Amphibious assault ship USS America (LHA 6) joined the U.S. 7th Fleet forward-deployed amphibious force in Sasebo, Dec. 6, after transiting the Pacific Ocean from its former homeport of San Diego.
This article about USS America was first published in Marine Corps Outlook: 2010-2011 Edition.
USS America (LHA 6) is the latest large-deck amphibious ship, the first of a four-ship class. Together with the eight LHDs (the last of which is Makin Island), her class will maintain the U.S. force of 12 such ships. Laid down on July 17, 2009, America is to be completed in 2012. She differs from the amphibious assault ships (LHAs) she replaces in that she has no well deck and no on-board parking area for vehicles. These changes can be traced to her origins.
America began as the LHA(R), the replacement for the five aging Tarawa-class LHAs built in the 1970s. Those ships were the first to combine a flight deck for helicopters, with internal vehicle parking leading to a well deck that could accommodate floating beaching craft to carry those vehicles to the beach. The follow-ons to the Tarawas were the Wasp-class LHDs. Although their designator was different, in effect they were second-generation LHAs with similar characteristics. An important difference was that the Wasps were conceived from the beginning to operate fixed-wing short takeoff/vertical landing (STOVL) aircraft (Harriers). Unlike the LHAs, they had a secondary sea control mission, for which they would have carried more Harriers and no transport helicopters. Another important difference was that the LHD well deck was proportioned to take three air-cushion landing craft; The differently shaped well deck of an LHA, designed for the slow LCU, could take only one. This limitation on the LHA could not, incidentally, be corrected in a refit. By the 1990s, it was therefore obvious that, given the opportunity, the Navy would opt for a replacement. Because the LHD series was extended to eight ships, and the total requirement was 12, current plans call for only four LHA(R)s.
The first study (1999) offered either a new design, a repeat LHD (although designated differently, the Wasp-class LHDs were, in effect, follow-ons to the LHAs built in the 1970s), or a service life extension program (SLEP) of the five existing LHAs. The SLEP was rejected at once because the five ships were in no condition for reconstruction (the well deck size issue was not raised publicly, but it must have been decisive). Unlike carriers, which were subject to SLEP, the LHAs had the much lighter structures of amphibious ships. Like other amphibious ships, they had been worked hard and had been subject to only limited upkeep.
The argument against building more LHDs was that the Marines were planning to deploy a new generation of shipboard aircraft, particularly the F-35B (JSF) – to replace the considerably smaller Harrier (AV-8B) – and the huge Osprey (MV-22). Not only were the new aircraft larger, they used much more fuel per flight hour, and the fighter was likely to carry more and heavier weapons. Experience with the LHAs also showed that the LHA(R) needed a much larger allowance for growth during her service life (the expenditure of their original margin was one justification for not modernizing the existing LHAs). An analysis of alternatives (AoA), the standard prelude to starting a new ship design, considered three possibilities: a repeat LHD 8 (Makin Island), a modified LHD 8, or an entirely new design.
The Marines first used their shipboard Harriers during the 1991 Gulf War. They were so satisfied that they decided to emphasize fixed-wing aircraft in their new large-deck amphibious ships. Thus the new JSF fighter/attack airplane was so important that at first, the Chief of Naval Operations favored an entirely new design called the “dual tramway.” It would have been about the size of a Forrestal-class carrier (like the previous USS America, about 60,000 tons). Her two separate runways (tramways) would have met at the bow, the island being set between them on the centerline. This arrangement had been considered decades earlier at the beginning of the design of USS Enterprise, the first U.S. nuclear carrier. In this case, one “tramway” would have been for helicopters, the other for JSFs. The dual tramway was too expensive, so the next best option was initially chosen: a stretched LHD 8 called the “plug plus.”
