Editing Kerrison Predictor (section)

==History==
By the late 1930s, both [[Vickers]] and [[Sperry Corporation|Sperry]] had developed predictors for use against high-altitude bombers. However, low-flying aircraft presented a very different problem, with very short engagement times and high angular rates of motion, but at the same time less need for ballistic accuracy. Machine guns had been the preferred weapon against these targets, aimed by eye and swung by hand, but these no longer had the performance needed to deal with the larger and faster aircraft of the 1930s.{{sfn|Bromley|1984|pp=1-4}}

The [[British Army]]'s new [[Bofors 40 mm Automatic Gun L/60|Bofors 40 mm gun]]s were intended as their standard low-altitude anti-aircraft weapons. However, existing gunnery control systems were inadequate for the purpose; the range was too far to "guess" the lead, but at the same time close enough that the angle could change faster than the gunners could turn the traversal handles.{{sfn|Bromley|1984|p=15}} Trying to operate a calculating gunsight at the same time was an added burden on the gunner. Making matters worse was that these ranges were exactly where the ''[[Luftwaffe]]''{{'}}s [[dive bomber]]s, which were quickly proving to be a decisive weapon in the ''[[Blitzkrieg]]'', were attacking from.

The problem was taken up by Major A.V. Kerrison of the [[British Army]], who had been working as the Army liaison at the  [[Admiralty Research Laboratory]], [[Teddington]], through 1930s. Kerrison had worked on several of the [[Royal Navy]]'s gunnery computers and took up the problem in the late 1930s.{{sfn|Bromley|1984|p=15}} After the war, Kerrison went on to become Director of Aeronautical and Engineering Research at the British Admiralty.

His solution was a calculator that dispensed with many of the corrections and timing issues seen in devices like the [[Vickers Predictor]] which were intended for high-altitude fire. Instead, it made a relatively simple calculation of the impact point based on relative motion as provided by the operator. Key to the concept was the use of two [[ball-and-disk integrator]]s, used in this case to maintain a constant rate of motion. On top of the motorized disk were two metal balls, set one on top of the other with the bottom one in contact with the disk and the second in contact with mechanisms that drove the Predictor's laying handwheels.{{sfn|Bromley|1984|p=15}}

The two balls were clutched so they could be separated or forced together. For the initial setup, the operator would declutch the balls and use the handwheels to bring the Predictor's telescope onto the target. This also moved the two balls across the surface of the disk, although they were not in contact with it. Once they had begun tracking it, the clutch would be moved to bring the two balls into contact with the disk, at which point the rotation of the disk would cause the balls to rotate and thus automatically move the telescope to stay aligned with the target.{{sfn|Bromley|1984|p=15}}

As the original inputs from the handwheels were unlikely to be perfectly accurate, the system would normally begin to "drift" away from the target. The operators would then move the handwheel to bring the target back into the center, which also slid the balls over the disk to a new location, changing their rotation speed, and thereby adjusting the rate of motion to properly track the target again. The position of the balls over the disk directly represents the rate of angular motion of the target. A third setting in the clutch reset the system to begin tracking a different target.{{sfn|Bromley|1984|p=15}}

The two rates, in azimuth and altitude, were used to calculate the angular rate of the target, and from that, the vector along which the target was moving relative to the gun. This does not provide a complete solution; the shell from the gun takes a certain time to fly to the target, during which time it moves. This requires the gun to "lead" the target to account for the motion during this time. Since the range to the target is independent of its motion, this value had to be input separately, initially by a separate crewman simply estimating the range or using some form of [[optical rangefinder]],{{sfn|Bromley|1984|p=16}} although small [[Gun Laying radar]]s for this task became common during [[World War II]].

The "output" of the device drove hydraulic servo-motors attached to the traversal and elevation gears of the otherwise unmodified Bofors gun, allowing it to follow the predictor's indications automatically without manual intervention. The gunners simply kept the gun loaded, while the three aimers simply had to point the Predictor, mounted on a large [[tripod]], at the target. The Kerrison predictor did not calculate fuse settings, as the shells fired by the 40&nbsp;mm Bofors gun, with which it was designed to work, were contact-fused.{{sfn|Bromley|1984|pp=15-16}}

The Predictor proved to be able to hit practically anything that flew in a straight line, and it was particularly effective against dive bombers. It was also very complex, including over 1,000 precision parts and weighing over {{convert|500|lb|abbr=on}}, even though much of it was made of [[aluminium]] to reduce weight. With the demands of the [[Royal Air Force|RAF]] for almost all light metals and machinists, the Predictor was far too difficult for the Army to produce in any quantity.

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 [[The Scott Motorcycle Company#Stationary engines|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.

The No.7 anti-aircraft composite predictor, also designed by Kerrison was similar in some ways. It was originally developed for the 6-pounder naval gun, for close-in defence and also against targets at intermediate altitudes of {{convert|6000|to|14000|ft|m|abbr=on}}. It was later adapted for use with the 40&nbsp;mm Bofors.{{sfn|Bromley|1984|p=16}}