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Type 095 - Integrated Electric Propulsion System (IEPS)

On the 30 May 2017, Chinese state broadcaster CCTV showed a new episode of its documentary program Focus Report. Chinese Navy Rear Admiral Ma Weiming made various remarks about developing a number of new systems over the years. Popular Science, on its Eastern Arsenal blog, wrote: " In the interview, he seemed to state that the PLAN is fitting its newest nuclear attack submarines with a “shaftless” rim-driven pumpjet, a revolutionary and silent propulsion system."

But in his CCVT interview, Radm. Ma does not discuss a rimdrive, but rather a third generation integrated electric propulsion system (IEPS) in regards to the next generation of Chinese nuclear submarines.

Steam turbines are most efficient when rotating at high speeds. For marine applications this is a problem since, on board a ship, the main goal is to rotate one or more propellers to create thrust to move the ship through the water. Counter to the high speed needs of the turbine, propellers are most efficient at lower speeds as they tend to cavitate (the quick forming and collapsing of bubbles) at high speeds. This decreased efficiency. Hence the eventual development of turbo-electric and geared turbine systems. In both of these systems, the turbine could run at its efficient higher speeds while the propeller ran at its lower speeds.

A turbo-electric propulsion system allows a submarine to eliminate the reduction gears that the driveshaft connects to, eliminating a significant source of radiated noise. The US Navy had barely adopted the oil-fired steam turbines for USS Nevada (BB 36) when another lead-ship of her class, Mexico (BB 40), was commissioned two years later. The new battleship featured the Navy’s first turbo-electric transmission, which eliminated the need for a gearbox to power high-speed turbines and the slow spinning propellers. Another advantage was increased energy efficiency; New Mexico provided enough power to run the ship’s electrical systems.

On Dec.14, 1927 the Lexington-class aircraft carrier, USS Lexington (CV 2) was commissioned. Originally designed as a battlecruiser, Lexington sported the bulbous bow that reduced water resistance, supported the forecastle and reduced bending stress on the hull, allowing the ship to reach speeds of 34 knots (38 mph). The carrier’s four propeller shafts were driven by eight electric motors powered by four turbo-electric generators. Two years after her commissioning, Lexington’s turbo-electric propulsion system provided supplemental electricity to the city of Tacoma, Washington during a drought between late 1929 to early 1930.

Turbo electric drive is slower. The Americans experimented with it with the Tubilee in the 60s and later Glenard P Lipscomb in the early 1970s but the speed penalty always ruled it out. The French never had the speed requirements of the US, hence turbo electric drive was a technologically much easier way to achieve quietness and with SSBNs in particular speed was not much of an issue. Turbo electric drive was clearly incompatable with the speed requirement of Los Angeles. Later developments like natural circulation [the ability for a reactor to circulate coolant without pumps at low speeds using convection] was used at low speeds on Trafalgar, Los Angeles, Akula, Sierra, providing silence at low speed.

Natural circulation clearly held promise to the Americans, vindicated in the all natural circulation reactor of the Ohio. Its use in Los Angeles allowed very quiet, sprint, drift operations. And Seawolf which boasted the previously low speed natural circulation range to over 20 knots. Although turboelectric drive reduced noise of gearing, improvements in manufacturing tolerances meant gear teeth could be made finer and with it, the noise of gearing much reduced. Thus the appeal of turbo electric (removing loud gearing) was diminished. The Soviets estimated that the Delta IV could not match Ohio in covertness largely because they could not making gearing to such fine tolerances as the Americans.




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