Power to Our (Naval) Elbow?

by Stuart Slade, Senior Naval & Marine Power Systems Analyst, Forecast International.

The last decade has seen the promise of a new generation of naval weapons and sensors.  The development of railguns and lasers has caused these weapons to slowly emerge from the pages of fiction and become an impending if not quite fully realized reality.  They are directed by equally new generations of active-array radars that provide a level of range and target discrimination that enables the long-sought defense against ballistic missile attack. These new systems have a major factor in common: they are all voracious consumers of electrical power.

Escalating requirements for electrical power are nothing new.  In the period since the Second World War, power demands on warships climbed steadily from the standard provision of two 100-kW diesel emergency generators up to power provisions in the megawatt range.  Gas turbine ships service generators replaced diesel engines in the U.S. Navy during the 1970s, a step forced by the need to provide quick-responding generating capacity within limited space.  The Spruance class destroyers carried three 2-MW gas turbine generators, two of which were in service at any one time, with the third acting as a hot spare.  This arrangement proved to be only marginally adequate; a series of improved generators, mostly based on the Allison 501K family, increased the power output in steps to the present level of 4 MW per generator.

Yet, this setup has remained barely adequate to meet demands.  The Japanese Navy is adopting the LM500 for its ships service diesel generators.  This is a small, efficient unit that produces between 4.5 MW and 5 MW and represents a significant technology advance over the aging 501K-based family.  The U.S. Navy has not followed the Japanese in shifting to the LM500, probably because it is heavily invested in logistics support of the older 501Ks.  Another reason, though, may well be that the relatively small increase in power offered by the LM500 doesn’t even begin to match the impending power demand increases.

If the demands of the new, power-hungry generations of weapons and sensors are to be met, then something more dramatic is required.  There are rumors in the naval sector that Rolls-Royce is planning to introduce a new generation of ships service gas turbine generators that will supply up to 10 MW each.  One of these generators will supply as much power as the existing triple-generator layout that is standard on American warships.  Using three such generators will offer a major increase in generating capacity.  As such, a new generator generation in this 10- 12-MW power range would seem to be a useful step forward.

But is it?  The demand for electrical power on the next generation of U.S. Navy warships is likely to be so great that even the putative 10-12-MW generators will be marginal, and something more will be needed if a decent reserve of generating capacity is to be maintained.  At present, the U.S. Navy already has a proven gas turbine generator in service across the Fleet and has a well-established logistics train to support it.  It’s called the GE LM2500.  At the moment it serves as a main propulsion turbine, but there is no logical reason why it should be restricted to that role.

A U.S. DDG-51 class destroyer currently has seven gas turbines on board.  Four are LM2500 propulsion units and three are 501K-derived ships service generators.  Could these not be replaced by a standardized outfit of six LM2500s?  This concept is made more attractive by the current development of integrated fully electric propulsion (IFEP) in which all the engines on the ship drive generators that feed current into a centralized pool.  Some of that pool is used to power electrical motors to drive the ship, some to operate its sensors, and more to operate its weapons.  The significant point is that because the gas turbines are not directly linked to the ship’s shafts, they can be situated anywhere in the ship instead of concentrated in the engine rooms.  It is easy to envision a layout in which those six LM2500s are distributed around the ship to prevent them all from being taken out by a single unlucky hit.

IFEP has the problem that it is an expensive solution to powering a ship.  It offers much but it has costs to match.  There is a less expensive option, a hybrid drive that incorporates an electric motor into an existing COGAG powertrain that offers some of the advantages of the IFEP system without its substantial cost.  This system will be tested on the DDG-103 Truxtun.

It is hard to avoid the impression that the traditional layout of separate generators and propulsion gas turbines on a ship is a holdover from the days when electricity generation was a relatively minor issue compared with the power demands of driving the ship.  This is no longer the case; the demands of the new generation of weapons and sensors make power generation at least equal in importance to driving the ship.  So why not use the same LM2500 turbines for both roles?

Stuart Slade is currently the primary analyst for Forecast International’s Warships Forecast and Industrial & Marine Turbine Forecast.  He is the author of United States Strategic Bombers 1945-2012, Littoral Warfare: Ships and Systems, Navies in the Nuclear Age and Multinational Naval Operations. In addition, he has been a regular contributor on the subjects of warships technology, military electronics, and naval systems to a number of leading journals.

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