US Army Korea Era 75 mm Gun, M51, Antiaircraft "Skysweeper"

Skysweeper, technically Gun, M51, Antiaircraft, was an American 75 mm anti-aircraft gun deployed in the early 1950s by both the U.S. Army and U.S. Air Force. 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 (the M10) 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 an airliner at cruise altitude. At very low altitudes there were only seconds in which to react, so a hand-swung weapon was the only possibility, no matter how inaccurate. This left an intermediate altitude at which a small gun could not be used because the range to the aircraft was too far, and a larger gun could not be used because the targets were moving too fast.

During World War II the U.S. Army filled this intermediate-range niche with the almost-universal Bofors 40 mm gun equipped with a U.S. version of the Kerrison Predictor the M5 director. 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; a 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 mm 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 ten-round revolver-type magazines and an auto-loader that allowed it to reach 45 rounds/minute—about one third of what 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 T-38 radar unit on the left. The T-38 radar had a range of about 30 km[1] and could track aircraft traveling at up to 700 mph (1100 km/h).

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 for airbase defense. 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 U.S. 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 favor of a much smaller number of missile sites. By the end of the year only three Skysweeper battalions remained in the U.S., one at Sault Ste. Marie, Michigan to protect the Soo Locks, two at the Savannah River Site, plus one 90mm and two Skysweeper battalions at Thule Air Base in Greenland. These too were removed by 1959.

General characteristics:

Caliber: 75 mm
Barrel length: 4,500 mm (60 caliber)
Magazine: 2 × 10 rounds
Maximum range: 13.5 km
Effective range: 6.3 km