Читать книгу Naval Anti-Aircraft Guns and Gunnery - Norman Friedman - Страница 11
ОглавлениеMAKING 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 HA director the Royal Navy wanted. The rangefinder had to be tilted so that it was at right-angles to the line of sight to the approaching aircraft (so that the angles to its two lenses were the same). The US Navy backed up its vertical rangefinder with a horizontal spotting glass, a stereo device a fire-control officer could use to see whether shell bursts were ahead of or behind a target. Although the spotting glass was not conceived as a rangefinder, in fact it could function as one. Experience with the spotting glass convinced the US Navy’s Bureau of Ordnance to adopt stereo anti-aircraft rangefinders in the Mk 28 and Mk 33 directors. The device shown, Rangefinder Mount Mk XXXII, was physically separate from the associated director (Mk 19), and this separation caused major problems. Although the problems were understood by about 1932, until 1939 there was no money available to correct them. Ultimately Mk 19 was packaged with a stereo rangefinder as Director Mount Mk I, which was widely installed by 1941. Director Mount Mk XXXII was manually stabilised by a leveller, who had his own eyepiece looking across the line of sight of the rangefinder. The device was operated by a separate pointer and trainer, each with a handwheel, and by a range taker and a spotter (spotting officer) operating the spotting glass (Mk V). The vertical rangefinder is the gun-like object, pointed skyward, in many pre-war photographs of modernised US battleships from the Nevada class onwards. The Royal Navy did not use spotting glasses, and it retained coincidence rangefinders in all its directors until well into the Second World War. Stereo rangefinders did not have to be vertical, because they relied on an observer’s ability to form a three-dimensional image in his head, comparing the apparent position of the target with a moveable mark (which the Germans, who first used such rangefinders, called a wandermark). This altiscope, with the rangefinder tilted, is from the cruiser Memphis.
Measurement
Range presented particular problems. Early in the 1920s, when navies began to seek mechanical fire-control solutions to the anti-aircraft problem, all but the Germans used coincidence rangefinders. A coincidence rangefinder has two lenses, which observe the target from slightly different angles. The operator matches the half-image from one lens with the other half as seen by the other. When the two half-images form a full image, the angles from the two lenses give target range by triangulation. The observer find the match using a line projecting through both half-images to check that they match up. For a surface ship that was typically a mast or funnel. Unfortunately the most visible line feature of an aircraft, the wing or wings, is horizontal. A horizontal rangefinder splits the image along roughly that horizontal line. Turning the rangefinder vertical split the image at right angles to the wings. The axis of the rangefinder had to be at right angles to the line of sight. The elevation angle of a vertical rangefinder (which the US Navy called an altimeter) gave the aircraft’s angle of elevation. At long range that did not work. It turned out that sight angle was difficult to measure at all until the aircraft was well above the horizon – which meant that attempts to measure elevation rate were frustrated.
The alternative was a stereo rangefinder. As in a coincidence instrument, a stereo rangefinder has two lenses, but instead of forming images they feed into the operator’s two eyes. The operator perceives objects in an exaggerated depth of field. He finds the range by moving a marker in depth to match the apparent distance to the target. The Royal Navy tested stereo rangefinders after the First World War, but became convinced that operators would lose their stereo capability under stress, for example during a protracted engagement. It is not clear to what extent this conclusion reflected Barr & Stroud’s desire to keep selling its coincidence instruments; it may be that stereo rangefinders did not become entirely reliable until the 1930s. The US Navy adopted them at about that time.
A spotting glass is a related instrument. It too has two lenses, one feeding into each of the operator’s eyes. In effect it deepens the operator’s field of view, so that he can see how far one object is from another – how far, for example, a shell burst is from the aircraft target. Spotting glasses and stereo rangefinders are so closely related that the latter was sometimes converted into combination rangefinders and spotting glasses. The US Navy used stereo spotting glasses well before it adopted stereo rangefinders, and their success may have inspired adoption of the stereo rangefinder. A gunnery school held on board Oklahoma in the spring and early summer of 1941 used a Mk V spotting glass atop No 2 turret as an instructional rangefinder, since by that time all the battleships in the Pacific had anti-aircraft directors using stereo rangefinders.
The observable data could, moreover, be misleading. To an observer on the ground (or on board a ship) an aircraft moving at steady speed on a steady course does not seem to be moving steadily at all. The observer sees the aircraft at an angle, and his impression of speed (in three dimensions) is given by the way that angle changes over time. The closer the aircraft, the faster the angle seems to change. To some extent range and speed are entangled, because if the gunner mis-estimates range he will also mis-estimate speed, given what he can see (the rate at which position angle is changing). Similarly, speed and range are entangled in the rate at which the deflection angle changes.
Wind affected anti-aircraft warfare far more than surface fire. Unfortunately wind affected aircraft and shell differently, and wind could also vary considerably between the surface and the altitude of an air target. For this reason, late in the 1930s US anti-aircraft officers wanted each task force provided with an aerological unit; but that was clearly impossible for an isolated unit or a small convoy escort. Even if the solution reached by a fire-control computer was perfect, gunners might still miss altogether if they misestimated the effect of wind on projectiles. In reality wind varied with altitude. To make calculations simpler, gunners and system designers generally used a fictitious ballistic wind, constant at all altitudes. Ballistic wind was a function of trajectory, i.e., of range.
