bearing: the locally-controlled high-velocity gun did better than a fully director-controlled low-velocity gun using RPC. By mid-1942 most 2pdrs in the fleet had been converted to fire high-velocity rounds as Mk VIII* guns.
Trials were run in 1938 to determine characteristics of a light eye-shooting close-range weapon (inside 1000 yds range), to be used particularly by ships too small for multiple pom-poms. At one end of the scale was the single 2pdr; other possibilities were a single or possibly twin 0.8in (solid or explosive round), quad 0.5in and finally multiple 0.303in. It emerged that the 2pdr was most effective, though it would need a new mounting. The 0.303in was completely outclassed. A drum-fed 0.8in offered tactical advantages, in that it could fire for 8 seconds and reload in 12 seconds, compared to 20 seconds for the 0.5in (45-second reload). It turned out that there was no time to develop a new gun.
Commercial Fire Control
In March 1920 Captain Usborne (president of the Naval Anti-Aircraft Gunnery Committee) proposed that DNO seek a fire-control system from Vickers, which was, thus far, the only British firm known to have make a complete study of the AA gunnery problem. Vickers was already working on a director and on fittings on gun mountings to be used for fire control, and it had experience in the design of periscopes. Work did not go entirely smoothly: within a few months Vickers was asked to stop work on the director while it was re-thought.
Vickers proposed its AA Predictor some time in the spring of 1920. The Committee concluded that if suitably modified it could calculate vertical and lateral deflection given measured vertical and lateral angular velocities, which in turn could be measured by Professor Sir James Henderson’s ‘rate of change of bearing and altitude meter’, which used his constrained gyros – it was a tachymetric device. Vickers submitted a modified design in October 1920. It incorporated the desired ability to predict fuse time, and the Committee thought that it might be worth adopting. Usborne asked DNO to pay for production of a prototype.
Vickers apparently soon abandoned work on the Predictor, resuming it in 1924; it produced a prototype in 1928. By this time it had abandoned measuring gyros in favour of comparing generated angles (produced mainly by integrators on the basis of estimates) with what was observed (a process called ‘tuning’). The Predictor also measured the range rate by tuning. As in the later HACS, the range operator adjusted the range rate to keep the target at the observed height as it moved (as the sight angle changed). Once rates were determined by ‘tuning’, they were fed into equations which connected vertical and horizontal deflections to the rates (the equations involved various trigonometric functions of the initial and final sight angles). Time of flight was a function of future sight angle and altitude (i.e., of future range). For any combination of these two parameters it could be obtained from a three-dimensional cam. Assuming constant rates, the deflections were simply the rates multiplied by time of flight. Vickers used two equations, each of which connected the sine of a deflection with time of flight and future sight angle. Instead of solving them directly, which would have been complex (because so many trigonometric functions were involved), it computed each side of each equation. The correct choices of time of flight and future sight angle would cause the two sides of each equation to match. The Predictor displayed the difference between the two sides of each equation, and the operator ‘balanced’ them by turning a wheel until the difference was zero.65
It took 3 to 4 seconds for the operators to reach a solution by tracking an aircraft. Once they had measured the horizontal and vertical rates and entered them into the Predictor, it could calculate future angles. Given height and vertical angle (sight angle), the Predictor calculated future range, which gave time of flight of a shell, hence fuse setting. This calculation was done using a three-dimensional cam. Below 10° or 15° angle of sight the system used range rather than height, because at low angles angle of sight did not change quickly enough. None of the rates was really constant, but (as in a Dreyer Table) they could be treated as though they were for a short time. By 1931 the British army was using the Predictor to control medium-calibre anti-aircraft guns. This Naval Predictor, which Vickers exported, was offered to the Naval Anti-Aircraft Committee. It formed the basis of Japanese wartime naval anti-aircraft fire-control systems. Vickers also exported the naval system to other countries, almost certainly including Argentina (for the training cruiser La Argentina) and Spain (for cruisers, presumably beginning with the Canarias class). Vickers also offered the committee a more elaborate device more like the HACS, but it was apparently never exported (and it may never have been built).66
More sophisticated devices used current angular velocities and range to estimate target course and speed, and then predicted on that basis. In 1932, when the Naval Anti-Aircraft Committee released its report, the more complex but more accurate technique was used by ARL, by Barr & Stroud, and by Sperry in the United States. Of these, the ARL Predictor was intended for use only ashore. Barr & Stroud’s system was rejected by the Admiralty as too complex.67 The Imperial Japanese Navy seems to have bought an earlier, simpler, Barr & Stroud calculator (see below). A majority of foreign armies had tachymetric systems, but in 1931 only Sperry in the United States and Hazemeyer in the Netherlands offered naval tachymetric systems for export.
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