Naval Systems: Electric Drive Becomes Reality

By Edward J. Walsh

On the systems side, the most decisive step forward is the introduction of an integrated electric drive (IED) power and propulsion architecture for a class of surface combatants.

The Navy has investigated forms of electric drive over many years, with the collier USS Jupiter (AC-3) often cited as the first electric drive ship in 1913. Design and weight problems worked against wider adoption of electric drive, however. IED brings the Navy up to speed with the U.K.’s Royal Navy, which is using IED in its Type 45-class destroyers and the 65,000-ton Queen Elizabeth carrier.

The Navy already has fielded a hybrid gas turbine/electric-drive system on board Makin Island (LHD-8), and is using the same system in the USS America (LHA-6) and Tripoli (LHA-7).

Technology breakthroughs and pursuit of new ship designs in the 1980s renewed interest in IED, both for propulsion and to power ship systems. The so-called “Revolution at Sea” studies in 1988 and 1989 postulated that in the 1990s the Navy would build only one new class of surface combatant, the Arleigh Burke (DDG-51)-class destroyers.At the time, the Arleigh Burke was under construction at Bath Iron Works, to be commissioned in 1991.

At that time, the Navy planned to build about 80 Arleigh Burke –class ships and introduce an upgraded Flight III variant with DDG-80. The Arleigh Burke design could not wait for electric drive technology; they were built with a conventional gas turbine plant similar to that used by the Spruance (DD-963)-class destroyers and Ticonderoga (CG-47)-class cruisers.  

In 1994, the Navy began its “Surface Combatant for the 21st Century” (SC21) study of a future destroyer, later dubbed DD-21. In 2001 this became DD(X), which evolved into the   class. Originally 32 ships were planned, but swelling research and development costs whittled the figure to 24, then to 7. In 2008, the Navy tried to kill the program to fund eight more Arleigh Burkes . In 2009, then–Secretary of Defense Robert Gates said the DDG-1000 program would end at three ships.

The Zumwalt -class IED architecture, the integrated-fight-through-power system (IFTPS), brings huge ship-design and performance advantages, all of which, the Navy postulates, will be reflected in greater reliability and lower fuel costs.

The IFTPS uses Rolls Royce MT-30 gas turbines as “prime movers,”with two Rolls Royce MT-auxiliary gas turbines to turn electric generators that are connected by cabling to motor drives and advanced induction motors (AIMs) provided by General Electric. The prime movers generate more than 75 megawatts of power. The AIMs turn the propeller shafts.

The arrangement overcomes the “tyranny of the drive shaft,” meaning ships no longer must be designed to allow for a long drive shaft linked to the prime movers. This frees hull space for weapons, stores, or other design innovations.

Power flows from the motors through a reduction gear, which adjusts the speed of the motor to the desired ship speed. Power can be redirected to ship’s services and combat systems, including lasers, railguns, and other directed-energy weapons now in development.

Much has evolved since the early visions. The AIM is a fallback; initially the Navy hoped to use an exotic high-torque permanent magnet motor, but this was not ready for DDG-1000 production schedules. The AIM, which is heavier and requires more space, was developed by the French company Converteam, acquired by General Electric in 2011.

The win for IED has not been easy. In December the Michael Monsoor cut builder’s trials short because of the failure of a harmonic filter, used to limit power fluctuations. Getting the system right for the lead ship, Zumwalt, took longer than planned, and the ship experienced several delays because of IFTPS problems. Nevertheless, the IED surface combatants are in the water.

 

 
 

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