The initial wartime versions of Mk IV were Mk 4 GB and Mk 4 GB AA/SU. Here G indicated the GRU and B indicated scooter control, for quick slueing to acquire a target. Typically one handle was provided for the director officer and a second for the GRU operator; in surface fire mode, the second was used by the rate officer. Because addition of scooter control to the existing oil motors proved lengthy and expensive, a few directors were fitted for electric control (‘E’) and were therefore designated Mk 4 GE. Here scooter control meant that by depressing the ‘scooter’ the control officer could quickly slew the director to a new bearing.
There was considerable interest in duplex rangefinders, but they made the directors heavy; the HACS was designed to work with one rangefinder. Plans called for duplex rangefinders in the 1937 (King George V class) battleships, the second barrel working separately from the HACS table except that the table would feed range to it to assist operators in keeping their cuts on the target.
Meanwhile a new Mk V director (tower) was developed with all possible improvements, such as a sliding roof, duplex rangefinder, and stabilisation for training and laying. With the advent of the Air Defence Officer (ADO), lookouts outside the director could cue it, so a clear overhead view seemed less important; weather protection became practicable. It seemed possible, in 1939, that earlier directors would receive roofs. At this time Coventry was testing gyros intended as the basis for a future tachymetric deflection control system. Mk V equipped the King George V class battleships and wartime aircraft carriers.37 Unlike all the earlier HA directors, it was cab-shaped rather than basically cylindrical. It was completely enclosed for weather protection. Because it controlled dual-purpose guns, Mk V had improved surface firing capability. That required more personnel, because surface firing required a spotter. Thus there was a seat for a rate officer (the GRU operator for AA fire) alongside the layer, with the trainer on the other side of the director cab. Between them was the raised seat for the control officer, with his binoculars and his HADES telescope. At the centre of the cab was the telephone operator, with the range-taker at the rear. The director controlled the ship’s HA/LA armament through an HACT for high-angle fire and through a separate AAFC (fire-control clock) for low-angle fire. Directors were fitted with a Radar Training Sight (RAC) after the war.
At the end of the war the Royal Navy introduced a new cylindrical Mk VI (typically written Mk 6) director designed to work with the Type 275 radar. It was designed to be operated from the ‘Tallboy’ (radar console) in the Control Position. The director was entirely electrically-powered. Mk 6 was designed specifically to provide a better view for the crew and to include arrangements for a local gun direction officer. It could be connected either to an existing computer (such as HACS 4) or to the new Flyplane introduced after the war. It and Flyplane are described in a later chapter. Unfortunately Mk 6 was not designed for tachymetric operation; it was not stiff enough to measure bearing rates as it tracked a target. That caused problems when it was used with the tachymetric Flyplane computer post-war, and it had to be rethought as the post-war Mk 6M.
All of this leaves the question of how effective the HACS/gun combination was, both in reality and in pre-war perception. In January 1938 DTSD tried to summarise the results of anti-aircraft practice in all the fleets.38 Assuming perfect prediction and fuse setting, guns could be expected to burst a third of their shells at the correct range. The standard for success was to burst a shell within 100 yds in front of the target. That was well beyond lethal range, but a pilot seeing such a burst would probably jink and thus ruin his aim. Some of the scores were unreliable because only the Home and Mediterranean Fleets had full facilities to film, and therefore triangulate, bursts. Tables of results showed that ships were much better at getting shells in the right direction (line) than in getting them within the right range. The right combination was achieved about 12 to 17 per cent of the time. In something less than a third of all practices, no bursts at all were placed within 100 yds of the target. In barrage fire, 6in and 4in guns placed their bursts within 100 yds 26.5 and 4.8 per cent of the time, respectively (4.7in guns failed altogether to do so).
The 1938 edition of Progress in Naval Gunnery included the comment that some form of tachymetric deflection measuring device was urgently needed; sleeve and Queen Bee targets, whose speeds were known within narrow limits, and which could not vary their speeds appreciably as they approached, were apt to breed false confidence in deflection control. The 1939 edition of Progress in Naval Gunnery reported that a new fully-stabilised high-angle control system was being developed for future construction (it figured in early plans for Vanguard). At the time (May 1939) it was planned for ships which would complete in 1942–3. The necessary stabilisation system was tested successfully in Coventry in 1938–9. At least initially a tachymetric unit was conceived as an add-on to the HACS. It could not completely replace the course and speed method at medium and long ranges, when vertical and lateral deflections were small (and presumably difficult to measure).
It was also obvious by early 1939 that diving targets would be more and more important.39 If constant height could no longer be the basis for fire control, the next possibility was to assume constant range rate as the basis for feedback. Tests of a duplex rangefinder were underway, one half operating on a constant-height basis and the other on a constant range rate basis. For the moment, since prediction would become impossible once dive bombers had broken formation, barrage fire was the only possibility; it had to begin as soon as the bombers broke formation, regardless of range. Accounts of wartime anti-aircraft action in the Mediterranean suggest that barrage fire was much preferred to aimed HACS fire.
Systems for Smaller Ships
By the 1930s the Royal Navy took the air threat seriously enough that it wanted destroyers to contribute to fleet air defence. Limited gun elevation made sense for a destroyer engaging bombers approaching a ship she was protecting. As the aircraft approached, it would spend very little of its time at high angles of elevation over the destroyer, which therefore would get very few high-elevation shots, however high it could point its guns. An attempt to build a 60° destroyer gun mounting having failed, 40° was selected as the highest for which a satisfactory mounting could be built. The first destroyers affected by the new requirement were the ‘E’ class of the 1931 programme, but they and their successors up to the ‘I’ class did not have special anti-aircraft fire-control arrangements, because the required systems did not yet exist. The 40° mounting was an acceptable compromise for a ship intended primarily to beat off torpedo attacks by enemy destroyers and to deliver its own torpedoes against the enemy’s capital ships.
Going to higher elevation entailed serious sacrifices. The higher the elevation of the gun, the higher its trunnions, which had to lift the gun high enough to allow it to recoil (at maximum elevation) without hitting the deck. Higher-powered guns recoiled further. Above a certain trunnion height, a man standing on the deck would be unable to serve the gun. The only solutions were to provide a pit under the gun mount (reducing deck strength) or to place the crew on a platform above deck, revolving with the gun, as in the contemporary US 5in/38. The Royal Navy rejected such complex arrangements (hoists revolving with the gun mount). DNO reported that the maximum gun which could be hand-loaded at all elevations, and which was really suited to anti-aircraft use from a destroyer, was the 4in (35lb shell), whose twin mounting weighed 14 tons, somewhat more than a 40° elevation single 4.7in (62pdr). Wartime experience showed that these assumptions were badly flawed, and eventually a 55° un-powered mounting was accepted (see a later chapter). Note that even a 40° gun needed an anti-aircraft fire-control system, not least in order to calculate fuse settings. The elevation argument did not apply to the relatively low-powered 4in HA gun, which could elevate to 80°.
A typical