Ohio Replacement Subs to Shift to Electric Drive
by Kris Osborn on September 27, 2013 DefenseTech
The U.S. Navy’s successors to Ohio-class submarines will feature an electric propulsion system, making them quieter and stealthier than today’s versions.
The technology for the ballistic-missile subs is being developed by the Navy and General Dynamics Corp. as part of the Ohio Replacement Program, Rear Adm. Richard Breckenridge told Military.com in an interview. Construction of the boats is set to begin in 2021, he said.
Unlike existing versions, which use mechanical propulsion technology, the replacement subs are designed to have an electric-drive system, Navy officials said. The technology still relies on a nuclear reactor to generate heat and create steam to power turbines, they said. However, the electricity produced is transferred to an electric motor rather than so-called reduction gears to spin the boat’s propellers, they said. “We just take the electricity from those high-speed turbines and use that electricity to drive an electric motor that propels the ship,” Breckenridge said. “It is quieter than a mechanical drive system.”
Evolving global threats require ever more quiet submarines, Navy officials said. The Navy decided to invest in the technology after reaching the limits of trying to silence mechanical propulsion, they said.
“Great minds have figured out how to get those gears whisper quiet,” one Navy expert said. “We did not have any more tools in the bag to get the stealth that we knew we needed for this national strategic imperative.”
The Navy has experimented with electric drive in the past, but it took 15 years for the service to perfect the technology, officials said. The system offers a number of potential advantages, including noise reduction, according to Bryan McGrath, managing director at FerryBridge Group LLC, a defense consulting firm based in Easton, Md. “When you have the motor tied directly to the propulsion shaft, that should eliminate some of the noise,” he said. Electric propulsion can also help ships generate more on-board power for electronics, sensors and weapons systems, McGrath said.
“Electric drive makes a lot of sense for submarines,” he said. “There is some technical risk in moving from mechanical to electric drive, but electric drive has been around for decades. The DDG 1000 (Zumwalt-class destroyer) surface ship is also electric drive — so you have two very big important ships are moving to electric drive.”
Other innovations in the submarine program include an X-shaped stern to improve maneuverability and stealth, officials said. As subs evolved from using propellers to more quieter propulsors, they lost some surface maneuverability, they said. “With the X-stern, the Ohio Replacement will regain some of that maneuverability and, as a side effect, will have improved flow characteristics in the stern area while submerged,” the Navy expert said. “This will improve quieting and it simplifies the hydraulic control layout in the engine room.”
Similar to the current Ohio-class submarines, the replacements will be equipped to fire the Trident II nuclear missile, Breckenridge said. The missile, designated D5, has proven reliable in testing, with all but one of its 149 test shots successful, he said.
“Last week we did another round of successful firings of that missile,” he said. “The performance of that strategic missile is just incredible. As we look to deter bad behavior from other countries, we’ve got this kind of reliability.”
The new subs will eventually be fielded with the successor to the D5, Breckenridge said. The program office is also working with officials in the United Kingdom to engineer a common missile compartment. General Dynamics’ Electric Boat unit in Groton, Conn., is building prototypes under a $770 million contract.
The Ohio Replacement Program aims to control costs in part by borrowing technology already in production on the Virginia-class attack submarine program, officials said. Examples of the technology include conformal plane array sonar, fiber-optic links between sail-mounted cameras and a control room and “fly-by-wire” digital controls that allow crews to use a joystick and touch-panel to control the boat, they said.
Sonar technology is of particular importance to a submarine platform whose mission depends upon quietness and detectability, Breckenridge said. “The SSBN has to have a capable sonar system with hull arrays,” he said. “We also stream along a towed array by putting out a string of transducers that give you that much more listening power. SSBN wants to detect an undersea adversary – if we can hear them further than they can hear us we have a tactical advantage in the undersea domain.”
In addition, the new submarines are being engineered with a new nuclear-reactor core designed to power the ships for 42 years. Unlike the current Ohio-class SSBNs, which require a multi-year refueling process halfway through their service life, the new Ohio Replacement boats will be able to continue their missions without needing a refueling pause, Breckenridge said.
The technology also allows the Navy to conduct the same mission with fewer submarines, service officials and analysts said.