Proportional Controller Throttle Testing of a Liquid Rocket Engine

Throttling results from a nitrous oxide and isopropanol rocket engine at California Polytechnic State University San Luis Obispo are presented. In this study the results demonstrate that smooth and accurate throttling can be achieved with a proportional gain controller that omits gains that react to and predict variance from predetermined thrust setpoints. The proportional gain control was tested to determine the viability of a system that requires little to no tuning while eliminating derivative and integral controller gain. Without the use of integral and derivative gain, instead the stability of the system is controlled by the relationship between the nonlinear flow characteristics of a ball valve and dynamic flow through the fixed volume of the engine, hose, and injector. The rocket engine operated at nominal thrust for approximately five seconds, before throttling down to 33% for two seconds, followed by a return to nominal thrust. The results from a throttled static fire with a peak thrust of 900 lbs and a minimum thrust of 300 lbs, demonstrated that the plant side’s response could match the modeled thrust within 10% error, while being controlled, smooth, and constant.