Design Overview of a Three Kilowatt Recuperated Ceramic Turboshaft Engine

A three kilowatt turboshaft engine with a ceramic recuperator and turbine has been designed for small unmanned air vehicle (UAV) propulsion and portable power generation. Compared with internal combustion (IC) engines, gas turbines offer superior reliability, engine life, noise and vibration characteristics, and compatibility with military fuels. However, the efficiency of miniature gas turbines must be improved substantially, without severely compromising weight and cost, if they are to compete effectively with small IC engines for long-endurance UAV propulsion. This paper presents a design overview and supporting analytical results for an engine that could meet this goal. The system architecture was chosen to accommodate the limitations of mature, cost-effective ceramic materials: silicon nitride for the turbine rotors and toughened mullite for the heat exchanger and turbine stators. An engine with a cycle pressure ratio below 2:1, a multistage turbine, and a highly effective recuperator is shown to have numerous advantages in this context. A key benefit is a very low water vapor-induced surface recession rate for silicon nitride, due to an extremely low partial pressure of water in the combustion products. Others include reduced sensitivity to internal flaws, creep, and foreign object damage; an output shaft speed low enough for grease-lubricated bearings; and the potential viability of a novel premixed heat-recirculating combustor.