Performance modelling and efficiency analysis of a micro turbojet engine under varying flight conditions

Abstract Micro turbojet engines play an increasingly important role in low-thrust, subsonic applications such as target drones and small unmanned aerial vehicles. Their performance, however, cannot be directly inferred from large-scale turbojets, as differences in scale lead to distinct thermodynamic and propulsive characteristics. In this study, a micro turbojet engine is analytically modelled to evaluate its performance through a stage-by-stage analysis of the inlet, compressor, combustion chamber, turbine, and nozzle. Thrust and exhaust gas temperature predictions are validated against literature data, and the model estimates a back work ratio between 0.81 and 0.67 over a speed range of 60,000–108,000 RPM. Performance parameters including thrust, exhaust gas temperature, thrust specific fuel consumption, specific thrust, thermal efficiency, propulsive efficiency, and overall efficiency are assessed across a range of Mach numbers and altitudes to capture different flight conditions. The efficiencies are further compared with those of an ideal large-scale turbojet and another micro turbojet engine. Results show that while thermal and propulsive efficiencies vary between engines of different scales, the overall efficiencies remain similar, with a near-linear dependence on Mach number up to the nozzle choking condition. Altitude has a moderate influence on thermal and propulsive efficiencies but only a minimal effect on overall efficiency.