Design and Development of an Axisymmetric Variable Area Nozzle for Small Turbojet Engines

This paper focuses on the design and development of a variable area nozzle for small turbojet engines, aimed at optimizing thrust performance and enabling rapid thrust modulation without engine spooling delays. For the purposes of this study, small turbojet engines include engines with a mass flow rate lower than five pound-mass per second. The primary objective is to induce thrust spoilage of eight pounds at 80% engine throttle and exceed 1900 pounds of thrust at full throttle while minimizing weight and size. A Jetcat P100-RX was used to develop and test the nozzle concept. In the initial phase of the project, an On-Design Parametric Cycle Analysis (PCA) and Off-Design Engine Performance Analysis (EPA) were conducted to characterize the engine’s behavior under different operating conditions. Using the Mass Flow Parameter (MFP), optimal and spoiled nozzle exit areas were determined to achieve an eight pound-force thrust range at 80% throttle. Several nozzle actuation concepts—including the feathered, iris valve, and slotted square designs—were evaluated based on manufacturability, precision, and thermal expansion considerations. Baseline testing of OSU’s JetCat P100 engines confirmed the theoretical thrust predictions, guiding material selection to high-temperature-resistant materials. Building on this foundation, the second phase of the project refined the actuation mechanisms, down-selecting to a circular rack-and-pinion feather mechanism as the most promising solution. Detailed computational fluid dynamics (CFD) and finite element analysis (FEA) were conducted to validate the design’s structural integrity, thermal performance, and aerodynamic efficiency. 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 successfully achieves the required actuation range while maintaining structural reliability and minimal weight. Future work includes physical testing under operational conditions to validate computational predictions and optimize nozzle responsiveness. This project demonstrates the feasibility of a variable area nozzle mechanism for small turbojet engines, providing valuable insights for future propulsion applications and potential commercialization through OSU. By enhancing the engine’s thrust modulation capabilities, the design offers improved performance for UAV applications and military aviation programs.