None of the information fed to the gunner was precise. All rangefinders, for example, had inherent inaccuracies. Shortly before the Second World War, an officer commenting on anti-aircraft fire control pointed out that because of inherent rangefinding (but not angle of sight) errors, an aircraft flying straight and level over a ship might seem to be climbing or diving at a slight angle. Gunners seeing up this nonexistent vertical motion might correct fire, only to discover that their corrections threw their fire off the moving target.
However good the solution or prediction a fire-control system produced, it had to contend with three other factors. First, there is a difference between gun (ballistic) and rangefinder range. For a variety of reasons, a gun aimed to fire (say) 6700 yds will generally throw its shell to a slightly different range. Second, the gun aimed at a point in space will distribute its shells around that point in a pattern. The size of the pattern is indicated by the distribution of half the shells (pattern size equates to the modern idea of Circular Error Probable, or CEP, which is frequently cited for long-range missiles). The US Navy referred to the Mean Point of Impact (MPI), in effect the centre of the pattern of bursts. Control was exerted to move the MPI onto the target. Pattern size corresponded to the spread of salvos in surface warfare. Third, to make matters even more interesting, the fuses themselves do not always go off at the intended time: there is a fuse pattern. The gun pattern is a distribution at right angles to the line of fire, so it is reflected in misleading results in elevation and bearing (deflection). The fuse pattern is a distribution in time of bursting, so it is a distribution in range. The US Navy referred to fuse range.
Gunners rely on observation to correct their fire. Patterns and range errors complicated spotting once fire began, because a spotter might correct aim based not on what the fire-control system was trying to do, but on outliers in all the patterns. On that basis corrections might throw aim further and further off – they were said to ‘pyramid’.
Spotting was complicated by the nature of anti-aircraft fire. Instead of making splashes at unambiguous ranges (short or over), an anti-aircraft shell was burst by a time fuse, which might use either a powder train or a clock mechanism. Either mechanism might not explode on time, creating an erroneous apparent range error. Virtually all the time-fused anti-aircraft guns described in this book were hand-loaded, so the time between fuse-setting and insertion of a round into the breech varied from round to round, further complicating system performance. Calculation had to take into account this dead time between calculation and firing. Small-calibre anti-aircraft guns were contact-fused (or did not explode at all).
In surface fire, gunners fired a salvo, spotted the fall of shot to see how well they were doing, then corrected and fired again. The anti-aircraft problem was complicated by the short time available and also by the limited lethality of each shell. Gunners could make up for limited lethality by putting up the greatest possible volume of fire, by firing continuously and as rapidly as possible. That greatly complicated the spotter’s task, because he could not be sure that the position of a burst he saw reflected the most recent corrections or previous ones. There was a real possibility that the spotter might call up corrections which would ruin aim. On the other hand, deliberate salvoes made for a much lower volume of fire, which in itself might fail despite accurate aim. These problems were difficult enough about 1930, when various navies began using automated fire-control systems, and aircraft were rated at perhaps 125kts. They were much worse on the eve of the Second World War, when aircraft speeds had roughly doubled.
Both in the Royal Navy and in the US Navy, and probably in others, there was considerable argument before the war as to whether all shells should be fired at the point indicated by the fire-control system, or whether they should be spread deliberately to make up for random errors. During the First World War the Royal Navy developed the ladder. Instead of firing one salvo, observing, and correcting, the British fired a quick series of salvos distributed in range (later also in bearing), the idea being that the different salvos or shots represented alternative possibilities which the spotter could then interpret. The usual delay between salvos was dramatically reduced, the gunner fixing much more quickly on the target. The technique was called laddering, because the salvos or shots at a series of ranges formed the rungs of a ladder. Laddering made it possible to get a few hits on a fast-moving, often manoeuvring, target. The Royal Navy developed it intensively between wars against fast surface targets.
Laddering seemed to be a way of overcoming uncertainties in anti-aircraft fire. At least the Royal Navy and the US Navy tried it. The US Navy tried ladders in aim and also in fuse timing (sometimes called mechanical fuse spotting). The US Navy found that laddering made it more likely that a target would be hit at least once, but much less likely that it would be shot down. Unfortunately individual shells turned out to be much less lethal than had been hoped. It took multiple hits to bring down an aircraft. Laddering badly diluted anti-aircraft shellfire. It worked much better in surface fire because the target was much larger, and because a single hit was much more likely to be effective.
Solving the Fire Control Problem
The gunner sees an aircraft moving up and down and from left to right – a pair of angular movements. He can measure the range to the aircraft. The basis of nearly all fire control was to measure or calculate the rates at which these key parameters changed. For example, speed is the rate at which range changes. Given known rates, range, bearing, and elevation could be calculated for a later moment, say when shells were to arrive at the target. The predicted bearing and elevation were the deflections a gunner had to use to lead the target so that it could be hit.
The only way to measure a rate is to see how something changes over time: that measurement takes time. The longer the time, the more accurate the measurement – if the rate is constant. If it is not constant, there is a premium on reducing the time of measurement (the rate is said to become stale) – but that makes for reduced accuracy. All of this is aside from the fact that a ship rolls and pitches (and moves ahead) while angles are being measured. The Royal Navy suffered because when its high-angle control system was being designed its scientists felt that directors could not be well enough stabilised to cancel out roll and pitch so as to isolate the aircraft’s motion.
An integrator fed with changing (or constant) rates calculated their total effect over time.1 It was said to generate future values of whatever it calculated. For example, integrating speed over time gives the predicted range.
