The 1944 instructions include anti-aircraft fire: barrage fire is to be used both against dive bombers and against low fliers. That is much the British doctrine of this period, and it suggests that, like the British, the Japanese did not expect to use aimed fire against other than high-flying level bombers. Ships escorting carriers were to concentrate on defending the carriers.
All of this meant that, as unpleasant as Midway had been, the Philippine Sea carried the additional message that the enemy would almost always be able to carry out his air attacks unhindered by any long-range Japanese strike. Anti-aircraft weapons suddenly became far more important, because the option of striking first at greater range was gone. That had already happened in 1942, but in 1944 the Japanese could hope to regain their range advantage with new carrier attack aircraft.
After Midway, Admiral Yamamoto issued new orders for ships under air attack. Battleships were taken as the basis for more general practices. The ship in the fleet closest to the attacking aircraft was to turn towards the enemy and emit specified smoke signals, firing its guns so as to direct Japanese fighters towards the enemy. Presumably smoke was to be used because the Japanese had taken from the British the idea that radio silence was golden. However, the orders also included flag and wireless signals to provide data such as the strength of the enemy force. Their list of ways of detecting incoming enemy aircraft consisted of radio intelligence, radio location (presumably radar), scouting aircraft, watching aircraft and fire-control predictors (presumably used to project forward the path of enemy aircraft).
Alternative means of distributing fire among ships of the fleet were given. The rules clearly envisaged British-style barrage fire by the main and secondary (LA) batteries, which could be used against torpedo bombers, long-range bombers, and bombers capable of strafing (presumably a literal translation), but primarily against torpedo bombers. Medium-calibre anti-aircraft guns would be used against bombers and dive bombers. Machine guns would be used against dive bombers and, according to circumstances, short-range torpedo bombers.
Special rules indicated when guns could open fire in the presence either of numerous or few or no Japanese fighters. For example, when there were numerous Japanese fighters, guns could open fire against torpedo bombers out to 15km range. They could open fire against dive bombers when they were running in – at an estimated altitude of 3000m (9840ft) and at a 50° vertical angle. Against low-level bombers, the range to open fire depended on whether there was an adequate patrol on the second warning line. If there was, fire could be opened not more than 5km (plan range) from the second line. When the patrol at the second line was inadequate, fire could be opened 6nm (unit given) from each ship. Fire could also be opened when the enemy aircraft were at an altitude of more than 6km (19,700ft) and 7km (7650 yds) from the ship (plan range).
An appendix warned that sights etc on all types of AA guns were unsuitable for use against fast aircraft moving at 200kts or more, and should be rebuilt.49 Simple unobtrusive sights suited to 300kt targets could be placed alongside the existing sights of 12cm and 7cm AA guns. The ordinary sight of the 8cm AA gun should be improved and a simple unobtrusive sight suitable for 300kt speed should be fitted. Measurement and gradation of the firing table for the Type 89 (12.7cm) AA gun and the time taken for communication were considered excessive; a simple and rapid type of measuring instrument should be made and distributed. Automatic weapon (25mm and 13mm) sights could not match target speed, as their capacity was too limited, and therefore they could not be used in combat. Either a prism should be inserted in the sighting telescope, or a simple 300kt sight should be installed.
A drawing of a typical battleship AA battery clearly showed a Yamato class battleship, but that must not have been evident at the time. Main and secondary gun calibres were not given, but the ship clearly had two main battery turrets forward and one aft, plus four secondary battery mounts in diamond arrangement. The diagram showed three AA guns (actually twin 12.7cm) on each side, numbered odd to starboard and even to port. Also on each side were two ‘concentrations’, each apparently corresponding to a pair of light anti-aircraft mounts, which were controlled together: one each at the ends of the row of medium AA guns. Another machine gun mount was on each side forward of the middle AA gun, for a total of ten machine gun mountings. All were mounted inboard of the medium-calibre guns.
