Antiaircraft (AA) guns were first used in World War I, when they were adapted from equipment designed for other roles. Their job was to prevent enemy aircraft from flying at such a height that they could observe, photograph, range artillery, bomb with accuracy, or attack troops at low level. They also prevented hostile aircraft from flying in formation, thus preventing the enemy from using the power of a combined defensive armament against counterattacking aircraft. This technology persisted until the end of World War II, by which time the great speed of jet aircraft made AA guns impractical against high-flying targets.
The AA problem
Once the AA shell has left the gun muzzle, it is set on an unalterable ballistic trajectory. At the same time, as it is traveling upwards toward the target aircraft, the target is itself traveling through the sky. For a guided missile, which can change course and maneuver as it homes in on its target, this travel distance is not of ultimate importance, but for a shell, it can affect the accuracy very considerably. For example, a target traveling at the now modest speed of 200 mph (320 km/h) would travel almost 1¾ miles (2.8 km) during the 30 second flight of a 3.7 in. (9 cm) AA shell. The position of the target is known at the moment the gun fires, but once the shell starts on its way, no further control can be exercised over it. Thus, certain assumptions must be made about the behavior of the target during the time of flight of the projectile: that the target will maintain a constant course, height, and speed shortly before and during the flight of the shell and that, if any of these variables change, it will be at a constant rate. The higher and faster the aircraft is flying, the longer the time of flight of the shell and the less likely are the assumptions to be justified.
Predictor
The apparatus developed to pinpoint the future position of the target is called a predictor. Although extremely complex in design, it is simple in principle. The predictor follows the path of the target and measures the bearing (direction) and elevation. The change in bearing and elevation in a short period of time enables the course and speed to be calculated and this information, with the height supplied by a modified range finder, gives all the target data. Initially an optical range finder was employed, but it could not be used at night and later was replaced by radar. The trajectory of the shell depends on its initial velocity, the retardation due to its shape and diameter, and its weight and stability in flight together with the meteorological conditions at various altitudes through which it passes. All these factors are fed into the predictor. The business of prediction is now carried out by computer with data-processing techniques so fast that it can deal with attack by the fastest low-flying jets.
In the U.S. Vulcan Air Defence System, the Vulcan 20 mm Gatling gun (which could spew out bursts of fire at 3,000 rounds a minute) used a computer to offset the optical sight to allow for the distance covered by the target while the shells were on their way to it. The computer gained its information from a range-finding radar and the movement of the gun as it tracked the target, with the gunner using optical sights. Because air attack is made at such speed that unaided target analysis is impossible, the Vulcan was designed to be used as part of a larger system in which early-warning radar gave notice of approaching aircraft and IFF (Interrogation Friend or Foe) radar could recognize signals emitted by friendly aircraft. This system relieved the gunner of the problem of identifying aircraft as hostile or not in the split second available to him.
The Vulcan’s sheer weight of fire gave it a high probability of hitting the target, but for guns that fired at a slower rate or for some missiles, the probability of achieving a kill was increased by a proximity fuse system that detonated the warhead in the target area. A proximity fuse works by using a radio device built into the shell to detect when it is near the target. The strength of the signal determines when the fuse should detonate the shell.
In the 1980s, it became increasingly clear that the Vulcan system could not compete with new technological developments and so was phased out by the U.S. Army Air Defense Artillery (ADA) and replaced with the Stinger missile.