Unfortunately neither angles nor range (slant range up to the aircraft) change at a constant rate, even if the aircraft is moving straight and level at a steady speed. For example, imagine the line pointing up at the aircraft (the angle up: sight angle or, in US parlance, position angle). As the aircraft approaches, that angle steepens, even if the aircraft is not climbing. Unless the aircraft is heading directly for the observer, the way in which the angle steepens depends on the changing bearing of the aircraft. Things become much more complicated if the aircraft is diving or climbing, because in that case the gunner sees is a combination of motion due to the approach of the aircraft and a separate component due to diving or climbing.
The angles and the slant range are all entangled. However, for short intervals the rates at which they change are nearly fixed. They can be measured, and they can be used directly for predictions (which become less and less accurate over time). Systems based on measured angular rates were called tachymetric, after the Greek word for speed, tachys.2 The simple multiplying approach might be called analytic, because it deduces what it needs by direct analysis of what the gunner can see. An analytic system can overcome the fact that the rates are not constant by measuring the way in which they change and using that data, too, but that involves a delicacy of measurement which is probably impractical.3
Tachymetric systems measured angular rates in various ways. For example, in the Vickers system sold to the Imperial Japanese Navy an operator set an estimated rate into an integrator, which generated angles on that basis. If the generated angle did not match what the operator saw, he changed the estimated rate until estimate matched reality. A more sophisticated technique employed a gyro forced to follow the target. The gyro would resist that movement. The force required to keep it on target was proportional to the rate at which the gyro was being forced to turn. In both approaches vertical and horizontal angles were handled separately.
Cancelling Ship Motion
Somehow the ship’s motion must be separated from that of the aircraft. The angle approach demands a stabilised line of sight, against which angles can be measured. Angles and rates had to be measured from a stabilised position. Otherwise the rates were entangled with the motion of the ship carrying the gyros. The higher the shooting angle, the worse the problem. The US Navy seems to have enjoyed great superiority due to its development of the stable vertical, a gyro motion sensor.
Post-1918 discussions of anti-aircraft fire control frequently refer to trunnion tilt. A gun or other pointing device elevates or depresses on trunnions. If it is pointed directly on the broadside, the ship’s roll simply changes elevation or depression. This motion can be cancelled out by elevating and depressing the gun as the ship rolls. However, if the gun points away from the broadside, then rolling tilts the trunnions, so that as the gun elevates or depresses not only does it not point up or down at the desired angle, it points to one side. The motion of the ship entangles the two key angles, elevation and bearing (azimuth). Some way has to be found not only to move the device up or down as the ship moves, but also from side to side. It must have been obvious fairly early that bombers tended to attack along the ship’s axis, just where trunnion tilt was worst, because that minimised timing errors which could cause bombs to fall long or short.
Synthetic Systems
If just multiplying rates was not good enough, what was? The alternative, adopted by the US Navy during the First World War and by the Royal Navy after it (and then by all other major navies) for surface warfare was to create a mechanical model of the engagement, based on assumed enemy course and speed.4 This analog computer could predict (generate) the bearing and range of the target based on the assumptions. Conversely, gunners could compare generated with actual bearing and range to correct their assumptions. Once the assumptions had been confirmed, the fire-control system would continue to predict range and bearing correctly until the enemy changed either. This technique had the incidental advantage that the ship could manoeuvre freely without losing the target.5 The US Navy and the Royal Navy both adopted this analog approach to air defence, albeit in very different forms. This form of control might be called synthetic, because its basis is a model created (synthesised) by the gunner. The system design problem is to make it possible for a gunner to correct for what he sees in such a way that the course and speed inside the system come closer to reality. The more direct the translation from what can be seen (in this case, rates) to prediction, the shorter (in theory) time to arrive at a fire-control solution. At least in theory, a quick solution might make it possible to deal with a manoeuvring aircraft, if it followed a more or less direct course for long enough at a time.
A drawing from a US Navy handbook shows the two kinds of levelling, level in the direction of the line of sight and cross-level across it. Trunnion tilt is a failure to cross-level. As the ship rolls and pitches, and as the director or gun is trained in some direction other than straight ahead or abeam, it is subject to a combination of errors in level and cross-level. The third dimension of stabilisation is yaw: the ship swings back and forth as it moves ahead. A tri-axially stabilised mounting cancels out all three kinds of error.
This diagram from a US Navy manual shows why stabilisation was so important in anti-aircraft fire. Although it was intended to show the effect of a ship’s roll on surface fire, the reader can easily imagine that there would also be errors in altitude. An anti-aircraft system had to compensate for roll, pitch and yaw. All of them caused the deck to tilt away from the horizontal. An anti-aircraft fire-control system tracked an aircraft as it moved in the sky, not as it was seen from the pitching, rolling ship. The motion through the sky was relatively simple (and in many cases system designers made further simplifying assumptions, such as that the aircraft was flying straight and level). Once its motion was measured, the system could project where it would be in the sky a few moments later, when shells should arrive. The same system ideally had to cancel out the ship’s motion, both in interpreting what was seen of the aircraft, and in aiming guns.
The livelier the ship motion, the more effort it takes to stabilise guns and directors, particularly those high in a ship (the distance from the waterline increases the linear motion, which is associated with the angular motion of the ship – this effect is used in inverse synthetic aperture radars). The Royal Navy approach, for small lively ships with primarily anti-aircraft batteries, was to stabilise the whole ship. The ‘Hunt’ class destroyers (originally called fast escort vessels) and the Black Swan class sloops (originally called escort vessels) were given fin stabilisers, a radical new technology at the time. Some of the ‘Wair’ conversions of ‘V&W’ class destroyers may also have been so fitted. Cleveland is shown in 1942, with a 2pdr in her bows to deal with E-boats (German MTBs) in the Channel.
