Recommendation. The U.S. Air Force should include research on propulsion systems for UAV applications in its long-term research program. The following general research topics should be included:

  • high-altitude propulsion technologies, which may include gas turbines, internal combustion engines, solar-powered motors, or fuel cells

  • propulsion systems for small, highly maneuverable vehicles, including vertical takeoff and landing (VTOL) capabilities

  • computational modeling capability to reduce the need for engine testing during development

  • cost-reducing technologies that, for example, reduce parts count and complexity

The following specific research topics should be considered:

  • low Reynolds number turbomachinery, which is very important for both high-altitude operation and very small vehicles

  • low Reynolds number heat rejection for high-altitude coolers and for cooling very small propulsion systems at lower altitudes

  • turbomachinery tip-clearance desensitization (for highly loaded engines, high-altitude operation, and very small systems)

  • desensitization to leakage and better, cheaper seals to reduce cost and enhance performance for highly maneuverable and very small vehicles

  • thrust vectoring for highly maneuverable vehicles

  • magnetic, air, and solid lubricated bearings to improve long-term storage, enhance high-altitude operation, and reduce complexity and cost

  • technologies for low-cost accessories, which tend to dominate the cost of smaller engines

  • low-cost vapor and liquid cooling schemes and affordable high-temperature materials (e.g., structural, magnetic, and electronic materials)

  • more effective cooling technologies for small engines

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