In January, Secretary of the Navy Richard Danzig announced that the coming DD-21 land-attack destroyer class would incorporate electric drive instead of the current geared gas turbines. The decision culminated more than a decade of work on a modern electric drive. The concept was first pressed in the late 1980s, ultimately in a study of a follow-on to the Arleigh Burke (DDG-51) class. It was dropped in the early 1990s, however, partly because the Navy had been given an object lesson in its problems, in the form of turbo-electric drive in the nuclear-powered attack submarine Glenard P. Lipscomb (SSN-685). Now it is back, at least for surface warships.
In the proposed plant, each of the ship's main gas turbines runs a generator. The generators are wired to motors driving the ship's propellers. There is no longer any need for long propeller shafts connecting the turbines directly (through gears) to the propellers; the motors can be quite close to the propellers, even in pods next to them. Moreover, there is no direct connection between any particular generator and any particular propeller. One turbine can drive both shafts, or several turbines can be brought on line to provide higher power. Moreover, the same turbines which drive the propellers can, in theory, drive the ship's electrically powered auxiliaries. The current distinction between prime and auxiliary power plants may no longer be meaningful.
Secretary Danzig's announcement recalls a major U.S. Navy success story of the early part of this century, turbo-electric drive for capital ships (five battleships and the battle cruisers completed as the carriers Lexington (CV-2) and Saratoga (CV-3). In that case, as in this one, a major attraction was flexibility of power plant arrangement. In the capital ships, the object was to place the turbines, the most crucial element of the power plant, as far as possible from the ship's side and hence from the threat of underwater damage. Turbo-electric power was abandoned only because the post-World War I naval arms treaties made it vital to save weight; geared turbines were far lighter. The U.S. Navy was unique in adopting turbo-electric power for major warships (some liners built after World War I also were turbo-electric), probably because the pre-1914 United States had the world's most advanced electric power industry.
The U.S. Navy revived turbo-electric plants during World War II for destroyer escorts, because U.S. gear-cutting capacity was insufficient. As in capital ships, turbo-electric machinery carried a considerable cost in weight and volume. As it turned out, however, the necessary lengthening of the ships' hulls reduced hydrodynamic (wave-making) resistance enough to balance off the extra weight, and the resulting Buckley (DE-51)- and Rudderow (DE-224)-class escorts were as fast as the geared turbine design would have been (which was not, in the event, built as planned). These ships apparently proved entirely satisfactory. Electric drive also was standard, of course, in U.S. diesel-electric submarines (the U,S. Navy had been the first to adopt diesel-electric drive in submarines in place of the earlier direct drive) and in some wartime auxiliaries whose power plants were adapted from those of submarines.
The submarine power plants offered one of the virtues driving the current program. At full power, a submarine needed all four diesels, each driving a generator. The submarine often would operate at much less than full power, however, in which case even one diesel might suffice. Although each diesel could be throttled back to low power, it was most efficient always to run diesels at maximum power. Thus it was far more efficient to run one of the four diesels at full power than to run all four at one-quarter power (in fact, in wartime, no commander would keep all but one shut down, but that is irrelevant here). Gas turbines are notoriously inefficient at less than full power, so by far the most efficient way to run a gas turbine ship at low power is to shut down most of the plant. Many current foreign gas-turbine ships use special cruising turbines for low-speed running. Even so, they need at least one turbine per shaft. A turbo-electric DD-21 could run on a single prime-mover turbine. In theory, it could achieve far better fuel economy than a conventional ship.
Turbo-electric drive became very attractive in the late 1980s for several reasons. The first was combat survivability. If the turbo-generators could be distributed widely through the hull, it would become extremely difficult to stop the ship with a single hit. For example, some turbo-generators could be placed well above the waterline, to avoid underwater damage, while others could be mounted deep in a ship to avoid damage by airborne missiles. Shock, which can bend a long propeller shaft, would be much less effective as a damage mechanism. Podded propellers, which become possible with electric drive, might add additional flexibility; one might envisage a combination of conventional podded propellers aft and tractor pods amidships or forward. With such propulsion, a ship might retain her mobility even if she lost her stern. Podding also offers, at least potentially, the possibility of steering (even stabilizing) the ship without using conventional rudders. Not only would a podded-propeller ship possibly be far more maneuverable, she might not be vulnerable to damage that would destroy her rudder.