A synthetic system could be considered tachymetric, in the sense that its outputs could include generated bearing and elevation. As in the much simpler Vickers approach, gunners would make adjustments (in this case, to assumed target course and speed) to cause generated data to match reality. This is the sense in which US synthetic systems such as Mk 37 can be considered tachymetric. They never measured rates directly, but they generated solutions which made the motion of the angle-measuring director match actual angles, hence the rates at which the director moved matched actual rates.
The two approaches largely correspond to the sort of co-ordinates the system uses. The gunner sees the situation in polar form: as a distance (slant range) and elevation and bearing (azimuth) angles. The rectangular approach concentrates on what the aircraft is actually doing. At the core of the calculation is assumed simple target motion – straight and at a constant speed. That motion is easy to project ahead. The aircraft’s steady motion is best expressed in rectangular co-ordinates (up and down, in and out, sideways). For example, in a system of rectangular coordinates centred on the aircraft, it is flying along one co-ordinate (dimension). The complicated part is to transform the solution so as to express that motion as a gunner sees it, first so that the gunner can correct the estimated target motion and then so that he can fire his guns.
An aircraft flying straight and level (the simplest situation) traces a corresponding straight course over the ground, what the British called a course in plan or a plan course. That was the course at which it proceeded at a steady speed. It could be deduced by cancelling out the sight angle. For an aircraft flying straight and level, the angular rate across (ultimately giving horizontal deflection) gave the speed across in plan. The vertical angular rate gave the speed along in plan. The ratio of the two gives inclination, the angle between the aircraft’s course and the line of sight.6 That is, inclination – enemy course – could be measured without knowing enemy range and speed. Similarly, given enemy speed and course (inclination), speeds along and across could be calculated without reference to range. The plan part of the aircraft’s motion could be treated like the motion of a ship along the surface, albeit at much higher speed.
If the aircraft could be seen far enough away, its speed could be estimated from observation. It would remain a long time at a low sight angle, so slant range would not be much different from plan range. Plan range gave the desired speed along the line of sight. The aircraft would also be flying more or less directly towards the observer; its speed across the line of sight could be neglected. A series of ranges would provide a reasonable estimate of its speed, until it rose far enough above the horizon that angle of sight made much of a difference. That is why, as described below, British systems were designed for rangefinding at low angles of sight (which meant long range) and for heightfinding (height could be calculated from slant range and angle of sight). Speed estimated at long range could be fed into a computer as an initial estimate. Another initial estimate might be that the aircraft was flying directly at the gunner. These estimates could be refined as the aircraft approached.
How good initial estimates were depended on the sensors available to the gunner. Although in theory the British could estimate the speed of an approaching aircraft based on a series of observed ranges, in fact their horizontal coincidence rangefinders were ill-equipped for this purpose. They relied instead on an estimate by the control officer, based on the type of aircraft involved. To the extent that speed was measured, that was by feeding assumed speed into the prisms of the rangefinder (which they called a height finder) and seeing whether the ‘cut’ stayed on the target. That was analogous to estimating a rate and checking the estimate against observed target motion, tuning the assumed rate until the two matched. In both cases the problem was that speed towards the gunner (the ship) would vary over time if the aircraft was not headed directly towards the ship (not to mention variation of measurable [slant] range due to changing sight angle). The US Navy was in a very different position once it adopted stereo rangefinders in its Mk 28 system in the early 1930s.
Imagine the enemy’s movement as a vector (an arrow) pointing along his course, its length corresponding to his speed. Any vector can be expressed as the sum of components, such as speed along and speed across, each at right angles to the other. As the target moves, the line of sight also moves, so the relationship between the speed along and the speed across changes. That is why, except in the unusual case in which the target is moving towards or directly away from the shooter, the range rate varies. A mechanical device can split a vector into the desired pair of components. The US Navy called its means of splitting a vector a component resolver. It and similar devices split motion into two rather than three dimensions, i.e., in a flat plane of some kind. The simplest solution to the problem was to divide the aircraft’s path into horizontal and vertical components, and to divide horizontal (plan) motion into along and across components.
A rectangular-co-ordinate computer worked in co-ordinates centred on the aircraft, in which rates along the different directions were fixed.7 It also traced the position of the ship, which gave changing angles of sight at which the aircraft was viewed. It used component resolvers or their equivalents to translate into ship co-ordinates. The resolvers worked in linear terms (such as knots), never in terms of angles.8 However the fire-control problem is handled, the gunner works in polar co-ordinates. In order to lead his target, for example, he needs to know how fast the enemy’s bearing is changing. In effect that is the speed across divided by the range.9 The fire-control computer can integrate the bearing rate to find the enemy’s future bearing.
Feedback: Spotting
The core of the synthetic approach is feedback while the target is being tracked, before opening fire. The initial set-up is a guess as to target course and speed in three dimensions. On that basis the computer generates future target position, and that generated data allows the gunner to make his corrections. The system employs a cycle of setting, observation, and correction. An aircraft can evade engagement by manoeuvring more rapidly than the cycle of observation and correction. For the US Navy, a crucial question on the eve of the Second World War was whether systems could obtain the desired solutions quickly enough.
The gunner or spotter can tell that the target is not where the computer says it should be, but he cannot always tell why. For example, imagine a target which seems to be climbing (elevation angle increasing) faster than expected. It may actually be climbing. It may also be approaching at higher than expected speed, without climbing at all – the closer the target, 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 by the Royal Navy in ‘anglicised’ form as MRS 3).
