The ZSU-57-2 Ob'yekt 500 is a Soviet self-propelled anti-aircraft gun (SPAAG), armed with two 57 mm autocannons. 'ZSU' stands for Zenitnaya Samokhodnaya Ustanovka (), meaning "anti-aircraft self-propelled mount", '57' stands for the bore of the armament in millimetres and '2' stands for the number of gun barrels. It was the first Soviet mass-produced tracked SPAAG. In the USSR it had the unofficial nickname "Sparka", meaning "twin mount", referring to the twin autocannon with which the vehicle is armed. Development history Origins During World War II, ground-attack aircraft emerged as a significant threat to mechanized units on the move. Conventional towed anti-aircraft artillery (AAA) was an inadequate response under such conditions owing to the time needed to bring anti-aircraft machine guns into action. This experience made it clear that an anti-aircraft tracked vehicle, armed with small-bore autocannons or heavy machine guns, was needed. Vehicles such as the German Wirbelwind and specialized variants of the US M3 Half-track had been used to good effect in the final battles of World War II, both by the US and nations which had received the M3 through Lend-Lease. In 1942, Soviet engineers developed the T-60-3. The vehicle, based on the T-60 light tank chassis, was armed with two 12.7 mm DShK heavy machine guns; but the prototype did not go into production because of flaws in the design. The SU-72 SPAAG and several other experimental vehicles based on the T-60 or T-70 light tank chassis and armed with 37 mm autocannon were also tested in 1942–1943. The ZSU-37 was based on the chassis of the SU-76M self-propelled gun (SPG) and armed with a 37 mm 61K anti-aircraft autocannon in an open-top rotating armoured turret. The vehicle entered production in February 1945 and was in small-scale production until 1948. After World War II it became clear that the firepower of a single 37 mm AA gun was insufficient to destroy high-speed, low-altitude targets. SPAAGs based on light tank chassis had poor maneuverability in difficult terrain, slow off-road speed and insufficient range in comparison with medium tanks and SPGs. Thus the ZSU-37s were retired from service by the end of the 1940s. ZSU-57-2 prototypes For several years after World War II there were no new SPAAG models in the USSR except for the BTR-152A (which were armed with two or four 14.5mm KPV heavy machine guns, designed ZTPU-4) and the BTR-40A (ZTPU-2) wheeled SPAAGs. Two of the