by ADM Frank L. "Skip" Bowman, USN

USS Virginia (SSN-774 will be the most technologically advanced submarine in the world. Future variants would benefit from the increased capability provided by an integrated electric power system. The Navy has told Congress that an integrated power system could be installed in a Virginia - class submarine authorized for construction around 2010.

Submarines today are working harder than ever. While our submarine force has been cut nearly in half since 1989, the number of intelligence, surveillance, and reconnaissance missions performed by submarines has nearly doubled. Rather than countering one monolithic force as we did during the Cold War, we are now responding to the multi-mission demands of dynamic trouble spots around the globe. Further challenging our submarine force is the accelerating pace of technology available to our potential adversaries. In order to develop and maintain the capabilities necessary to ensure national security, we must innovate in four key areas to face the challenges of the 21st century:

• Improve our connectivity with the Battle Group, Joint Forces, and National Command Authority
• Develop and employ disruptive technologies and payloads
• Extend modularity to allow a more "tailorable" submarine
• Incorporate integrated electric power systems

In the Spring 1999 issue of UNDERSEA WARFARE, I touched upon these four areas of innovation. Today, I want to focus on integrated electric power systems. 

The Continuing Role of the Reactor 

Although our modern nuclear reactors offer enough stored energy to power the ship throughout its lifetime, their mechanical drive propulsion systems immediately reserve 75-80 percent of useful reactor power output exclusively and unalterably for propulsion. If we incorporate an integrated electric power system, large amounts of reactor power output could be placed on a common electrical bus, allowing allocation of that energy as determined by tactical requirements, whether that be answering a maneuvering bell or delivering payload or a combination of the two. The ability to redistribute power on demand will give the commanding officer enormous operational flexibility. 

A Description of an Integrated Electric Power System 

In today's submarine propulsion systems, power generated by the reactor is transferred to a prime mover - a steam turbine - which mechanically powers the propeller shaft through a series of reduction gears. Ship's service turbine generators convert mechanical energy into electrical power for combat systems and other loads. In an integrated electric power system, the propulsion and auxiliary steam turbines are replaced with steam-driven main turbine generators as the prime movers. These main turbine generators convert all the available reactor power into electrical power, which is then sent to a common electrical bus for allocation. 

Through flexible distribution and switching architecture, the common electrical bus can supply electrical power to both non-propulsion and propulsion electrical loads and instantly redistribute power as necessary. For propulsion, electrical power from the bus is sent to a motor drive (often referred to as a motor controller), where the voltage and frequency of the electrical energy are modified to operate the propulsion motor at a desired speed. The propulsion motor then converts the electrical energy delivered by the motor controller into mechanical energy to rotate the propeller shaft. 

As you can see from the diagrams, the rigid alignment of reduction gears and a long shaft required by mechanical drive is replaced by the flexibility of an electric motor and a short shaft. Cables, instead of a shaft, connect the turbines to the propulsion motor and provide architectural flexibility for the designer to make space available for payloads, improve the submarine's stealth, and ease construction and maintenance. More importantly, be-cause the power of an integrated electric system is consolidated on a common electrical bus, the power previously reserved exclusively to propel the submarine at high speeds is now available to the commanding officer for other uses when high-speed propulsion is not an operational requirement.

The Technology of Electric Drive 

Electric drive has a long history dating back to the early 1900s. Electric drive was pursued and used on several platforms, including submarines. The diesel-electric power plant was widely used on submarines prior to the development of naval nuclear propulsion, allowing the boats to propel themselves underwater for short periods while on battery power. The Navy built two nuclear-powered attack submarines to experiment with the technology of electric drive: Tullibee, commissioned in 1960 and decommissioned in 1988, and Glenard P. Lipscomb, commissioned in 1974 and decommissioned in 1990. However, the electric motor and power conversion technology of the 1960s and 1970s was not sophisticated enough to compete with the advances made in mechanical drive. As a result, these early electric-drive submarines were much slower and more difficult to maintain than their mechanical-drive peers. 

Since the 1980s, many improvements have been made in electric motors, motor drives, and semi-conductor conversion devices. Modern high-strength magnet materials allow new compact permanent magnet motor designs to provide power sufficient for ship propulsion in a package small enough for submarine use. Permanent magnet motors are being developed for a variety of ship propulsion applications, including future Virginia- class submarines. New solid-state power electronic switching devices allow electric propulsion systems to achieve a level of stealth not possible with even the most advanced mechanical drive. The rapid pace of improvement in these devices will further allow the Navy to upgrade the motor control capabilities installed in tomorrow's electric-drive submarines without having to replace at great cost the entire propulsion motor and drive.

The Bottom Line: The technology necessary for a submarine integrated electric power system exists today. The engineering challenges that face us are scaling this technology up to meet the power demands of a warship, integrating and interfacing systems and controls, and meeting acoustic quieting and shock requirements. Fortunately, the submarine community is not facing these challenges alone. 

