The new Zumwalt-class destroyer is described as an “electric ship.” The next-generation carrier is more “electric” by far than any predecessor. Probably the same will be true of the coming cruiser. There has also been talk of electric (nuclear) submarines. What is happening, and why? What is so special about electric drive and electromagnetic catapults? And why is this happening right now?
Electric drive was important to the U.S. Navy about the time of World War I, but it entailed a significant weight penalty, which killed it during the period of treaty limitation between the World Wars, because those treaties limited the tonnage of individual ships.
The Tennessee-class battleships – USS Tennessee (BB 43) and USS California (BB 44) – as well as the follow-on Colorado-class battleships – USSColorado (BB 45), USS Maryland (BB 46), and USS West Virginia (BB 48) – all used turbo-electric drive, although USS Washington (BB 47) was canceled due to the Washington Naval Treaty. In accordance with the same treaty, the USS Lexington (CV 2) and USS Saratoga (CV 3), both of which also used turbo-electric drive for their propulsion, were converted from battle cruiser hulls then building.
The idea was revived in the late 1980s, for example, by a panel on the desirable characteristics of next-generation surface warships. It then briefly died again, because it had been applied very unsuccessfully to the nuclear submarine Glenard P. Lipscomb. However, it was revived again in the 1990s, and it has now been applied to new U.S. warship designs. The key technological considerations included the advent of new lighter-weight motors and generators, but it is also the case that ships are now so large that a bit of extra weight is of limited consequence.
In each case, the ship’s prime mover drives a generator rather than a propeller shaft. The propeller (or propulsor) is driven by a motor wired to the generator. In modern cruise ships, this kind of arrangement makes it possible to place the propellers in pods, which can rotate to turn the ship – to give it unusual maneuverability for its size. In a warship, electric power in itself might have several consequences. One would be that the ship would not have to run all of her engines all the time, because electric power from any of them could drive all of her propellers. At the very least, that would make for much better efficiency and longer range at a given speed. That increased efficiency alone justified electric drive for several new major amphibious ships. USS Makin Island (LHD 8), first of the class to use a gas-turbine/electric drive, made news in 2009 for saving $2 million during her transit from Pascagoula, Miss., to San Diego, Calif. The Navy’s Lewis and Clark-class T-AKE dry cargo and ammunition ships also use an integrated propulsion system. The ships’ four MAN diesel generators serve two electric propulsion motors driving a single shaft. The first of class Lewis and Clark was launched in 2005.
In the 1980s, the Royal Navy adopted partial electric drive for its new Type 23 frigates because electric drive isolated the quiet propeller from the ships’ inherently noisy diesel engines used at low speed. Because the diesel did not have to be connected directly to the propeller, it could be sound-mounted. In the past, frigates, which had to be silenced, were generally powered by relatively inefficient, but inherently quiet, gas turbines. Diesels were far more efficient, but also produced far more noise, and silencing them (e.g., by sound-mounting and hooding) was expensive and elaborate, mainly because the ships’ propeller shafts still had to be connected directly to their engines. Modern diesel-electric submarines do not have this problem because their diesels drive generators, hence, are easier to sound-mount and hood. The argument favoring diesel efficiency may become important to the U.S. Navy in a future of expensive fuel. In the late 1980s, the Royal Navy generally espoused electric drive in future surface warships on economy grounds.
Because the engines did not have to be placed in line with the motors actually driving the propellers, they could be spread out in such a way that the ship could not be disabled by a single hit. For example, the power plant could be divided (at least in theory) between a portion in the conventional below-water position (where it was reasonably well protected against above-water attack) and a portion above water, hence better protected against underwater attack. Such dispersion did entail a larger ship and some problems of routing uptakes and downtakes, and it has not yet been attempted – but it is probably in the cards if navies become more interested in survivability. Electric power could be routed to the motors along several paths, further improving the ship’s ability to survive damage. This seems to have been done in the new Zumwalt. Quite aside from that, eliminating a long propeller shaft would eliminate an important vulnerability, in that the shaft itself could be distorted by underwater damage (such as shock). If it kept turning, it would tear up the ship’s bottom. If that seems a remote consideration, remember that a bent shaft that kept turning helped considerably to sink the British battleship Prince of Wales in 1941 by opening up the ship’s hull like a sardine can.
