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