The Kamikazes emphasised the great difference between hitting the oncoming aircraft and demolishing it thoroughly enough that what was left of it would not badly damage a ship. This distinction applies to current anti-ship missiles.
Anti-Submarine Attack
Nearly all submarines had to charge batteries on the surface, and they could make good a reasonable distance only on the surface. They were reasonably invisible to any but the nearest surface ships, but they were easily seen from the air. The usual defence was a crash dive, but many First World War submarines could not dive very suddenly. For them an anti-aircraft gun might mean survival. In some navies submarines had more powerful anti-aircraft guns than most surface warships. Most Second World War submarines mounted a few heavy machine guns, but they relied mainly on crash dives. That was one reason US and Royal Navy submarines had air-search or air-warning radar. The Germans, however, lacked the necessary technology, and also feared that Allied antisubmarine aircraft might home on their radars. They tried to provide submarines with receivers which could pick up the emissions of Allied radars on board anti-submarine aircraft. The failure of this equipment led them to equip some U-boats, which they called ‘Flak U-boats’, with unusually powerful anti-aircraft batteries, with which they hoped to fight off air attacks. These weapons had no special fire-control equipment, and they proved ineffective. The ultimate defence against air attack was not to surface at all, which is why the Germans introduced the snorkel in 1944 (it bred a new kind of attack using sonobuoys and homing torpedoes).
MAKING ANTI-AIRCRAFT FIRE EFFECTIVE
Throughout the inter-war period, navies used two or three classes of anti-aircraft guns. The heaviest (medium calibre, compared to battleship and heavy cruiser armament) guns, from 3in calibre up, required full fire-control systems which could predict target position and set fuses accordingly. They are the elaborate fire-control systems described below in principle and also described in detail for the different navies. Ships could not accommodate many such systems, hence could not use them to engage many separate targets at any one time. Typically not all of a ship’s anti-aircraft directors could bear in any one direction. This was apart from the issue of how many of a ship’s guns were needed to develop effective-enough fire against a single aircraft. Hitting was always a matter of statistics. The number of directors and the number of guns needed to produce an effective volume of fire determined how many targets the ship could engage at the same time. Engagement range was generally such that it was difficult for a battery to destroy one target and then shift effectively to another which co-ordinated its attack with the first. In more modern terms, the ship’s heavy anti-aircraft battery could be saturated at a relatively low level. This saturation problem was appreciated, at least by the US Navy, by 1942. The navy’s solution was to provide each 5in gun mount on board heavy ships with its own short-range director (in addition to the long-range directors), so that the battery could be split up as desired. That did not solve the problem of how many guns were needed to deal with each aircraft. However, the splitting was made more effective by the advent of the proximity fuse, which made it possible for fewer guns to kill each aircraft. It does not seem that other navies made similar efforts to provide local director control for medium-calibre anti-aircraft guns.
At the other end of the spectrum were machine guns controlled entirely by those firing them. The British called these ‘eye-shooting’ weapons. Often fire control amounted to providing them with the usual ‘wheel’ sights, the rings of which represented different speeds. They could be used to estimate deflection at a standard range. Only during the Second World War did anything more elaborate appear, in the form of the US Navy’s Mk 14 gyro-sight.
Between the heavy and very light weapons was a third category, typified by the pre-war British 2pdr pom-pom and the US 1.1in, and by the wartime Bofors gun. Most of these weapons were in heavy power mountings and were controlled by external directors. External control moved the gunner away from the noise and distraction of the gun mount; this was the same argument which led the Royal Navy to introduce director control for destroyer guns during the First World War. The director typically embodied very limited prediction of target position, and the rounds were fused to explode on hitting. Fuse-setting was not envisaged.
Ship motion could include violent evasive action. In effect navies balanced what they hoped their anti-aircraft batteries could do against evasive action which they hoped would defeat attackers. The Imperial Japanese Navy seems to have had little faith in the former. It adopted high-speed circling as a way of defeating dive bombing, as circling would, it was hoped, frustrate a pilot trying to keep his aircraft pointed at the target ship. The same technique was proposed for the US Navy, but it was rejected because it would defeat anti-air gunnery. Shokaku is shown at Coral Sea in May 1942.
All of the automatic weapons were aimed by gunners following a stream of tracers. It was discovered early in the Second World War that pilots generally did not see the stream heading for them, so that the automatic guns had no deterrent effect (pilots did see the bursts from medium-calibre shells exploding short). The pilots did see explosions when the small-calibre rounds self-destructed at a fixed altitude. Prewar developers did not realise how important self-destruction was as a deterrent; to pilots it was like seeing the bursts of medium-calibre shells. This conclusion seems to have been drawn by Americans comparing the British 2pdr (which self-destructed) with their own 1.1in gun (which did not).
To a far greater extent than in surface-to-surface gunnery, gun and fire control – and, usually, projectile – were elements of a single more or less integrated system. The fire-control element was intended to bring the projectile into lethal range of the air target (direct hits, except by light automatic weapons, were unlikely) and to set the fuse to burst the projectile near the target. Lethal range depended on the details of the projectile (the reliability of the fuse had to be taken into account). Thus fire control had to predict target position in three dimensions. It aimed a gun in bearing and in elevation, the latter based on a combination of target motion and predicted range. Fuse timing was based on calculated range, translated into time of flight (taking into account dead time between prediction and firing). The US Navy wrote of fuse range spotting, not range spotting; shells could fail to damage a target both due to errors in aim (due to predicted range errors) and to errors in fuse timing. Only fuse timing could, in effect, be seen directly. During the Second World War proximity fuses in the US and to an extent the Royal Navy simplified the prediction problem by removing the element of fuse timing.
The core anti-aircraft gunnery problem was prediction: where would the aircraft be when the shell arrived? Gunners could not directly see the speed, course and altitude of the air target (radar changed this situation). They had to rely on what could be seen: the elevation (or sight angle or position angle [US terminology]), the bearing (deflection), and the range to the target. To some extent the rates at which elevation, bearing, and range were changing could also be measured. Fire-control systems turned these observed data into predictions on the basis of which guns could be aimed.
The measurement ‘mil’ was often used in fire control as a measure of precision or tracking success. A mil is a thousandth not of a degree but of a radian, a measure given by the circumference of a circle, hence approximately 57.3°; so a mil is about 0.057°. Also approximately, the distance subtended by a mil is a thousandth of the range. Thus a shot one mil off at 10,000 yds would be 10 yds off laterally or vertically.
Until well into the 1930s the US Navy used coincidence rangefinders to measure the range to aircraft. Unlike the Royal Navy, it set them vertically (they were called altiscopes), because a rangefinder arranged that way ‘cut’ the image of the aircraft so that the wings did not line up. That made it relatively easy to get a range quickly, but a rangefinder arranged that way was a poor fit for the sort of weather-proof