The argument in favour of the analytic (tachymetric) approach is that, as in a flyplane system, calculation is very rapid. If the target manoeuvres, or if it is in sight for only a very short time, the inaccuracies that build up may not matter very much. The simpler system may be good enough, even better than one involving a great deal of spotting and feedback. That was certainly the case for the US Navy’s short-range tachymetric systems of the Second World War.
A diagram from a US handbook shows the simplicity of the flyplane idea – and the complexity of translating it into practice. The gunner is at O. The aircraft flies in the ‘true elevation plane’. Measured in that plane, it moves along a straight line. That motion has to be translated into deflection and elevation as seen at the gun on a rolling, pitching ship. If the ship were not rolling and pitching, the situation would be simple. The motion of the aircraft would be projected down onto the ‘true traverse plane’ and across onto the ‘cross-traverse plane’, and the two motions involved could be observed directly. Given these motions and a range, the fire-control system could deduce what was actually happening (in the flyplane) and thus predict where the aircraft was going. The solution would be instantaneous, because the rates would translate directly into predictions. Aside from the problem of ship motion, the rub in a flyplane system is that it is impossible to measure rates instantaneously: measurement means that a change in, say, elevation is measured over a given time. The faster the aircraft, the higher the rates, the shorter the time needed for a sufficiently accurate measurement. The slower the rates, the more difficult quick measurement can be. This diagram omits the additional problem of translating between the flyplane fixed in the earth and the actual planes of measurement used by a rolling, pitching ship.
Targets
Most Second World War anti-aircraft fire-control systems were developed during the inter-war period, when none of the major navies fought real aircraft. Much of their perception of what was and was not likely to work depended on the devices they used for practice firing. Until the late 1930s the usual target was a sleeve (or banner, in British practice) towed by an aircraft. The aircraft could dive gently, but it could simulate neither a manoeuvring bomber nor a dive bomber. There were also even simpler targets, such as balloons and there were a few gliders in British service. Both the Royal Navy and the US Navy deployed drone targets in the late 1930s, although only the US drones could simulate dive bombing. That they did not appear any earlier was probably due to a combination of immature radio control technology and the financial impact of the Depression. Once the drones did appear, both navies were shocked to discover how ineffective their fire-control systems – and, in the US Navy, their anti-aircraft shells – were. Only a drone could manoeuvre evasively, like a real aircraft, and only the US drone could dive like a dive-bomber. Japan also operated target drones (powered gliders). It is unlikely that they could simulate dive-bombing. They appear to have entered service only in 1940.
US pre-war experience showed that without drones it was unlikely that anti-aircraft fire control could be sufficiently tested to be perfected. Conversely, drone firings convinced many in the US Navy that fast high-altitude bombers were nearly impossible to shoot down. That is probably why the standard US counter to such attacks, which the Japanese attempted early in the Second World War, was violent evasive manoeuvring, which might well ruin anti-aircraft fire control, but would also ruin a bombardier’s aim.
None of the other major Second World War navies appears to have employed drone targets. That may well have caused them to underestimate the difficulty of engaging realistic targets, both manoeuvring bombers in level flight and dive bombers.
Data Transmission
Whatever the fire-control system estimates has to be passed quickly and accurately to the guns, which may be aimed manually (to match pointers, for example) or automatically, by power. That requires some form of data transmission. At the least, operators will move equipment in response to the messages indicated on their dials, but the faster they have to move, the less accurately they will follow those pointers. The idea is therefore some way of using transmitted data to move the masses of guns and directors. That is not simple, because the masses being moved have inertia: once they are moving, they have to be stopped at the desired point. Transmission applies not only to guns and their auxiliary equipment, but also to any remote means of stabilisation, which has to cause masses such as guns and directors to move to counteract a ship’s motion.
The best (smoothest and quickest) means of data transmission was the synchro, developed independently by Germany (during or before the First World War) and by the United States (after the war). It was later adopted by the Japanese, the French and probably the Italians. Synchros exploited transformer technology, which in turn required AC power (either from a ship’s main supply or from a motor-generator driven by a DC system). The synchro was a superior alternative to the earlier step-by-step transmitter. It was simple but anything but smooth, and its fidelity was limited by the size of the steps. The main application of step-by-step data transmission in this book was the British High Angle Control System (HACS).
The British Admiralty Research Laboratory independently discovered the synchro principle in the early 1930s to produce a British equivalent, Magslip. The later British high-angle control systems received and transmitted their data by Magslip.
By the mid-1930s the US Navy was linking its synchros with electronic amplifiers (thyratrons) to move masses in response to synchro movements. Aside from making it possible for a director to control guns, this kind of remote control made it possible for a stable vertical deep in a ship to stabilise both director and gun mountings, as in the combination of Mk 37 and fully-enclosed 5in/38 mountings. The initial US system (on board five heavy cruisers, beginning with Portland and Indianapolis) used thyratrons and electric motors. About 1936 an acceptable electro-hydraulic system was devised. It was considered lighter, more compact, and more rugged.10
The synchros of the successful US system tied the director to the computer and to the gun mount, but they did not connect the director to the source of target designation. For that the US Navy of 1945 relied on the same means it had used much earlier, the sound-powered telephone. That had the advantages of simplicity and flexibility, but it also imposed unacceptable delays. The limitation of such target designation was a major reason the US Navy dramatically decentralised the control of its 5in guns from 1943 onwards, seeking to provide each mounting with its own Mk 51 director. On the other hand, the telephone system was easy to adapt as more and more gun mounts were added to US ships. There was no US equivalent to the British Target Indication system.
