Because the director was not integral with the HACT, a director could be connected with any of various HACTs. Because HACS was so expensive, initially ships had only one. It could engage only a single target. By 1931 policy envisaged adding a second HACS for capital ships, typically to be mounted aft with its own calculating position below decks. Guns would have duplicate receivers (for follow-the-pointer instructions), so they could follow whichever director was desired. Installations began in 1931 as ships refitted.
As of 1940, six systems were in service: HACS Mk I, II, III, III*, III** and IV, each with its own type of director and its own type of computer (HACT). HACS I was fitted to all ships up to 1935; as of 1940 it was being replaced by ‘ended’ HACS III (the resulting surplus sets were installed as second HACS in heavy cruisers). Each HACS Mk I director was associated with a particular HACT (ships had several). To be able to switch directors a ship needed duplicate wiring from each HACT to the guns, each gun having a change-over switch from one set of receivers to the other. HACS I** had improved fuse-setting gear. In 1935 arrangements were made for ships’ staffs to modify HACS to accommodate an enemy speed of 250kts. Later they would be modernised with 350kt settings, and errors in the deflection gear of HACS I and II would be corrected. This took time, so as of 1940 it was still planned ultimately to modify all sets for increased target speed (350kts) and for gyro roll correction (HACS I***).33
HACS II was a slightly improved HACS I* fitted only in Repulse and in the Leander class cruisers (one per ship). By 1939 gyro correction replaced an earlier failed liquid pendulum, whose float did not exert sufficient force. The gyro corrector stabilised the director telescopes in the vertical, and removed the effect of roll from measured director setting. The layer no longer had to follow the target as the ship rolled, but instead concentrated on the angle of sight. It employed a single gyro mounted athwart the line of bearing to the target. It did not provide cross-level (that was incorporated only in HACS IV). Gyro roll correction was exerted by a separate unit in the HACP. The Mk II director had its windscreen shape altered to accommodate an electric junction box inside its screen, and to suit its new rangefinder.
HACS I, II and III all used step-by-step transmission. Elevation was transmitted to the director in 3 minute (of arc) steps, at up to 100 steps/sec; train was in similar steps, but 130/sec. In HACS I and II (including improved versions of I), power follow-ups used M type transmission (motors and hunters) with small DC motors. For HACS I and II, target speed (as reflected in deflection gear) was normally - 45 to 200.4kts; own-ship speed limits for all versions were 0 to 40kts. Steps for speed transmission were 0.25ft/sec per step, with a maximum of 45 steps/sec. APV limits were -870 to 2100ft/sec. Elevation angles in all versions could be 10 to 90° (HACS Mk I* was limited to 9° 30’ to 89° 30’).
By 1938 HACS III was in production at three firms. It equipped some cruisers prior to the Birmingham class and some modernised battleships and carriers. It could provide deflection for barrage fire (see below). A 15ft (rather than 12ft, as in earlier systems) rangefinder was fitted in an anti-vibration cradle. Range conversion (to height) was limited by stops to between 2000 and 20,000 yds. The director had an entirely new roll corrector adjacent to but separate from the HACT proper, employing a gyro (in US terms a stable vertical) driving an oil motor connected to the HACT. This device was trained in the direction of the director, so its correction was applied to the director’s line of sight (a new gyro roll corrector was designed for retrofit to HACS I and II). Angle of roll could be sent (by ‘M’ transmitter) up to a corrector sight in the director, if that was fitted. Users particularly liked it and the anti-vibration rangefinder mounting. HACS III embodied considerable mechanical improvement, including ABC transmission and oil motor follow-up (maximum motor speed of 400 RPM corresponded to 6°/sec motion by the director).34 Target speed limits were 35 to 350kts. Each transmission step was 0.25kts, and the device could handle 100 steps/sec. This version could handle 4in and 4.7in ballistics (APV 870 to 2100ft/sec and 1000 to 2230ft/sec, respectively). The destroyer depot ship Woolwich and the anti-aircraft cruisers all had the C version with provision for low-angle fire.
The associated Mk III director had its plating thickened to 0.08in, presumably to deal with more powerful aircraft guns (later this thickness was described as merely weatherproof). Mk III* had an additional range-taker’s position, and the control officer’s window was enlarged so that an eye-shooting High Angle Director Forward Area Sight (HADFAS) could be installed. The directors in Penelope and the forward one in Malaya did not have this sight.35 Mk III** had a completely round windscreen but no additional range-taker position, and it was reinforced against shock.
By 1938 HACS IV was being installed in all new-construction and also in rebuilt battleships with dual-purpose secondary batteries. It was a modified Mk III with magslip (synchro) transmission and with roll compensation; provision was also made for cross-level. The design was cleaned up to reduce loads on shafts and pinions to reduce maintenance and backlash. Mk IV* was designed specifically to support the longer-range 5.25in gun. It had improved plotting arrangements. When used with 5.25in guns it was fitted for salvo fire. Target speed limits were 0 to 350kts, and the Magslip transmitter could turn at 400 RPM (one revolution was equivalent to 650kts). At least initially, HACS IV was fitted for 4in and 4.5in ballistics (1050 to 2350ft/sec and 1050 to 2250ft/sec). HACS IV* was designed to handle longer-range 5.25in guns on board Dido class cruisers and battleships. It and HACS IV were modified to incorporate drives which could be linked to an Admiralty Fire Control Clock (AFCC) for surface fire. HACS IV could operate both with various versions of the Mk IV and Mk V director and with the later Mk VI (which had been conceived for the Flyplane computer described below). HACS IV and IV* were designed specifically to deal with both air and surface targets, the air-only version being designated AA and the dual-purpose version AASU.36
Ships completed in 1940 had a new Gyro Rate Unit (GRU) mounted on the director, to measure horizontal and vertical angular rates directly. Although this measurement might be described as tachymetric, it could not transform the HACS into something capable of dealing with climbing or diving targets. Nor did it solve the problem of measuring target inclination. That was still fed in by the control officer. The Gyro Rate Unit Box (GRUB) in the calculating position below decks received the two rates and the angle of presentation from the GRU, ultimately to feed data into the HACT nearby. This was still a feedback process. The assumed target ground (plan) speed was set on the GRUB. Given an angle of presentation and a target height and range, this speed implied particular vertical and horizontal rates. It was adjusted until these rates matched the observed rates, the GRU acting in effect as a feedback mechanism. The resulting checked ground speed was designated the GRU ground speed and fed into the HACT. According to the 1939 HACS manual, ‘at long ranges this information should afford a valuable source of information to the Control Officer, but will not be accurate enough to be followed blindly. At short ranges it should be quite accurate enough to be accepted.’ At long ranges and low sight angles the rates would be too small to be reliable, and might even be grossly misleading (according to the GRU handbook).
The layout of the HACT from the 1930 handbook for HACS I and II.
When HACS IV was being designed, director production lagged, so the Mk III director was adapted to Magslip transmission and stabilised in elevation as Mk IV. This director was completely round, larger in diameter, and had the additional range-taker position. By 1945 the typical crew was seven rather than the five of the original HA director: a director officer, a rate officer (GRU operator for AA), a director layer, a director trainer, a phoneman and a local gun direction officer or rating. The director officer still had the angle of presentation binoculars. Note the absence of a range-taker due to the use of radar.
Some ships had HA/LA Mk IV directors, similar to Mk IV but wired also to control the low-angle armament (in 1940 they were in the Birmingham and Liverpool classes and Aurora). This director was modified during the Second World