3D-Printed cellular reinforcements: Mechanical and ballistic responses of paraffin-based armored grains

Additive manufacturing enables the fabrication of complex open-cell geometries that can be used as mechanical reinforcements in paraffin-based fuels for hybrid propulsion. In this study, heterogeneous fuel grains are characterized in terms of their mechanical response and burning behavior. Tested formulations feature a 3D-printed reinforcement in polylactic acid, and are named armored grains. Four strengthening reinforcements are produced by fused deposition modeling: two triply periodic minimal surface structures (gyroid and Schwarz-P), and two honeycomb-based reinforcements (straight and twisted). Gyroids are contrasted considering different infills (i.e., volume fraction of the 3D-printed part), while different structures are contrasted for a single infill. The results show that all reinforcements markedly increase the yield stress and strain energy of the pristine paraffin, while enhancing the regression rate (r˙f). This result is noteworthy, since reinforcements typically suppress the fast r˙f of waxes. The uneven surface of the burning grain yields increased heat transfer from the flame to the surface, thus providing faster r˙f. Regression rate enhancement for gyroids and twisted honeycombs is in the range 25%–27% for oxidizer mass flux ∼40 kg/(m2·s). Finite-elements analyses on the reinforcing structures capture the observed collapse modes. The performed screening provides structure-property-performance details for the design of 3D-printed reinforcements in paraffin-based fuels for hybrid propulsion. Gyroids emerge as suitable structures for grain reinforcement given their stress distribution and small-scale r˙f performance.