Propulsion Systems for Navy Ships and Submarines

GAO-06-789R: Published: Jul 6, 2006. Publicly Released: Jul 6, 2006.

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Janet A. St Laurent
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In recent years, the Navy has used nuclear propulsion systems for its submarines and most aircraft carriers and conventional propulsion systems that rely on fossil fuel for its surface combatants and amphibious warfare ships. As the Navy looks to design an affordable force that is capable of meeting future security challenges, some of the assumptions and factors that have guided past Navy decisions on propulsion systems may require reassessment. For example, technological advances have enabled greater efficiency in both nuclear and conventional propulsion systems. Moreover, the cost of fossil fuel has risen sharply in recent years. Congress requested that we review the Navy's assessment of alternative propulsion methods for submarines and surface combatants. Our objectives were to determine (1) the status and scope of key Navy studies on alternative propulsion methods, (2) the major improvements to existing propulsion systems, (3) near-term and future ships' propulsion systems, and (4) the various ship propulsion related technologies the Navy is pursuing. In March 2006, we provided you with a briefing of our findings regarding propulsion systems for Navy ships and submarines. This report summarizes the results of that briefing as well as additional work we performed since that time.

The Navy has completed one study, and is in the process of completing two other studies on alternative propulsion systems for surface combatants, amphibious warfare ships, and submarines. The completed study is a "quick look" analysis of comparative life cycle costs of nuclear and fossil-fueled surface combatants and amphibious warfare ships. Although the study attempted to examine the fiscal break-even point for nuclear and conventional propulsion systems, it had several limitations. Specifically, it did not consider the operational requirements or advantages of nuclear and conventionally powered propulsion systems, nor did it undergo a high-level, Navy-wide review. According to Navy officials, the second study, required by the 2005 Chief of Naval Operations guidance, will be similar, but will provide in-depth analysis covering costs and operational factors for surface combatants as well as submarines. The Navy anticipates that the third study, required by the National Defense Authorization Act for Fiscal Year 2006, will build upon the Chief of Naval Operations study. Our limited review indicates that while the planned methodology for this study, as described by Navy officials, appears reasonable, its usefulness will depend on the extent to which the Navy uses accurate, reliable data and reasonable assumptions for its modeling and considers all relative costs. Nuclear and conventional propulsion systems for Navy ships and submarines have been improved in recent years. According to Navy officials, nuclear power plants are now simpler and smaller with reduced maintenance and personnel requirements, and their life span has also been increased. These reported improvements have eliminated the need for refueling newer submarines, such as the Virginia class submarines. Improvements have also been reportedly made to conventional propulsion systems, such as the Integrated Power System, which produces electrical power for both the propulsion system and ship's support systems. Ships being developed in the near term and long term will have a variety of newly designed propulsion systems depending on their size, mission, and ship characteristics. For example, the Littoral Combat Ship will have two diesel engines for low-speed operations, which will be augmented by two gas turbine engines for high-speed operations. The next-generation destroyer, DDG 1000, will have an Integrated Power System consisting of four gas turbines and two advanced induction motors, which will supply electrical power for the propulsion and ship support systems. The first aircraft carrier to be built under the CVN 21 program will have a newly designed nuclear power plant, and the Navy's amphibious replacement ship, LHA 6, will utilize a combined gas turbine and electric propulsion system instead of the steam propulsion systems now used in many amphibious warfare ships. The Navy spent over $212 million from fiscal years 2003 through 2005, and plans to invest an additional $264 million from fiscal years 2006 through 2011 to develop propulsion and ship support technologies designed to make future ships more fuel efficient and mission effective. These technologies, which are at various levels of maturity and not yet ready for implementation, focus on making electric motors smaller but more powerful, using high-speed generators without reduction gears, and using fuel cells. These technologies will still require fossil fuel as an energy source, but Navy officials stated they have the potential to reduce the amount of fossil fuel needed and improve ship operations.

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