Ironically, the addition of the human pilot—turning the torpedo into a seaborne kamikaze weapon—appears to have made it less effective.

During the opening years of the Pacific War, Japan operated what was arguably the world’s most lethal naval torpedo: the Type 93 “Long Lance,” which consistently outperformed Allied torpedoes in range, speed, and destructive power. The capability of the Type 93 caught Allied commanders by surprise, forcing adjustments during early naval battles.

Over time, US forces improved their torpedoes, while industrial shortages inhibited Japan’s ability to construct more of its own. The broader war turned against Japan as well, and after losses at Midway, Tokyo grew increasingly desperate to restore a naval advantage. One shocking, but unsurprising, innovation was to add a cockpit and pilot to the Type 93, turning it into a suicide bomb. The resulting Kaiten, or “Turning of Heaven,” was less well known than Japan’s kamikaze pilots—and ultimately far less effective in combat.

How Japan’s “Long Lance” Revolutionized Torpedo Warfare

In the early 20th century, the Washington Naval Treaty limited Japanese battleship strength—restricting the Imperial Japanese Navy to three-fifths the size of the US Navy and the Royal Navy. Smarting at these restrictions, Japanese war planners sought asymmetric advantages as a way to dull the Allied advantage in tonnage. Accordingly, naval engineers invested heavily in torpedoes, which could give destroyers and cruisers the striking power to threaten much larger vessels. The Type 93 Long Lance emerged from this development program after years of experimenting in secret. 

What made the Long Lance special was its propulsion system, which used compressed oxygen instead of the conventional compressed air/steam system. Oxygen propulsion produced greater range and higher speed than air/steam, while allowing the torpedo to carry a larger warhead and produce almost no visible wake. With new possibilities available, Japanese engineers crafted an enormous torpedo, resembling a small submarine—measuring a whopping 30 feet long and weighing 3 tons. The warhead itself weighed over 1,000 pounds. Despite the massive weight, however, the Long Lance could travel for miles at speeds of up to 49 knots (56.3 mph). 

Japan was the only nation to field a compressed oxygen torpedo. Britain experimented with oxygen torpedoes but abandoned them after several fatal accidents. The US never bothered to adopt oxygen propulsion, citing the volatility (and extreme flammability) of pure oxygen. The Japanese overcame these safety issues through careful engineering: redesigning fuel lines, eliminating grease contamination, and using compressed air during startup before switching to oxygen. Japan kept these details, and its overall program, highly classified for years. 

As the Pacific War began in earnest in December 1941, the Long Lance shocked the Americans, sinking or damaging dozens of Allied warships in the first months of the war. Japanese tactical doctrine called for every destroyer and cruiser to carry the Long Lance torpedoes, with rail systems that allowed for faster reloading during battle. This facilitated repeated torpedo attacks in single engagements, and helped create Japanese night-fighting superiority early in the war.

The Torpedo Was Less Effective as a Manned Weapon Than an Unmanned One

After the Japanese setback at Midway, Tokyo searched for new ways to gain an edge over the strengthening Americans. Two young naval officers proposed inserting a cockpit into the Type 93 Long Lance to create a human-guided suicide weapon. Of course, accommodating a human pilot would require major redesigns to the weapon; even if that pilot was never expected to return from the mission, he would need to be able to see his surrounding environment and steer the torpedo mid-course, developments that were never an issue with the original Long Lance.

Once the redesigns were complete, the resulting “Kaiten” stretched to 54 feet in length, with a warhead of 3,000 pounds. The newly-minted human torpedo could travel up to 40 miles at 40 knots (46 mph). Pilots could view their surroundings through a small periscope and steer with basic controls before deliberately crashing into enemy ships, blowing themselves up in addition to the target. Despite the macabre nature of the weapon, Imperial Japan had no shortage of volunteers; hundreds of fanatical sailors joined the program.

Early Kaiten tests were plagued by unreliability and accidents, and dozens of test pilots drowned. By the end of 1944, the IJN began to employ it in combat, alongside traditional torpedoes. In November, a salvo of eight Kaiten torpedoes launched at the US Navy oiler USS Mississinewa successfully sank it, killing 66 American sailors. In July 1945, in the final weeks of the war, another Kaiten struck the USS Underhill destroyer escort near the island of Luzon in the Philippines, killing 113 sailors and sinking the ship.

Yet generally, the Kaiten failed to work. At that point in the war, industrial shortfalls in Japan meant persistent quality issues, and mechanical failures on the torpedo were very common. In particular, water leakages often disabled their batteries, leaving them unable to move and extremely vulnerable to American fire.

The United States calculated that it had lost 187 men to the Kaitens in their various successful strikes, while 106 Japanese Kaiten pilots died during the war—in addition to more than 150 support personnel and hundreds of other crewmen on the lost submarines carrying Kaiten torpedoes into battles. Ultimately, the program is little more than a footnote in military history, speaking to the desperation of Japan in the final year of the war.

About the Author: Harrison Kass

Harrison Kass is a writer and attorney focused on national security, technology, and political culture. His work has appeared in Tablet, City Journal, The Hill, The Spectator, and The Cipher Brief. He holds a JD from the University of Oregon and a master’s in Global & Joint Program Studies from NYU. More at harrisonkass.com.

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