Teething Problems for Electric-Drive?
Delft Dynamics
Two of the world’s foremost navies—the U.S. Navy and the Royal Navy—seem to be having trouble with new warships using all-electric drive in place of more traditional geared steam or gas turbines. The new U.S. aircraft carrier Gerald R. Ford (CVN-78) reportedly needs to have her main generators replaced. The new destroyer USS Zumwalt (DDG-1000) reportedly suffers from serious cooling problems. The British Type 45 air defense destroyer reportedly has insufficient generator power, particularly in hot climates. What is happening? Electric power is nothing new, and the U.S. Navy operated turbo-electric warships in the past.
Something is new, however, and the new approach offers worthwhile potential benefits. The new idea, under development since the 1980s, is to unify main and auxiliary power systems. In nearly all current warships, the two systems are separate. A ship’s main engines move her through the sea. Her auxiliary generators power everything else, from radars to the refrigerators which keep food fresh. In many ships, the main engines power hydraulic pumps which provide some auxiliary power. Many ships, particularly major combatants such as missile destroyers, require so much electrical power for their weapon systems that their generator power approaches the power the ship actually needs to cruise.
Gas turbines, which power most major warships today, are inefficient except at their designed maximum power (in a few cases efficiency is gained by some form of regeneration). Ideally a ship should use only a fraction of full power when she cruises. However, it is difficult to power a ship with two or more propeller shafts from only one turbine. In the 1960s, the U.S. Navy considered powering the Spruance-class (DD-963) with three rather than four gas turbines, using electric drive to connect one, two, or three of them to the two shafts. That proved impractical, and the Spruances ended up with four turbines geared to their two shafts. Their successors, the Arleigh Burke-class guided-missile destroyers are similarly powered.
If all the engines in a ship were rigged as generators, at least in theory, power could flow from any combination of them to the propellers. Proponents of this kind of electric drive cited two different advantages. One was survivability: generators could be distributed around a ship so that she would be difficult to immobilize with an attack on one area of the ship. A second advantage was better fuel economy. Not only could the ship be driven by any combination of her gas turbines, but at low speeds she might even be driven by diesel engines which are normally auxiliary generators. A third potential advantage, which has not been realized yet, is that the connection between generators and motors driving propellers is wiring (which can be multiplied to make it difficult to destroy) rather than conventional propeller shafts. Shafts take up valuable hull volume and are inherently vulnerable to shock.
Against all of these advantages were two major problems. One was the need to be able to reroute electric power from different generators quickly and seamlessly. In the 1980s, the desired means to do so was a software-controlled high-capacity switchboard. A second issue was weight: the combination of generators and propulsion motors was substantially heavier than the usual gearing and propeller shafts. That had been true of the steam turbo-electric plants the U.S. Navy adopted for capital ships from 1916 on and then for destroyer escorts in World War II. Weight is the main reason the U.S. Navy abandoned turbo-electric drive for its capital ships despite its promise of superior underwater protection. (It also turned out that ships with such drives were more vulnerable to shock than their more conventional counterparts).
For the U.S. Navy, by the early 1990s enough progress had been made in developing components like the software switchboard to make a switch to unified electric power a practical proposition. Then-CNO Admiral Frank Kelso was skeptical, having personally experienced the many crippling problems of the Navy’s turbo-electric submarine Glenard P. Lipscomb (SSN-685). But what sold Admiral Kelso on the idea was the prospect of future electric weapons which would require a ship to pour almost all of her power into from time to time. Future weapon concepts at the time meant high-powered lasers and particle beams; later it meant rail guns, too. When the Zumwalt was conceived, fuel was expensive, and fuel economy was a key consideration (overall, the design of the ship was intended to minimize operating cost, including personnel cost). Electric drive was a natural choice. The Royal Navy, too, apparently chose electric drive for its Type 45 destroyer, primarily to improve fuel economy.
Unified electric drive offers further advantages. It is relatively difficult to exert fine control over a hydraulic system based on computers, but computer control of electric motors is relatively simple and has interesting implications for combat maneuvering and for damage control. For example, in theory a ship facing an antiship missile attack should turn to present a minimum radar cross-section while using a deception jammer and launching decoys such as chaff. The jamming and decoy launching can be automated under the control of the ship’s combat system, but steering is a separate matter. The combat system may display a recommended course, but it cannot impose one.
That may or may not be a good idea, but it would take unified electric power to provide a ship with a unified reaction to missile attack. It can also be argued that a fully electric system might be able to react quickly to some forms of battle damage. Again, just how desirable that is depends on how much automation and electric power are to be trusted under battle damage conditions.
Electric drive also offers a path to future propulsion. Because the connection between prime movers and propellers is cut, it should be easier to replace a ship’s prime movers with alternative power plants, such as more efficient gas turbines. At some point the turbine-generator combination might be replaced by fuel cells. In the case of a nuclear carrier, at some point it may be possible to generate electric power directly from a ship’s reactor.
So much for possibilities, but what is going wrong now? There seem to be two kinds of problems. One is probably underestimation of power losses in a unified all-electric powerplant, which would explain why the generators which are supposed to drive a Type 45 at low speed are not up to the job. Underestimated losses could also affect the required output of the generators in the Gerald R. Ford, as would any underestimate of the electric power required by her catapults. Unexpectedly high losses between generators and motors would have a second effect: they would generate more heat than planned. Hence the Zumwalt problem. In her case, it is also possible that efforts to make the ship stealthy led to insufficient cooling area underwater.
All these issues seem to be teething problems for a technology which offers a great deal—if it can be fully exploited.
Dr. Friedman is the author of The Naval Institute’s Guide to World Naval Weapon Systems, Fifth Edition, and Network-centric Warfare: How Navies Learned to Fight Smarter Through Three World Wars, available from the Naval Institute Press at www.usni.org.