The transition from diesel to nuclear powered propulsion systems in submarines was not only about efficiency as it had been in large surface vessels like carriers — it was also a significant leap ahead in stealth capability. Submarines traveling through enemy waters maintain careful sound discipline, aware that the noise produced by the vessel can carry for miles beneath the surface of the ocean and allow for detection.
Nuclear propulsion is far quieter than the diesel power plants employed by many national navies to this day, but the systems still utilize a steam powered mechanical drive line not unlike those employed in a diesel-powered vessel. In America’s fleet of Ohio class submarines, for instance, the nuclear power plant generates heat, turning water to steam which then turn turbines to produce electricity as well as to provide propulsion through a series of reduction gears. These reduction gears transfer the high-RPMs produced by steam turbines into the appropriate shaft revolutions for the speed set by the bridge — they’re also rather noisy.
In the new Columbia class of submarines, however, this mechanical drive is set to be replaced by electric shaft motors — eliminating the noise produced by metal gearing engaging and disengaging. Much like many electric cars don’t have traditional transmissions like you might find your old Camaro, these electric drive submarines will be able to utilize the electricity produced by the steam turbines to propel the ship at whatever rate is appropriate. This design not only eliminates many noisy moving parts, but it also frees up a great deal more of the power produced by the onboard nuclear power plant for other shipboard uses.
According to a paper written in 2001 by Joel Harbour for the Naval Postgraduate School, an electric drive system (which at the time was only just beyond theoretical for applications like an attack submarine) would offer a far more efficient use of the power produced aboard the vessel. Mechanical drive lines require as much as 80% of the electrical output produced by the submarine at all times, reducing the ship’s speed via gearing, not a reduction in energy transferred into the drive line itself. An electrical motor could utilize only as much power as necessary for propulsion while allowing the remainder to be diverted to other applications. Harbour explained,
With an electric drive submarine, the installed reactor power of the submarine is first converted into electrical power and then delivered to an electric propulsion motor. The now available electrical potential not being used for propulsion could easily be tapped into for other uses.”
In the 1990 thriller “The Hunt for Red October,” the Russian Navy fielded a new submarine with a “caterpillar drive” that used nuclear-produced electricity to create magnetic fields that pushed water to the rear of the vessel, propelling it forward without any moving parts in the drive line. While China has since claimed to have actually put this theoretical concept into practice on small applications, the U.S. Navy’s Columbia class of submarines will likely be the closest approximation to this electrically powered “silent drive” in use anywhere in the world when they first take to the seas.
A stealthier submarine armed with the Navy’s forthcoming (and recently revealed) Sea Dragon anti-ship missiles could provide the Navy with a powerful weapon against rapidly developing naval powers like China, while also giving the U.S. the advantage once again in its efforts to keep Russian submarine activities at bay. Russia publicly announced a training exercise in which they dispatched an unknown number of nuclear attack submarines to U.S. Navy ports on the East Coast last year, where they loitered apparently undetected for some time before returning home. In response to the Russian Navy’s activities in the Atlantic, the U.S. Navy announced that they would be standing up the Second Fleet once again — tasked specifically with countering Russia’s aggression in the region.
The Columbia class of submarines are slated to enter operational service in 2028, and replace America’s aging fleet of Ohio class submarines in the years thereafter.
Image courtesy of the Dept. of Defense