Impact load reduction in water-entry vehicles enabled by Re-entrant Hexagonal lattice structures

Cross-medium vehicles, as a new class of platforms capable of high-speed operation in both air and water, face a critical challenge of excessive peak impact loads during oblique water entry. Conventional cushioning materials generally suffer from low energy absorption efficiency and insufficient structural stability under such extreme conditions, rendering them inadequate for engineering applications. To address this issue, this study proposes a load-mitigation strategy based on a Re-entrant Hexagonal (REH) lattice structure. Benefiting from its low weight, high strength, high specific energy absorption, and geometric design flexibility, the proposed lattice effectively attenuates water-entry impact loads. The oblique water-entry load characteristics were first captured using a dedicated cross-medium experimental platform, while the lattice parameters were optimized through a particle swarm optimization–support vector machine (PSO–SVM) algorithm. Subsequently, a finite element model employing an equivalent loading method was developed to elucidate the dynamic response mechanisms, leading to the design of an embedded lattice-based load-mitigation nose cap. Experimental validation demonstrated that the proposed structure achieved a 50.6% reduction in peak load through progressive plastic buckling, multi-hinge formation, and energy dissipation. Overall, this study clarifies the water-entry load-mitigation mechanism of REH lattices and provides theoretical and technical support for the impact protection design of cross-medium vehicles.