The Navy's Corporate Structure for Integrated Electric Power Systems 

Integrated electric power systems will benefit a variety of ship types throughout the Navy. In January of this year, the Secretary of the Navy announced that the Zumwalt-class (DD-21) Land Attack Destroyer would be equipped with an integrated electric power system. In June, the Navy testified before Congress that an integrated electric power system could be installed in a Virginia-class submarine authorized for construction around 2010. To ensure the Fleet gets the greatest return on the investment in integrated electric power systems, the Navy plans to take a "corporate" approach to development where appropriate. The Navy will make sure the technologies developed for one platform are applied to other platforms when they are needed. 

Advantages for Submarines 

Stealth Enhancement: Stealth is the single most important operational advantage of any submarine. It permits use of the submarine in politically sensitive or militarily contested areas to gain and sustain access for the rest of the battleforce. It creates uncertainty, fear, and disproportionate diversion of resources on the part of the adversary. It allows submarines to be used in many covert and non-provocative, intelligence collecting operations. An integrated electric power system will enable necessary changes to key propulsion plant parameters, afford more flexibility in equipment selection and location, and permit use of other quieting methods. Electric drive opens the door to new methods for improving stealth by leveraging state-of-the-art technology with good future growth potential.

Power and Energy Availability: As I mentioned earlier, with mechanical drive 75-80 percent of the useful power produced by the reactor is available exclusively for propulsion. An integrated electric power system, on the other hand, puts power on a common electrical bus and gives the commanding officer operational flexibility in how this energy is distributed to suit the range of payloads, sensors, and propulsion needs for a given tactical situation. An integrated electric power system will allow tomorrow's submarines to make greater use of rechargeable off-hull vehicles, payloads, and sensors to extend the submarines' tactical reach and safeguard operations in high risk and restricted areas.

Payload Volume and Ship Reconfiguration Flexibility: The Virginia-class modular construction practices, and design features, such as a reconfigurable torpedo room to accommodate underwater vehicles and special operating forces, represent a significant improvement in the ability to modernize and reconfigure our submarine force in a more affordable and timely manner. Submarines, in the confines of littoral waters, could be able to deploy a wide array of unmanned electric vehicles and remote devices to gather intelligence; disable mines; provide early detection of enemy vessels; and deploy countermeasures against incoming weapons. This capability would increase the submarine's range within the battlespace while reducing potential detection by minimizing ship maneuvering. To do this, another step change in the degree of modularity will be necessary to accommodate more (and as yet unidentified) payload and sensors and to achieve a wholly reconfigurable multi-mission fleet, a fleet that must meet emerging and growing demands to make more efficient use of naval platforms. An integrated electric power system will allow increased flexibility and decentralization of ship systems and component arrangements, as well as decentralization and elimination of hydraulic and pneumatic fluid systems, to further enhance modularity in design, construction, operation, training, and maintenance.

Two early nuclear-powered electric drive submarines: USS Tullibee (SSN-597, top), and USS Glenard P. Lipscomb (SSN-685, bottom). Immature electric-drive technology in the 1960s and 1970s limited the performance of these boats in comparison to their mechanical-drive counterparts.

Reduced Logistics Dependency: Modern Space Age and Information Age tools available in the commercial sector, together with proliferating precision-guided munitions and missiles in the hands of adversaries, will put non-stealthy and non-mobile logistics assets at risk. Additionally, the political realities of reduced forward basing and reduced military support infrastructure greatly complicate the business of logistics. Today's nuclear-powered submarines have a demonstrated record of being self-sufficient in terms of fuel, food, equipment reliability, and so forth. Direct use of electrical power extends this self-sufficiency because it results in more efficient use of payload space, such as delivery of payload with reduced (or no) attached propellant or propulsion system. For example, with larger quantities of electrical power available, hydrogen and oxygen can be made from seawater to act as fuel for a fuel cell. It also offers the prospect of regenerative, vice expendable, weapons and countermeasures, such as directed energy, limited only by the stored energy of the reactor. 

Surface ships can also benefit from electric-drive. The Navy recently announced that the Zumwalt-class (DD-21) Land Attack Destroyer will be an electric-drive ship.

Conclusion 

With all the advantages an integrated electric power system has to offer the submarine force, it is critical that we take this next step in advancing the design of our submarine propulsion plants. Our unparalleled history of technological innovation foreshadows this next step. Our role in national security demands it. Our dedicated Sailors deserve it. Or, as Secretary Danzig stated last January, "Changes in propulsion systems are fundamental and of fundamental importance." Thus, "we are moving forward to embrace a technology, electric drive technology, and ... the integrated power system that comes with it, to drive Navy ships. This is a very fundamental step.... We're taking it because we've judged that the technology is ripe enough to reach it...."

ADM Bowman is the Director, Naval Nuclear Propulsion.