The IPS Advantage

Electric Drive: A Propulsion System for Tomorrow's Submarine Fleet?

By EDWARD C. WHITMAN

Dr. Edward C. Whitman, the technical director of the Center for Security Strategies and Operations (CSSO) at Anteon Corporation in Arlington, Va., is former deputy assistant secretary of the Navy (C4I, electronic warfare, and space).

Integrated Power Systems--Electric Drive--Integrated Electric Propulsion--the names are different, but the underlying concept is the most promising development in ship engineering plants since the advent of nuclear reactors and gas turbines 50 years ago. And submarines are no exception. As Adm. Frank L. Bowman, director of Navy Nuclear Propulsion, predicted (in the Spring 1999 issue of Undersea Warfare Magazine), "... our Navy's ships--particularly our nuclear-powered warships with their 'unlimited' reservoirs of stored energy--stand at the threshold of access to remarkable capabilities that Jules Verne could barely have imagined."

In conventional mechanical-drive propulsion systems, the prime mover--a steam turbine, diesel engine, or gas turbine--powers the propeller through the use of reduction gears and a drive shaft. Separate auxiliary diesel or turbo- generators provide the electrical power needed for ship's services and combat-system loads. In contrast, the prime movers in an integrated power system (IPS) generate a pool of electricity that is made available, through a flexible distribution and switching architecture, as a shared resource for all shipboard needs. Under IPS, the actual propulsion is implemented with electric drive--electric motor-driven propellers, pump-jets, or even external pods that draw their power from the common supply.

Thus, instead of having separate power systems, each with its own load limitations, for mobility and ship's services, the total generating capacity of the ship can be traded off among functions and mission areas in accordance with changing tactical demands. In a nuclear-powered submarine, for example, instead of having 75 to 80 percent of reactor power reserved only for propulsion, virtually any amount of power could be used for any of numerous shipboard needs.

The New Power Standard

High power density; more sensitive control; greater flexibility in the internal arrangement of machinery spaces; a higher modularity potential; the ability to allocate power at will among propulsion, hotel load, and weaponry; lower total ownership costs--all of these advantages have convinced the Navy that integrated power must ultimately become the standard for modern warships.

For submarines, the shift to all-electric power would permit the building of a new generation of high-energy, onboard mission systems whose greater power requirements could never be accommodated within today's architecture. These include electromagnetic weapons, advanced active sensors and countermeasure systems, and deployable off-board systems that could be charged and recharged as required. Thus, to the extent that future weapons can be propelled by electrical means or made to deliver destructive force in the form of electrical energy, a nuclear-powered submarine would never "run out of bullets."

IPS brings other significant advantages. Transferring power wherever possible as electric current breaks the "tyranny of the shaft line" and introduces new flexibility into internal design and space utilization. Even more importantly, a ship's inherent survivability is enhanced by two key factors. First, eliminating reduction gears; that significantly reduces radiated tonals, and that translates into substantial increases in acoustic stealth. Second, the relative ease of redirecting electrical power --by using zonal power distribution, automatic fault detection, and high-speed switching --to bypass damaged compartments and subsystems dramatically increases the ability to "fight while hurt." Furthermore, the greater freedom available to distribute critical systems throughout the ship minimizes the possibility of multiple failures and/or major collateral damage from a single hit.

Finally, significant savings are expected in total ownership cost. IPS engineering plants may at first cost somewhat more than current conventional plants, but over a ship's total life cycle their greater fuel efficiency--estimated by the Navy to be from 15 to 19 percent higher than the conventional plants--will more than offset the initial outlay. In addition, the substitution of standardized nonrotating components for today's mechanical counterparts will significantly reduce maintenance requirements and the associated manpower and training devoted to maintenance.

Even more life-cycle savings can be gained downstream by replacing, with electrical equivalents, many of the shipboard auxiliary systems now powered by steam, hydraulics, or compressed air. This next logical step in the ongoing IPS revolution will eliminate even more onboard "clutter"--compressors, pumps, steam lines, etc.--and further reduce cost, weight, and complexity. In short, if IPS becomes a reality, the "all-electric ship" cannot be far behind.

The Historical Challenge Posed by Electric Drive

Fielding full-scale electric propulsion motors of sufficient power and reliability for today's warships is the greatest challenge facing IPS today. But electric drive is not new. As far back as 1913, the collier USS Jupiter--later converted to America's first aircraft carrier, USS Langley--used steam-powered turbo-generators and alternating-current propulsion motors. When they proved to be operationally successful, USS New Mexico and five successor battleships were configured the same way.

Slightly later, when the Navy's second and third aircraft carriers, USS Lexington and USS Saratoga, were built on the hulls of cancelled battle cruisers, they retained the turbo-electric plants of the original design.

Ultimately, though, improvements in reduction-gear technology made possible the use of geared-turbine machinery that was both smaller and lighter than the electric-drive systems needed to meet the Navy's requirements for ever-higher speed, so in the 1930s geared turbines became standard for large combatants.

In the submarine world, battery-powered, direct-current drive motors had been used underwater for decades, and, as the fleet boats of World War II evolved, the obvious next step was to adopt diesel-electric drive for surface propulsion. The Navy's oceanographic survey ships and other small vessels also have used diesel-electric propulsion for many years.

Two nuclear-powered submarines of the 1960s and 1970s, USS Tullibee and USS Glenard P. Lipscomb, also used turbo-electric drive for certain purposes --primarily to achieve quieting for ASW (antisubmarine warfare) missions. Both boats were somewhat under-powered in comparison with their contemporaries, though, and the carbon brushes of their direct-current, mechanically commutated propulsion motors created recurring maintenance problems. For that reason, the Lipscomb was retired early. Meanwhile, the rest of the submarine force retained geared turbines.

