Dr. Friedman is a longtime U.S. Naval Institute author and Proceedings columnist. He has written on a wide variety of military and maritime subjects. This article was originally prepared in 2000 under a contractual arrangement with American Superconductor, and he has updated it prior to publication at the request of the company.
The recent announcement of the DD(X) award brings the Navy another step closer to a return to what used to be seen as the inevitable future of American warships-electric drive propulsion. What is new is the possibility that more advanced technology, perhaps incorporating superconductivity, will solve the problems of the past, so that we can fully realize the advantages which have long been associated with electric drive.
There are several. The one usually associated with electric drive as part of an Integrated Propulsion System (IPS) is better survivability. Because no long shaft need connect motor to prime mover, the ship is likely to survive shock far better. Too, there is no propeller shaft to occupy the valuable space abaft the power plant. For that matter, the power plant can be located where it can best survive damage, rather than in a place dictated by the position of the propellers. None of this is new. Before World War I, the U.S. Bureau of Engineering developed turbo-electric power plants for capital ships. The turbo-generators were placed on the ship’s center line, the least vulnerable location, surrounded by boilers and then by layered torpedo protection. This type of machinery was installed on board five battleships and the carriers LEXINGTON and SARA TOGA. The Bureau rightly considered electric propulsion its greatest triumph, and the U.S. Navy wanted to use it in the new battleships designed in the 1930s. Unfortunately, turbo-electric plants weighed considerably more than conventional geared ones, and the new battleships were designed under naval arms treaties which specifically limited the size of new capital ships. Every ton added for propulsion would have been subtracted from armament and armor.
In the case of the DD(X), there is further interest in integrating the prime mover with the ship’s auxiliary power system. Generators for the integrated system can be distributed around the ship, so that no single hit can disable her. Integrated drive has the additional virtue that it can provide the pulses of power which future weapons, such as laser and rail guns, may need, as they need it. However, to realize that sort of advantage the ship’s propelling motors must be able to accept sudden changes in power, as power is siphoned off for other purposes. This is known as transient stability.
A further advantage of an all-electric ship is controllability. If every shipboard power function is controlled by the same system, then the ship can be integrated much more effectively. For example, it may be very advantageous to unify the ship’s combat and propulsion systems. As an enemy missile approaches, for example, the appropriate response is a combination of hard-and soft-kill systems and evasive maneuvers. One control system would be able to apply both, if all shipboard systems were electrical (currently, many systems are hydraulic and thus are separately controlled). Note that a distributed and unified power plant virtually implies the transition to all-electric control. The logic of the usual hydraulic shipboard power system is that power can easily be distributed by a pump in the centralized machinery space. Without such centralization, hydraulic power becomes a major dead weight in a ship.
The combination of full integration and distribution of genera-tors requires the most compact possible generators. At least some generators will necessarily be located fairly high in a ship (to resist underwater damage). The Jess they weigh, the less they will affect the ship’s stability.
Another virtue of electric drive was that the prime mover could be decoupled from the propeller. For example, diesel engines operate most efficiently at an optimum speed, and indeed they have resonant speeds at which they break down. Having triumphed with battleship electric drive, in the 1930s the Bureau of Engineering applied the same idea to U.S. submarines. One result was that much lighter diesels, running far faster than propellers, could be used. Another was that, for the first time, submarines did not have to avoid running at speeds equivalent to resonant diesel speeds.
This kind of diesel-electric propulsion was then unique to the U.S. Navy. It is now virtually universal for non-nuclear submarines. One advantage, realized only postwar, is that the submarine becomes much quieter, since the propellers no longer carry the noise generated by the diesel out into the water. Another is that even though the best submarine design employs only a single large propeller, the submarine can still use multiple diesels to run it, via their generators and propulsion motor. The submarine can continue to operate even if one of her diesels cannot run. By analogy, an electrically-powered surface ship might connect multiple prime movers to the same set of propellers, and run all of them on any number or combination of prime movers. Some navies currently use this sort of arrangement to run two propellers on a single gas turbine.
Adopting electric drive in a submarine would have implications beyond better silencing of the main propulsion. As in a surface ship, the after part of the submarine could be rearranged, possibly to the submarine’s hydrodynamic advantage. Because the ship’s power output would be entirely electrical, there would be an incentive to rethink the ship along electric lines. For example, at present the pumps used in torpedo tubes are a source of noise. For some years NAVSEA has been working on electromagnetic catapults as an alternative. Given sufficient electric power, they would become a useful alternative to the current water pulse tubes. Such electromagnetic launchers might make super cavitating and supersonic underwater projectiles (on which NUWC has been working) much more practical. Such a development would parallel the long-standing surface community interest in electric power as a prerequisite for a variety of electric weapons, such as rail guns. Too, the controls of an all-electric submarine might be easier to control electronically, and they might be more responsive. That in turn might be very important as a way of gaining maneuverability, for example to evade an incoming torpedo.
