The Arleigh Burke–class destroyer engineering plant has been the gold standard of naval propulsion for the past 30 years. What makes it uniquely successful is its power density, simplicity, and reliability. So why not use this proven engineering plant for all future naval warships? The answer is that future combat systems will need significantly more electric power.
Studies of integrated power systems (IPS), such as on the Zumwalt-class destroyer, make clear that adding more electric power without sacrificing propulsion power density, simplicity, and reliability is nearly impossible. This is because of the addition of large electronic cabinets for motor-propellor speed control, switchboards, and other controls needed for integration with the ship’s distribution system, as well as the weight and space of the main propulsion motors and generators. However, based on its historical experience with electric propulsion and its recent investments in power-dense propulsion motors and generators, the Navy can get the electric power needed for future ships while approaching the power density, simplicity, and reliability of the Arleigh Burke class.
Navy Turbo-Electric Propulsion Experience
Before the availability of sophisticated electronic motor speed controllers that now dominate commercial, industrial, and naval applications, the Navy operated hundreds of battleships, destroyers, aircraft carriers, and oilers with electric-propulsion motors that did not have engineering spaces full of electronic control cabinets. This method—“turbo-electric propulsion”—controlled the speed and direction of the propulsion motor and propeller by directly transferring the electric power and unaltered frequency of the generator to the motor. This direct-cable connection resulted in coupling the generator and motor electrical frequencies so an increase in generator rotation speed resulted in a proportional increase in motor and propellor speed without the use of power electronic control devices.
While turbo-electric propulsion proved successful, the motors were large and heavy, and in those days the Navy did not need large amounts of electricity for ship service and combat system loads. When power-dense naval reduction gears were introduced, the Navy transitioned to smaller and lighter mechanical propulsion systems for its next-generation warships.
Synchronous Electric Propulsion
The turbo-electric propulsion plants were simple and reliable, but because of the size of the motors, they were not as power dense as main reduction gear (MRG) propulsion systems. Today, however, high temperature superconductor (HTS) and permanent magnet (PM) synchronous motors are about one-third the size and weight of the early naval main propulsion motors. This advancement provides the opportunity to reconsider the demonstrated power density, simplicity, and reliability benefits of turbo-electric propulsion by solving its biggest problem—the huge size and weight of the main electrical machinery.
By combining the turbo-electric propulsion approach with modern HTS and PM motors and the Arleigh Burke class’s controllable pitch propeller (CPP), the technical building blocks would be in place to create a highly power-dense, simple, and reliable all-electric engineering plant. A more descriptive name for this modernized version of turbo-electric propulsion is synchronous electric propulsion (SEP), because the operational mode of the propulsion system is the direct synchronization of the generator output to the propulsion motor without the need for any power electronic speed-control devices.
With SEP, a gas-turbine generator (GTG) is electrically locked or geared to the propulsion motor for speed control. The SEP generator motor combination electrically replaces the mechanical DDG-51 MRG. A DDG-51 SEP would function like a mechanical plant with the combination of GTG speed and propeller pitch controlling ship speed. In fact, the SEP plant is effectively an electric gear twin of a DDG-51 mechanical plant with an identical MRG ratio. This change would preserve Arleigh Burke–class simplicity but with the benefit of four 19.5 megawatt GTGs that could be used for either propulsion or ship service and combat systems power.
DDG-51 Power Density, Simplicity, and Reliability
The Arleigh Burke class’s space constraints, particularly in the engine rooms, have been well documented by the Navy. Packed in two engine rooms is more than 100,000 shaft horsepower (78 megawatts) for propulsion. A SEP system with equivalent power could be installed in the two engine rooms without any changes to the main bulkheads. Removing the two MRGs and replacing them with two HTS main propulsion motors linked to four GE LM2500 HTS GTGs would provide the equivalent power of the mechanical plant. Location of the GTG switchboards could remain in their respective engine room divisions.
Engineering spaces full of power electronic cabinets requiring continuous chilled water have proven to be significantly less reliable in a marine environment compared with traditional and mechanical motors and generators. The system reliability can only be as good as the reliability of the lowest subsystem, which has a significant effect on the ship’s availability. By eliminating the motor drive and control cabinets and directly connecting the SEP GTGs to the propulsion motors via a standard switchboard, the SEP reliability would approach Arleigh Burke–class mechanical plant reliability metrics.
DDG-51 All-Electric Ship
The Zumwalt-class integrated power system delivered the benefits of an all-electric ship but with significant penalties in engineering plant power density, simplicity, and reliability. By eliminating most of the power electronics and control systems the Zumwalt-class design requires, an all-electric SEP engineering plant for the Arleigh Burke class with comparable power density, simplicity, and reliability is within reach.
Even if, given the age of the Arleigh Burke class, an electric propulsion upgrade were not an attractive backfit option, all future warships and submarines would benefit from the power density and reliability of synchronous electric propulsion.
