mm) gun became the preferred solution, but it took six years to gain funding. Production of the QF 3.7-inch gun began in 1937; this gun was used on mobile carriages with the field army and transportable guns on fixed mountings for static positions. At the same time the Royal Navy adopted a new 4.5-inch (113 mm) gun in a twin turret, which the army adopted in simplified single-gun mountings for static positions, mostly around ports where naval ammunition was available. The performance of the new guns was limited by their standard fuse No 199, with a 30-second running time, although a new mechanical time fuse giving 43 seconds was nearing readiness. In 1939 a machine fuse setter was introduced to eliminate manual fuse setting.[41]
The US ended World War I with two 3-inch AA guns and improvements were developed throughout the inter-war period. However, in 1924 work started on a new 105 mm static mounting AA gun, but only a few were produced by the mid-1930s because by this time work had started on the 90 mm AA gun, with mobile carriages and static mountings able to engage air, sea and ground targets. The M1 version was approved in 1940. During the 1920s there was some work on a 4.7-inch which lapsed, but revived in 1937, leading to a new gun in 1944.[42]
While HAA and its associated target acquisition and fire control was the primary focus of AA efforts, low-level close-range targets remained and by the mid-1930s were becoming an issue.
Until this time the British, at RAF insistence, continued their use of World War I machine guns, and introduced twin MG mountings for AAAD. The army was forbidden from considering anything larger than .50-inch.[citation needed] However, in 1935 their trials showed that the minimum effective round was an impact-fused 2 lb HE shell. The following year they decided to adopt the Bofors 40 mm and a twin barrel Vickers 2-pdr (40 mm) on a modified naval mount. The air-cooled Bofors was vastly superior for land use, being much lighter than the water-cooled "pom-pom", and UK production of the Bofors 40 mm was licensed. The Predictor AA No 3, as the Kerrison Predictor was officially known, was introduced with it.[43]
The 40 mm Bofors had become available in 1931. In the late 1920s the Swedish Navy had ordered the development of a 40 mm naval anti-aircraft gun from the Bofors company. It was light, rapid-firing and reliable, and a mobile version on a four-wheel carriage was soon developed. Known simply as the 40 mm, it was adopted by some 17 different nations just before World War II and is still in use today in some applications such as on coastguard frigates.
Rheinmetall in Germany developed an automatic 20 mm in the 1920s and Oerlikon in Switzerland had acquired the patent to an automatic 20 mm gun designed in Germany during World War I. Germany introduced the rapid-fire 2 cm FlaK 30 and later in the decade it was redesigned by Mauser-Werke and became the 2 cm FlaK 38.[44] Nevertheless, while 20 mm was better than a machine gun and mounted on a very small trailer made it easy to move, its effectiveness was limited. Germany therefore added a 3.7 cm. The first, the 3.7 cm FlaK 18 developed by Rheinmetall in the early 1930s, was basically an enlarged 2 cm FlaK 30. It was introduced in 1935 and production stopped the following year. A redesigned gun 3.7 cm FlaK 36 entered service in 1938, it too had a two-wheel carriage.[45] However, by the mid-1930s the Luftwaffe realised that there was still a coverage gap between 3.7 cm and 8.8 cm guns. They started development of a 5 cm gun on a four-wheel carriage.[46]
After World War I the US Army started developing a dual-role (AA/ground) automatic 37 mm cannon, designed by John M. Browning. It was standardised in 1927 as the T9 AA cannon, but trials quickly revealed that it was worthless in the ground role. However, while the shell was a bit light (well under 2 lbs) it had a good effective ceiling and fired 125 rounds per minute; an AA carriage was developed and it entered service in 1939 as the 37 mm gun M1. It proved prone to jamming, and was eventually replaced in AA units by the Bofors 40 mm. The Bofors had attracted attention from the US Navy, but none were acquired before 1939.[47] Also, in 1931 the US Army worked on a mobile anti-aircraft machine mount on the back of a heavy truck having four .30 calibre water-cooled machine guns and an optical director. It proved unsuccessful and was abandoned.[48]
The USSR introduced a new 76 mm M1931 in 1937, an 85 mm M1938[49] and developed the 37 mm M1939 (61-K), which appears to have been copied from the Bofors 40 mm. A Bofors 25 mm, essentially a scaled down 40 mm, was also copied as the 25 mm M1939.[50]
