Ballistic Characterization of Armored Grains with Vortex Flow Pancake Hybrid Rocket Engine

This work deals with a combustion behavior assessment of paraffin and paraffin-based fuels burning in a vortex flow pancake (VFP) hybrid rocket engine. Blending of paraffin wax with reinforcing polymer is a commonly implemented strategy pursuing the design of fuels with fast enough regression rate and suitable mechanical properties. Being the wax regression rate strictly related to the entrainment of melted fuel droplets, blending has the drawback of limiting the solid fuel ballistic response. A new reinforcing strategy based on 3D-printing of scaffold structures to be embedded in the solid fuel grain has been proposed by the Space Propulsion Laboratory (SPLab) of Politecnico di Milano and is here applied to the VFP engine configuration. This reinforcing strategy enables a reduction of the polymer mass fraction in the blend, thus enabling a more tailorable combination of ballistic and mechanical properties. In this paper, a microcrystalline wax is blended with 5 wt% of a styrene-based copolymer; the blended fuel is used as the embedding matrix for gyroid structures serving as reinforcement for the fuel grain. The ballistic response of the reinforced fuel is characterized under quasi-steady and forced transient operating conditions. Ballistic performance of the gyroid-reinforced fuel is contrasted with those of paraffin-based blends where mechanical properties improvement is afforded loading the wax with a styrene-based reinforcing polymer. Overall, the testing campaign consisted of nearly 29 firings under quasi-steady conditions, and 9 tests with forced transient operating conditions. The main observable parameters of interest for the quasi-steady campaign were the solid fuel regression rate and the engine propulsion efficiency (evaluated in terms of both characteristic velocity and thrust coefficient). For forced transient analyses, the engine ballistic response was characterized by pressure trace and overall system behavior. The proposed reinforcing strategy provided encouraging results, and the paraffin-based fuels showed suitable response to the forced transient operating conditions.