Professor Sir James Henderson, the Royal Navy’s gyro expert, pointed out that the force required to keep a gyro pointing at a target (moving it from its preferred position) measured the rate at which the direction of the target changed. This measuring device was called an angle gyro. The committee liked Henderson’s design of a two-man director with stabilised layer’s and trainer’s sights. Given Henderson’s angle gyro, the committee favoured a tachymetric computer, which would rely on it to measure directly vertical and horizontal target motion (in terms of angles, not distances). It hoped that such a system could largely dispense with rangefinding, which was difficult and unreliable (given the need to set fuses, and the way in which target range affected gun elevation, it rangefinding could not have been avoided altogether). The Admiralty Research Laboratory (ARL), which developed anti-aircraft fire-control systems for the British army, produced a tachymetric system. So did Vickers.
The rub was that shipboard sights had to be stabilised. Otherwise the ship’s motion would be entangled with the aircraft’s apparent motion. High-angle firing required much better stabilisation than low-angle: the Master Gunnery Gyro being developed for surface fire was not good enough. Unfortunately trials conducted in 1926–7 on board the cruiser Dragon and the battlecruiser Tiger failed. It proved too difficult to maintain constant gyro speed (gyro speed affected the force needed to move the gyro), and existing gyros took too long to get up to speed. The gunnery school HMS Excellent proposed moving the gyros below, but nothing was done, probably because the stepping motors used by the Royal Navy to transmit data could not operate smoothly enough. Several foreign navies, including the US Navy, adopted tachymetric approaches based on gyro-stabilised directors.
During the 1930s the Royal Navy considered long-range fire the dominant means of air defence, so it emphasised sophisticated fire control, just as surface gunnery entailed elaborate means of control. At the very least, heavy anti-aircraft fire could force bombers to keep high and/or jink, which would ruin their aim. It could break up formations, rendering attacking aircraft more vulnerable to fighters and also ruining the cohesion flight commanders needed if they were to make many hits. Aircraft should be attacked before they could get close enough to attack.7 The Royal Navy thought that its investment in fire control and guns would enable it to destroy many attacking aircraft.8 War experience showed that actual destruction was rare, but that gunfire often accomplished the other goals. Lighter guns, which were more likely to destroy attackers – particularly dive bombers – became far more important, a reversal unexpected before the war.
A ship’s manoeuvres under air attack could complicate fire control. As of 1931 the Royal Navy view was that last-minute manoeuvres to evade bombs were seldom worthwhile. Large turns would ruin gunnery. Fire control (as described below) worked best if a ship followed a more or less straight path.9 However, it might be well worth while to manoeuvre upon sighting aircraft, both to bring maximum anti-aircraft fire to bear and to make it difficult for the aircraft to approach on a course giving it the best chance of hitting.
Rangefinding
The committee soon realised that rangefinding such a difficult major problem that it sought a system based entirely on angular rates. The ARL developed just such a tachymetric system for the British army, but as noted that was impossible on board ship. After tests of Barr & Stroud coincidence rangefinders and a German stereo type modified for anti-aircraft use, the committee reported in January 1920 that the army’s UB 2 coincidence unit ‘shows the greatest promise, and meets Naval AA requirements better than any other type’.10 UB 2 automatically read out target height based on sight angle. Differential gearing connected range and height-reading scales. If the aircraft flew at a constant height, and the rangefinder was kept ‘on’ the target, it read out change of range. UB 2 had a special presentation which apparently made it easier to get a ‘cut’ on any part of an aircraft. To get it onto a target more quickly it had four sets of open sights usable by range-taker and trainer, and also by an officer helping get the rangefinder onto the target. The general arrangement of the navy’s wartime FT 29 precluded open sights. Against an aircraft flying at a steady height of 3000 to 10,000ft, UB 2 could get about eight good cuts (ranges) per minute. This number was roughly halved when the aircraft manoeuvred evasively. Generally it took half a minute to a minute from sighting the aircraft to making the first cut (range measurement). Errors were about 300 to 500ft; cuts were best against higher-altitude targets because they appeared to the range-taker to be moving more slowly. UB 2 became the basis of future Royal Navy anti-aircraft rangefinders, beginning with UB 4, which had been adopted by 1925.
Given the discovery during the First World War that it was usually better to concentrate on target height, the new rangefinders were described as heightfinders. The committee decided that they should be mounted horizontally rather than vertically, to protect its crew. It turned out that if the appropriate rate of change of range was applied to the instrument, the eyepieces were held on a target flying level. Even if the target was moving rapidly, the cut would be less transitory, and an operator could get more frequent cuts. That made the range- or height-finder a potential means of measuring a fire-control solution against reality – a means of feedback.
The main subsequent development was to optically convert slant range (which the device measured) into plan range, the quantity fire-control systems used. Barr & Stroud tested this mechanism in a modified UF 1 rangefinder about 1931. The success of these trials showed that it was possible to produce a dual-purpose (HA/LA) rangefinder, a significant step towards a dual-purpose battery.
Calculation
The 1919–21 sub-committee followed much the same path as surface fire control developers. Initially it hoped that guns could be aimed on the basis of plots of what could be seen, much as in the Dreyer Table used for wartime surface fire control. That proved impossible; it was necessary to predict based on estimated target movement, correcting by comparing estimate with reality. That approach was embodied in the AFCT (for surface fire). It had been pioneered before war by Arthur Hungerford Pollen, and adopted during the First World War by the US Navy. Pollen’s engineer, Henry Isherwood, was helping develop the new surface fire-control system.
The one major success of the simpler approach was a means of estimating time of flight, hence fuse setting. Major A V Hill, who had had considerable wartime anti-aircraft experience, based estimates on current height and sight angle (equivalent to range). Given these data, he could draw a curve, which in turn could be represented by a suitably graduated slide rule. It was not precise, but errors were quite small in most cases (they were worst at low angles of sight and at extreme height, i.e., at long range). Prototypes were made by HMS Excellent. In October 1919, the sub-committee recommended that Hill’s plotting board be issued to the fleet as an interim device, to be made either by HMS Excellent or by the commercial fire control maker, Elliott Bros.
Initial experiments suggested that it was easy to estimate target plan course, but unless an aircraft was near the ground it was virtually impossible to say whether it was climbing or descending. The subcommittee (and others) realised that if an aircraft flew level, the key to prediction was target plan course, the course the aircraft flew over the ground, not taking diving or climbing into account. As in surface gunnery, that meant plan range and also plan inclination (the angle between the observer’s course and the aircraft’s course, in plan form). Speed could be estimated to within about 20 per cent from a plot of plan position. Creating one required time, and speed plotting required a skilled operator.11 Also, range and bearing could not be measured quickly enough or accurately enough. It seemed simpler to guess based on knowledge of the type of aircraft.
By 1920 the sub-committee had concluded that an analog computer like the evolving surface-fire AFCT was needed. The inputs (set-up) would be guessed target speed and course (inclination), and the target would be assumed to fly level. These choices were inescapable unless the director was fully stabilised. The computer would compensate for own-ship manoeuvres. Errors in the set-up would be corrected based on observation. Once the set-up had been confirmed, the computer could use it to calculate deflections (horizontal and vertical) and fuse timing. Given the correct set-up, the computer would continue to predict target position even if the aircraft disappeared in cloud. A correct set-up could be upset only if the target manoeuvred.
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