During World War II, the United States was badly short of gear-cutting capacity. Normally gearing is used to reduce the speed of a fast prime mover, such as a turbine, to the point where it can efficiently drive a propeller. It is often possible to build a slower turbine, but such a machine will be far larger and far less efficient than a fast one. Electric drive can have much the same effect as gearing. During the war, many U.S. auxiliaries, and also many destroyer escorts (frigates) had various forms of electric drive.
Electric motors are, moreover, inherently quiet. There are no gear teeth meshing into each other to make recognizable sounds. When the U.S. Navy decided, in 1955, that it wanted to build fast but very quiet nuclear submarines, the obvious solution was to replace the existing mechanically driven geared drives with turbo-electric drive.
With all of these advantages, it is surely a distinct surprise that electric drive has not taken over the naval world. It pops up here and there-recently, for example, as a component of the machinery in the British Type 23 frigate-but it is hardly the dominant force that might have been imagined in, say, 1920.
The main reason why is that the combination of generator (for the prime mover) and motor can be massive. During World War II, when many destroyer escorts were given turbo-electric power plants, the price was 26 feet more length. As it happens, a longer hull encounters less hydrodynamic resistance, so the added resistance due to the added displacement (due to the weight of the power plant) was balanced off by the added length. Even so, designers generally felt that they would prefer to use added length and space for other purposes.
As for nuclear submarines, initially the project stalled because no existing motor could produce enough power. Instead, gearing and other noise-making elements of the power plants were sound-isolated on rafts. Over forty years later, sound isolation is still the main means of silencing nuclear submarine power plants, and it is still quite expensive. One of the main advances made between the Seawolf and Virginia classes is a better and less expensive means of sound isolation, but the technique is still much less than ideal. As a veteran of earlier Bureau of Engineering electric propulsion triumphs, Admiral Hyman Rickover pressed hard for electric submarine propulsion. He managed to have a prototype, GLENARD P. LIPSCOMB, built, but the technology proved less than successful. The submarine was too large and her machinery was too unreliable. Yet Rickover’s reasoning is still valid, to the point that the French adopted turbo-electric machinery for their nuclear attack submarines (the Soviet Alfa class [Project 705] appears to have been similarly powered). The main difference between the French and Soviet submarines and their unhappy U.S. counterpart was that they used much more efficient AC power. The U.S. submarine used DC because a DC motor has an inherent ability to reverse (if the polarity of the current reverses), an ability which may be extremely valuable in an emergency or during rapid maneuvering situations. The simplest way to make a reversing AC power plant would be to combine a pair of windings (one for each direction), but with conventional motor design that would be unacceptably massive. The alternative, using controllable pitch propellers, adds additional complexity and weight.
The great barrier to electric propulsion, then, is that electric motors and generators based on today’s technologies are large and heavy. For electric power to be realty widely used in future warships, it must become more compact. Is that possible? In 1911 a new electrical phenomenon, superconductivity, was discovered. At very low temperatures, in some materials, it was found that electricity suddenly flowed without encountering any resistance.
Electric motors or generators based on superconductivity could be dramatically shrunk. Unfortunately, for years superconductivity was essentially a laboratory stunt It worked only very close to the absolute zero of temperature. Indeed, much of the effort of superconductivity experimenters went into building complex and expensive cooling systems which could reach the requisite ultra low temperature (in the range of 0 to 5 degrees Kelvin -or 0 to 5 degrees above absolute zero). Physicists spent their time trying to understand why superconductivity occurred. There seemed to be little chance that it would have any very practical applications. Even so, the promise. of low temperature superconductivity was such that in 1980 the Navy installed a 400 HP low temperature superconducting motor on a research craft, following it up with a 3000 HP motor in 1983. To operate, the motors had to be bathed in liquid helium at 4.2 degrees Kelvin.