That this was a serious gap became evident during the Kamikaze campaign against the US fleet, because the telephone switchboard and phone operation slowed a ship’s response. The late 1945 Pacific Fleet Board on ordnance lessons learned pressed for an automated means of slewing directors onto targets found by CIC. By that time the Bureau of Ordnance had already developed a special target indication device which could display a radar picture in PPI form. An operator could designate a designator to a target by turning a dial and throwing a switch indicating a particular director. The Mk 37 director was given a special receiving panel for this purpose. The board was unenthusiastic, because there would still be a time lag as those in the director read the receiver dial and reacted accordingly. They wanted a device which would slew the director itself. It materialised after the war as a first-generation target designation unit. The Royal Navy seems to have had a much better understanding of the need for target designation. Partly because its air warning radars had such broad beams, late in the Second World War it fielded a special target designation radar (Type 293) and a Target Indication Unit to match. Type 293 and the TIU could trace their lineage back to pre-war interest in an Aircraft Direction Officer (ADO) organisation, for which there was no direct US equivalent.
By the late 1930s the British were well aware of the advantage of this kind of control, to the point where they considered anti-aircraft control without it useless at short ranges, where gun mountings had to turn very rapidly. The British termed the systems involved Remote Power Control (RPC), and developed several types denoted by RP numbers.
The Germans used a form of RPC based on magnetic amplifiers rather than vacuum tubes. The US Navy’s Bureau of Ordnance found magnetic amplifiers particularly interesting, and it used them in at least one aircraft fire-control system.
A quadruple 40mm gun is shown in action on board the battleship West Virginia, 22 July 1944. The smaller splinter shield above and on the right side of the main gun tub contains a Mk 51 director controlling the gun mount. Although the trainer and pointer are in their seats, the mount is fully controlled by the Mk 51, training and elevating under power. Note the telephone head set worn by the Mk 51 operator. Although the director was wired directly to the gun mounting, its operator received target designation information by sound-powered phone from the ship’s Combat Information Center. This was standard US practice.
Saturation
Only towards the end of the inter-war period did navies apparently begin to face the problem of multiple attacks against one ship. It was one thing to concentrate a ship’s fire using a single director-controlled system focussed on a single attacker. It was another to recognise that an intelligent enemy would try to attack at least from both sides of a ship. To some extent barrage fire was a means of handling massed attackers approaching from roughly one direction.
No one used modern terms such as ‘channels of fire’ or ‘saturation’, but they were the issue. One US Navy approach was to estimate the unit of fire necessary to engage a single aircraft with a reasonable chance of success. At one time that was four 5in guns backed by one fire-control system. In theory, each unit of fire represented a channel of fire (the ability to deal with one target), although in practice a ship would be unlikely to be able to bring all her channels of fire to bear against an attack from one side. As the number of attackers mounted, there was a strong temptation to split up a ship’s anti-aircraft battery as finely as possible. For example, in 1943 the US Navy decided to assign a short-range director to each 5in mount. That did not change the statistical character of anti-aircraft fire: the fewer the guns firing at an aircraft, the less the chance of shooting it down.
Both the US Navy and the Royal Navy liked open anti-aircraft mountings, on the theory that the gunner needed a clear overhead view as a backup if director control failed. That did not allow for splinter or strafer protection, and during the war such guns were increasingly shielded. This 4in Mk V gun was on board an Australian cruiser; the crew is shown at action stations. The large disc on the left-hand side of the gun is the fuse dial. Note the angled eyepieces for the layer’s and trainer’s telescopes, on either side of the gun, so that they did not have to squat as the gun elevated to track a HA target. Note the fixed 4in round held by the loader. It was about the largest single item of ammunition which a man could quickly load. The US 5in/38 had semi-fixed ammunition, with shell and cartridge separate so that they could be handled easily. The fuse-setting machine is on the platform on the left side of the gun (a shell is being inserted into it). Note the handle: the fuse machine operator turned it to match a dial setting transmitted by the fire-control system. (State Library of Victoria)
Placing Guns
Somewhere else in the equation was the choice and placement of the guns and fire controls, which varied considerably from navy to navy, and the relation between guns and fire controls. Placement was chosen both for tactical reasons and due to the limitations set by ship real estate. For example, it was generally much easier to place anti-aircraft guns on a ship’s sides than on her centreline. The best centreline positions were occupied by a ship’s main battery, generally of anti-ship guns. Bridges and machinery occupied most of the remaining usable centreline of the ship. On the other hand, aircraft might often attack from ahead or astern, flying down the length of the ship. Some of the guns might bear, but a ‘sided’ battery would find it particularly difficult to track an aircraft crossing the ship’s track. That applied not only to the guns but also to their directors.
A ship’s captain had to balance his manoeuvring choices in the knowledge of the arcs of both directors and guns. He might find it difficult to turn so as to unmask the battery on one side, because that might make the bombers’ task easier. The ship would present a larger target. How important that was depended on how the captain judged the relative threats presented by level, dive, and torpedo bombing. Level bombers might prefer to fly down the ship’s hull, because they were more likely to err in range than in line (in reality they tended to spread their bombs both ways). Dive bombers could attack from any direction, because they were relatively accurate. A torpedo bomber preferred to attack more or less abeam, because range errors line were insignificant compared to errors in line (bearing).
