Closed-Loop Precision Throttling of a Hybrid Rocket Motor
A closed-loop throttle controller for a lab-scale N2O and HTPB hybrid rocket motor is presented. Closed-loop throttling was achieved using commercial-off-the shelf valve hardware, and a commercially available motor case adapted for hybrid rocket testing. Multiple and open and closed-loop tests were performed to demonstrate that closed-loop control can significantly reduce the run-to-run burn variability typical of hybrid rocket motors. Closed loop proportional/integral (PI) control algorithms featuring thrust or pressure feedback, are used to track prescribed step and linear ramp profiles. Because the relationship between the selected throttle control-valve position and the effective valve flow area was highly non-linear, the effect of valve position on motor trust/chamber pressure was measured open-loop and curve fit to allow direct command of either total thrust or chamber pressure. Control law gains were tuned a priori using a numerical model, and then adjusted using the actual test hardware. Response profiles were optimized according the integral absolute error (IAE) criterion. Control law tuning examples are presented. Test results indicate that to a 95% confidence level, closed-loop throttling significantly reduces the mean run-to-run thrust variability from +9.1% to less than +3.9%. When effects of nozzle erosion are accounted for, the closed-loop thrust variability reduces to +1.5%.