Anti-aircraft warfare

• 1. Introduction

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  On the 30th of September 1915, troops of the Serbian Army watched three enemy aircraft approach the city of Kragujevac. Soldiers fired at them with shotguns and machine guns and could not stop them. The aircraft dropped 45 bombs over the city, striking military installations, the railway station, and many civilian targets. Then a private named Radoje Ljutovac aimed a cannon at the planes and brought one down. It crashed in the city, and both pilots died from their injuries. The cannon was not built for the job. It was a slightly modified Turkish cannon, captured during the First Balkan War in 1912. That moment was the first time in military history that a military aircraft was shot down by ground-to-air artillery fire. Anti-aircraft warfare is the counter to aerial warfare, and it grew out of moments exactly like this one. How do you hit a target moving through three-dimensional space, at the precise instant it occupies a point in the sky? How did gunners learn to aim not at an aircraft, but at where it would be? And how did a field improvised with shotguns and propped-up field guns become a world of radar, guided missiles, and lasers? The answers run from a besieged Paris to a battleship's deck to the edge of stealth.

• 2. The Aiming Problem

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  The essence of air defence is to detect a hostile aircraft and destroy it, and the hard part is the geometry. An attack must match three coordinates in space, and it must match them at the exact time the target is at that position. A projectile must either be guided to the target or aimed at the target's predicted future position. That prediction has to account for the speed and direction of both the target and the shell. In the First World War this was called deflection gun-laying. Off-set angles for range and elevation were set on the gunsight and updated as the target moved, so that when the sights rested on the target, the barrel pointed at where the target would be. Range and height together determined the fuse length. The British attacked range first, building the height/range finder, beginning with the Barr and Stroud UB2, a two-metre optical coincident rangefinder on a tripod. It measured distance and elevation angle, which together gave the aircraft's height. A companion device, the height/fuse indicator, let an operator read off the necessary fuse length from the reported height. The remaining puzzle was 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, with displays at the guns, and was used with the 75 mm. The British Wilson-Dalby gun director used a pair of trackers and mechanical tachymetry. By the late 1930s the British had a working definition of how high a gun could reach, calling it the effective ceiling. They set it as the height at which a target approaching directly at 400 mph could be engaged for 20 seconds before the gun reached 70 degrees of elevation.

• 3. The Machine That Aims

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  In 1925 the British adopted a mechanical analogue computer built by Vickers and called the Predictor AA No 1. Given the target's height, its operators tracked the aircraft, and the predictor produced bearing, quadrant elevation, and fuse setting. Those values were sent electrically to the guns and shown on repeater dials, where layers matched pointers to lay the guns. This repeater-dial arrangement built on a system British coast artillery had used since the 1880s, and many anti-aircraft officers came from a coast artillery background. The deeper lesson arrived with the Bofors 40 mm gun, where service trials showed that ranging and tracking the new high-speed targets was nearly impossible. At short range the target looks large, the trajectory is flat, and the flight time is short, so a gunner can correct lead by watching the tracers. At long range the aircraft stays in firing range for a long time, but small errors in distance cause large errors in shell-fall height and detonation time. For the Bofors, neither situation gave a good enough answer, and the solution was automation through the Kerrison Predictor. Operators kept it pointed at the target, and it calculated the aim point automatically, displaying it as a pointer on the gun. The gun crew simply followed the pointer and loaded the shells. The Kerrison was fairly simple, yet it pointed toward later generations that added radar, first for ranging and later for tracking. Germany introduced similar predictor systems during the war and added radar ranging as the fighting continued. From the early 1930s eight countries developed radar, and by the late 1930s this work was advanced enough that development of sound-locating acoustic devices was generally halted, though the equipment was kept. The German Wurzburg radar, put into use in 1940, could provide data suitable for controlling guns, and the British Radar, Gun Laying, Mark I, was in use by 1939.

