Type 93 torpedo

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The Type 93 was a 610 mm (24 inch) diameter torpedo of the Imperial Japanese Navy. Called the "Long Lance" by US sailors (a nickname attributed to Samuel E. Morison, a historian who spent much of the war in the Pacific theater), it was the most advanced torpedo in the world at the time.

A Type 93 torpedo modified into a Kaiten manned torpedo, Tokyo Yasukuni War Memorial Museum.

The Type 93 was quietly developed in Japan in the 1930s. At the time, the most powerful potential enemy of the Japanese Navy was the United States Navy's Pacific Fleet. US doctrine, presuming a move by Japan against the Philippines (then a US possession), called for the US battle line to fight its way across the Pacific Ocean, relieve or recapture the Philippines, and destroy the Japanese fleet. Since Japan possessed fewer battleships than did the United States, the Japanese navy planned to use light forces such as cruisers and destroyers to whittle down the US fleet in a succession of night actions. After US numbers were sufficiently reduced, Japan would commit her own fresh and undamaged battleships to finish off the US remnants in a climactic engagement.

The only naval weapon of the time that could do material damage to battleships - other than the guns of other battleships - was the torpedo. Consequently, the Japanese Navy invested heavily in developing torpedoes. Japan's research focused on using compressed oxygen instead of compressed air for its propulsion oxidizer, feeding this into an otherwise normal wet-heater engine. Air is only about 21% oxygen, so a torpedo using compressed oxygen instead of air would hold about 5 times as much oxidizer in the same size tank. This meant that the torpedo could travel further and faster. Additionally the uncombusted air, principally nitrogen, bubbled to the surface and left a trail pointing back at the launcher. With oxygen, the gas was almost completely burned and left an almost invisible bubble trail.

However, compressed oxygen is more dangerous to handle and it required lengthy testing and experimentation for it to be possible to use operationally. Finally it was discovered that by starting the engine with compressed air and gradually switching over to pure oxygen, engineers were able to overcome the uncontrollable explosion that hampered its development. To conceal the use of oxygen, the oxygen tank was named Secondary Air Tank. It was first deployed in 1935.

Type 93, "Long Lance" torpedo, on display outside U.S. Navy headquarters in Washington, DC, during World War II. This torpedo was recovered from Point Cruz on Guadalcanal.

The Type 93 had a maximum range of 40,000 meters at 38 knots with a 1,080 lb (490 kg) warhead. By contrast, the standard US destroyer-launched torpedo of World War II, the Mark 15, had a maximum range of 15,000 yards (13,500 meters) at 26.5 knots, or 6,000 yards (5,500 meters) at 45 knots, with a 825 lb (375 kg) warhead. Too large to fit in the standard 21-inch torpedo tubes on submarines, the Type 93 was usually launched from 24-inch (610mm) tubes mounted on the decks of surface ships, although some submarines also had deck-mounted launchers.

The Japanese Navy outfitted many of its destroyers and cruisers with the Type 93 torpedo. The long range, speed, and heavy warhead of the Type 93 gave these warships a formidable punch. Most also carried reloads and equipment for rapidly inserting them into the tubes - a practice unique among navies of the era.

In early battles, Japanese destroyers and cruisers were able to launch their torpedoes from over 20,000 metres out at unsuspecting Allied ships that were attempting to close to gun range, expecting torpedoes to be fired at less than 10,000 metres, the typical range of that era. The losses sustained in such engagements led to a belief among the Allies that the torpedoes were being fired from submarines operating in concert with the surface ships but at much closer ranges. On rare occasions, the very long range of the torpedo caused it to strike a ship that was far behind the intended target. The Type 93's capabilities were not recognized by the Allies until one was captured intact in 1943.

A 17.7 inch (450 mm) version designated the Type 97 torpedo was later developed for use by midget submarines, but it was not a success and was replaced operationally by the Type 91 torpedo. A 21 inch (533 mm) version for use by submerged submarines was designated the Type 95 torpedo and was highly successful, but did not result in the advantage it might have owing to the Japanese tactic of using submarines to engage warships rather than merchantmen.

The Type 93 was not without faults. They were highly unstable compared to the compressed air versions and a single explosion from one was enough to sink destroyers or heavily damage cruisers that carried them. As air raids became common, captains of destroyers under attack were faced with the decision of whether to ditch the torpedoes to better survive the air attack, or carry them to have much better odds against heavier or outnumbering opponents in surface battles. In surface battles, by the time the deck-mounted torpedo tubes were reloaded ready to launch a second wave of torpedoes, Allied ships were close enough to be able to use their torpedoes as well and the advantage of reloads was reduced. The reloading systems were not mechanically perfect causing occasional jams to become a hazard.