Trajectory Optimization And Guidance Method For The Recovery Of Reusable Launch Vehicles

Recently the rise of vertical takeoff and vertical landing(VTVL)launch vehicle has demonstrated its ability to promote reusability,reduce cost and improve launching efficiency.Among the core technical modules related to VTVL rocket,trajectory planning and guidance is one of the key aspect which reflects the level of intelligence of the rocket and will play a crucial part to a successful landing.A rocket during recovery has to go through extreme conditions and uncertainties while meeting strict accuracy requirements,which pose challenges to the design of the algorithm.In this paper,the dynamic optimization method is used for trajectory planning and online guidance.The optimization problem for rocket landing is a multi-stage,nonlinear,non-convex problem with complex constraints,thus startegies to improve the convergence and efficiency should be studied.Also measures should be taken to increase autonomy and adaptability of the guidance algorithm.Focusing on the above difficulties,this paper discusses the trajectory planning and guidance of the rocket recovery problem,and the main research work is carried out as follows:(1)The rocket model and environmental conditions of different flight phases during the rocket recovery process are introduced in detail.Based on the analysis,two trajectory optimization problems are studied in the paper,including the global trajectory planning of the whole recovery flight using 3-degree-of-freedom model,and the local motion planning of a single phase using 6-degree-of-freedom model.(2)For the flight outside the atmosphere,a trajectory tracking guidance law based on fast non-singular terminal sliding mode is proposed to achieve the finite time convergence.It's also combined with the super-twisting control theory to attenuate chattering effect.(3)For the flight in the atmosphere,the aerodynamic characteristics can't be ignored and guidance method should be more robust to uncertainty.A guidance and control integration scheme is designed based on pseudospectral feedback framework,in which the trajectory optimization is performed online repeatedly.Since guidance performance is vulnerable to the perturbation of aerodynamic parameters,the coefficient model is modified in real time through parameter estimation method based on Extended Kalman Filter.In case the optimization problem is difficult to converge,the paper also proposes two convergence enhancing strategies to improve the convergence and efficiency of the algorithm.(4)For the final vertical landing phase,the rocket might encounter emergencies where no feasible solution for the optimization problem exists.A strategy of constraints relaxation is designed to provide an emergency response.Simulation results show that,comparing with traditional separated guidance control scheme,the proposed integrated guidance control scheme has advantages in guidance performance and flexibility in case of emergencies.