Prediction and Suppression of Liquid Propellant Sloshing Induced Oscillation in RLV Terminal Flight

During reentry terminal flight of lifting-body Reusable Launch Vehicles (RLVs) propelled by liquid fuel, the sloshing of liquid propellent presents new features, and if neglected, could lead to adverse flight oscillations or even worse. This paper focuses on the liquid sloshing coupled flight dynamics, sloshing effect prediction and suppression of adverse flight oscillations. First, the transfer function model for unsteady aerodynamics is improved and applied to describe the sloshing force effect, being included in the rigid-liquid-control coupled flight dynamics model. Frequency domain analysis results show that liquid sloshing tends to degrade the closed-loop stability margin of vehicle, and even induce less damped oscillations which can be predicted through the frequency characteristics with sloshing force effect included. Furtherly, three suppression control measures to mitigate the adverse oscillation are addressed, which are enhancing the trajectory tracking loop damping, separating the frequencies of the rigid body motion and the liquid sloshing, and especially introducing a compensation loop to counteract the sloshing effect. Simulations demonstrate that all the provided approaches help mitigate the sloshing effect, while the compensation control with sloshing frequency characteristics included, works best.