A modified regression rate formula for the uppermost stage of CAMUI-type hybrid rocket motor is proposed in this study.\nAssuming a quasi-steady, one-dimensional, an energy balance against a control volume near the fuel surface is considered.\nAccordingly, the regression rate formula which can calculate the local regression rate by the quenching distance between the\nflame and the regression surface is derived. An experimental setup which simulates the combustion phenomenon involved in\nthe uppermost stage of a CAMUI-type hybrid rocket motor was constructed and the burning tests with various flow velocities and\nimpinging distances were performed. A PMMA slab of 20mm height, 60mm width, and 20mm thickness was chosen as a sample\nspecimen and pure oxygen and O2/N2 mixture (50/50 vol.%) were employed as the oxidizers. The time-averaged regression rate\nalong the fuel surface was measured by a laser displacement sensor. The quenching distance during the combustion event was also\nidentified from the observation.The comparison between the purely experimental and calculated values showed good agreement,\nalthough a large systematic error was expected due to the difficulty in accurately identifying the quenching distance.
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