During the 1930s solid-fuel rockets were under development in the Soviet Union and Britain. In Britain the interest was for anti-aircraft fire, it quickly became clear that guidance would be required for precision. However, rockets, or "unrotated projectiles" as they were called, could be used for anti-aircraft barrages. A two-inch rocket using HE or wire obstacle warheads - the Z battery - was introduced first to deal with low-level or dive bombing attacks on smaller targets such as airfields. The three-inch was in development at the end of the inter-war period.[51]
Naval aspects
WWI had been a war in which air warfare blossomed, but had not matured to the point of being a real threat to naval forces. The prevailing assumption was that a few relatively small caliber naval guns could manage to keep enemy aircraft beyond a range where harm might be expected. In 1939 radio controlled target drones became available to the US Navy in quantity allowing a more realistic testing of existing anti-aircraft suites against actual flying and manoeuvring targets.[52] The results were sobering to an unexpected degree.
The United States was still emerging from the effects of the Great Depression and funds for the military had been sparse to the degree that 50% of shells used were still powder fused.[52] The US Navy found that a significant portion of its shells were duds or low order detonations (incomplete detonation of the explosive contained by the shell). Virtually every major country involved in combat in World War II invested in aircraft development. The cost of aircraft research and development was small and the results could be large.[53] So rapid was the performance leaps of evolving aircraft that the British High Angle Control System (HACS) was obsolete and designing a successor very difficult for the British establishment.[54] Electronics would prove to be an enabler for effective anti-aircraft systems and both the US and UK had a growing electronics industry.[54]
In 1939 radio controlled drones became available to actually test existing systems in British and American service. The results were disappointing by any measure. High-level manoeuvring drones were virtually immune to shipboard AA systems. The US drones could simulate dive bombing which showed the dire need for autocannons. Japan introduced powered gliders in 1940 as drones but apparently was unable to dive bomb.[55] There is no evidence of other powers using drones in this application at all. It may have caused a major underestimation of the threat and an inflated view of their AA systems.[56]
Second World War
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Poland's AA defences were no match for the German attack, and the situation was similar in other European countries.[57] Significant AAW (Anti-Air Warfare) started with the Battle of Britain in the summer of 1940. QF 3.7-inch AA guns provided the backbone of the ground-based AA defences, although initially significant numbers of QF 3-inch 20 cwt were also used. The Army's Anti-aircraft command, which was under operational command of RAF Fighter Command within Air Defence GB, grew to 12 AA divisions in three AA corps. Bofors 40 mm guns entered service in increasing numbers. In addition, the RAF regiment was formed in 1941 with responsibility for airfield air defence, eventually with Bofors 40 mm as their main armament. Fixed AA defences, using HAA and LAA, were established by the Army in key overseas places, notably Malta, Suez Canal and Singapore.
While the 3.7-inch was the main HAA gun in fixed defences and the only mobile HAA gun with the field army, the QF 4.5-inch gun, manned by artillery, was used in the vicinity of naval ports and made use of the naval ammunition supply. The 4.5-inch at Singapore had the first success in shooting down Japanese bombers. Mid war QF 5.25-inch naval guns started being emplaced in some permanent sites around London. This gun was also deployed in dual-role coast defence/AA positions.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/0/00/Bundesarchiv_Bild_101I-635-3999-24%2C_Deutschland%2C_Flak-Batterie_in_Feuerstellung.jpg/170px-Bundesarchiv_Bild_101I-635-3999-24%2C_Deutschland%2C_Flak-Batterie_in_Feuerstellung.jpg)
Germany's high-altitude needs were originally going to be filled by a 75 mm gun from Krupp, designed in collaboration with their Swedish counterpart Bofors, but the specifications were later amended to require much higher performance. In response Krupp's engineers presented a new 88 mm design, the FlaK 36. First used in Spain during the Spanish Civil War, the gun proved to be one of the best anti-aircraft guns in the world, as well as particularly deadly against light, medium, and even early heavy tanks.
