the steeper the target angle. Even an error in range itself will affect the apparent elevation rate. In theory, the gunner can tell the difference because he can measure the range rate, hence the speed towards him, but in fact ranging on aircraft using coincidence rangefinders was difficult at best (those, like the US Navy, which shifted to stereo rangefinding, had an easier time). Much the same might be said of entanglement between speed across the line of sight and deflection. If the gunner applies correction to the wrong parameter, say to climb rate rather than speed, his errors can ‘pyramid’, the solution becoming less and less accurate just as it matters more and more, as the aircraft approaches.
Matters worsen when the guns begin to fire. Now spotters have to work in three dimensions: fuse timing (which is not quite the same as range), elevation angle, and deflection (bearing). The spotter sees bursts near or far from the target. If he has a stereo spotting glass, he can tell whether they fall short or long, but even then he cannot be sure of whether a burst beyond the target is due to excessive fuse timing or to a shot aimed too long. The spotters are also working inside a cycle of correction, firing, and observation. It incorporates a time lag between solution and the moment a shell bursts. Dead time was often equated to the time of flight plus the time between fuse-setting and firing. However, there were at least two other elements: time between observation and entry into a fire-control system, the time the system took to change its setting, and the time it took for the guns to react. The more automated the system, the less these latter delays counted.
The essence of fire control was spotting: correcting fire to bring it onto the target. That was particularly difficult against air targets because where a shell burst depended both on how well it was aimed and how well its fuse was timed. Spotters watched the pattern of bursting shells. Proximity (VT) fuses presented a problem, because they burst only when they were near the target; they gave no hints of errors in a long-range fire-control solution. If the solution was bad enough, no bursts would be seen, except for shells self-destroying at a set range. These two US carriers, the primary targets of Japanese Kamikazes, are firing at incoming aircraft. One has already been splashed. The large smoke puffs are the bursts of 5in shells. The small ones are 40mm shells self-destroying so that they do not fall on other ships. At the end of the Second World War, US proximity fuses still lacked a self-destroying feature, which made them a danger to ships in company with the shooter. After the war, the Royal Navy decided to shift to all-VT fuse firing, which the fleet disliked because it left no scope for spotting (an important issue given the problems of British medium-calibre anti-aircraft fire-control systems).
Firing and spotting could be seen as a cycle, the time intervals of which were set by projectile time of flight plus the dead time required for fuse-setting and loading. At least one pre-war US officer wrote in terms of moves in the game between gunner and aircraft, the length of the move being the cycle time. An aircraft might evade the fire-control system if it could zigzag within the move time. For the US Navy, this connection emerged forcibly only after the service received manoeuvrable drone targets in 1938. One conclusion was that time of flight should be minimised. This does not seem to have fed into the decision to approve development of the 5in/54 to succeed the slightly lower-velocity 5in/38, but it did highlight the inadequacy of the much lower-velocity 5in/25.
Pilots were well aware that successful gunnery depended on how steady their course was; under fire they could jink violently to frustrate the gunners. That might save them, but it would also ruin their aim (guided anti-ship weapons changed this situation). To some extent, then, anti-aircraft fire was as much a means of protecting a ship from aerial weapons as it was a means of destroying attacking aircraft. This dual role makes it difficult to evaluate anti-aircraft fire: in how many cases did aircraft survive without hitting their targets?
One unhappy conclusion the US Navy drew from its late pre-war exercises was that horizontal bombers could straighten up for their final runs unpleasantly close to the target. The US Navy described the situation in terms of what it called position angle – the angle up from the surface to the target (the Royal Navy called the same quantity the angle of sight or sight angle). Initial pre-war US Navy practice with drones was for them to straighten out for simulated level bombing runs when they reached a 45° position angle (the angle increased as the bomber closed with its target). For a screening destroyer some distance from the target, the position angle on straightening out might be greater. By way of contrast, the pre-war Royal Navy considered a maximum elevation of 40° sufficient for destroyers screening major fleet units. In the US view the destroyers would have had little or no chance of hitting their air targets, because they would not have straightened out by the time the ships had to cease fire. Later pre-war exercises featured faster drones, which began their bombing runs at a considerably lower sight angle (32°), within the range of pre-war British destroyers. The straight run was determined not by distance flown but by the time a bombardier needed to steady out and prepare to drop bombs.
It was sometimes argued that the higher the muzzle velocity of the anti-aircraft gun, the better the chance that its shells would arrive at the aircraft before the latter could jink away from the shell. In any case the shell was unlikely to hit the aircraft directly; the key issue was whether it would explode within lethal range of the aircraft. On this basis a heavy high-velocity shell was best, at least against an aircraft flying at medium altitude. The other side of the argument was that heavy shells with large cartridges could not be loaded very rapidly (except by power), and that a larger gun with a longer barrel could not be manoeuvred as quickly as a smaller one (again, unless power was applied).
The Flyplane
About 1925 a fire-control designer working for the British firm of Barr & Stroud discovered a further possibility, in effect a compromise between the polar and rectangular approaches. He focussed on a plane containing the aircraft’s course and the gunner. In this plane, the target moved in a steady way. The plane itself did not move, except to the extent that the gunner moved. The plane contained both the present position of the aircraft and its position when a shell arrived (assuming the aircraft flew a straight course at constant speed, which all calculations required anyway). The orientation of the plane and the target’s course could be deduced from range and current rates, using simple geometry. It did not matter that the rates would be different a short time later, because they were being used to set up a solution in which future rates did not figure. The current rates also gave the steady angular rate of the aircraft’s motion across the special plane in space defined by its course. Thus the calculation gave an accurate fire-control solution without any need to integrate range or to deal with the way in which angular rates and range rate were entangled. Because the whole calculation was geometric, it could be carried out extremely quickly. Note, however, how delicately the approach depends on measuring angular rates, hence on stabilisation and precision of measurement.
A quick solution was particularly attractive if targets manoeuvred. Just how quick it was in practice depended on how quickly the relevant rates were measured for translation into the right-angled triangles this type of system solved to provide its predictions.
The Royal Italian Navy adopted this approach, almost certainly having taken it from Barr & Stroud. It seems to have been adopted by the Germans, one of whose Dutch front companies hired a Barr & Stroud expert in 1926. The Imperial Japanese Navy also used this technique or, more likely, a simplified version which assumed that the target was flying straight and level.
Later this plane (and, by extension, this approach) was called a flyplane. The idea was apparently rediscovered (and named) about 1931 by a British engineer at the Admiralty Research Laboratory. The British did not adopt it at the time because it could not handle a curved aircraft path, which was apparently considered very important at the time. After the Second World War the British adopted the flyplane approach – which they found unhappily complicated. The US Arma company rediscovered this approach in the 1930s and applied it to an experimental fire-control system. The later US Mk 56 was a flyplane director, though that word was not used for it (Mk 56 was adopted
