applied sciences  
Editorial  
Editorial for Special Issue: Advanced Fault Diagnosis and Fault-Tolerant Control Technology of Spacecraft  
[PERSON],*(c), [PERSON], [PERSON] and [PERSON]  
In order to complete the various tasks in advanced space missions, such as communication, navigation, and remote sensing, single complicated spacecraft and many distributed spacecraft systems have been launched into orbit. Due to the extremely harsh outer space environment, with various categories of hazardous radiation, huge temperature variations and the aging of components to a certain extent, there may be various malfunctions/failures in components or subsystems of the spacecraft. To improve the reliability, safety, and maintainability of spacecraft systems, fault diagnosis and fault-tolerant control (FTC) for spacecraft systems subject to space disturbance and internal physical constraints have become interesting research topics in recent years. Therefore, alongside recent developments in various learning algorithms, advanced observer design, intelligent control, etc., the application of advanced fault diagnosis and FTC techniques to single complicated spacecrafts or distributed spacecraft systems (spacecraft formation flying system, spacecraft cluster, etc.) will be extensively investigated in this Special Issue. This Special Issue aims to highlight the state-of-the-art techniques that are used for advanced fault diagnosis and the fault-tolerant control technology of spacecraft, especially for intelligent fault diagnosis algorithm development and sliding mode fault-tolerant control. This Special Issue received 20 manuscripts, 15 of which have been accepted, and 5 of which were rejected by the peer-review process. In [1], to perform indicator selection and verification for the on-orbit fault reconstruction of a giant satellite swarm, a hybrid multi-objective fault reconstruction algorithm is proposed and then verified by Monte Carlo analysis. The simulation results not only show the algorithm's validity but also reveal the relationship between the number of satellite faults and the health of the satellite swarm. From this, the maximum number of faulty satellites allowed in the giant satellite swarm is calculated, which is significant for assessing the swarm's health. In the work [2], free-floating flexible-joint space-manipulator dynamic modeling is studied, and a state-observer-based robust backstepping fault-tolerant control is proposed for the system joint actuator failure. For the slow subsystem, a state observer-based robust backstepping fault-tolerant controller is designed to eliminate the angle error, compensate for the uncertain parameter and the external disturbance, and achieve joint-trajectory asymptotic tracking. The use of a speed filter makes it inappropriate to measure and provide feedback about the system's velocity signals, so the controller is simpler and more precise. In [3], the TWR characteristics and stress distribution characteristics of a high-speed level gear in an aero-engine are analyzed in detail by means of experiments and simulations. Based on the acoustic waveguide system and dynamic stress test system, the TWR fatigue failure monitoring experiment of the central drive level gear in an aero-engine is carried out, and the TWR frequency, dangerous speed, dynamic stress and fatigue fracture characteristics of a driven level gear are obtained. Based on the transient dynamic analysis method and Hertz contact theory, the stress distribution characteristics of the driven level gear, which cannot be obtained in the test under the condition of TWR, are analyzed. The influence of the changes in the working temperature and the thickness of the spoke on the TWR characteristics and the stress distribution characteristics are discussed. In [4], the fault-tolerant control problem of hypersonic vehicles (HSVs) in the presence of unexpected centroid migration, actuator failure, and external interference is studied in depth.