Later, the Germans hosted the entire crew at Château de Trévarez before sending them by special train to Paris. While his crew enjoyed the City of Light, Kinashi traveled to Berlin to be decorated with the Iron Cross by the Führer. As the Japanese relaxed, their hosts removed four outdated anti-aircraft guns from their submarine and replaced them with heavier 37mm Krupp anti-aircraft guns and one 20mm Mauser. In addition, a HWK 509A-1 rocket motor that was used on the Messerschmitt Me-163 Komet rocket interceptor and a Jumo 004B axial-flow turbojet used on the Messerschmitt Me-262 jet fighter were loaded aboard. I-29 also received drawings of the Isotta-Fraschini torpedo boat engine, a V-1 buzz bomb fuselage, acoustic mines, bauxite ore and mercury-radium amalgam. There is also evidence to suggest that I-29 carried a quantity of U-235 uranium oxide, which after refining could have been used in an atomic bomb. Two officers were entrusted with blueprints of the Me-163 and Me-262 jet fighters and plans for rocket launch accelerators. They also received plans for a glider bomb and radar equipment. Finally, 20 more Enigma coding machines were stuffed aboard. Loaded with vital military plans and hardware, I-29 departed Lorient on April 16.

On June 11, the sub passed I-52 in the South Atlantic. The submarines did not communicate, but Kinashi picked up some German radio traffic addressed to I-52. Eighteen days later, I-29 entered the Indian Ocean and on July 13 rendezvoused with its air escort. The next day the sub passed through the Straits of Malacca and arrived safely at Singapore. At the former Royal Navy base I-29’s passengers disembarked with their plans and documents and proceeded by air to Japan. Most of the scientific cargo, however, remained aboard.

Anxious as to the exact whereabouts of the Japanese sub, Allied code-breakers were greatly relieved when they intercepted a signal that indicated its arrival in Singapore. Relief quickly turned to alarm when, on the heels of the first intercept, a message from Berlin to Tokyo was received that gave the details of the submarine’s strategic cargo. Aware of the frightening potential of what was being carried in I-29’s hold, Allied intelligence began working around the clock to devise a way to intercept the submarine before it could reach Japan.

Their prayers were answered on July 20, when Kinashi transmitted his proposed route for the last leg of the trip. The U.S. Navy’s Fleet Radio Unit, Pacific (FRUPAC) intercepted and deciphered the message. FRUPAC alerted Vice Adm. Charles A. Lockwood of I-29’s planned route, cargo and schedule from Singapore to Japan. The admiral then sent a top-secret signal to Commander W.D. Wilkins with that information. Wilkins commanded a wolf pack that included his own USS Tilefish as well as the submarines Rock and Sawfish. He was told that it was imperative that I-29 be intercepted and its cargo prevented from reaching Japan.

Unaware that his travel plans had been discovered, on the morning of July 22, Kinashi left Singapore. Three days later, he reported sighting a surfaced enemy submarine. The next afternoon, as I-29 was itself running on the surface through the western entrance of the Balintang Channel in the Luzon Strait, a lookout on Commander Alan B. Banister’s Sawfish sighted the sub. Banister fired four torpedoes at I-29. Three of them hit, and the Japanese submarine exploded and sank almost immediately.

Three of the Japanese crew were blown clear of their doomed boat, and one of them managed to swim ashore to a small Philippine island and report on I-29’s fate. The loss of the aircraft engines slowed the Japanese jet program, but their blueprints, flown to Tokyo, arrived safely. They were used immediately to develop the Nakajima Kikka (orange blossom) based on the Me-262 and the Mitsubishi J8MI Shusui (sword stroke) based on the Me-163.

Hope for additional technological treasures now rested on Commander Kameo Uno’s I-52, which had left Kure on March 10, 1944, while I-29 was making its dash for Brest. I-52 was a Type C-3 attack submarine, much slower than the more nimble B-1. In its hold, Uno’s submarine carried strategic metals including molybdenum, tungsten and 146 bars of gold packed in 49 metal boxes, as well as opium and some caffeine. I-52 also carried 14 passengers including engineers and technicians who were to study German weapon systems.

Upon arrival in Singapore, Uno added tin, rubber and quinine to his cargo. On April 23 he departed Singapore for Lorient via the Sunda Strait and the Indian Ocean. His plan was to sail around the Cape of Good Hope. To avoid Allied spotter planes, he traveled submerged during the day and only surfaced at night to recharge his batteries.

