Closed-loop throttle control of a hybrid rocket motor.

Abstract

Hybrid rocket motors produce thrust by reacting a solid fuel with a liquid oxidizer inside a combustion chamber. This approach has certain advantages over conventional solid propellant rockets including improved safety and the potential for thrust control, while also being less expensive than liquid propellant engines. Liquefying hybrid fuels, such as paraffin wax, regress at a faster rate than the conventional solid fuels like HTPB that are dominated by vaporization at the solid-gas interface. Non-classical theory is still in its infancy, however, and more work is required to validate performance models experimentally, especially where throttling of the oxidizer mass flowrate is incorporated. While hybrid motor throttlabilty remains a subject of considerable interest, there has been little investigation of throttling in motors that use high regression rate, liquefying fuels such as paraffin wax. This study proposes a closed-loop thrust control scheme for paraffin wax/nitrous oxide hybrid rocket motors using a low-cost ball valve as the controlling hardware element. There are a number of advantages to throttling hybrid rocket motors but the most important is to enforce a constant thrust curve throughout the burn. A test facility and laboratory scale hybrid rocket motor utilizing paraffin wax as fuel and nitrous oxide as oxidiser were used for experimental testing. Using a mathematical model of a laboratory-scale hybrid rocket motor, the controller constants for a PID controller were obtained and tested through experimental testing. Open-loop testing was first done in order to determine the control authority of the ball valve over the oxidiser mass flowrate, as well as characterize the oxidiser mass flowrate in relation to each valve angle value. Closed-loop testing was undertaken to verify and refine the controller constants obtained via the laboratory-scale model. The tests prompted a redesign of the injector and additions to the LabVIEW™ controller regime. Using results from the open-loop tests a feed-forward lookup table was developed to allow for the controller to move to a specified angle quickly and thereby remove nonlinearities present in flow control using ball valves. Three successful closed-loop tests were done where the controller causes the thrust of the motor to track a predetermined thrust or chamber pressure set point with a reasonable degree of accuracy. The set-point profile of the first test was a constant thrust throughout the burn while the second test had a ramp set-point profile. The final test used chamber pressure as the feedback variable and had a step-down set-point profile. This study demonstrates that thrust control can be exercised over a paraffin wax/nitrous oxide hybrid rocket motor, using a low-cost ball valve as the control element to modulate the oxidiser mass flowrate.