It took a combination of light and medium-calibre anti-aircraft guns to protect a large ship under air attack, but she might be fighting a surface action at the same time. The Japanese seem to have been alone in providing blast shields for their light anti-aircraft guns. This photograph was taken on board the super-battleship Musashi when the Emperor visited on 24 June 1943. At upper centre is one of her shielded triple 25mm mounts, the shield being about the size of that of a 12.7cm gun like the ones visible on the right. Note that the 12.7cm shields are considerably more streamlined (and more complete) than those on board other Japanese warships, possibly as an anti-blast measure. The photograph was taken on the ship’s port side, looking aft. When the ship’s anti-aircraft battery was upgraded with many more 25mm guns, the anti-blast shields were apparently no longer in production, so the additional guns were unshielded, like all others in Japanese naval service.
The obvious place to put anti-aircraft guns is on the sides of the superstructure, but that makes it easier for an attacker to fly down the centreline of the ship. When it designed new cruisers, unconstrained by treaty, from 1939 on, the US Navy adopted a new arrangement including centreline 5in guns, as shown by Pittsburgh off Boston on 1 November 1944. It is not clear whether the designers were trying to increase end-on firepower or were forced to place the mounts on the centreline because there was no other space along the ship’s length. The evaluation by the immediate post-war Pacific Fleet board on wartime ordnance experience (the Kraken Board) favoured the latter explanation, and argued further that the centreline positions exposed control positions to excessive gun blast. On the other hand, the US Navy was probably unique in placing its long-range anti-aircraft directors on the centreline, where they could follow a target across the ship’s beam. Late pre-war exercises against simulated torpedo bombers certainly demonstrated that ships lacked the ability to handle such targets.
Savannah is shown as rebuilt after battle damage by a German guided bomb at Salerno in September 1943. The line of her blister is evident roughly abreast No 4 turret. The new gun mounts were controlled by new Mk 37 fire-control systems. Clearing the sides between the mounts left space for more Bofors guns. She had four quadruple mounts and six twin mounts (two on her quarters are hidden by her catapults), a total of twenty-eight guns.
Gun placement could have subtle implications. With a total of five main-battery turrets, the Brooklyn class cruisers had their batteries of four 5in/25s on each side squeezed into a smaller space than on the previous New Orleans class heavy cruisers. When the ships entered service, it turned out that their guns fired on average only slightly more than half as fast as those of the heavy cruisers, the result of crowding. Boise is shown about 1943–4, her sides even more crowded by adding two twin and four quadruple Bofors guns plus many Oerlikons. The last two ships of the class, St. Louis and Helena, had four twin 5in/38s instead of the single 5in/25s, which presumably solved the crowding problem. Remarkably, plans drafted in 1940 for anti-aircraft improvement did not apparently envisage exchanging the cramped single 5in/25s for twin 5in/38s. Instead, the existing guns would have been exchanged on a one-for-one basis for single 5in/38s. During the war Savannah and Honolulu were rebuilt with four twin 5in/38s. They had to be blistered to maintain stability.
For example, in 1940–1 the US Navy ran simulated torpedo attack exercises, the attackers’ tactics being modelled on those demonstrated during the ongoing European war. The attackers flew a course parallel to that of the ship and then turned suddenly towards the target, glided down to attack height (diving at an angle of 15° to 30°), and released their (notional) torpedoes. One US cruiser captain pointed out that such tactics would place the burden of defence on the only guns which could bear along the ship’s centreline, the newly installed 1.1in machine cannon. If he tried to unmask his main anti-aircraft battery of 5in guns he would present his broadside to the attackers, much simplifying their task. The perception of this problem probably explains why the US Navy chose to place two single 5in guns on the centreline of its last pre-war heavy cruiser (Wichita) and why the new cruisers it designed in 1939–40 (Clevelands, Brooklyns and Alaskas) all had twin 5in guns on their centrelines, fore and aft.
Guns
The guns in this book may be characterised as quick-firing (QF in British terminology, RF in US), semi-automatic, and automatic. Both QF and semi-automatic guns fired one shot at a time; they had to be reloaded between shots, and the gunner had to pull the trigger (or some other firing device) for each shot. Automatic guns fired continuously, as long as the firing device was engaged and ammunition lasted. All of these guns used brass cartridge cases, because it was impossible to load a gun with bag ammunition at high elevations. In this regard what mattered about the cartridge case was that it was rigid.
Firing at a high angle, a gun had to have enough space under it to recoil. To do that, its mounting had to have its trunnions, about which the gun elevated, as high as possible. That in turn made it difficult to load the gun when it was at a lower elevation. High-angle targets also complicated sight design, because the gunner ideally could stand or sit while aiming. This 3in HA gun is on board a British First World War battleship. Note how far down the loader has to kneel. Some inter-war Italian medium-calibre mountings were designed to raise their trunnions as the gun elevated, so that it could be loaded equally easily at high and low angles.
Cartridge cases also made it possible to use simple quick-acting breech mechanisms, because when the gun fired the case expanded to help create a gas seal. The case then quickly contracted as it cooled, so it could easily be extracted. In a QF gun, extraction was done manually. A semi-automatic gun used the recoil energy of the gun to eject the cartridge case and to prepare the breech for the next round. That made for faster fire. Semi-automatic guns existed in small numbers during the First World War, but became common afterwards. The US Navy’s 5in/38, its standard heavy anti-aircraft gun of the Second World War, was a typical semi-automatic weapon. Movies of anti-aircraft action make it obvious that the gun threw out its spent cartridge cases upon recoiling.