After passing the Cape of Good Hope and entering the South Atlantic, on May 15 Uno sent his first message to Germany. By this time the British and Americans had broken the military codes of both the Germans and Japanese. Allied intelligence intercepted and deciphered Uno’s reports to Tokyo and Berlin, including his daily noon position reports. When I-52 entered the South Atlantic, the code-breakers quickly relayed its position and predicted route to a U.S. navy antisubmarine-warfare (ASW) task force.

On June 6, Rear Adm. Hideo Kojima, Yokoi’s replacement as the Japanese naval attaché in Berlin, informed Tokyo and I-52 that the Allies had landed at Normandy. He advised I-52 that it might have to divert to Norway and instructed Uno to rendezvous with a U-boat on June 22. He then signaled Tokyo what I-52’s position would be. That radio transmission was intercepted, decoded and passed by special-intelligence Ultra signals to an American ASW group operating near the Azores.

On June 16, I-52 sent a coded transmission that its position was off West Africa. Captain A.B. Vosseller’s small escort carrier USS Bogue, with 14 aircraft, was ordered to hunt down and destroy I-52. After arriving in the area where the Japanese sub was supposed to meet the U-boat, Vosseller began around-the-clock flights of Grumman TBF Avenger torpedo bombers to search for the Axis submarines.

Although the skies were filled with American aircraft, Uno rendezvoused with Kurt Lange’s U-530 about 850 miles west of the Cape Verde Islands as scheduled. The Japanese commander welcomed a Lieutenant Schäfer on board to help navigate the last leg of his journey. Schäfer was accompanied by two petty officers who carried improved Naxos FuMB7 radar with them. During the exchange, the radar detector fell into the sea, but a Japanese seaman jumped in and retrieved it. About two hours after meeting I-52, U-530 submerged and headed for Trinidad, leaving the three German officers aboard the Japanese sub.

The day after his rendezvous with U-530, Uno, believing he could take advantage of a dark moonless night during stormy weather to cloak his location, traveled along the surface in order to reach the safety of a German-held port sooner. At about 11:40 in the evening, an Avenger piloted by Lt. Cmdr. Jesse D. Taylor picked up I-52 on its radar. Taylor dropped flares that illuminated the area around the submarine and then dropped two 354-pound bombs that barely missed its starboard side. Uno crash-dived. Although he had successfully evaded Taylor’s attack, he now had been located.

Taylor and his crew dropped sonobuoys over a square mile of ocean. The buoys transmitted any underwater sounds. Within minutes the Avenger’s crew heard I-52’s propellers in their headsets. Taylor maneuvered his plane into position and dropped a new top-secret Mark 24 Fido acoustic homing torpedo into the water. The torpedo was designed to home in on the propeller noise that had been picked up by the buoys. After a long wait, Taylor’s crew heard a loud explosion.

On June 24, another Avenger, this one piloted by Lieutenant William D. Gordon, arrived and dropped more sonobuoys. Gordon’s crew picked up the sounds of the damaged submarine’s propellers. At about 1 a.m., Gordon launched another Fido toward the submarine. Gordon and his crew soon heard the submarine breaking up underwater. The next day Janssen, one of Bogue’s destroyer escorts, found a large oil slick at the site of Gordon’s attack and salvaged more than a ton of raw rubber bales floating amid other debris on the surface.

Back at Lorient, a German ship stood by to escort I-52, and diplomats scheduled to return to Japan waited anxiously for their ride home. With them at the dock were tons of secret documents, drawings and strategic cargo, which included T-5 acoustic torpedoes, a Junkers Jumo 213-A engine used on the long-nosed Focke Wulf Fw-190D fighter, radars, vacuum tubes, ball bearings, bombsights, chemicals, alloy steel, optical glass and 1,000 pounds of uranium oxide. The Germans also intended to equip I-52 with a snorkel.

On August 30, the Kriegsmarine finally declared I-52 presumed sunk in the Bay of Biscay as of July 25. With the Americans closing in on the Home Islands and the final showdown of the war in the Pacific fast approaching, the IJN needed to husband every available resource. After the failure of I-52’s mission, it no longer tried to send its submarines to Europe.

Although the Yanagi missions are today little more than historical footnotes, they serve as an example of what the Axis powers might have been able to accomplish with greater coordination. The prospect of an exchange of nuclear weapon and jet engine technology was sufficiently alarming to Allied officials that they devoted considerable energy to finding and sinking the Japanese submarines before they could deliver their most important cargoes. It is fortunate that they did.

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This article was written by Bob Hackett and originally appeared in the October 2005 issue of World War II magazine. For more great articles subscribe to World War II magazine today!

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