In the 1980s, however, experimenters discovered that some ceramic materials could become superconducting at much higher temperatures. These were nothing like room temperature-the room temperature superconductor is stilt a kind of holy grail, probably unreachable-but they were within the range which quite conventional and relatively low cost refrigeration equipment could reach. Suddenly very small, inexpensive, and essentially loss-less motors could be built. Moreover, higher-temperature superconductivity emerged at about the same time that the Navy began to tum back towards electric propulsion for all the reasons which had made it attractive in the past. American Superconductor Corporation of Westborough, Massachusetts recently completed an $80 million HTS wire manufacturing plant in Devens, Massachusetts that will allow its wire manufacturing capability to grow from the present 500 Km per year to 20,000 Km per year. In 1999, the Office of Naval Research (ONR) awarded the company an initial $1.5 million contract to design a 33,000 SHP motor using a conventional AC stator and a superconducting DC rotor. In February 2002, ONR awarded an $8 million dollar contract to build and deliver a 5 MW, 230 RPM marine motor to the Navy in July 2003. Superconducting technology makes for a very compact and extremely power-dense machine. The combination, then, over-comes past problems in applying electric propulsion to, for example, nuclear submarines.
The superconducting motor is, moreover, substantially quieter than a conventional electric motor. A conventional electric motor develops a high concentration of magnetic flux, which is concentrated in iron teeth, and hence is not perfectly uniform around the motor and thus causes vibration and therefore noise. Superconducting motors can be designed as air-core machines without iron teeth, hence drastically reducing the concentration of flux normally associated with the high currents in motors. The magnetic field can be made far more uniform, so operation is inherently quieter. Too, in the past sheer motor size has generally been associated with motor speed: the slower the speed, the more massive the motor. Propellers are most efficient (and, incidentally, quietest) when they tum slowly. Thus designers could choose between relatively lightweight motors coupled to propellers by inherently noisy gearing, or large and very heavy direct-drive motors. Because a superconducting motor can develop high power at low speed within much more compact dimensions (it is typically a third the size of an equivalently-rated conventional motor), it should resolve this dilemma.
American Superconductor offers a wire (ceramic filaments in a silver alloy matrix) which reaches superconductivity at approximately 110 deg K. Although this is hardly what a layman might consider high temperature, it is well within the range reached by cooling systems already used in, for example, medical magnetic resonance imaging (MRJ) systems-that is, in normal industrial practice.
From the Navy’s point of view, perhaps the most important aspect of the new high-temperature superconductivity technology is that it has numerous commercial applications. The Defense Department is no longer so wealthy that it can afford to develop as many special technologies as it likes. It is far better to put some seed money into a technology which is likely to take off in the commercial sector, after which defense can reap some of the dividends. This is hardly a new idea. For example, in the 1930s the Navy badly wanted a new high-speed submarine diesel, but it was building so few view submarines annually that no company was likely to develop such an engine. More to the point, even if a satisfactory engine was developed, no company would invest enough to bring it to the degree of reliability the Navy needed.
The then Bureau of Engineering well understood the problem. Fortunately, in that Depression time General Motors was interested in a new potential market, diesel railroad engines. The Navy realized that the engine it wanted would also be suitable for a railroad engine. If it paid for a prototype, GM would market the engine to the railroads. Within a few years, as some bought it, GM would find itself paying for developing the sort of reliability the Navy needed, even if the Navy bought only a few engines. The idea paid off; the resulting World War II submarine engines performed brilliantly (another manufacturer, Fairbanks-Morse, developed a competing engine for the Navy and then marketed it to the railroads, too).
Conversely, when defense spending is down, it is difficult to get anyone to invest in specifically military technology. The Navy learned as much after World War 11, when it tried to develop closed-cycle submarine engines, which had no obvious commercial application. At a 1948 Submarine Officers’ Conference, those running the various closed-cycle programs all complained that the companies were reluctant. They preferred to put their better engineers into programs for commercial products, which had much higher payoffs. Then one officer suggested the only submarine propulsion system which did have a major civilian application. He was Captain Hyman Rickover, who was running the nuclear program at a time when civilian nuclear power seemed to be both close and extremely attractive. Reading the minutes of the meeting, one can almost hear the officers cheering.
High-temperature superconductivity seems to have very important civilian applications, because it can replace many existing electrical devices (including transmission cables) and drastically improve their efficiency. That is likely to be very attractive if energy prices continue to rise. Since July 2000, Wisconsin Public Service Corp has operated six superconducting magnetic energy storage (SMES) units built by American Super-conductor in its 200 mile Northern Transmission Loop. Detroit Edison is installing superconducting cable in one of its inner city substations, to carry three times the power of their conventional predecessors. Again, they are using American Superconductor’s new wire. In 2001, the company successfully completed the testing of the world’s first 5000 HP, 1800 RPM commercial scale HTS motor. In effect, American Superconductor is where GM’s diesel division was about 1932, on the eve of its very successful railway dieselization program. Once again, the Navy may be poised to jump aboard a commercially attractive technology, gaining large benefits from a very limited initial investment. Conversely, that investment may help develop technology the country at large will find very useful as we enter into the 21st century.