Putting the System Together: The ADO
British experience during the First World War showed the importance of designating targets. It was also clear that anti-aircraft fire had to be concentrated on the most threatening air target. By the late 1930s the British therefore became interested in the step before a director and guns were assigned to a target: the choice of target by a designated Air Defence Officer (ADO) supported by dedicated look-outs. The ADO would also be responsible for choosing the appropriate means of resisting air attack. The ADO concept seems to have been considerably in advance of that of any other navy.
In effect the ADO organisation was designed to maintain awareness of the air situation around a ship so that targets could be prioritised and fire assigned to them. The ADO organisation seems to have evolved into the Gun Direction Room (GDR), which was an element of the wartime Action Information Organisation (AIO). The AIO was often considered equivalent to the US Navy’s CIC, but the Royal Navy emphasised the split between ship self-defence (as organised in the GDR) and functions such as fighter control, the latter organised on a much larger scale. The US Navy tended to combine the two functions.
Ramillies shows the air defence position built up from her bridge in this late pre-war photograph. The objects visible atop it are ADO sights, which were used to indicate incoming targets. In effect they were the distant forebears of the later British ‘Eversheds’ and the US Target Bearing Transmitters. (Naval Institute by Ted Silberstein)
The ADO concept seems to have originated in the Home Fleet. During the latter part of 1934 the fleet convened a committee to consider fleet anti-aircraft defence. It concluded that the captain of a modernised battleship could simultaneously engage as many as six air targets, not counting those engaged by Lewis guns. With a modern closed bridge, he had little or no view of the sky from which synchronised air attacks could come at great speed, with little warning. Air defence was essentially different from other kinds of action, in that it would develop suddenly. Moreover, the relative importance of different attacks could shift instantly, a previously innocuous aircraft becoming a priority threat, and vice versa. The committee recommended that the captain be relieved of some of his many activities so that he could concentrate on developing the offensive power of his ship. He would of course retain a veto. However, reaction time was so short that the ship could not afford wasted time.
The alternatives were decentralisation to the weapons or groups of weapons, and centralisation under an ADO. Rigid centralisation might preclude quick reaction in a close-in melee, but there had to be some coordinating authority to collect and disseminate information and to insure that the various weapons were used most effectively. To direct the batteries most effectively there had to be someone not immediately concerned in the actual control of fire, hence able to take a general view of the situation and keep the captain informed. He would control the air lookouts, the most important of which would be located near him, and also in direct contact with the captain. He also needed adequate weather protection. The proposed organisation was immediately tested on board Nelson and Rodney. To accommodate the ADO, ships under refit were being given suitable open bridges. Unfortunately that was impractical in some ships, including Warspite, then under reconstruction. By mid-1936 the ADO idea had been accepted throughout the fleet.
The ‘Ideal System’
In 1931 the Royal Navy convened a new Naval Anti-Aircraft Gunnery Committee to devise an ‘ideal system’ to replace the existing one. In effect it was a critique of the HACS; as in the past, it advocated a tachymetric system.46 Also as in the past, the main issue was whether the director sights could be stabilised well enough. The committee pointed out that if all ship motion could be cancelled out, the predictor part of the system would function at sea as on land, so the Royal Navy could use the same system the army used (and the predictor part could be tested on land). As in HACS, the director of the ideal system would have an ‘undisturbed’ line of sight, tracking the target without shifting away to compare actual and calculated target motion. As in the existing HACS, the calculating element should be separate from the director, protected by armour both from enemy fire and from the blast of a ship’s guns. It could also be given collective gas protection (an important consideration in the inter-war period). The director should be capable of handling a target moving at a relative speed of 250kts (high for 1931) at 2000ft (greater altitude would imply slower angular rates) and also with an aircraft passing at 2000 yds on a flank or at a maximum angle of sight of 60° when approaching. The system should handle targets at all elevations up to 70°.
The system would measure rates at a particular time, projecting them ahead based on deduced target motion (including whether the aircraft was flying a straight or steadily curved path, and whether it was flying level or climbing or diving). Hopes for a curved-path predictor (assuming that the aircraft was flying steadily along an arc of a circle) seem to have foundered within a few years. DNO accepted this recommendation, and began work on a system called TS I (Tachymetric System 1). Unfortunately this project was incomplete when war broke out; the Royal Navy fought the Second World War with HACS, which had been considered inadequate nearly a decade earlier. It did try to patch on a tachymetric element, in the form of GRU and GRUB, but that was not nearly enough, and it did not solve the basic limitation of assuming that the target was flying straight and level.
The anti-aircraft battery had to develop the maximum possible rate of fire. In practically all cases (except perhaps destroyers) a ship rolled more slowly than the possible interval between shots. Surface ships timed their shots so that they were always at the same point in the roll, but in this case that was impossible: the guns had to be stabilised so that they could fire any time in the roll. Past practice, in which the gun crew kept elevating and training the gun to match pointers controlled remotely by the fire-control system, was inadequate. The crew just could not move the gun quickly enough, since they would be tracking a fast target while compensating for the ship’s motion.
The only solution was to have the fire-control system remotely control the gun: what the Royal Navy later called Remote Power Control (RPC). In 1931, as in the past, the Royal Navy transmitted dial settings using the ‘M’ transmitter, a stepping motor. Although reliable, it transmitted motion as a series of separate jerks, rather than the required smooth control. The ARL proposed a new rotating-field transmitter (later called ‘Magslip’), broadly equivalent to the US Selsyn. The motion involved would be anything but smooth; the mass of the gun had to accelerate and decelerate rapidly and stop as desired. The available alternative means of powering a gun mount were oil engines (as in car transmissions) and electric motors, the committee favouring the first because they contributed much less inertia to the mechanism as a whole. The Royal Navy tried both alternatives when it began to adopt RPC in 1939. The committee urged development of a new medium-calibre anti-aircraft gun suited to RPC, with a high inherent rate of fire.
Guns
As the first Naval Anti-Air Gunnery Committee began its deliberations, officers of the Grand Fleet asked whether new battleships could have secondary batteries combining anti-air and anti-torpedo (i.e., anti-destroyer) functions. The Naval Staff wanted an anti-destroyer gun of at least 5.5in calibre (as in Hood), and there was some question as to whether so heavy a gun could be loaded rapidly enough. The issue could be resolved only by examining a design, so the committee recommended asking the three mounting suppliers (Vickers, Elswick and Coventry Ordnance Works) for sketch designs of 5.5in dual-purpose mountings (Woolwich was later added). Examination of existing sketch designs of 6in, 5.5in and 4.7in dual-purpose mountings the previous year had shown that the 4.7in gun was the largest whose fixed ammunition (shell and case in one) could be manhandled. On this basis the Committee decided to order six 4.7in guns for trials (the order was soon cut to four). The Committee suspected that the 4.7in would fire so much more slowly than the existing 4in gun that it was already somewhat too large. Heavier guns had to be power-loaded. Separating shell and cartridge would slow the rate of fire, since the loading cycle would involve two separate operations. In any case the trunnions would have to be at the rear of the cradle, near the breech, to limit the height of the mounting