Interim requirements stated in November 2002 included 10 vertical take-off and landing deck spots, to operate a notional mixture of 12 Ospreys, six (objective 8) fighters, four CH-53E heavy-lift helicopters, four AH-1Z gunships, three UY-1Y light troop carriers, and two MH-60 ASW self-defense helicopters. At this stage, the ship was required to have the same floodable well deck as her predecessors (to take three LCAC or two LCU), the same troop capacity (1,687), at least the same vehicle capacity (“square,” meaning square feet of vehicle deck), more cargo (to meet expected future requirements), and an increased service life allowance (including 7.5 percent growth in displacement). The ship should have maximum survivability, including reduced signature and rapid recoverability. All of this required a larger, beamier (for more reserve of stability) ship. The plug plus design revealed in 2003-2004 was 77 feet longer than an LHD (with two separate plugs, one immediately forward of the island structure) and was 10 feet beamier. The extra beam on each side would have made it possible to take on seawater to compensate as oil was burned without mixing the two in the same tanks. The ship also would not have to dump oily seawater when she fueled (the new San Antonio-class LPD is similarly equipped).
All of this made for a large ship, displacing about a quarter more than the old LHAs: 50,125 tons fully loaded (921 feet overall x 116 feet wide at the waterline [128 at the flight deck] x 26.4 feet draft). By way of comparison, Makin Island displaces 41,335 tons fully loaded (847 feet overall x 106 feet at the waterline [110 feet at the flight deck] x 27 feet draft). Not surprisingly, the plug plus was dropped as unaffordable. In effect the effort to design it showed that the Marines could not have both the expanded aviation facilities they wanted and the sort of cargo-carrying and -handling facilities that had marked previous LHAs and LHDs. Which was more important?
America is variously described as the aviation variant of the LHA(R) and as LHA(R) Flight 0. Her hull and propulsion duplicate those of Makin Island. Her command and control system is essentially the same. The well deck and vehicle cargo areas of previous ships are eliminated altogether (the ship still carries her full complement of Marines, which her helicopters can bring ashore with their equipment and light vehicles). In return for the cuts, she has 42 percent more hangar (it is extended fore and aft) and 1.3 million rather than 600,000 gallons of jet fuel. The hangar has two (rather than one) high-hats (extra height areas), specifically to service the big Osprey. Magazines are armored like those of a carrier (apparently a first for an amphibious ship), and she has extra longitudinal structure for greater hull strength and survivability. Medical spaces have been resized, since the ship is particularly well adapted to receive casualties by air. All of this makes America a kind of potential STOVL carrier, albeit a slow one (maximum speed is 24 knots or less, as in the other large-deck amphibious ships). In the past, when the Navy has felt the need to disperse its fighting power, one option has always been to use the large-deck amphibious ships as second-line carriers, and the more carrier-like design of America may encourage such use.
The LHA(R) design matured roughly in parallel to the concept of the Sea Base, whose planned ship complement now includes an LHD and two LHA(R). Given the considerable cargo capacity of the other ships in the sea base, and plans for high-capacity craft to bring that cargo ashore, the well deck built into earlier large decks may not seem as important. The big CH-53E helicopters assigned to the LHA(R) can shuttle some cargo between it and the other Sea Base ships. The three large decks are envisaged as the key command and control elements of the Sea Base: the LHA(R)s to command the Marine Expeditionary Brigade associated with the Sea Base, and the LHD to command the air element. Having three large decks working together would seem to provide what was lost with the dual tramway LHA(R) – which was presumably expected to work by itself as part of an Expeditionary Strike Group (ESG). The ESG was conceived around a single large-deck amphibious ship and a Marine Expeditionary Unit – the Sea Base is a very different proposition. Obviously the air and ground components of the force on board the Sea Base have to work together, and in the past that would have mandated combining all the main command and control elements into one hull (which is why the specialized amphibious flagships always had air-control facilities). Now it is possible to link the three using reliable high-capacity communications. Another difference from the past is that the Marines and their air support will have to be projected well beyond the horizon, so that the three command and control ships will be depending in large part on what national-level sensors, mainly in space, can tell them about what is happening beyond the horizon.
On a more mundane level, America duplicates the new gas turbine-diesel power plant introduced in Makin Island. It combines gas turbines for main propulsion with electric motors for cruising. The earlier ship has, somewhat incorrectly, been described as a “seagoing Prius,” i.e., a ship with the sort of hybrid power plant that makes some cars far more economical. In a hybrid car, the wheels are normally driven by electric motors, which take their power from the car’s battery. The battery is normally topped up by the car’s internal combustion engine, which can operate at its most economical power while doing so. At higher speeds the internal combustion engine is clutched directly to the wheels, for maximum efficiency. Because motors can be used as generators, at times the motors in the wheels may pour energy back into the battery. In an extreme version of this idea, the energy typically expended in braking the car may be converted, at least in part, into battery power to be used later for propulsion. None of this quite applies to a ship.