Automatic guns take the process a step further, using the energy of firing not only to eject a cartridge and to recock the firing mechanism, but also to reload the gun and then fire again. The two main sources of energy for all this are the gun’s recoil and the gas produced when the gun fires. For example, the widely-used Bofors 40mm gun was recoil-operated. As the gun recoiled, it compressed a heavy spring wound around its barrel (and very visible in photographs). The spring returned the gun to firing position, and the energy of the recoil was used to eject, reload, and recock.
The energy of recoil or gas is limited, so it is not possible to produce a fully-automatic gun above a particular calibre (the limit is probably about 57mm, as in the post-1945 Bofors replacement for the 40mm gun). Larger weapons can eject and reload automatically, but it takes external power to supply rounds to the breech. Work on such powered automatic guns seems to have begun before the Second World War, and the first such weapons appeared at the end of the war. The first US example was the power-operated 3in/50, which was the earlier semi-automatic gun with a power loader and a slightly modified breech. One of the less happy lessons of post-war anti-aircraft development was that power-loading systems often jammed, and that gun mountings had to be de-rated (slowed down) if they were to function effectively. An alternative approach was to abandon really heavy anti-aircraft calibres in favour of smaller rounds more amenable to power operation.
The US Navy’s solution to the trunnion-height problem was to place the gun on an elevated platform, like that shown on this 5in/38 aboard the carrier Yorktown, at Newport News (newly completed) on 27 September 1937. Loaders on the platform could easily load the gun at any elevation. The structure in the foreground is the fuse-setting machine; loaders moved shells onto the platform, placed them nose-down in the machine (three at a time), then withdrew them and loaded them. The Royal Navy disliked this solution for high-powered destroyer guns, presumably because it raised the centre of gravity of the gun mounting. Much of the structure visible around the breech of this gun is a massive counterweight, moving the centre of gravity of the gun so that its trunnions can be placed nearer the breech, limiting the necessary trunnion height. The Germans went furthest in this direction, their trunnions being nearest the breech, and their barrels elevating through the roofs of their gun houses.
This book is about the gun defence of surface ships, but fleet air defence also involved fighters, both carrier- and land-based. Situations alternately favoured or precluded much fighter air defence of ships. Throughout the war, it seems to have been clear that fighters and guns were complementary, but that guns could never exact enough of a toll on attacking aircraft to end the problem. It seems clear that guns tended to beat off attacks and to shake attackers sufficiently to ruin their aim, thus saving ships. That was much the lesson the Royal Navy much later drew during the Falklands War of 1982, when surface defences, fighters and attackers were all much effective than they had been during the Second World War.
Another solution to the trunnion-height problem was to provide a pit into which the gunner could move as he elevated his gun. The Royal Navy tried this solution in a HA destroyer mounting in the 1920s, but disliked it for its complexity. Portable deckplates had to be provided for gunners when the gun was at a shallow elevation. Aside from that, opening the main deck of a small ship to provide such a pit cut into the ship’s girder strength. This 20mm Oerlikon is on board the Canadian armed merchant cruiser Prince Robert. (RCN)
Some designers overcame the trunnion height problem by detaching the gunner from the guns, giving him a separate sight which he could use while standing. This quadruple Vickers 0.5in gun was aboard HMAS Perth. Unlike the Oerlikon, it was not free-swinging; the gunner turned the mounting with his body, but he elevated using a wheel and gearing. It turned out that a gunner could manipulate a free-swinging mounting, like that of the Oerlikon, much more easily than he could manipulate his wheel. During the Second World War the US and Royal Navies both devised one-man controls for powered anti-aircraft mountings, replacing separate pointer’s (in British parlance, layer’s) and trainer’s wheels. The wartime ones were called ‘scooters’, because their horizontal bars resembled the handlebars of scooters. After the war the US Navy preferred joysticks. In each case, it was vital to find a mechanism a gunner could handle intuitively. (State Library of Victoria)
The trunnion-height problem also applied to small-calibre guns like this 20mm Oerlikon. Even at a moderate elevation, the gunner has to squat back uncomfortably, leaning back into the strap supporting him, because he has to look directly at the target through his sight. This Mk 4 Oerlikon mounting incorporated a column which could be raised or lowered so that the gunner did not have to squat too far (its operator is on the left of the gun; the gunner on the right reloaded when the ammunition drum was empty). Note the Mk 14 gyro gunsight atop the gun, a standard feature when this photograph was released in January 1944.
At Scapa Flow in 1918, Bellerophon displays the standard battleship anti-aircraft battery of two 4in HA guns. She also had a 6pdr. Most battleships had 3in guns (excepts were the 15in battleships and Centurion). The standard 4in gun was a Mk V on a HA Mk III mounting. This gun was also mounted in ‘D’ and ‘E’ class cruisers (the ‘C’ series had 3in guns). Many light cruisers carried the gun on a 60° HA mounting. In 1918 some cruisers were assigned 3in rather than 4in anti-aircraft guns because these weapons were also used against submarines, which might suddenly be encountered on the surface; the 3in was considered much handier, and it could be mounted in more convenient places. By late 1917 the 4in Mk V was favoured for anti-aircraft fire because its ceiling (30,000ft) was considered essential if British warships were to beat off future air attacks. Plans called for mounting one gun each on board capital ships, cruisers and the carrier Argus, a total of eighty-three guns, none of which had yet been delivered (109 had been ordered). Deliveries began in December 1917.