At the least, the generator-motor combination eliminates the gearing standard in gas turbine ships. Gearing is essential because a gas turbine spins at high speed, far above the speed at which a propeller can turn (the U.S. Navy initially adopted turbo-electric drive for this reason, as an alternative to the gearing that other navies tried). It is also inherently noisy. That noise can be eliminated by isolating the gearing from the hull, using the sort of raft that submarine power plants have, but any such installation is expensive and space-consuming. The French adopted electric drive for their nuclear attack submarines specifically to avoid the size (hence cost) penalty associated with silenced geared turbines. For that matter, the U.S. Navy built the unfortunate Glenard P. Lipscomb for silencing, but she turned out larger, rather than smaller, than geared turbine submarines of similar type (and also far less reliable, hence the prejudice against electric drive in the 1980s).
Without the usual propeller shafts, there would be less reason to locate all of a ship’s propellers at the stern. Thus it would be possible to spread out the ship’s propulsion so that, for example, she could keep running even if her stern were blown off. This was a serious idea in the late 1980s, when survivability in the face of Soviet attack was considered essential, but no navy has taken so radical a step since then. For example, the Zumwalt design shows conventional propeller shafts driven by motors in more or less the positions typically occupied by a ship’s prime movers.
Protection was why the U.S. Navy became interested in electric drive just before World War I. Instead of placing a ship’s steam turbines in tandem with her propeller shafts, the Navy was able to place them on the centerline, as far as possible from any underwater hit. They were surrounded by spaces containing the boilers, and then by layers of underwater protection. The result was probably the best degree of underwater protection in the world, although it was vulnerable to shock, which might cause circuit breakers to jump (the carrier Saratoga was once disabled that way).
Electric drive also simplifies the ship’s internal arrangement. When the Spruance-class destroyers were fitted with vertical missile launchers, they received cells only forward of their bridges. The considerable deck space aft could not be used because the bottoms of the launch cells would have blocked the ship’s propeller shafts. The ship received about half as many missile launchers as a Burke-class destroyer of very similar size. This consideration does not apply to the Zumwalt, which carries its missile cells along the sides of its hull, but it was an important argument when the idea of electric drive was first being pressed within the U.S. Navy in the late 1980s.
Electric drive also offers a subtler advantage. By disconnecting the prime mover from the propeller or propulsor, it much simplifies the replacement of the prime mover. For example, from time to time, it is suggested that fuel cells can be far more efficient than gas turbines; they also seem not to entail the same sort of thermal signature. It would be relatively simple to replace gas turbines driving generators with fuel cells, assuming that the latter could provide a similar output.
In a submarine, the seal between hull and propeller shaft is difficult to design, particularly if the submarine is to operate at greater and greater depths. At least in theory, an electric motor could be designed that could be mounted entirely outside the submarine’s pressure hull. Instead of a penetration for a propeller shaft, cables could be passed through the hull from generator to motor. Non-nuclear submarines have used electric motors for years for underwater propulsion, but not in this way; the great bulk of the world’s nuclear submarines are driven by geared turbines (the French seem to be the only exception, and they use motors driving shafts passing through the hull in the usual way). If deeper diving is important in future, or even if it makes sense to simplify hull design by eliminating the penetration for the propeller shaft, then this kind of electric drive becomes attractive. It becomes even more interesting when it is pointed out that modern periscopes no longer penetrate the hull, since they send their data in via fiber optics. Again, this is not an academic point. In the late 1950s, the diving depth of U.S. submarines was set by the extent to which hull penetrations, particularly that for the propeller shaft, could be made to work at great depths (hence great pressures).
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.
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