A second factor was the predicted rise of electrically powered weapons such as rail guns and electric lasers. It seemed likely that from time to time a ship would have to devote much more than the usual auxiliary load to such weapons. If the prime and auxiliary plants were integrated, the ship could shift most or all of her power to the auxiliary circuits to power weapons. More than a decade later, the electric weapons still seem quite futuristic, but they still figure prominently in projections. Ship hulls may last as much as 40 or 50 years, and it would seem foolish to foreclose their potential. From a less grandiose point of view, one might observe that over time ships need more and more auxiliary power; virtually all modernizations entail installation of new generators. A ship with, in effect, an open electrical architecture might be much easier to modernize.
A third factor is internal volume. The historical cases quoted certainly do imply that the turbo-electric plants of the past needed more space and weight than their geared-turbine equivalents, but in some cases what matters is not so much total space required as the way that space is distributed. The 1980s studies were much affected by experience with the Spruance (DD-963) class. It turned out to be impossible to install vertical launchers aft because there was too little hull depth. Because the electrical plant is broken up into smaller individual packages, at least in theory it is easier to arrange as desired.
Finally there is acoustic signature. Gears are inherently noisy. Silencing is expensive. In modern U.S. destroyers and cruisers, designers had to resort to submarine techniques, placing the gears on rigid rafts and suspending the rafts from the ships' hulls. This type of construction is expensive, and to make it effective the designers have to exercise great care to avoid acoustic "short circuits." Electric motors are inherently quiet, which is why diesel-electric submarines operating on battery always are described as very quiet (they still have major sources of noise, but that is another story). This is not a new idea. In 1956, when the U.S. Navy decided to build a class of fast antisubmarine warfare submarines, it was assumed that they would be turbo-electric. That did not happen only because there was no existing motor nearly powerful enough. Admiral Hyman Rickover developed a turbo-electric plant for the Glenard P. Lipscomb specifically to replace what he considered the poorly-engineered raft concept. As it turned out, the result was disastrous, and it made the naval leadership of the early 1990s very leery of turbo-electric plants for surface ships.
Another possibility is the all-electric ship, replacing much of the hydraulically operated devices on today's vessels. The distributed power plant requires very reliable, very robust wiring. There is little point in retaining any other means of remotely powering, e.g., valves; the ship's electrical system must be duplicated so that it cannot be put out of action. If valves and even watertight doors are electrically actuated, then they can be software-controlled more easily. Such control would take advantage of modern systems that use sensors throughout a ship to determine its state, and which then offer advice to the damage controllers—who can do much of their job remotely. It is at least arguable that without some such mechanism it is impossible to reduce manning as drastically as seems to be necessary.
If turbo-electric plants are so promising, why are they still in the future? First, they demand the ability to switch back and forth between individual generators at very high power. The power use profiles of the prime and auxiliary power systems are radically different, for example, quite apart from whatever electric weapons would require. A software-controlled switching system is needed, because there cannot possibly be sufficient manpower to place an operator at each switch (assuming that manual operation could possibly be quick and safe enough anyway). The necessary high-power switching technology apparently does not yet exist. Second, fully distributing the generators requires up- and down-takes for each generator, i.e., considerable trunking running through the hull. It is not clear to what extent such trunking would affect a ship's watertight integrity. It may be that it is acceptable only if the ship has an automated damage control system capable of shutting off trunks before they become dangerous. At the least, it may be that distributed power is practicable only in a very large ship, like DD-21. Quite possibly the current Arleigh Burke (DDG-51)-class just is not large enough. Once a ship is large enough, however, the space and weight sacrifices involved may no longer be very costly—just as the U.S. capital ships of World War I were well suited to the turbo-electric plants introduced at that time.
Consider the future. Today, electric drive means changing the relationship between the current prime mover, the gas turbine, and the propeller, replacing gears and a shaft with a generator, wiring, and a motor. At least in theory, however, one might substitute some alternative prime mover for the gas turbine itself. If we look ahead several decades, that might mean a fuel cell, or even some sort of nuclear plant converting energy directly into electricity. Thus one might see electric drive as a necessary step in opening up new possibilities for power at sea.