A US Navy evaluation of Japanese AA fire in mid-1944 was that medium-calibre guns were being used for barrage rather than aimed fire. Most aircraft were being damaged by guns in the 20mm to 40mm class, the 25mm Hotchkiss being the most effective. Guns of 20mm to 40mm calibres had caused three times as many casualties as those of heavier calibres and six times as many as many as guns of lighter calibres. That was contrasted with US experience in which 5in guns had overtaken the lighter weapons in lethality. A captured document gave ranges to open fire for various calibres: 9900 yds for the 12cm (4.7in), 7700 for the 3in, 6600 yds for the 8cm, 2750 for the 25mm, and 2200 yds for the 13mm. All but the last were in line with ranges at which the British and the US Navy expected fire to become effective; the 13mm figure was more than twice that adopted by the Allies. It seemed that the Japanese were relying on a course and speed sight (like the Le Prieur sight of the 25mm gun and its director) to an unrealistic degree. The same document stressed the need to conserve ammunition, hence to limit the number of rounds fired at any one target. Limits given were six rounds for a 12cm gun, ten for 8cm, and one magazine (fifteen rounds for a 25mm gun) for machine guns. Automatic weapons were not to fire at retiring targets (a policy also followed by the Allies). The severe restriction on numbers of rounds to be fired reflects production problems even before Japan began to suffer strategic bombing. The figures were far below the RPB estimated for US guns.
After Midway the Imperial Japanese Navy decided that its Type 94 fire-control system was inadequate even with planned improvements, so work began on a new Type 3 (1943) system. Like Type 94, it had its rangefinder in the director, which was arranged to insure that layer, trainer and control officer were all observing the same target. Like many wartime British systems, it had scooter control for rapid slewing by the control officer. Very rapid development, and many system features, suggest that Type 3 was inspired by British systems such as the FKC, details of which were probably captured at Singapore. Like the British systems, Type 3 worked in terms of plan motion, the target being handled as though it was flying at constant altitude. Thus, unlike Type 94, Type 3 used rectangular co-ordinates. Also like the British systems, this one included a height plot intended to allow an operator to estimate aircraft height from a scatter of observed points. Unlike British systems, the director was sufficiently stabilised (by leveller and cross-leveller, not gyros) that it was expected to provide accurate bearing data. The Japanese later said that Type 3 was designed to provide rapid solutions. Initial inputs were estimated target course and speed (as in British systems). Unlike Type 94, Type 3 worked in rectangular co-ordinates, decomposing target speed into across and along components. To avoid the use of three-dimensional ballistic cams, it employed a British-style roller on which firing table data were engraved. The computer turned the roller, and an operator found the appropriate tangent elevation on it, sending it to the guns by means of a follow-up. A similar roller was used to enter wind corrections. Ballistics could be changed simply by replacing the rollers. Type 3 was never completed, although manufacture of a prototype was well underway at the end of the war. It was not related to the Type 3 developed for use ashore.
There was also an attempt to produce a dual-purpose destroyer system to control 12.7cm/50 guns. This Type 2 (1942) system replaced pre-war LA fire-control systems. Unlike Type 3, it entered service, but was never considered satisfactory for HA fire. The Japanese described it as grossly over-complicated, because its designers refused to compromise by emphasising either HA or LA fire (i.e., large or small angular rates). Instead, the same mechanism was used for both high and low angles, with change-over gears and clutches to shift function. Change-over required a complicated lining-up procedure. The associated Type 2 director was fully enclosed and trained hydraulically, but the optics were not cross-levelled (director outputs were adjusted for cross-level). On top it carried a 3m rangefinder which could train independently. In addition to the usual pointer and trainer it carried a target inclination operator (Japanese surface fire-control systems included elaborate inclination devices). The computer maker, Aichi, considered the associated Type 2 computer the most complicated it had ever made. Prediction was based on rate integrators. The computer used a three-dimensional cam to correct LA elevation to super-elevation for HA fire.
By 1944 there was an urgent requirement for a radar director to replace the Type 2 director; the result was the Type 5 (1945) director, which was intended as a minimum modification to Type 2 for destroyers and light cruisers. The Japanese described it as a means of blind fire, but that was not true in Western terms, since their radars did not provide good enough bearing and elevation data. Type 5 never entered service.
The standard Type 95 machine-gun director was modified with scooter control (probably based on British technology acquired when Singapore fell). By the end of the war the associated ring sights provided for target speeds of 900, 800 and 700km/hr (900km/hr equated to 492kts). Given production problems, a simplified version was produced, designated Type 4 (1944) Mod 3. It had range rings only for 800 and 700km/hr (800km/hr is 437kts), with a central area to be used for speeds of less than 600km/hr (328kts).53 Because the new device was simpler, it was available in larger quantities, and it could be used more extensively, and also ashore. Initially it was intended for 12cm rockets (see below) in addition to 25mm machine guns, but use was later extended to the war-built Matsu and Tachibana class escort destroyers.
