| Summary: | This article explores the throttling aspect of the hybrid rocket motor through experiments using a lab-scale motor. The lab-scale motor utilizes a wax-Al based fuel and compressed air as the oxidizer. The oxidizer flow rate was modulated using a PID controller to study the closed-loop thrust control performance of the motor. Numerical simulations and cold flow tests were carried out to identify the suitable gains for the PID control algorithm. Pressure feedback was used in the control algorithm to obtain the closed-loop thrust control. The resultant closed-loop system followed the reference pressure accurately during the step input response test of the system. The maximum error in the observed chamber pressure was <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>1.86</mn></mrow></semantics></math></inline-formula>% for a reference pressure of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>4.69</mn></mrow></semantics></math></inline-formula> bar, which corresponds to a reference thrust of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>117.6</mn></mrow></semantics></math></inline-formula> N. The response of the system for a ramp input, with linear thrust variation from <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>78.4</mn></mrow></semantics></math></inline-formula> N to <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>127.4</mn></mrow></semantics></math></inline-formula> N, showed that the measured thrust followed the desired ramp profile with a root-mean-square error of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>1.99</mn></mrow></semantics></math></inline-formula> N. A ramp-down test with the same thrust range produced a root-mean-square error of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>6.2</mn></mrow></semantics></math></inline-formula> N.
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