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New Attack Submarine: Kurohana



The New Attack Submarine (NSSN) –codenamed Kurohana (Black Flower)- is an advanced stealth, multi-mission, nuclear powered attack submarine for blue ocean anti-submarine warfare and for littoral (shallow or brown water) operation. The NSSN will be quieter than anything the U.S. or the Soviet Union can build, at speeds higher than that of the Oyashio class,  and at costs two-thirds less than a Tenshio class.


Much of the security of the Japanese Empire lies in the hands of the IJN submarine force. The original and ultimate stealth weapon, modern Japanese submarines have preserved Japanese naval dominance in the western Pacific for more than forty years. The current primary attack submarine is the Kuroshio class. The first of the Kuroshio class boats was commissioned in 1976, with twenty-one more planned in the series to follow. For the last 20 years, the Kuroshio class has remained unmatched in stealth capabilities. But three of these submarines have been decommissioned and the rest are scheduled to follow around 2006 at a rate of four a year as they reach the end of their thirty year service life.


By 2013, the attack submarine fleet will fall below the numbers thought to be the minimum consistent with future national security requirements. In the eighties, the Tenshio Attack Submarine was born on the architect’s table, designed to be a superior replacement to the Kuroshio class. It was bigger, faster, had more firepower, more advanced tracking and surveillance gear, and was widely reputed to be "as quiet at cruising speed as a Kuroshio class at the pier." There was a catch, however. After years of testing, tooling up, and construction it became evident that the Tenshio class boats would exceed cost expectations. Indeed, the prohibitive unit cost of approximately 400 billion yen effectively limited the Tenshio class to a run of three units. The dilemma: how to design and construct a fleet of high-tech attack submarines in numbers sufficient to ensure the security of the country and still not squander regional stability.


Then the Kurohana appeared. Tenshio class quieting is being incorporated in a smaller hull while military performance will be maintained or improved. With a focus on the littoral battlespace, the Kurohana will have improved magnetic stealth, sophisticated surveillance capabilities, and Special Warfare enhancements. The Kurohana is being engineered for maximum design flexibility. Its responsiveness to changing missions and threats, and the affordable insertion of new technologies, ensures that it will be a potent warship well into the 21st century. Integrated electronic systems with Commercial-Off-The-Shelf (COTS) components facilitates state-of-the-art technology introduction throughout the life of the class and avoids unit obsolescence.


Revolutionary cost controls have been arranged to build this new class at reasonable prices. In 1995, the Diet directed the Imperial Navy to submit a plan which would allocate the first four New Attack Submarines to the Mitsubishi Boat Company and the Hitachi Shipbuilding on an alternating basis. Mitsubishi Boat Corporation and Hitachi subsequently announced that they had reached a teaming agreement to cooperatively build the Imperial Navy's new class of attack submarines (NSSN). The teaming was later approved in the Diet.


Based on the demonstrated success of the design/build process, the Kurohana design will be far more mature than other submarine designs at the beginning of construction. This greatly reduces the risk of design changes during ship construction -- traditionally, a major cause of cost increases. The nuclear reactor core has been designed to power the submarine throughout its entire operational life without refueling. This will result in a factor of two decrease, compared to previous classes of submarines, in the amount of spent nuclear fuel requiring disposal. Not having to refuel will also result in multi-million yen cost avoidance for each submarine. Powered by a Toshiba Pressure Water Reactor R10H, 2 turbine engines with one shaft and a pump jet propulser, the Kurohana will retain the high speed characteristics of the Tenshio class. The claimed speed is 28 knots dived. Additionally, this powerplant is possibly the quietest of its kind in the world. The noise level of the Kurohana is equal to that of the Tenshio, with a lower acoustic signature than the Soviet Improved Akula class and U.S. 4th Generation attack submarines currently under construction.


The Kurohana will be armed with a variety of weapons; carrying the most advanced heavyweight torpedoes, mines, Long Lance cruise missiles, and Unmanned Undersea Vehicles (UUVs) for horizontal launch. In addition, Long Lance missiles will be carried in vertical launch tubes. It also features an integral special lock-in, lock-out chamber for special operations and will host a special operations mini-sub.


The Kurohana will be the first Japanese submarine built with a visual imaging capability that does not require a mast to penetrate the pressure hull. The sail configuration houses two new photonics masts for improved imaging functions, an improved electronics support measures mast, a HF sonar array, and multi-mission masts that cover the frequency domain for full-spectrum, high data-rate communications. The sail is also designed for future installation of a special mission-configurable mast for enhanced flexibility and combat performance.


It's sonar system will include a 15-foot bow array, a HF array in the sail, and a UC-30 thin-line towed array which will improve detection of threats in shallow waters or in areas with high background noise. The plans could also call for a UC-61 towed array, a bow conformal array and/or a wide aperture array. Using modular hull-section design, the boats could be more easily configured for special operations, including the ability to carry 200 troops for covert action, in excess of 100 cruise missiles, or perhaps even the older M-IV missiles.


The submarine's Command Center will be installed as one single unit resting on   cushioned mounting points. The submarine's control suite is equipped with computer touch screens. The submarine's steering and diving control is via a four-button two axis joystick. Advanced design, state-of-the-art technology, multi-faceted operational capability; the Kurohana will continue a proud tradition and ensure Japan's dominance in the Pacific ocean.



Kurohana's  Characteristics:



115 meters


10.5 meters


7,700 tons


28+ knots


38 weapons, including Vertical Launching System and Special Operations Forces


Four 21-inch torpedo tubes

12 Vertical Launching System tubes

Long Lance Land-Attack Cruise Missiles

NL-84 Advanced Capability Torpedoes


Spherical Active/Passive Array

Light Weight Wide Aperture Arrays

UC-30 and future towed arrays

High-frequency Active Arrays


Internal (reloadable)

14 external (non-reloadable)


R10H pressurized water nuclear reactor

Steam turbine, single shaft