Perceptions and Priorities

Beginning approximately 15 years ago, however, technical advances in high-power alternating-current motors led to their growing use in cruise ships, cargo carriers, and cable-layers. Even earlier, the U.S. Navy had begun a series of exploratory and advanced development efforts to investigate the use of superconducting and homopolar motor technology to increase the power density of shipboard electrical machines.

In 1982, the Navy succeeded in demonstrating a 3,000-horsepower, superconducting electric-drive system on a 65-foot test craft. That approach was abandoned, though, because of its perceived technical risk, concern about the robustness of the cryogenics that would be required in a shipboard environment, and the lack of a commercial market to provide economies of scale. Instead, because of the increased industry interest in more conventional electric-drive machinery, the Navy also turned there for key components, and in 1995 subsumed all the predecessor efforts into the Integrated Power System program at the Naval Sea Systems Command (NAVSEA).

The capstone of NAVSEA's IPS effort has been the creation of a Land-Based Engineering Site (LBES) at the Naval Surface Warfare Center facility in Philadelphia, Pa., where a gas turbine-generator, a 25,000-horsepower induction motor, and 1-1/2 zones of direct-current power conversion and distribution hardware have been installed to demonstrate various ways in which IPS can be used in surface ships, especially for electric drive and survivable zonal power distribution.

Because of the confidence gained from both this facility and the technology programs of the Blue and Gold industry teams competing for the Zumwalt-class land-attack destroyer, the Navy was able to announce a year ago that DD 21 will incorporate both IPS and electric drive; the first ship is scheduled for completion in 2010, but that date could change if the current Pentagon leadership reallocates funding priorities in favor of other programs.

Extending IPS Concepts To Future Submarines

The Navy's decision to commit to IPS for the Zumwalts will provide a significant impetus to electric-drive systems for submarines as well. On top of the work already accomplished by the Office of Naval Research and NAVSEA, the experience gained in fielding IPS in surface combatants should create a solid foundation for the application of similar concepts in later variants of the Virginia-class SSNs.

For an attack submarine the size of the Virginia, the engineering benchmark discussed in the open literature for at least a decade calls for a propulsion motor capable of delivering 25,000 shaft horsepower--the electrical equivalent of 19 megawatts. This is approximately the per-shaft requirement of a destroyer-sized surface ship, and the prototype induction motor at Philadelphia was sized with that application in mind.

Because of its large size and inherent acoustic noise, however, the induction motor is a poor choice for submarines, and for that reason the community has investigated several other alternatives. The most promising to date seems to be the permanent-magnet synchronous motor, the rotor of which carries a dense assemblage of rare-earth magnets, which interact with rotating magnetic fields generated by a fixed stator. The "magnet" approach avoids the need for brushes or slip-rings and facilitates cooling, since electrical currents flow only in the stator itself. Several contractors are working on the concept to scale it up to full power, and there is high confidence that such a system can be fitted into a Virginia-sized hull.

"The Broadest Range of Ships"

The Navy Department agrees. In a March 1999 report to the Congress on integrated electric-drive systems, NAVSEA noted that the Navy has concluded that "the radial-gap permanent-magnet motor possesses the power density, acoustic performance, and maturity of technology to be a viable propulsion motor common to the broadest range of ships." Accordingly, NAVSEA's Advanced Submarine Technology program has funded several permanent-magnet efforts, and is supporting further research on superconducting, direct-current homopolar designs.

The first actual "submarine" application of these investments was the installation of a 6,000 horsepower permanent-magnet motor in the Navy's Large-Scale Vehicle (LSV)-2, a quarter-scale model of the Virginia used for hydroacoustic experiments at Lake Pend Oreille, Idaho.

The decision is currently on hold, but when the Navy selects the winning DD 21 contractor team several key features of the associated IPS approach are likely to emerge. The submarine community should be ready at that time to take full advantage of the lessons learned from the DD 21 propulsion plant and to augment those lessons with submarine-specific technology and with the development of components not supported in the surface ship program.

For this and other reasons, the fielding of an integrated electric-drive system for nuclear-powered submarines can be expected to lag behind the DD 21 effort by approximately five years. By 2010, though, the experience accumulated with DD 21, combined with steady progress in developing submarine-specific IPS components, should reduce technical risk sufficiently to fund the first electric-drive variant of the Virginia-class SSNS, and to put that ship to sea by 2015 or so.

Promising Future Technologies

For the more distant future, there are a number of more advanced technologies that hold significant promise for even greater efficiency, higher power density, and quieter running. As noted earlier, the Navy and industry are showing renewed interest in superconducting synchronous machines because of their potential to reduce the size of key system components. By incorporating supercold, zero-resistance conductors into field windings, their size could be reduced substantially without incurring excessive heat dissipation. Ultimately, main propulsion motors might be designed that would be compact enough to mount in external pods.

Prime movers using rotating machinery--turbines or combustion engines--may eventually be supplanted by high-power direct-energy conversion --which could transform the heat of a nuclear reactor into electrical energy through the use of advanced semiconductor materials or fuel cell technology. Again, by eliminating all the rotating mechanical components --except for those actually needed to drive the propeller --reliability, efficiency, and quieting would be increased dramatically while cost, weight, and maintenance requirements would be reduced.

Currently, the generation of sufficient output power for naval propulsion through these approaches would still be a daunting challenge, but the steady progress achieved to date in fielding all of the other components of an integrated power system bodes well for reaching this next and higher plateau as well.