Type 93 Torpedo - History and Development

History and Development

The Type 93's development (in parallel with a submarine model, the Type 95) began in Japan in 1928, under the auspices of Rear Admiral Kaneji Kishimoto and Captain Toshihide Asakuma. At the time, by far the most powerful potential enemy of the Japanese Navy was the United States Navy's Pacific Fleet. The U.S. Navy's doctrine, presuming an invasion by Japan of the Philippines (an American commonwealth at that time), called for the battle line to fight its way across the Pacific Ocean, relieve or recapture the Philippines, and destroy the Japanese fleet. Since the IJN had fewer battleships than the U.S. Navy, it planned to use light forces (light cruisers, destroyers, and submarines) to whittle down the U.S. Navy's fleet in a succession of minor battles, mostly at night. After the number of American warships were sufficiently reduced, IJN would commit its own presumably fresh and undamaged battleships to finish off the U.S. remnants in one huge climactic battle. (This was essentially what the U.S. Navy's "War Plan Orange" expected.)

The Japanese Navy invested heavily in developing a large, heavy, and long-range torpedo, the Type 93. Torpedoes were one of the few naval weapons enabling small warships, such as destroyers, to damage battleships. IJN torpedo research and development focused on using highly-compressed oxygen instead of compressed air (which is about 21% oxygen) as the fuel oxidizer in the torpedo's propulsion system, feeding this into an otherwise normal wet-heater engine burning a fuel such as methanol or ethanol. Pure oxygen provides five times as much oxidizer in the same tank volume, increasing speed and range; and the absence of inert nitrogen reduced the gases emitted to carbon dioxide, which has significant solubility in water, and water vapor, much reducing the tell-tale bubble trail.

Compressed oxygen is dangerous to handle and required lengthy research and development, and extra training for the warship's torpedomen, for safe operational use. Eventually, IJN weapons development engineers found that by starting the torpedo's engine with compressed air, and then gradually switching over to pure oxygen, they were able to overcome the problem of uncontrollable explosions which had hampered it before. To conceal the use of pure oxygen from the ship's crew and any potential enemy, the oxygen tank was named the secondary air tank. The pure oxygen torpedo was first deployed by IJN in 1935.

The Type 93 had a maximum range of 40 km (21.6 nmi; 24.9 mi) at 38 knots (70 km/h; 44 mph) with a 490 kg (1,100 lb) high explosive warhead. Their long range, high speed, and heavy warheads provided a very formidable punch in surface battles. In contrast, the U.S. Navy's standard surface-launched torpedo of World War II, the 21 in (53 cm) Mark XV, had a maximum range of 15,000 yd (14 km; 7.4 nmi) at 26.5 knots (49.1 km/h; 30.5 mph), or 6,000 yd (5.5 km; 3.0 nmi) at 45 knots (83 km/h; 52 mph), with a significantly smaller 375 kg (830 lb) warhead. Torpedoes of other Allied nations did not have longer range. The Type 93 was launched from 61 cm (24 in) torpedo tubes mounted on the decks of IJN destroyers and cruisers; some Japanese destroyers, unlike ships of other navies, mounted their banks of torpedo tubes in turrets offering protection against splinters, and had tube loaders. IJN armed nearly all of its cruisers with Type 93 torpedoes.

In the early surface battles of 1942–43, Japanese destroyers and cruisers were able to launch their torpedoes from about 20 km (11 nmi; 12 mi) at the unsuspecting Allied warships attempting to close to gun range. Allied warships expected, that if torpedoes were used, they would be fired from not more than 10 km (5.4 nmi; 6.2 mi) away, their own typical torpedo range. The many torpedo hits suffered by Allied warships in such engagements led their officers to believe torpedoes had been fired from undetected Japanese submarines operating in concert with the surface warships. On rare occasions stray very long-range Type 93s struck ships much further away than their intended targets, leading the Allies on occasion to suspect their ships had been mined. The capabilities of the Type 93 went mostly unrecognized by the Allies until examples were captured intact in 1943.

A 17.7 in (450 mm) version, the Type 97 was later developed for midget submarines, but was not a success, and was replaced operationally by the Type 91. A 21 in (53 cm) version for use by a few IJN submarines was designated the Type 95, and it was ultimately successful.

A disadvantage of the Type 93 was that it was far more likely to detonate due to shock than a compressed-air torpedo. The explosion from one Type 93, with its heavy warhead, was usually enough to sink the destroyer, or heavily damage the cruiser, carrying it. As American air strikes against IJN ships became more common, the captains of destroyers and cruisers under air attack had to decide whether to jettison torpedoes to prevent them from being detonated during the attack.

In one instance heavy cruiser Chikuma was fortunate to have jettisoned her Type 93s just before being hit by bombs from several USN dive bombers at the Battle of Santa Cruz Islands. During the Battle off Samar (in the eastern Philippines) a five-inch (127 mm) shell from destroyer escort USS Samuel B. Roberts struck heavy cruiser Chokai. While in most circumstances a shell of this size would not seriously damage a heavy cruiser, this shell detonated the cruiser's torpedoes, disabling her rudder and engines; she was scuttled the next day.