## 1 Introduction
In [24], based on the type-II fuzzy logic, a robust adaptive fault diagnosis and FTC scheme was proposed for multisensor faults in the variable structure hypersonic vehicles with parameter uncertainties. In [25], indirect passive compensation factors shielded system faults, and an observer with adaptive learning rates that combined the fault magnitude and fuzzy premise variable was then designed to estimate actuator faults, where a novel bionic variable parameter algorithm improved the spacecraft sensitivity of estimation to the incipient fault deviations. In [26], an FTC scheme was considered for a cascade nonlinear spacecraft system with mismatched uncertainties and unknown actuator faults. However, the above-mentioned FTC methods did not consider active flexible variable structure and actuator saturation, and thus, it is difficult to meet the space warfare requirements for dexterous maneuvering. This paper improved the adaptive control on the basis of finite-time control in [23] or the methods of other references mentioned above so that the spacecraft can achieve attitude fault-tolerant tracking in a multi-mechanical configuration and create conditions for avoiding enemy interception, grabbing, and space junk. This paper focused on an active adaptive FTC for the attitude systems of a flexible variable structure spacecraft to tackle multiple actuator faults and saturation. The main contributions are as follows:  
1. A fault diagnosis approach was developed for the attitude systems with variable structure parameters, detects the occurrence time and amplitudes of all faults in different actuators: partial loss of effectiveness (LOE), bias, and complete failure. Finally, the FTC under the complete failure of general spacecraft has been solved firstly in the world. 2. According to fault estimation, an adaptive fault-tolerant controller without actuator saturation was designed by utilizing the fast terminal sliding mode control technique, such that all the closed-loop signals of the attitude systems are finite-time convergence. Then, the FTC scheme under the actuator saturation case was further proposed; it has good tolerance capability to multiple actuator faults;
3. A variable structure improved adaptive scheme was proposed where the harmonic functions based on variable structure parameters were designed. It was combined with the fault-tolerant algorithm so that the fault repair process can maintain the stability under multiple mechanical configurations and multiple faults. Finally, the control of a universal variant spacecraft is first achieved in the world. The rest of the paper is organized as follows. Section 2 provides the attitude system mode of a flexible variable structure spacecraft and the actuator fault mode. In Section 3, a fault detection observer detects the occurrence time of an unknown fault. Then, a fault estimation observer estimates the amplitude of the actuator fault. In Section 4, a fault-tolerant controller with variable structure harmonic functions self-repairs actuator faults, guaranteeing the finite-time convergence. Then, an improved adaptive FTC scheme is developed to deal with the actuator saturation. Section 5 verifies the effectiveness of the developed FTC method by simulation. Finally, Section 6 provides the conclusion.