• 4. Words For The Sky

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  Flak comes from the German Flugzeugabwehrkanone, or aircraft-defence cannon, and it is only one of the many names this field has gathered. The term air defence was probably first used by the UK when Air Defence of Great Britain was created as a Royal Air Force command in 1925. Yet British arrangements were also called anti-aircraft, abbreviated AA, a term that stayed in general use into the 1950s, sometimes prefixed by light or heavy to classify a gun or unit. Ack-ack came from the spelling alphabet the British used to transmit AA by voice. Archie was a First World War British term, probably coined by Amyas Borton and believed to derive, by way of the Royal Flying Corps, from the music-hall comedian George Robey's line, Archibald, certainly not. The flaming onion was an Allied flier's name for a German revolving cannon with five barrels that quickly launched a series of 37 mm shells. Other languages added their own words. The Russian term is Protivovozdushnaya oborona, abbreviated PVO, and Russian anti-aircraft systems are called zenitnye, meaning pointing to zenith. In French, air defence is Defense contre les aeronefs, abbreviated DCA. The names also carried doctrine. NATO defines weapons control status in three levels: weapons free, where weapons may be fired at any target not positively recognised as friendly; weapons tight, where they may be fired only at targets recognised as hostile; and weapons hold, where they may be fired only in self-defence or under a formal order.

• 5. First Guns Skyward

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  After the disaster at Sedan in the Franco-Prussian War of 1870, Paris was besieged, and French troops outside the city tried to communicate by balloon. Gustav Krupp mounted a modified one-pounder, a 37 mm gun, on a horse-drawn carriage to shoot those balloons down, calling it the Ballonabwehrkanone, the balloon-defence cannon. This is the earliest known use of a weapon made specifically for the anti-aircraft role. The first ever anti-airplane operation came later, during the Italo-Turkish War, when the Turks, lacking anti-aircraft weapons, became the first to shoot down an airplane by rifle fire. The first aircraft to crash in a war was that of Lieutenant Piero Manzini, shot down on the 25th of August 1912. Before that, balloon and airship guns drew growing attention in Europe, but by 1910 only Krupp, Erhardt, Vickers Maxim, and Schneider had published any information. Krupp's designs included adaptations of their 65 mm nine-pounder, a 75 mm twelve-pounder, and even a 105 mm gun. Erhardt offered a twelve-pounder, Vickers Maxim a three-pounder, and Schneider a 47 mm. The French balloon gun appeared in 1910, an eleven-pounder mounted on a vehicle with a total uncrewed weight of two tons. Since balloons moved slowly, the sights were simple, but the challenge of faster aeroplanes was already recognised. By 1913 only France and Germany had developed field guns suitable for engaging both balloons and aircraft. The first US anti-aircraft cannon was a one-pounder concept by Admiral Twining in 1911, designed against the threat of airships, which became the basis for the US Navy's first operational anti-aircraft gun, the 3-inch/23 caliber gun.

• 6. The Improvised War

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  On the 8th of July 1914, the New York Times reported that the British government had decided to dot the coasts of the British Isles with towers, each armed with two quick-firing guns of special design, with a complete circle of towers around naval installations. By December 1914 the Royal Naval Volunteer Reserve was manning guns and searchlights at some nine ports, and the Royal Garrison Artillery had been given responsibility for anti-aircraft defence in the field, using motorised two-gun sections first formed in November 1914. Most armies simply propped up their field pieces, like the French 75 mm and the Russian 76.2 mm, on embankments to point the muzzle skyward. These ad hoc solutions proved largely useless. Gunners had little experience, no way to measure a target's range, height, or speed, and great difficulty observing where their shell bursts fell relative to the target, so most rounds exploded well below their targets. The exception was guns protecting spotting balloons, where altitude could be measured from the length of the cable. German air attacks on Britain grew in 1915, and the AA effort was judged ineffective, so a Royal Navy gunnery expert, Admiral Sir Percy Scott, was appointed to build an integrated defence for London. Zeppelins, filled with hydrogen, became targets for incendiary shells fired with airburst fuses, and the British fitted tracers for night use. By December 1916 there were 183 AA sections defending Britain, 74 with the British Expeditionary Force in France, and 10 in the Middle East. By the war's end, a pattern had been set. Anti-aircraft warfare would use heavy weapons against high-altitude targets and lighter weapons when aircraft came lower, and the Red Baron is believed to have been shot down by an anti-aircraft Vickers machine gun.