Yet for decades, navies have been painfully aware that engines designed to be efficient at full power are anything but efficient at the much lower power needed for cruising. As a ship moves faster, the power needed rises roughly as the cube of the speed: Doubling speed requires roughly eight times as much power. Turbines, whether steam or gas, are most efficient at maximum speed. That is why many gas turbine warships have lower-powered cruising turbines, or else cruise on diesel power. Combinations that run on gas turbines or diesels are common; they are called CODOGs. The British Type 23 (“Duke” class) frigates have a variation of CODOG in which the diesels run the propellers at low speed via electric generators and motors, which might be considered not too different from what is being done in the new U.S. ships. In the British case, what might seem a complicated cruising power plant was adopted for quietness, so that the ship could make efficient use of passive towed-array sonar. Diesels, which are inherently noisy, could be isolated from the outside of the ship and also from the propellers. The U.S. Navy has long sought similar silencing in its destroyers and frigates by avoiding diesels altogether, accepting inefficient propulsion.
Earlier LHAs and LHDs had 70,000 SHP steam plants. They were the last U.S. Navy non-nuclear steam warships (nuclear submarines and carriers have steam plants, but they are quite different). That in itself presented problems, since to maintain and operate their steam plants, the Navy had to maintain a separate training cycle. Also, it is apparently difficult to automate a steam plant. The U.S. Navy (like all Western navies) is under intense pressure to cut personnel costs, which by some estimates account for 70 percent of its costs. Anything that could eliminate those unique
power plants would be welcome. The obvious answer was to adopt the same sort of gas turbines that power U.S. cruisers and destroyers – General Electric’s LM-2500. In a Burke-class destroyer, these engines nominally produce 25,000 shaft horsepower each. An LHD needs 70,000 horsepower, and to use three such engines would have entailed undue redesign of the entire hull (for three propeller shafts instead of the current two). Instead, the engine was boosted to 35,000 horsepower output; Makin Island has the first such LM-2500s. Aside from lower manning, gas turbines get under way much more quickly than steam plants. Endurance at high speed (20 knots) is reportedly somewhat less than on the earlier steam turbines.
The LM-2500s could not be a complete solution, because gas turbines are so inefficient when they operate at low power. Nor, it seems, was there enough internal space for separate cruising engines. Fortunately, the ship was being designed as electric power of various kinds was enjoying a renaissance in the U.S. Navy.
For entirely different reasons, there was and remains intense interest in using electric power to unify a ship’s primary and secondary power plants – her propulsion engines and the plants that provide auxiliary power. The unified power plant was initially attractive because its output could be poured into a bank of capacitors, to provide a burst of power for an electric weapon – like the recently demonstrated anti-aircraft laser, or like the long-promised rail gun. Without a great deal of electric power, always on tap but usually not needed, such weapons cannot be placed aboard a ship. Because shipboard space and weight are limited, they cannot have dedicated sources of power. Instead, they must somehow benefit from the fact that a ship constantly produces an enormous amount of power. In a conventional ship, most of that power is transferred mechanically to the ship’s propellers, hence cannot be poured instead into the capacitors. If instead the propellers are driven by motors, and the prime movers drive the generators, which in turn supply the motors, enough electric power is constantly available to power the next-generation weapons. Doing that is not a trivial proposition, but this potential explains why electric drive was an important feature of the Zumwalt-class destroyer design.
Electric power plants are also attractive because they can be made very survivable. They can be split into multiple elements, each of which can have its own power bus. Although one bus may be destroyed by enemy fire, enough redundancy can be built into the system for it to survive considerable damage. This potential seems not to have been as important in the Zumwalt decision as the future weaponry.
Electric propulsion became part of a much wider electric revolution. In the past, warships have relied on a combination of electric and hydraulic power for their auxiliaries. Hydraulic power relies on pumps driven by the main engines. Electric power can be controlled much more easily, because computers can control switches and because commands to motors are easy to transmit. The U.S. Navy has a long and successful history of electric command transmission, dating back to just after World War I. What is new is the use of computers to set controls.
