All of these considerations made electric drive attractive, but not so much so that it was worth rethinking ship design, particularly in the post-Cold War period. The decisive argument for electric drive was that a new generation of weapons, in prospect but hardly yet in service, might need massive amounts of electric power: electric lasers, other directed energy weapons, and rail guns. In ships, a distinction is typically made between auxiliary and propulsive power. The underlying assumption is that far more power is needed to drive the ship, particularly at maximum speed, than to operate auxiliaries such as, say, pumps and even radars. That might be true on average even for an electric weapon, which would use a burst of power but would not fire continuously. However, to build up enough energy for that burst would hardly be a trivial proposition. An extremely powerful radar, which might (for example) be needed to detect a distant missile, might present the same sort of problem.
Electric lasers in particular were attractive because the alternative, chemical lasers, were generally considered too dangerous. In one study, a chemical anti-missile laser proposed for use on board a frigate was rejected because each burst of fire would have been accompanied by a toxic exhaust capable of killing everyone on the ship’s bridge. Yet as anti-ship missiles became faster, it seemed less and less likely that the usual defensive missiles could defeat them.
The promise, thus far unfulfilled, of the rail gun is that the ship’s energy provides the propulsion. Only the warheads, about the size of conventional shells, must be stowed.
Similarly, since the late 1990s, (electric) rail guns (linear motors) have attracted intense naval interest. The missile capacity of a surface ship is quite limited because each missile takes up so much space. Much of that space goes to propulsion. The promise, thus far unfulfilled, of the rail gun is that the ship’s energy provides the propulsion. Only the warheads, about the size of conventional shells, must be stowed. The usual claim is that projectiles can be delivered quite precisely at ranges out to about 400 miles. Only a rail gun, if one can be built, offers the combination of compactness and precision and great range. For ships that might be expected to last 30 or 40 years, the future promise of the rail gun seemed to justify a new kind of propulsion architecture. Rail guns may have seemed particularly close in the early 1990s because they were being developed actively as part of the Star Wars program – a rail gun is the only kind that can probably fire effectively despite the airlessness of space, because any other kind of gun involves ignition, and existing gun propellants do not include their own oxidizers.
As these possibilities began to seem important in the 1990s, the idea grew that a ship should be able to divert her considerable propelling power to other uses. That was impossible so long as propulsion meant driving a propeller directly from a gas turbine, whereas auxiliary power was electric, taken from an auxiliary generator. If propulsion were electric, however, both main and auxiliary systems would be fed by the same kind of power, and (at least in theory) they no longer had to be separate. Instead of a prime mover and some generators, the ship might use several generators of the same type, ideally spread so that at least some would survive any sort of damage.
This was never a trivial proposition. Somehow the ship had to be able to move large amounts of power quickly and smoothly from one role to another. That required a computer-controlled, high-power switchboard. Storing enough power for the bursts that a weapon might need required some kind of high-capacity storage device, such as a bank of capacitors. In a ship with electric weapons, the capacitors would replace the usual magazine – and, because they could store so much energy, they would present much the same sort of explosive hazard to an enemy hit (or to some kind of shipboard disaster).
There were other interesting possibilities, too. In the past, ships actually used three different kinds of power: prime power for propulsion, driving propellers; auxiliary electric power; and hydraulic power taken from pumps generally driven more or less directly by the prime mover. Electric power is relatively easy to control using electronics; hydraulics and propeller shafts are far more difficult to control in this way. If everything in a ship was electrically powered, it could be controlled electrically, and the electric controls could, in turn, be controlled by computers. That would include damage control devices such as pumps and vents and even watertight doors.
In the early 1990s, the David Taylor Model Basin, the U.S. Navy’s main experimental ship establishment, suggested that as computers became more compact, it might be useful to imagine a kind of “self-aware” ship, in which computers and sensors embedded in the fabric of the ship constantly monitored (and reacted to) her condition. They would, for example, sense damage. In that case, they might be programmed to recognize what was happening – and, via the electronically controlled devices in the ship, react to it. Since the ability to handle battle damage is a major factor in crew numbers, the Model Basin suggested that the self-aware ship could operate with a dramatically smaller crew. Manpower now accounts for a surprisingly large fraction of the lifetime cost of a ship. The Model Basin thought that self-awareness could more than halve the crew of a destroyer. Its report became a major factor in the concept of the next-generation destroyer, which became the Zumwalt. Although the design of the ship seems not quite to have achieved what was hoped for, it seems likely that some form of self-awareness is involved. It, in turn, makes sense only if the ship has very survivable pathways both for electric power and for data. The former is most likely to be part of a redundant electric propulsion power net, because the usual auxiliary power net is probably not redundant enough in itself (although considerable attention is paid in all designs to power survivability).
The new carrier goes a step further. Her electromagnetic catapult is a kind of low-velocity rail gun. In addition, she has electromagnetic arresting gear – the energy of a landing airplane is converted into electric power instead of being dissipated as heat, as in the past. As in a hybrid electric car, this energy is stored and can be applied to the ship’s power needs. Making the catapult electromagnetic simplifies the ship’s design and arrangement, because it does away with considerable piping and with the steam accumulators under a conventional steam catapult.
This article was first published in The Year in Defense Naval Edition, Spring 2010