• 7. Guns Of The World War

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  Krupp's engineers presented an 88 mm design, the FlaK 36, after specifications for a planned 75 mm gun were amended to require much higher performance. First used in Spain during the Spanish Civil War, it proved to be one of the best anti-aircraft guns in the world and was particularly deadly against light, medium, and even early heavy tanks. The British backbone was the QF 3.7-inch gun, whose production began in 1937, joined by the QF 4.5-inch gun near naval ports where naval ammunition was available. The 4.5-inch at Singapore had the first success in shooting down Japanese bombers. The 40 mm Bofors had become available in 1931, developed for the Swedish Navy, and was adopted by some 17 nations just before the war. The British arranged licence building of it, and the gun mattered so much that they produced a film, The Gun, to push assembly-line workers to work harder. The Americans first produced their own unlicensed copy before moving to licensed production in mid-1941. American forces answered Germany's quadruple-20 mm Flakvierling with the Maxson M45 Quadmount, four heavy-barrel M2 machine guns mounted together, often carried on a half-track as the M16 Multiple Gun Motor Carriage. The larger American 90 mm M3 gun, like the eighty-eight, proved an excellent anti-tank gun. The 120 mm M1 stratosphere gun was the most powerful AA gun, with a 60000 ft altitude capability, yet no 120 mm M1 was ever fired at an enemy aircraft. At sea, the US Navy adopted the Swiss Oerlikon 20 mm in exchange for M2 machine guns, and by the end of 1942 the 20 mm accounted for 42% of all aircraft destroyed by the Navy's shipboard fire. The 5-inch/38 naval gun, mated with the Mark 37 director and the proximity fuse, could knock drones out of the sky at ranges as far as 13,000 yards. The Germans also built massive reinforced-concrete flak towers, some more than six stories high, and several in Berlin were among the last buildings to fall to the Soviets during the Battle of Berlin in 1945.

• 8. From Gun To Missile

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  Post-war analysis showed that even with the newest systems on both sides, roughly 90% of bombers reached their targets, a figure made intolerable by the nuclear bomb, where a single bomber arriving was no longer acceptable. That shift drove the move from guns to guided missiles. Germany had been desperate to field anti-aircraft missiles, with sophisticated designs like the radar-guided Wasserfall rocket, but none became operational before the war ended. After several years of post-war development, such systems matured into viable weapons. The US upgraded its defences with the Nike Ajax missile, and the larger guns soon disappeared. The same happened in the USSR after the SA-2 Guideline arrived. By 1955 the US military deemed the 40 mm Bofors obsolete against jet aircraft and turned to surface-to-air missiles. Missiles took over more and more roles, shrinking until they could ride on armoured cars and tank chassis, and man-portable systems, MANPADS, were introduced in the 1960s. The 1982 Falklands War put this arsenal into the field, where the Argentine forces used the 35 mm Oerlikon GDF-002 twin cannon and the Roland missile, while the British relied on the Rapier system, with a few brand-new FIM-92 Stinger missiles used by special forces, and naval systems including Sea Dart, Sea Slug, SeaCat, and Sea Wolf. The newest challenge runs in the opposite direction. Stealth designs cut detection ranges so far that an aircraft is often never even seen, and when it is, an intercept may come too late. Anti-stealth technology answers with bistatic and low-frequency radars, advanced thermographic cameras, and high-sensitivity radars. The Russian S-400 is claimed to detect a target with a 0.05-square-metre radar cross-section from 90 km away.