Unifying a ship’s prime mover (main propulsion engine) and her auxiliary plant required a new kind of power switchboard. It had to react rapidly to commands, automatically translating demands for power into loads on multiple engines. Success in creating just such a switchboard made a new kind of all-electric ship possible, beginning with the Zumwalt-class destroyer and including the new Ford-class carrier. The carrier, for example, has an electric catapult instead of the previous steam catapult. However, the ability to switch power between applications may well also provide the ship with a new kind of self-defense weapon in the form of electric lasers. In the carrier, electric control of the catapult makes possible far more delicate control, and that in turn will make it easier for the ship to launch a wider variety of aircraft in quick succession. Rapid automatic switching makes it possible to reverse the field of the motor driving the catapult to brake it, without using the sort of physical brake now required.
All-electric auxiliaries include the ship’s pumps and can also include motors on her watertight doors. They are naturally adapted to a computerized damage control system. Ships normally have sensors distributed throughout their compartments, reporting to the ship’s damage control center. Traditionally the information from those sensors has been plotted on special charts (which are familiar to anyone visiting a U.S. warship). Those in the damage control center use the plotted data to understand what damage the ship is suffering, and they react accordingly. To a limited extent they can remotely control doors and pumps, but a great deal depends on their ability to get to the damage and fight it by hand. When the frigate Stark was hit by an Iraqi Exocet in 1987, most of her damage control specialists (her “khaki”) were killed. Errors made by the survivors almost sank the ship (they used far too much firefighting water). One reaction was to ask whether a computer could make sense of the mass of ship-status data, much as a combat system computer makes sense of the mass of radar and other sensor data to understand the world outside the ship. Early attempts to do just that were initially rejected, but the idea is now accepted.
Given a computer system that can understand the ship’s status and recommend reactions, the obvious step is to make it possible for that computer, with human assistance (and vetoes) to control the ship’s pumps and watertight doors and some firefighting systems. Computers send electrical signals, and the all-electric ship is peculiarly well adapted to make use of them.
America is not an all-electric ship. Her two big gas turbines are geared to her propeller shafts. In effect she has a separate unified (primary and secondary) power plant to drive her at lower speeds (12 knots and less), using 5,000 horsepower electric motors also able to drive the shafts. At such speeds the big gas turbines are de-clutched. The motors take their electric power from her oversized auxiliary plant, consisting of four 4,000-kilowatt Fairbanks-Morse diesel generators. At higher speeds, they feed power to the ship’s electric grid and thus to her all-electric auxiliaries. The computerized switchboard balances the demands of propulsion and that grid. It is assumed that the ship will run at low speed about 75 percent of the time. On her initial run to San Diego, Calif., from her builders on the Gulf Coast, Makin Island saved about 90,000 gallons of fuel (the Naval Sea Systems Command expects her to save about $250 million in fuel costs over her lifetime, at 2010 prices). Her crew points out that she can run about a month between refuelings. Efficiency at lower speed may be particularly important for a ship conceived to spend substantial time in the low-speed environment of the Sea Base, which may be poorly adapted to underway replenishment from tankers.
The combination of gas turbines and diesels and motors is complex. To make it work, the ship has a new automated machinery control system, which works with her computerized electric power switchboard.
Cutting the Navy’s fuel requirements is much more than a financial advantage. When the crew notices that the ship can run much longer without refueling, it is noticing, in effect, that the Navy can do with fewer tankers and with fewer fueling visits to ports. When the destroyer Cole was attacked in Aden, she was in port to fuel. It might be argued that she was actually there to show the flag, and to demonstrate that the United States was engaged in the area, but longer endurance would have made it possible to heed warnings of a terrorist attack and postpone the visit. A combination of gas turbines and compact motors driven by the ship’s normal electric power grid is inherently quiet, in a way that no normal CODOG plant can be, and it clearly pays much of its way by also being highly efficient. The experience gained in automated management of a hybrid power plant and with the ship’s automated power switchboard, is applicable not only to other large amphibious ships but also to combatant ships.
This article was first published in Marine Corps Outlook: 2010-2011 Edition.