Kerrison Predictor

The Kerrison Predictor was one of the first fully-automated anti-aircraft "predictors", fire-control systems which would attempt to predict the correct aim at a plane based on simple inputs like the observed speed and angle to the target. Such devices had been used on ships for gunnery control for some time, and versions were also available for larger anti-aircraft guns, but the electromechanical Kerrison was the first to be fast enough to be used in the demanding high-speed low-altitude role.

The Kerrison Predictor was developed after it had been realized that modern aircraft on the attack flew too quickly for existing traversal systems on medium-sized guns to work. Smaller guns could be aimed by hand due to the short ranges at which they operated, allowing the operator to simply "guess" the required lead. Larger guns shot at targets so far away that the change in angle was low enough to calculate the required "lead" using a simple slide rule device in the gunsight.

However there was a middle range, served by the British Army's new Bofors 40 mm guns, where existing systems simply didn't work well. In this case the range was too far to "guess" the lead, but at the same time close enough that the angle changed so fast that the gunners were hard pressed simply turning the traversal handles, let alone trying to operate a calculating gunsight at the same time. Making matters worse was that these ranges were exactly where the Luftwaffe's dive bombers were attacking from, which were quickly proving to be a decisive weapon in the Blitzkrieg.

The Predictor solved the problem by doing all of the calculations mechanically through a complex system of gears. Inputs to its calculations included wind speed, gravity, ballistics of the gun and the rounds it fired, angle to the target in azimuth and altitude, and a user-input estimated speed. Some of these inputs were fed in via dials, which turned gearing inside the Predictor to calculate the range (from the change in angle and estimated speed) and direction of motion. The "output" of the device drove motors attached to the traversal and elevation gears of the otherwise unmodified Bofors gun. The gunners simply kept the gun loaded, while the three-man aimers simply had to point the Predictor, mounted on a large tripod, at the target.

The Predictor proved to be able to hit practically anything that flew in a straight line, and it was particularly effective against dive bombers. However it was also very complex, including over 1,000 precision parts and weighing over 500 pounds even though much of it was made of aluminum to reduce weight. With the demands of the RAF for almost all light metals and machinists, the Predictor was far too difficult for the Army to produce in any number.

Meanwhile in September 1940 the US Army's Coast Artillery Corps had become unhappy with their existing 37mm guns, and General Marshall asked the British to lend him four of their Bofors guns and Predictors for testing. They were more than impressed with both, and started plans to produce them in the USA. They obtained Imperial measurement engineering diagrams of the gun and Predictor from the British, passing the gun plans to Chrysler, and the Predictor to Sperry. Sperry was just starting production of their own horribly complex high-altitude system, the Sight, Computing, M7, and had no excess capacity to produce the new design as well. Instead they completed changes needed to US production and sent the plans back to the Army for production elsewhere.

To produce the Predictor, Singer Corporation was contracted in December 1940 to produce 1,500 a month to equip the Army's existing 37mm guns while production of the Bofors ramped up. Singer required massive changes in the company in order to ramp up production, including building new factories and the switching of a foundry from steel to aluminum. Production didn't start until January 1943, but the production line proved to be sound, and the entire order was filled for their Director, Antiaircraft, M5 by the middle of 1944. For a brief time some of the Army's Bofors guns were equipped with the Sperry M7, but these were replaced in the field as soon as M5's became available.

While the Predictor proved to be an excellent addition to the Bofors, it was not without its faults. The main problem was that the system required a fairly large electrical generator in order to drive the gun, increasing the logistics load in supplying the generators with fuel. Setting the system up was also a fairly complex task, and not something that could be done "on the fly". In the end they were used almost entirely for static emplacements, field units continuing to rely on their original iron sights or the simple Stiffkey-Stick sights that were introduced in late 1943.

With aircraft speeds increasing dramatically during the war, even the speed of the Kerrison Predictor proved lacking by the end. Nevertheless the Predictor demonstrated that effective gunnery required some sort of reasonably powerful computing support, and in 1944 Bell Labs started delivery of a new system based around an analog computer. The timing proved excellent. Late in the summer the Germans started attacking London with the V-1 flying bomb, which flew at high speeds at low altitudes. After a month of limited success against them, every available anti-aircraft gun was moved to the strip of land on the approach to London, and the new sights proved to be more than capable against them. Daytime attacks were soon abandoned.

Long after the war, US M5's started appearing in surplus shops in the late 1950s. John Whitney purchased one (and later a Sperry M7) and connected the electrical outputs to servos controlling the positioning of small lit targets and light bulbs. He then modified the "mathematics" of the system to move the targets in various mathematically controlled ways, a technique he referred to as "incremental drift". As the power of the systems grew they eventually evolved into what is today known as motion control photography, a widely used technique in special effects filming.

External links

* [http://www.bbc.co.uk/ww2peopleswar/stories/98/a7120298.shtml Memories of Frank Yates]
* [http://home.cfl.rr.com/featherweight/m5direct.htm Singer in World War II, 1939-1945]