Experimental and analytical investigation of effect of pressure on regression rate of axial-injection end-burning hybrid rockets

The regression characteristics of axial-injection end-burning hybrid rocket (EBHR) fuels having numerous small ports were experimentally investigated for the first time using a laboratory scale motor. In this paper, three requirements for EBHR fuel grains are explained in detail. High accuracy 3D printing allows for the production of fuel that satisfies the requirements for EBHR as defined in this paper. A data reduction method that overcomes the problem of multiple solutions to the c* equation is used to determine fuel regression rate with less than 10% error. Results of fifteen static firings tests show that fuel regression rate increases as the chamber pressure increases, which agrees with the trend revealed in previous studies (pressure exponent n is close to unity). No difference in combustion characteristics was found by comparing results of multi-port and single port fuel firing tests conducted in this and previous studies. A fuel regression model based on the Granular Diffusion Flame (GDF) model and Matthew and Frederick’s method is developed to investigate regression characteristics. Results calculated with this model agree with experimentally observed values, as well as the results calculated by Matthew and Frederick. However, this does hold true in tests with varying oxidizer port velocity. A GDF model only takes into account solid propellant regression, neglecting the effects of oxidizer velocity, and is shown in this study to be inappropriate for evaluating EBHR regression characteristics.