Tubular Equivalent Regression Rate in Hybrid Rockets with Complex Geometries

A new performance parameter titled “tubular equivalent regression rate” is introduced to evaluate burning rates in hybrid rockets with geometrically complex solid propellant grains. Tubular equivalent regression rates are calculated for eight previously reported CAMUI-type hybrid rocket firing tests and compared with extrapolations of previously reported empirical correlations for classic, swirl and vortex hybrid rockets. A non-dimensional number titled “CAMUI Number” is introduced to evaluate how CAMUI-like a solid propellant stack is. The CAMUI Number ranges from 0-1: 0 means no CAMUI-type blocks are used, 1 means only CAMUI-type blocks are used. The results show that the tubular equivalent regression rate increases logarithmically with CAMUI Number, and approaches a value of around 3 [mm/s] for a CAMUI Number of 1. This increase in tubular equivalent regression rate is shown to correspond to an increase in performance range from a classic (tubular) hybrid rocket at low CAMUI Numbers (0.1) to surpassing a vortex hybrid rocket for high CAMUI Numbers (>0.7). Furthermore, through the block-by-block analysis of tubular equivalent regression rate in a fuel stack with a CAMUI Number of 0.71, it is shown that maximum burning rates were achieved in blocks under slightly oxidizer rich conditions.