Defense Technology International - April 2008 - (Page 42) ENGINEERING SUBMARINE more energy-e cient at partial loads. A loitering submarine using less than 30-40% of the HTS motor’s potential shaft power will consume far less energy than one with legacy motors. This has a significant impact on a submarine’s indiscretion rate, reducing the amount of time needed for snorting. HTS motors are also acoustically quieter and achieve power densities previously available only by direct shafting to a nuclear-driven steam turbine. The U.S. Navy is testing an HTS motor for installation on the third Zumwaltclass destroyer (DDG 1002). American Superconductor (AMSC), a strategic partner of Northrop Grumman, has developed the 36.5-megawatt motor. AMSC delivered a 5-megawatt motor to the U.S. Navy in 2003, and is working on concepts for submarines. Matching the development of motor and generator technology are major improvements in energy-storage batteries. Lithium-ion (Li-ion) batteries o er four times the energy density of lead-acid batteries and better cycle durability, allowing for more frequent charging, draining and recharging. Yardney Technical Products of Pawcatuck, Conn., has developed a warship-ready, 1.2-megawatt-hr. Li-ion battery for Northrop Grumman’s Advanced SEAL Delivery System midget submarine. The battery weighs 8 tons, compared to the 380 tons of lead-acid batteries on the Victorias that store around 15 megawatt-hr. Siemens is also developing HTS marine applications and will have a 4megawatt generator available by 2010. Rolls-Royce is marketing a molten salt battery using the South Africandeveloped Zebra (Zeolite Battery Research Africa) technology that is 2.7 times more energyefficient than lead-acid batteries. Molten salt batteries are self-contained units that don’t require the maintenance and emissions monitoring of lead-acid batteries. Zebra batteries have been developed to replace lead-acid batteries in SSKs and are used in NATO’s Submarine Rescue System. The power and energy e ciencies of HTS and advanced batteries provide AM ER A IC N SU PE ON RC DU OR CT IN C. N CO CE PT surplus space for AIP systems. While they don’t have the power density to offer more than sustained loiter speeds, AIP systems can be important tools for submarines facing a “hold down” by overhead threats. Despite power-density improvements, future diesel-electric submarines will still need to snort. Snorting compromises stealth by increasing broadband acoustic emissions and exposing a mast above the waterline. E orts are ongoing in Australia to counter the broadband acoustic signature of the Collins class. This capability has been recognized by Singapore, which acquired Australian signature-management technology for its Challenger-class SSK boats (formerly the Swedish Sjoormen-class). Reducing noise emitted by diesel motors and active cancellation has reduced the snorting signature of these submarines. Radar cross-section (RCS)-reducing designs are also applied to above-water elements of masts. The U.S. Navy’s Virginia-class Kollmorgen Photonics Mast System has a geometrically shaped casing that reduces RCS against periscopehunting radars. Bringing together HTS and advanced batteries, which will be proven and mature technologies by the mid-to-late 2010s, will produce an SSG boat that can compete with SSNs in many operations. With Virginia-class technology as the source of the hull and mission systems (identical CMS, sensors and weapons), an SSG sketch design could have the following characteristics: length 260 ft., beam 34 ft., displacement 4,250 tons surfaced (with 10% reserve of buoyancy), 12-megawatt motor, three 4-megawatt diesel generators and a 60-megawatthr. battery. With today’s Li-ion battery and HTS performance, the boat would have a sprint speed of 25-27 kt., sustainable for 5 hr., and transit speed of 16 kt. over 10,000 naut. mi. while submerged (30% indiscretion). With a tactical speed of 4 kt., such a boat would only need a daily indiscretion of less than 2.5% (around 30 min.) to keep the battery at full charge. It could stay submerged at tactical speed over eight days without snorting, if necessary. Northrop Grumman and U.S. politicians have been leading the push to the Australian government for the new power and energy technology with the intent to establish the U.S. as the leading exporter of diesel-electric submarines. British ship designer BMT Defense Services is also working on SSG concepts, and will unveil a design incorporating HTS and advanced batteries at the Underwater Defense Technology Europe conference and exhibition in Glasgow, Scotland, in June. The technology is driving the consideration of Australia’s replacement SSG, but it may also find a home in the U.S. Navy and the Royal Navy. Both services are struggling to sustain their SSNs amid growing costs and budget limitations. Mixing the fleet with SSGs could be a way to resolve this dilemma without significant capability compromises. Traditional SSK users are also looking at longer ranges due to a move from regional to global security. Long-range, high-speed diesel-electrics may soon be the fleet submarine of choice. I High-temperature superconductor motors yield space, weight and energy savings over legacy motors. AviationWeek.com/dti 42 DEFENSE TECHNOLOGY INTERNATIONAL APRIL 2008 http://AviationWeek.com/dti
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