After the Dambusters raid in 1943 an entirely new system was developed that was required to knock down any low-flying aircraft with a single hit. The first attempt to produce such a system used a 50 mm gun, but this proved inaccurate and a new 55 mm gun replaced it. The system used a centralised control system including both search and targeting radar, which calculated the aim point for the guns after considering windage and ballistics, and then sent electrical commands to the guns, which used hydraulics to point themselves at high speeds. Operators simply fed the guns and selected the targets. This system, modern even by today's standards, was in late development when the war ended.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/f/f0/Wehrmacht_Flugabwehr_Airdefence.jpeg/170px-Wehrmacht_Flugabwehr_Airdefence.jpeg)
The British had already arranged licence building of the Bofors 40 mm, and introduced these into service. These had the power to knock down aircraft of any size, yet were light enough to be mobile and easily swung. The gun became so important to the British war effort that they even produced a movie, The Gun, that encouraged workers on the assembly line to work harder. The Imperial measurement production drawings the British had developed were supplied to the Americans who produced their own (unlicensed) copy of the 40 mm at the start of the war, moving to licensed production in mid-1941.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/3/37/B-24_hit_by_Flak.jpg/220px-B-24_hit_by_Flak.jpg)
Service trials demonstrated another problem however: that ranging and tracking the new high-speed targets was almost impossible. At short range, the apparent target area is relatively large, the trajectory is flat and the time of flight is short, allowing to correct lead by watching the tracers. At long range, the aircraft remains in firing range for a long time, so the necessary calculations can, in theory, be done by slide rules—though, because small errors in distance cause large errors in shell fall height and detonation time, exact ranging is crucial. For the ranges and speeds that the Bofors worked at, neither answer was good enough.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/c9/Hyde_Park_Anti-aircraft_guns_H_993.jpg/220px-Hyde_Park_Anti-aircraft_guns_H_993.jpg)
The solution was automation, in the form of a mechanical computer, the Kerrison Predictor. Operators kept it pointed at the target, and the Predictor then calculated the proper aim point automatically and displayed it as a pointer mounted on the gun. The gun operators simply followed the pointer and loaded the shells. The Kerrison was fairly simple, but it pointed the way to future generations that incorporated radar, first for ranging and later for tracking. Similar predictor systems were introduced by Germany during the war, also adding radar ranging as the war progressed.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/3/39/20-mm-AA-cannon.gif/220px-20-mm-AA-cannon.gif)
A plethora of anti-aircraft gun systems of smaller calibre was available to the German Wehrmacht combined forces, and among them the 1940-origin Flakvierling quadruple-20 mm-autocannon-based anti-aircraft weapon system was one of the most often-seen weapons, seeing service on both land and sea. The similar Allied smaller-calibre air-defence weapons of the American forces were also quite capable. Their needs could cogently be met with smaller-calibre ordnance beyond using the usual singly-mounted M2 .50 caliber machine gun atop a tank's turret, as four of the ground-used "heavy barrel" (M2HB) guns were mounted together on the American Maxson M45 Quadmount weapon (as a direct answer to the Flakvierling), which were often mounted on the back of a half-track to form the M16 Multiple Gun Motor Carriage. Although of less power than Germany's 20 mm systems, the typical four or five combat batteries of an Army AAA battalion were often spread many kilometres apart from each other, rapidly attaching and detaching to larger ground combat units to provide welcome defence from enemy aircraft.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/e/e4/Anti-aircraft_BrenGun.jpg/220px-Anti-aircraft_BrenGun.jpg)
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