Neural-network-based terminal sliding-mode control for thrust regulation of a tethered space-tug

This paper studies the thrust regulation of the tethered space-tug in order to stabilize the target towed by a flexible tether.To compromise between model accuracy and simplicity,a rigid-flexible coupling multi-body model is proposed as the full model of the tethered space-tug.More specifically,the tug and the towed target are assumed as rigid bodies,whereas the flexible tether is approximated as a series of hinged rods.The rods are assumed extensible but incompressible.Then the equations of motion of the multi-body system are derived based on the recursive dynamics algorithm.The attitude motion of the towed target is stabilized by regulating the thrust on the tug,whereas the tether-tension-caused perturbation to the tug's attitude motion is eliminated by the control torque on the tug.The regulated thrust is achieved by first designing an optimal control trajectory considering the simplified system model with constraints for both state variables and control input.Then the trajectory is tracked using a neural-network based terminal sliding-mode controller.The radial basis function neural network is used to estimate the unknown nonlinear difference between the simple model and the full model,while the terminal sliding mode controller ensures the rapid tracking control of the target's attitude motion.Thrust saturation and tether slackness avoidance are also considered.Finally,numerical simulations prove the effectiveness of the proposed controller using the regulated thrust.Without disturbing orbital motion much,the attitude motion of the tug and the target are well stabilized and the tether slackness is avoided.

The New Generation of Cryogenic Liquid-Fueled Medium-Lift Launch Vehicle

The maiden flight of LM-8 performed perfectly on December 22,2020.The design concept of modularization,seriation and combination has been perfectly exhibited in LM-8.The four main technical innovations,including rapid integrated design based on modularization,engine thrust regulation,modal parameters acquisition technology based on numerical simulation,and flight load control,were verified during the maiden flight.LM-8 is now positioned to be the main force in China’s medium launch vehicles for commercial launch.In the future,the mission adaptability of LM-8 will be improved to provide efficient and low-cost launch services.In addition,new technologies to allow repeated use and autonomous flight will be validated.