Naval Anti-Aircraft Guns and Gunnery. Norman Friedman
across the dial. When it met the fuse curve, it indicated the elevation which would cause a shell to burst on the line of sight. To keep the required fuse curves always under the pointer, the sight had to be readjusted constantly as the angle of sight changed. The complicated dial required its own highly-trained sight-setter in addition to the usual sight-setter. A simpler dial was therefore developed, to give as nearly as possible the same results with only a single sight setting for each fuse.7
To compensate for target motion, the British simply drew two more curves, one for an aircraft approaching at what seemed a reasonable average speed (in 1916, 80mph), and one for the same aircraft receding at the set average speed (the three curves were all marked on the dial, coloured black [static target], red [approaching] and green [receding]). For other speeds, small angular corrections sufficed. Once shells could reliably be fired to burst on the line of sight to the target, it seemed that gunnery could be reduced to lengthening or shortening the range. Spotting for line proved relatively simple, presumably because aircraft tended to fly directly towards the ship.
Cards and curves were needed because it became clear that few officers could calculate the change of range during the time the shell was in the air, let alone the dead time during which fuses and sights were being adjusted and also the loading interval at the gun (time between fuse setting and firing). A shell might take 20 seconds to go from gun to burst. During that time an aircraft might well fly 1200 yds (108kts). It seemed much simpler to concentrate on target height, which would not change much while the shell was in the air. For example, high-flying aircraft were unlikely to climb as much as 1000ft in a minute, and they proved unwilling to descend while under fire. Zeppelins did climb and descend more, but by 1917 they were apparently less important targets.
The combination between fuse timing, gun elevation and angle of sight was complicated enough to preclude, at least for the moment, any kind of dynamic fire control. A control officer could set fuses and aim a gun to create a burst at a predetermined place, but the set-up was too cumbersome to keep changing to deal with a moving aircraft. Multiple guns could create a zone of fire. By late in the First World War the British advocated creating multiple zones of fire through which the aircraft was expected to fly. This barrage was a burst of rapid fire (several rounds per gun), all on one line of sight, and all with the same fuse setting. In theory the barrage created a curtain of fire into which the aircraft would fly. Fuses would be set somewhat short for approaching aircraft. For slow targets, a variation of the ‘ladder’ technique used against ships was recommended. Burst or zone fire required that guns have ready-use ammunition at hand.8 In 1917 the Royal Navy introduced a fuse-setting instrument to guarantee that successive zones of fire would have a reasonable relationship to each other. It was based on the assumption that aircraft would fly level. The spotting officer had to estimate the speed of approach of the target (the instrument was set for 100kts, but could be adjusted).9 This zone approach had already been developed to deal with fast surface torpedo craft. No one considered it satisfactory. The main lesson of the dials and curves and zones was that calculation should be automatic and rapid.
By 1917 anti-aircraft organisation had been formalised. The control party consisted of a spotting officer; two men to work the fuse range indicator or card; a rangefinder operator; a rangefinder layer (pointer); a range scale reader; a communications party; and an observer to identify aircraft (typically with a high-powered telescope). Ships with two different high-angle (HA) guns (typically one 3in and one 4in) needed fuse calculators for each gun, preferably both using the same data. Each gun’s crew should include two sight-setters, one to keep the sights set to the chosen fuse setting, and one to apply vertical and horizontal deflection. Two more crew members were to set fuses and to raise ammunition from the ready-use boxes.
Perhaps the most important wartime lesson was that it was difficult to measure the range of a fast aircraft. Naturally the Royal Navy turned to its rangefinder maker, Barr & Stroud. That company sold coincidence rangefinders for surface and therefore now for air targets. Initially Barr & Stroud simply redesigned its rangefinder mounting so the instrument’s line of sight could be tipped up towards the aircraft. The French army was the first customer, with a 1911 request. Barr & Stroud delivered its first tippable MT mounting in 1913.10 The rangefinder itself was the standard portable army-type FT, which could have a base length of up to 2m. The Royal Navy bought some tippable rangefinders for trials, but disliked the technique.
For the Royal Navy, Barr & Stroud modified the 2m version of its FT rangefinder to swing into the vertical. It could measure both sight angle and slant range, and the combination gave target height. The rangefinder could swing back into the horizontal to give target bearing. In the latter half of 1914 the Admiralty ordered high-angle FT28 rangefinders (1m base) for ships armed with HA guns.11 Initially instruments read out only true (slant) range, but they were later modified to give horizontal range as well. Early in 1916 a 2m rangefinder was made, and after July 1916 trials, these instruments (FT29 on MT6 mounting) were supplied to capital ships and light cruisers towards the end of 1916 and early 1917.
Thus by late 1916, ships had 1m or 2m rangefinders with vertical bases, each with three operators: (i) trainer and operator; (ii) elevator; (iii) reader. In January 1917 it was proposed that a heightfinder be attached to the 2m rangefinder as a basis for sight-setting and fuse-setting. This new feature was embodied in the MT10 mounting, which was being supplied at the end of the war. MT10 presumably embodied a new feature Barr & Stroud offered in a new specialised anti-aircraft rangefinder supplied to the army (UB2, 1917). Using differential gearing, the instrument automatically converted target range and elevation into height. As with the FT series, there were three operators. Initial production went to the British army, which ordered 400 UB2s in November 1917.
Each measurement with a coincidence rangefinder took time; the faster the aircraft, the less accurate a series of ranges. It was impossible to use that series of ranges to deduce the speed of an aircraft. Post-war development was shaped by the realisation that although it might be difficult to measure range quickly enough, most aircraft flew straight and level. Rangefinders could measure height, using trigonometry. Although each rangefinder-derived height might be inaccurate, successive observations could be averaged. The exception was at low altitude, where even a large change in angle would not be associated with much change in height. At such altitudes ranges would have to be taken directly.
Guns
The Royal Navy first experimented with guns on HA mountings to defend fixed shore installations, particularly magazines. Experiments began at the end of 1912. At the end of 1913 the navy decided to go a step further and mount HA guns on board ships. It opted for a 3in 20 cwt semi-automatic QF gun on a roller-path mounting (the British Army 4in QF Mk IV proved unsuccessful). After successful trials, orders for 3in mountings were placed late in 1913 for the latest battleships, the Iron Duke and Queen Elizabeth classes (two 3in each). This weapon became the standard wartime Royal Navy HA gun. It combined a large enough shell with a high muzzle velocity (2600ft/sec) and a high rate of fire (twenty-two to fifteen rounds per minute); effective range was about 6500 yds. The gun was modified (in non-semi-automatic form) for destroyers and submarines with its sight and recuperator under the gun, and with a mounting which could better survive immersion in salt water. By November 1914 all battleships of the Grand Fleet had two HA guns of some type. About November 1914 some 3in HA guns were withdrawn from the fleet to supplement the anti-aircraft defence of London.
In May 1915 specially-designed HA mountings became generally available to the fleet, and they began to replace the locally-converted improvised mountings. The latter now armed auxiliary patrols. By the spring of 1916 all ships had their designated anti-aircraft weapons.
The standard late-war 4in Mk V gun (4in/45) equipped the inter-war Royal Navy. This one was photographed aboard Exeter when she visited the United States in the 1930s.