Massive anti-aircraft rearmament began in the spring of 1944. The two superbattleships had their wing 6.1 in anti-destroyer mounts replaced by anti-aircraft weapons. Many 25mm guns were added. For example, in the superbattleships the original 25mm mountings were in closed shields to protect them from the blast of their 18.1in guns. The new mountings were the standard unshielded type. The big fleet destroyers had their after superfiring twin 5in guns replaced by triple 25mm guns. Note that, unlike fleet destroyers, the Matsu and Tachibana class escort destroyers all had 12.7cm/40 guns, which were truly dual-purpose.
Two new weapons were deployed. After a short development programme, 12cm anti-aircraft rockets were deployed in 28-round launchers on modified 25mm machine-gun mounts, controlled by standard 25mm machine-gun directors. These launchers were installed on board the battleships Ise and Hyuga and on board several carriers including Zuikaku. These shrapnel incendiary weapons were used at Leyte Gulf, but results were not recorded. The Japanese did say that they valued them as a deterrent and as a way of increasing anti-aircraft firepower at relatively low cost – much as the Royal Navy had adopted rocket weapons in 1940.
The second new weapon was the Model 3 incendiary anti-aircraft shell, which was fired by low-angle guns up to and including the 18.1in guns of the two superbattleships. Shells were filled with steel tubes containing an incendiary mixture. The shell was burst by a mechanical time fuse, the tubes igniting about half a second later, burning for 5 seconds. An alternative Model 4 was phosphorus-filled. Much effort was directed at production of these shells during the run-up to the Guadalcanal campaign. Gunnery officers considered these shells more effective than the usual common shells when fire was directed at approaching targets, because the tubes and fragments formed a cone beyond the point of burst. The post-war US view was that the officers were misled by the impressive appearance of bursts; the projectiles were apparently ineffective. Among other problems, the ballistics of the special shells was different from that of standard HE. Moreover, the shells should have been burst higher than HE shell, because shrapnel drops as it is ejected by the shell. Gunnery officers were given special ballistic charts and cards giving the necessary corrections. In many ships, some turrets were loaded with HE and some with incendiary shrapnel, to be prepared to engage either approaching or retiring targets.
There was also an attempt to improve the performance of anti-aircraft guns by improving and streamlining the shells. By the end of the war, tests had been completed on the destroyer 5in shell, the standard 4.7in shell, and the 3.9in shell, of which the 4.7in had gone into production.
Under post-war interrogation, the Japanese professed themselves satisfied with their anti-aircraft weapons. Few records of shipboard performance had survived, so most naval records were of the air defence of Japan itself. The subject is complicated further by the fact that, during and after the Bougainville Island engagement, the Imperial Japanese Navy was extremely short of ammunition. As a result, it shot down many fewer aircraft. For example, 25mm machine guns were limited to ten rounds per plane against diving targets, fire being held until the aircraft closed to 1000 metres. The Japanese claimed that US aircraft were so predictable that such figures were adequate, but it turned out that they grossly overclaimed aircraft shot down. The one ship figure which emerged in interrogation was that the carrier Zuikaku, armed with three twin 12.7cm guns and sixty to seventy machine guns required 150 RPB with her 12.7cm and 1000 RPB with her 25mm at ranges of 1000 to 2000m in the South Seas Battle (presumably Philippine Sea). These were not far from generally accepted figures, which may represent hoped-for rather than achieved standards. The Japanese also stated that effective range for the 12.7cm anti-aircraft gun was 8000m and below 3000m altitude, and for the 25mm machine gun, 2000m range and 1000m height (1500 RPB). Attempts were made to predict the effectiveness of various weapons, but they were not backed by operational data of the sort used by the US Navy. Other remarks made under interrogation were that no planes were claimed by 10cm and heavier batteries for ranges beyond 8000m, and the best results were obtained at 4000m and below. For medium ranges between 4000 and 7000m, the 10cm high-velocity gun was considered the best medium-calibre weapon. No kill claims were made for ‘jinking’ targets.
