Computational fluid dynamics optimisation of grid fin aerodynamic design for reusable launch vehicles

Abstract

Grid fins are an unconventional flight control surface utilised by SpaceX for its Falcon 9 reusable launch vehicle upon re-entry. Previous literature surrounding grid fins has used computational fluid dynamics (CFD) to investigate sensitivity factors of the design to vary drag or maximise hinge moment but often suffer from limitations (e.g., insufficient modelling of the boundary layer, or no evidence of sensitivity studies/validation).Due to the lack of literature a systematic CFD-based method is employed in which grid fin geometry is simplified to a 2D flat plate and validated against Tekure (2021) and the simulation verified using oblique shock wave theory (White, 2009; NACA and NASA, 2017). The method increases in complexity as it progresses to a 2D lattice and subsequently a 3D cell (1 portion of a grid fin), investigating the impact of plate spacing, thickness and material selection on the total drag and maximum temperature. In line with the presented methodology each simulation undergoes mesh and domain studies to ensure sufficient convergence of the solution and to certify independence of the solution. Subsequently a design is suggested that increases drag by 21.7% whilst maintaining the original designs measured maximum temperature. By considering the effects of the increased drag and varying the grid fin material, a simplified grid fin geometry is applied to analytical beam bending theory to provide an estimated factor of safety (FoS) and suggest the validity of composite integration.