Fixed-Time-Synchronized Nonaffine Attitude Control for Reusable Launch Vehicles

This study investigates a fixed-time-synchronized nonaffine attitude tracking control scheme for reusable launch vehicles (RLVs). The nonaffine characteristics, which are often overlooked and simplified into an affine model during the controller design, significantly impact both the precision of the RLV models and the stability of the systems. To address these nonaffine characteristics, as well as system uncertainties and disturbances, a fixed-time-synchronized controller with a time-varying parameter is designed to generate a virtual control signal. The time-varying parameter helps reduce control energy consumption and mitigates saturation issues by moderating the initial control input. Subsequently, a dynamic inversion subsystem is incorporated to generate the control input, ensuring fixed-time tracking of the virtual control signal. A closed-loop stability analysis of the system governed by the controller is provided, demonstrating fixed-time-synchronized convergence for the nonaffine RLV system. Finally, numerical simulations are presented to illustrate the effectiveness and advantages of the proposed method.