Kaiten (literal translation: "Turn the Heaven") were manned torpedoes and suicide craft, used by the Imperial Japanese Navy in the final stages of World War II. Research on the first Kaiten began in February 1944, followed on 25 July of the same year by the first prototype. By 1 August, an order for 100 units had been placed.The very first Kaiten was nothing much more than a Type 93 torpedo engine compartment attached to a cylinder that would become the pilot's compartment with trimming ballast in place of the warhead and other electronics and hydraulics. The original designers and testers of this new weapon were Lieutenant Hiroshi Kuroki and Lieutenant Sekio Nishina. In total six models of Kaiten were designed, Types 1, 2, 4, 5, and 6 were based on the Type 93 torpedo. Type 10 was the only model based on the Type 92 torpedo. Types 2, 4, 5, 6, and 10 were only manufactured as prototypes and never used in combat. Kaiten were designed to be launched from the deck of a submarine or surface ship, or from coastal installations as a coastal defence weapon. In practice, only the Type 1 craft, using the submarine delivery method, were ever used in combat. In late 1944, Type 2 prototypes were tested in dry-dock and in controlled submerged conditions; however, they never met a standard satisfactory for open water running.The Type 2 was fueled by diesel fuel oxidized by the product of the reaction between hydrogen peroxide and hydrazine hydrate catalyzed by potassium cuprocyanide; water was also used as a diluent in the combustion cycle. The engine was a custom design specifically created to satisfy the requirements of the Kaiten. It was of a U8 configuration and was in the aft compartment of the vessel. This compartment was open to the sea, with seawater used as coolant. The complete operation of the engine had to be simple for a pilot who was already under immense stress, so it was designed to be controlled by a single lever.The general construction of the Type 2 is substantially different than that of the Type 1. In the forward part of the vessel is the warhead, followed by the fuel, trimming and 2 steering air tanks. The forward part of the midsection is occupied by the 2,600 l (570 imp gal) hydrogen peroxide chamber. This cylindrical tank required a complicated construction of welding 5 ribs evenly spaced around the circumference, then coating the inside of the tank with tin, then asphalt and finally with a thick tin plating. Construction of each tank took on average 2 weeks and the skill required was very high.Behind the peroxide chamber is the pilot's compartment. This is much more complex with more controls than that of the Type 1. From fore to aft controls available to the pilot are: depth control gear, trim tank controls, electric switches, gyroscope, rudder control, engine control lever, gauges for various applications, warhead safety gear, engine starter, fuel flow control, and lights with their respective switches.In the aft quarter of the vessel is found the battery for the gyroscope and other electrics, the aft trim tanks and 2 hydrazine hydrate containers. These containers are made of steel lined with a rubber bag to prevent sea water (used to force the hydrazine hydrate to the engine) and the hydrazine hydrate from coming into contact with each other. Immediately behind these tanks is an auxiliary water tank, then the engine compartment.The engine compartment is always completely filled with sea-water, so it only has mechanical components. Immediately after the midsection is the generator, the engine, the reduction gears, and finally the propeller and control planes. The Type 2 had several improvements over the Type 1. It could stay relatively reliably on course with no pilot input and could withstand depths twice as deep as those of the Type 1.
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