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

This study investigates a fixed-time-synchronized non-affine attitude tracking control scheme for reusable launch vehicles (RLVs). The non-affine 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 non-affine characteristics, as well as system uncertainties and disturbances, a fixed-time-synchronized controller (FTSC) 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 non-affine RLV system. Finally, numerical simulations are presented to illustrate the effectiveness and advantages of the proposed method.