Investigation of the Lightweighting Potential of Hierarchical Honeycomb Core Topologies for Spaceborne Primary Mirrors

This study investigates the lightweighting potential of hierarchical honeycomb core architectures for spaceborne primary mirrors while preserving the optomechanical performance of a conventional honeycomb reference mirror. Three hierarchical honeycomb core topologies were considered: spiderweb hierarchical honeycomb (SWHC), diamond hierarchical honeycomb (DHC), and triangular hierarchical honeycomb (THC) cores. To evaluate their mass reduction potential, several series of mirror configurations with nominal masses ranging from 10 kg to 5 kg were generated using three independent mass reduction strategies and evaluated using finite element analysis. The first strategy relies on wall thickness reduction while maintaining a constant cell size, the second increases the cell size while maintaining constant wall thicknesses, and the third combines both approaches. For each topology and mass reduction strategy, a two-stage parametric search was conducted. An initial search using 0.5 kg mass increments was first performed to identify the lightest configuration exhibiting optomechanical performance equal to or better than that of the conventional honeycomb core reference mirror. A refined search using 0.1 kg mass increments was then performed between the lightest acceptable configuration and the immediately lower mass level identified in the initial search, allowing the lightest acceptable configuration to be identified more precisely. The results demonstrate that the THC topology consistently exhibits the greatest lightweighting potential among the investigated hierarchical architectures across all three strategies. Moreover, the first two mass reduction strategies achieve the highest mass reduction of 46.37%, corresponding to a lightweight ratio of 80.37%, compared with 63.40% for the conventional honeycomb reference mirror while maintaining equivalent optomechanical performance. However, the second strategy emerged as the most practical, combining substantial mass reductions with wall thicknesses that remain potentially more manufacturable using conventional manufacturing processes. Overall, the study demonstrates that hierarchical honeycomb core architectures, particularly the THC topology, provide an effective solution for the mass reduction in lightweight spaceborne primary mirrors.