Abstract: During spacecraft missions such as in-orbit ignition and maneuvering, propellant in tanks is prone to sloshing under variable thrust, which in turn exerts reaction forces on the spacecraft, inducing rigid body-liquid coupled oscillations and increasing the difficulty of coordinated and high-precision control. A rigid body-fluid coupled sloshing solver for propellant tanks was developed based on the open-source CFD (Computational Fluid Dynamics) platform OpenFOAM, in which the rigid body quaternion equations and the fluid Navier-Stokes equations are iteratively solved at each time step to achieve a two-way coupled simulation of the system’s six-degree-of-freedom dynamics. Numerical simulations were conducted to investigate liquid settling in cylindrical propellant tanks with elliptical end caps under various external forces. The effects of filling ratio, settling acceleration, and the presence of coupling on the gas-liquid interface morphology, fluid distribution, center-of-mass oscillation, evolution of the angular velocity response, and settling efficiency were analyzed. Results indicate that fluid oscillation and settling efficiency do not change monotonically with the filling ratio. Increasing the settling acceleration effectively enhances the settling rate and efficiency but amplifies fluid oscillations at high filling ratios. Coupling effects are significant at high filling ratios, mainly inducing six-degree-of-freedom (6-DOF) disturbances, exciting new oscillation frequencies, and slowing the decay of oscillations, which is unfavorable for liquid settling. However, at low filling ratios, coupling effects are weaker, favoring overall system stability, although the gas-liquid interface tends to destabilize, which leads to gas-liquid mixing. In summary, this study suggests that both residual propellant volume and external loading conditions, along with rigid body-liquid coupled disturbances, should be considered in assessing system stability and robustness. The present work reveals the dynamic evolution characteristics of coupled sloshing in partially filled tanks, provides theoretical and numerical tools to study system coupling effects that are rarely addressed, and offers insights for optimizing fluid management strategies in spacecraft.