Feasibility Study of an Inverse Brayton UAV Propulsion System

The advancement of currently available propulsion systems, as well as new technologies, is necessary for the improvement of current and future Unmanned Aerial Vehicle (UAV) range and performance. The traditional gas turbine provides high reliability and power density, beneficial to aircraft applications. However, at the small power requirements of typical UAV's, the gas turbine suffers from significantly increased fuel consumption. A potential alternative to the traditional thermodynamic operation of the gas turbine is an Inverse Brayton cycle (IBC). The sub-atmospheric operating pressure of an IBC increases the size of the engine components in comparison to a traditional Brayton cycle. This increased size enables improved turbomachinery efficiencies. It is the goal of this study to investigate the potential for increased gas turbine-powered UAV range through the use of an IBC. Numerical Propulsion System Simulator (NPSS) was used to perform thermodynamic design point calculations for cycle comparisons, and literature sources were used to estimate compressor efficiency and overall engine weight scaling. Results show that linearly scaling the geometry of engine components for their use in an IBC results in an increase in component weight that counteracts the improvement in overall efficiency.