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Skysweeper, technically Gun, M51, Antiaircraft, was an American 75 mm anti-aircraft gun deployed in the
early 1950s. It was the first such gun to combine all of the various systems needed for effective use against high-speed aircraft
into a single carriage, namely radar, an analog computer for calculating "lead", and an autoloader for high-speed fire.
Anti-aircraft guns naturally fall into several categories, each for a different altitude and speed requirement. High-altitude
targets require very large guns to get the needed power into the shell to reach those altitudes, but at the same time have the
advantage of not needing to move very fast because at that range the change in angle of the target was small – consider the
seemingly slow motion of airliner at cruise altitude. At very low altitudes you had only seconds to react, so a hand-swung weapon
was the only possibility, no matter how inaccurate. This left an intermediate altitude at which you couldn't use a small gun
because the range to the aircraft was too far, and you couldn't use a larger gun because they were moving too fast.
During World War II the US
Army filled this niche with the almost-universal Bofors 40 mm gun equipped with a
US version of the Kerrison Predictor. But even by the end of
the war the performance and accuracy of newer bombers demanded a much higher
performance system that was able to attack targets at longer ranges. The Army eventually formulated the definitive requirements;
an new gun was needed to defeat aircraft flying at 1,000 mph (1,600 km/h) at altitudes up to 20,000 feet (6 km). Above that the
existing radar-equipped WWII era 90 mm and 120 mm guns would continue to serve well, with
the 120 able to reach as high as 80,000 ft (24 km).
Development of what would become the Skysweeper started in 1948. A new 75 mm gun, known as the T83E1 or
M35, was developed that had excellent muzzle
velocity, along with two cylindrical magazines and an auto-loader that allowed it to reach 45 rounds/minute – about the
same as the much lighter Bofors had managed. This was mounted on a large box-shaped powered traverse, which also mounted the
computer and manual gunsights on the right side, and the radar unit on the left.
Deployment started in the early 1950s and a number were rushed to Korea. Field units
used the system widely, although its size meant it was largely static and was primarily used at airbases. It proved to be an
excellent system and was found in various places until the early 1970s.
Skysweeper was also integrated with the massive anti-aircraft systems under the command of Army's Anti-Aircraft Command
(ARAACOM) in the US. Most ARAACOM deployments were around cities and used the 90 mm and 120 mm guns, but the Skysweeper was used
near targets that would have to be attacked by low-altitude aircraft. However the development of missile systems like the
Nike Ajax meant that the days of the anti-aircraft gun were soon over, and in
1957 the ARAACOM started to dramatically reduce the number of guns in favour of a much smaller number of missile sites. By the
end of the year only three Skysweeper batallions remained in the US, one at Sault Ste. Marie, Michigan to protect the locks on the Saint Lawrence Seaway, two
at Savannah River, plus one 90mm and two Skysweeper battalions at
Thule, Greenland.
These too were removed by 1959.
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