A self-propelled anti-aircraft weapon (SPAA, also self-propelled air defense, SPAD, or self-propelled anti-aircraft gun, SPAAG) is an anti-aircraft gun or surface-to-air missile launcher mounted on a mobile vehicle chassis. The Russian equivalent of SPAAG is ZSU, for zenitnaya samokhodnaya ustanovka, ("anti-aircraft self-propelled mount").
Specific weapon systems include machine guns, autocannon, larger guns, or missiles, and some mount both guns and longer-ranged missiles. Platforms used include both trucks and heavier armored fighting vehicles such as APCs and tanks, which add protection from aircraft, artillery, and small arms fire for front line deployment.
Anti-aircraft guns are usually mounted in a quickly-traversing turret with a high rate of elevation, for tracking fast-moving aircraft. They are often in dual or quadruple mounts, allowing a high rate of fire. Today, missiles (generally mounted on similar turrets) have largely supplanted anti-aircraft guns, particularly in the minds of generals and military planners. Anti-aircraft guns are still deadly against aircraft at short ranges (within a few kilometers). This is evidenced by:
Anti-aircraft machine guns have long been mounted on trucks, and these were quite common during World War I. A predecessor of the WW2 German "88" anti-aircraft gun, the WW1 German 77mm anti-aircraft gun, was truck-mounted and used to great effect against British tanks.
Larger guns followed on larger trucks, but these mountings generally required off-truck setup in order to unlimber the stabilizing legs these guns needed. One exception to this rule was the Italian Cannone da 90/53 which was highly effective when mounted on trucks, a fit known as the "autocannoni da 90/53". The 90/53 was a feared weapon, notably in the anti-tank role, but only a few hundred had been produced by the time of the armistice in 1943.
Among early pre-WW2 pioneers of self-propelled AA guns were the Germans. By WW2, they fielded the SdKfz 10/4 and 6/2, cargo halftracks mounting single 20mm or 37mm AA guns (respectively). Later in the war similar German halftracks mounted quad 20mm weapons.
Other nations tended to work on truck chassis. Starting in 1941, the British developed the "en portee" method of mounting an anti-tank gun (initially a 2 pounder) on a truck. This was to prevent the weapon from being damaged by long-distance towing across rough, stony deserts, and it was intended only to be a carrying method, with the gun unloaded for firing. However, crews tended to fire their weapons from their vehicles for the mobility this method provided, with consequent casualties. This undoubtedly inspired their Morris C9/B (officially the "Carrier, SP, 4x4, 40mm AA"), a Bofors 40mm AA gun mounted on a chassis derived from the Morris Quad Field Artillery Tractor truck. Similar types, based on 3-ton lorries, were produced in Britain, Canada and Australia, and together formed the most numerous self-propelled AA guns in British service.
Interest in mobile AA turned to heavier vehicles with the mass and stability needed to easily train weapons of all sizes. Probably the desire, particularly in German service, for anti-aircraft vehicles to be armoured for their own protection also assisted this trend.
The concept of an armored SPAAG was pioneered mainly by Germany during World War II, with their "flakpanzer" series. German World War II SPAAGs include the Möbelwagen, Wirbelwind, Ostwind and Kugelblitz. Other forces followed with designs of their own, notably the American M16 created by mounting quadruple M2 machine guns on a M3 Half-track. The British Army also introduced their own SPAAG late in the war, the Crusader III AA Mark I, which mounted the excellent Bofors 40 mm gun. On occasion SPAAGs have been used as very effective direct fire weapons against infantry, for example by American forces during late World War II, in Korea against mass infantry assault, and extensively during the Vietnam War, where for example the U.S. M42 Duster SPAAG (based on a light tank) was employed purely for this purpose.
Modern weapons include the feared Russian ZSU-23-4 Shilka and Tunguska-M1, Chinese Type 88 SPAAG, Swedish CV9040 AAV, Polish PZA Loara, American M6 Bradley Linebacker and M1097 Humvee Avenger, Yugoslavian BOV-3, Canadian ADATS, aging German Gepard, Japanese Type 87 SPAAG and similar versions with the British Marksman turret (which was also adapted for a number of other users), Italian SIDAM 25 and Otomatic, and versions of the French AMX-13. Older post-war examples include the ZSU-57-2, M163 VADS (mounting the Vulcan cannon) and failed M247 Sergeant York.