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SS-501 Soryu / 16SS / SS 2,900 ton Class

The Soryu class are the largest submaries Japan has operated since the end of World War II [the I-400 boats were larger]. At 84 m long and 4,200-tons submerged, Soryu is significantly larger than the original 11 Oyashio-class boats (81 m and 3,500 tons), in order to incorporate a Kockums Stirling air independent propulsion (AIP) system. With the Kockums Stirling AIP system the submarine doesn't have to surface to charge the batteries and thus increases the submerged endurance from days to weeks. Components for the AIP, which will enhance the vessel's stealth and special operations capability, were supplied by the Swedish company for assembly in Japan.

A follow-on to the Oyashio class, the design features higher automation (particularly in combat systems) and computer-aided X control planes. The revolutionary X rudder configuration was first developed by Kockums for the Gotland. The X rudder provides the submarine with extreme manoeuvrability and also enables it to operate very close to the seabed. The Soryu's hull is clad in anechoic coating and the interior features sound isolation of loud components. An anechoic coating has a major effect on the avoidance of detection. The hull is HY-80 alloy (the same as the USN's Los Angeles class SSN) steel. They are being built at the Kobe shipyard.

The first of three improved Oyashio-class attack submarines on order for the Japanese Maritime Self-Defence Force (JMSDF) was launched at Mitsubishi Heavy Industries' Kobe shipyard on 05 December 2007. It was formerly known as the "SS 2,900 ton" and the "16SS" project as it originated in the 16th year of the current Emperor's reign (2003).

The Stirling engine is a heat engine. Heat is produced in a combustion chamber separated from the actual engine. The heat is tranferred to the engine's working gas, operating in a completely closed system. The working gas forces the pistons in the engine to move, thus producing mechanical energy. In other words, all the Stirling engine needs is heat and it doesn't really matter how the heat is produced. It can come from anything that burns: oil, diesel, petrol or gas. Or the heat may come from the sun. Letting a concave mirror concentrate the sunbeams on a Stirling engine, driving a generator makes for the present the most efficient conversion of solar energy to electrical power. In addition, the Stirling engine is flexible, silent and practically vibration-free. All these characteristics open a wide field of applications.

A submarine disappears from sight when it dives, but it can still leave a trail of sound, heat, sonar reflections and magnetic anomalies. For the submarine to remain concealed, all of these signatures must be minimized. All emitted sound that can be detected by enemy hydrophones must be suppressed.

In the 1960s Kockums became involved in the development of a Stirling engine and in 1968 Kockums assumed responsibility for continued project planning. In the mid-1980s Kockums installed Stirling engines and a LOX (Liquid Oxygen) system from AGA Cryo aboard the French 500 ton civilian research submarine Saga, which has an operating depth of 600 meters and is fitted with a diving system for work at depths as deep as 450 meters. Intense R&D and the experiences gained from the Saga project paved way for the installation of a Stirling engine in the Royal Swedish Navy submarine Ncken in 1988. The submarine was placed in dry-dock and cut in two. A fully outfitted eight-meter AIP section was then inserted.

The AIP system of the SS-501 Soryu comprises the virtually vibration-free and silent Stirling engine. The fact that the submarine can operate at great depths for long periods of time is also an important stealth factor. The hydrodynamic design of the hull, rudder and propeller is of vital importance. The flow sound when the submarine travels through the water can disturb the submarine's own hydrophones and can also be heard by the enemy.

They will probably build around 5 or 6, the JMSDF is looking for a fleet of about 20 SSK's. The Japanese have an incredibly rapid turnover rate compared to international standards when it comes to her submarine fleet, with most boats being retired after around 20 years in service.

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