Investigation of paraffin combustion in a hybrid rocket slab burner

<ns3:p>Hybrid rocket engines (HREs) are a category of chemical rocket propulsion systems in which the two propellants are stored in different states of matter. They have been often overlooked due to their lower performance with respect to conventional liquid rocket engines, and increased complexity over solid rocket motors. Recently, HREs are gaining new momentum for small launcher applications, as their performance is coupled with intrinsic safety, throttle-ability, flexibility and low cost, promising advantages when aiming to build a cost-effective space transportation system. Taking advantage of a hybrid rocket slab burner, an experimental HRE where a two-dimensional solid fuel grain is tested in a combustion chamber with optical access, the combustion behavior of paraffin wax with oxygen has been investigated, measuring the regression rate of the fuel and acquiring high-speed video recordings of the combustion. Multiple tests have been performed under different oxidizer mass flux and combustion chamber pressure conditions, grouped into four main experimental campaigns, providing insights into the fuel regression rate dependence on these parameters, allowing to trace the experimental correlation between oxidizer mass flux and fuel regression rate. Then, taking advantage of the video recordings of the combustion, phenomena such as liquid droplets entrainment have been visualized, as well as a qualitative description of the combustion behavior at different combustion chamber pressure and oxidizer mass flux. The effect of the oxidizer mass flux on the regression rate of the paraffin slab has been assessed, and the experimental data proved to be in accordance with the predictions coming from literature, as an increase in mass flux correlates with an increase in the measured regression rate. The effect of pressure at low mass fluxes, however, has not been clearly assessed in the study, and further investigations are required for future studies.</ns3:p>