A "clean sheet" perspective unbound by current design programs has revealed a critical finding in determining ship power needs. In discussing the results of a comprehensive study of ship propulsion alternatives completed and released this spring, Howard Fireman, the Navy's chief naval architect, said that missions and required operational performance, not ship displacement, are the most important factors in evaluating options for propulsion architectures that produce the right "energy demand signal" for various ship types.
By that measure, nuclear power may be most efficient for ships with constant high demands for energy. Ships with lower energy requirements may operate more efficiently with a hybrid architecture such as a combined diesel-gas turbine plant and a hybrid transmission system-for example, a single shaft with a secondary propulsion unit.
The study, supported by the Naval Surface Warfare Center's Carderock Division and the Office of Naval Research, builds on two 2005 studiesan economic analysis aimed at reducing ship energy consumption and a detailed CNO-directed analysis of various ship technologies-neither of which looked at missions.
Rear Admiral Kevin McCoy, Naval Sea Systems Command's (NAVSEA) deputy commander for ship design, integration, and engineering, said that the study "brings in the needs of the warfighter," by looking at missions and operational requirements for speed, endurance, survivability, and operational tempo that could be achieved by a range of power-plant designs.
He pointed out that the alternative propulsion study did not consider current programs. Instead, it examined 23 notional ship "synthesis models": 12 representing small-surface combatants of about 7,500 tons; 7 models of medium-surface combatants of about 20,000-ton displacement, and 4 of amphibious assault ships in the 35,000 to 38,000-ton range.
In focusing on acquisition and life-cycle costs as well as mission effectiveness, the study evaluated several power-generation sources-nuclear, gas turbine and diesel engines, and combined diesel-gas turbine plants-as well as different power technologies-mechanical transmissions, hybrid power transmission systems, integrated power systems (IPS), and several propulser types. It determined that ship vulnerability can be reduced by power system redundancy including zonal power distribution, longitudinal separation of engines, use of auxiliary propulsors, and flexible power conversion by means of an IPS that generates power both for propulsion and ship systems.
Among the study's findings were that the number of refuelings and fuel capacity needed for high-speed surge is reduced by efficient energy conversion and high-energy densities, which are best provided by nuclear power; drives that minimize drag such as podded propulsors fitted to the ship's hull; and large tank capacities. Efficient power architectures based on nuclear, diesel, or combined diesel-gas turbine systems also enhance sustainability and fuel-cell technology is a promising future alternative.
It also determined that existing nuclearpower reactor designs are technically feasible for small- and medium-surface combatants and amphibs, but did not examine the costs of modifying construction capabilities. Nuclear power for propulsion and ship's power also offers significant advantages for surge to theater and time-on-station for all ship variants. These advantages will become even more important as the Navy fields new highenergy weapons, such as electro-magnetic guns and advanced high-power sensors. Future ships fitted with such systems will require "significant growth" in electrical loads that will impact power architectures and fuel requirements.
The study recommends that the Navy continue to use parameters of mission performance-for example, vulnerability and sustainability-when seeking options for future ship power systems. This approach will be especially important as the Navy considers adopting new commercial propulsion technologies. Further, the Navy is urged to invest more resources in research and development focused on affordability, improvements in power density, and efficiency of new power and propulsion technologies such as fuel cells, power distribution, propulsion transmissions, reduced hull drag, and advanced propulsors.
McCoy pointed out that NAVSEA is working with Navy contractors on a next-generation IPS study of technologies that could be incorporated into future power systems, with a goal of achieving commonality of architectures aboard carriers, surface combatants, amphibs, and submarines. Some early feedback is expected this fall, he said.
Mr. Walsh is a veteran reporter of Navy-Marine Corps news and former senior editor of Sea Power magazine.