Naval Anti-Aircraft Guns and Gunnery. Norman Friedman
supported a proposal for such a computer from Commander D T Graham-Brown.12 His parameters give an idea of what was expected at the time: own ship speed of up to 40kts, aircraft speed up to 130kts, and wind speed up to 60kts, and for altitudes up to 30,000ft. Graham-Brown’s proposal did not quite get all the way to a computer with feedback, because it did not really model target motion. It was complicated because target motion as seen from a ship was complicated. Even if they could be observed directly, rates were not constant, particularly for a crossing target. The only simple motion was the target’s. Once that motion was known, it could be translated into terms of what would be seen from a ship.13
The Committee realised that Graham-Brown’s proposal was the germ of a much better future system, so it referred his design to Henry Isherwood, who had designed the pre-war Pollen fire-control computer.14 Isherwood explained what was wrong: Graham-Brown solved the problems separately, then connected the solutions. To do that Graham-Brown required far too much mechanism and too many operators. ‘Anti-aircraft fire control must be exceedingly easy and rapid in operation: the fewer the operators, the better the chance of success. Commander Graham-Brown’s system is grafted on to the existing naval fire control, which in my opinion condemns it at once.’
An Interim System: STS and SGU
For the moment, ships received the Standard Temporary System (STS). The Royal Navy later saw it almost as a pre-automated equivalent to the HACS. STS survived alongside HACS on board ships not deemed worth fitting with the full system, such as the First World War-built ‘C’ and ‘D’ class cruisers. This system was in service by 1925, and probably much earlier.15
Like the later HACS, STS was conceived to support aimed rather than barrage fire. Elements were a UB-series rangefinder (heightfinder), an anti-aircraft Dumaresq, a Hill fuse predictor using a plot of angle of sight, and a deflection calculator. On sighting an enemy aircraft, the control officer estimated its speed and inclination. The anti-aircraft Dumaresq resolved that into virtual inclination and speed along the line of sight. Tangent elevation was applied using a fuse curve or cosine sight. Deflection was obtained from calculators set with height (or range), angle of sight, and apparent relative speed and inclination (obtained using the AA Dumaresq). All of this assumed that the target would continue at constant height, course and speed. The resulting deflections were passed to the guns. Once bursts appeared, the control officer could change inclination and speed to bring the bursts ‘on’. For example, if the bursts seemed slightly high and astern of the target, he could alter the inclination accordingly and increase set target speed. In theory, inclination and speed had to be kept up to date by adjusting the AA Dumaresq, but experience showed that was difficult once guns began to fire. Targets were designated from STS to guns by an Evershed (bearing transmitter) of the same type used for surface fire. Fuse and deflection were apparently communicated by voice.
As a result of sea experience and also of trials on board Tiger in July and August 1926, a standard control procedure was drawn up (issued 30 November 1926). Gun output was deliberately limited to avoid confusion in fuse setting, the rate of fire being governed by dead time. To avoid confusion, for example, the setting for the third round to be fired could not be made until about 2 seconds after the first had been fired. The prediction interval was therefore dead time plus 2 seconds divided by two, giving 8½ seconds between rounds, a maximum rate of 7 rnds/min, well short of what guns could actually do. The only way to do much better was to reduce dead time by adopting an automatic fuse-setter on the gun mounting, which could be kept continuously up to date. That in turn had to be associated with a mechanical predictor. What was done, by 1930, was to set the fuse on the gun mounting rather than some distance away, so as to minimise the delay between setting and firing.
Apparently the combination of AA Dumaresq and deflection calculator was less than effective, because in 1927 there was interest in replacing both with a simple Aldis tube sight (a telescope, rather than the usual simple tube) incorporating deflection rings.16 Accepting the use of a very simple deflection sight was, in effect, an admission that nothing short of the elaborate measures involved in the new HACS was worthwhile at anything but very short range.
By 1927 all battleships except the Iron Dukes, all battlecruisers, carriers, and cruisers down to and including the Centaur class had the STS. It seems to have survived during the Second World War on board ‘C’ and ‘D’ class cruisers. For a time the STS was associated with a dual-purpose (HA/LA) director developed specifically for the fast minelayer Adventure, which was armed with 4.7in HA guns.17 Unlike the director associated with the HACS, the Adventure director had no rangefinder. These directors and STS were installed on board battleships (Queen Elizabeth and later classes), the battlecruisers, the carriers and large cruisers (Hawkins, ‘E’ and Kent classes). Installation of HACS in these ships released some for other ships, so in 1926 they were proposed for the Iron Duke class battleships and ‘D’ class cruisers (no installations were proposed for the ‘C’ class). However, the following year it was decided that the Iron Dukes (except Marlborough, which was already fitted) would not be fitted with STS due to their age and the cost involved. Installation of Adventure-type directors was, however, being considered for the ‘D’ class cruisers. This installation was soon rejected due both to topweight and to the need to cut spending (1928).
Roughly parallel with STS was the Single Gun Unit (SGU). In 1924, when SGU was first discussed, it was planned for earlier cruisers (Caroline class and before), for leaders armed with 3in HA guns, and for large gunboats which could have no HA control positions due to limited space and weight. SGU would consist of a fuse plate and a deflection calculator, existing sights being used. By 1925 the fuse plate had been tested successfully and manufacture was proceeding. The fuse plate could also be used with the existing deflection cards. The supply of fuse plates and deflection calculators began in 1927. At this point it was decided that SGU would be fitted to the Iron Dukes (except Marlborough), to Cambrian and Caroline class cruisers, to older cruisers, to gunboats, leaders and new-construction destroyers, and to some auxiliaries. It might also become a backup system for ships with two or more guns on each side but with only a single HACS. The Aldis ring sight was scheduled for trials in 1926, and by 1927 it was expected to replace the deflection calculator pending successful trials. Production of the Aldis sights was badly delayed, although by 1928 supply of fuse plates and deflection calculators had begun.
The prototype SGU was installed on board the cruiser Calypso for comparative trials (against smoke burst targets) against STS in the 3rd Cruiser Squadron during 1930. They were successful enough that it was suggested that SGU was preferable to STS in ships in which one gun could be brought to bear on an air target; to see whether that was true further trials were ordered in Curlew in 1931. Final Aldis ring sight telescope trials were conducted at about the same time, and it was reported to compare very favourably with deflection calculating arrangements in other systems. The first seventeen Aldis telescopes were expected to become available for trials during 1931.
After all of this optimism, 1933 trials showed that SGU worked only under ideal conditions. No more were to be fitted, but ships which already had it were to retain it until something better became available. Two alternative systems were under consideration, either as primary controls for ships without HACS or as alternatives for cruisers with HACS, to engage targets on the opposite side or as fall-backs. Nothing seems to have come of these projects, because it was so much more urgent to improve other kinds of anti-aircraft fire.
The Computer System: HACS
The Committee completed its work before any new computer could be designed, but it appears that Isherwood’s comments led him or others to develop a High Angle Calculating Table (HACT) analogous to the AFCT, using somewhat similar mechanisms. It entered service in 1928, and it remained in use in modified form after the Second World War. The HACT was the computing element of the HACS, the others being the director aloft and the connections to the guns. The HACS was conceived specifically to defeat high-altitude bombers, controlling long-range anti-aircraft fire (defined in 1933 as anything from 3000 yds out).18 Permissible errors were set by the lethal radii of different shells.19 Later it was also described as a means of defeating formations before they broke up to deliver dive, torpedo or other