OTHER SYSTEMS

Kurai Aerial Torpedo No.6/No.7

In April 1944, the UN initiated a design for an air dropped, anti-submarine torpedo. While not a tme guided weapon, what made the design unique was that the torpedo was winged and once in the water, entered a cir­cular pattern as it dived down. Work on the torpedo was carried out by the First Technical Arsenal Branch at Kanazawa under Com­mander Fukuba. The torpedo body was made of wood with the exception of the metal nose. The torpedo had no means of propulsion, either for flight or in the water. The wooden wings, each 1.5m (4.9ft) in length, were glued to the main torpedo body at a 20° upward angle. The wing span was approximately.9m (2.6ft) and the nose car­ried a 100kg (2201b) warhead and the total weight was 271kg (597 lb). The wood rudders were fixed at an 8° angle which, once in the water, imparted the counter-clockwise circu­lar path. The rudder was covered with a wood fairing to stop it affecting the freefall glide of the torpedo. Once the torpedo entered the water, the aluminium pin holding the fairing on sheared off and the fairing came free.

The first tests were carried out to evaluate the gliding properties of the torpedo. In all, forty drops were made of which fifteen were complete failures, the torpedo tumbling or spinning out of control. The wings were mod­ified according to the specifications given above, resulting in the Kurai No.6 which improved the gliding ability but not to a satis­factory level. Underwater testing was con­ducted by releasing the torpedo from a boat and the results showed a 17° dive angle at a speed of 5-6 knots. Colour dye released from the nose of the torpedo assisted the evalua­tors in determining how the weapon worked underwater. The maximum depth the tor­pedo could reach before the pressure over­came it was 100m (327ft). From the tests, it was determined that a form of gyrostabilisa – tion would be required.

The designated carrier plane for the Kurai was the Nakajima B6N1 and B6N2 Tenzan (known as Jill to the Allies). No modification of the plane’s torpedo rack was required. In operation, the pilot had to visually sight the target and release the torpedo from a height of 100m (327ft). An air spun vane would arm the weapon after it was released and the tor­pedo would go into a 20° downward glide. Once in the water, it would enter its circular dive with a diameter of 79m (260ft), making one revolution through a depth of 79m (260ft) after which it would continue to circle and dive until it was crushed under pressure or had struck the submarine before reaching 100m (327ft). A magnetic proximity fuse was to be used on the torpedo.

In all, 100 of the Kurai No.6 were built with many of them expended in testing. Kugisho constructed the rudders and the metal com­ponents (nose and wing braces) while the Marunimoko Company (located in Fut – sukaishi, Hiroshima Prefecture) produced the torpedo body and wings.

The poor results of the KQrai No.6 resulted in slight modifications to the design. The wing span was increased and the rudders made taller and set at 6° rather than 8°, and the nose was thickened to enhance the ability of the torpedo to penetrate a submarine hull. This new model was designated the Kurai No.7 and in January 1945, eleven test drops were made, but the weapon showed little improve­ment over the Kurai No.6. The end of the war brought the entire project to a close, the engi­neers having run out of time to solve the poor gliding performance and finalise the special fuse.

Two Kurai No.6 mock-ups were captured and shipped to the US for delivery to TAIC. What their fate was is unknown.