Design and development of an axisymmetric variable area nozzle for small turbojet engines

This paper presents the design, manufacturing, and testing of a variable area nozzle for small turbojet engines, specifically defined in this study as engines with a mass flow rate lower than five pound-mass per second. The goal is to optimize thrust performance and enable rapid thrust modulation without engine spooling delays, addressing a critical need for precise thrust control in unmanned aerial systems and other propulsion applications. The JetCat P100-RX turbojet engine was selected as the baseline for analysis, with performance targets including a thrust spoilage range exceeding 8-lbf at 80% throttle and a maximum thrust exceeding 19-lbf at full throttle, all while minimizing weight and size constraints. To characterize the performance of the engine, an On-Design Parametric Cycle Analysis and Off-Design Engine Performance Analysis were conducted to determine the optimal and spoiled nozzle exit areas using the Mass Flow Parameter. Several nozzle actuation concepts, including feathered, iris valve, and slotted square designs, were evaluated based on manufacturability, precision, and thermal expansion considerations. Baseline testing the JetCat P100 engines confirmed theoretical thrust predictions and guided the selection of high-temperature-resistant materials. The down-selected concept, a circular rack-and-pinion feather mechanism, was selected for further refinement. Finite Element Analysis and Computational Fluid Dynamics were performed to validate the structural integrity, thermal performance, and aerodynamic characteristics of the design. An iterative prototyping approach was implemented, incorporating outsourced components such as ball linkages, servo motors, and metal-printed nozzle feathers to ensure robust actuation. The final design achieves the required thrust modulation range while maintaining structural reliability and minimal weight. This study demonstrates the feasibility of a variable area nozzle for small turbojet engines, offering insights into future propulsion applications and potential commercialization. By enhancing thrust modulation capabilities, the system provides improved performance for unmanned aerial system applications and military aviation programs. Future work includes physical testing under operational conditions to validate computational predictions and further optimize nozzle responsiveness.