Effect of grain length on GOx-paraffin hybrid rocket engines performance and regression rate

An experimental campaign was carried out on a hybrid rocket engine, using gaseous oxygen and paraffin wax-based fuel, to investigate the effect of fuel grain length on motor performance and internal ballistics. The results of the current work offer the reader a comprehensive overview of the pros and cons of reducing the fuel grain length and the difficulties in optimizing combustion efficiency. Steady-state numerical analyses were also performed to support the experimental findings. The motivation for the current work originates from a previous explorative test campaign performed with a 220 mm fuel grain length where only fuel-rich conditions were experienced. A trade-off analysis was carried out by introducing two additional configurations with grain lengths of 130 mm and 70 mm, since the grain length directly affects the mixture ratio and engine performance. Propellant mixing, combustion instability, entrainment, and regression rate are the main topics discussed in the paper. The results show that the reduction of the grain length increases the mixture ratio close to the optimum value, but mixing and combustion instability issues are promoted. No significant effects of the grain length on the regression rate have been found, leading to a discussion on the physics of the mass transfer mechanism of paraffin wax.