The Effect of the Presence of Grid Fins on the Aerodynamic Coefficients During Landing of the First Stage of Falcon 9 Rocket

Abstract

Reusable first-stage boosters have sharply reduced the cost of orbital launch, but recovering them depends on precise aerodynamic control during atmospheric descent. During the unpowered coasting phase, with the main engines off, grid fins are the primary means of steering the vehicle: the lift and drag they generate adjust its trajectory and orientation. How much their presence changes the booster’s aerodynamic coefficients is therefore central to descent control, yet few studies isolate that effect across a range of angles of attack. This investigation is a numerical, qualitative assessment of it for a Falcon 9 first stage at a fixed altitude of 12 km and angles of attack from 0° to 18°. A simplified three-dimensional computational fluid dynamics (CFD) model was used, approximating the booster as a right circular cylinder with an idealized lattice representing the fins, held at zero deflection to isolate the effect of their presence alone. Drag and lift coefficients (Cxa, Cya) were compared with and without the fins. Their presence increased both coefficients, with the largest effect on lift — a ≈20% increase at 18°, compared to only ≈5% for drag — and raised the efficiency parameter K (Cya/Cxa) from 0.89 to 1.01 at 18°, indicating improved control authority. These results show that grid fins meaningfully enhance the booster’s aerodynamic controllability during descent, particularly at higher angles of attack, and demonstrate that simplified CFD modeling can capture these effects without the cost of a full-fidelity simulation.