Trajectory Planning And Guidance System Design For An Unpowered RLV In The Terminal Area

With the development of space transportation technology,the second-generation reusable launch vehicles(RLVs)have been designed aiming at reducing the costs while improving the safety,autonomy,and adaptability of the vehicles.Advanced guidance & control technology is well recognized as one of the effective means to achieve these objectives.This dissertation focuses on the study of trajectory planning and guidance problem for an unpowered RLV during the Terminal Area Energy Management(TAEM)phase.Firstly,the guidance-oriented mathematical model is constructed.Then,gliding capability of the unpowered RLV is analyzed.Furthermore,different guidance methods are developed,including a hybrid guidance strategy based on offline trajectory planning and online ground-track prediction/correction,a rapid trajectory-planning-approach-based onboard TAEM guidance scheme,and a sliding-mode-based autonomy TAEM guidance method.The main results achieved in this dissertation are listed as follows:1.With the consideration of the limited gliding capability of the unpowered RLV,an energy-tube concept based strategy is developed to study the gliding capability of the unpowered RLV during TAEM phase.A unified algorithm is designed to generate longitudinal profiles,based on which the maximum dive and maximum glide profiles are created.The energytube for TAEM flight is constructed,on which the impact of various uncertainties is studied.The conceived energy-tube describes the gliding capability of the studied RLV,which is also considered as the evaluation index for the efficiency of the after-mentioned guidance methods.2.To improve the guidance precision of the traditional decoupling guidance method,a hybrid guidance strategy based on offline trajectory planning and online ground-track prediction is proposed.A longitudinal trajectory generation strategy based on dynamic pressure profiles is firstly designed.A trajectory database is therefore constructed using these longitudinal profiles.Then,an onboard ground-track predictor is developed to correct a RLV's position and generate lateral commands online,which improves the lateral guidance precision.Finally,a closed-loop guidance law is designed to track the reference commands,ensuring the robustness to system model uncertainties.Numerical simulations demonstrate that the generated smooth trajectories are easy to be tracked with acceptable precision,which shows the high potential to be applied in engineering.3.Considering the poor precision obtained by the traditional fixed-ground-track-based guidance method,an online TAEM guidance strategy is developed based on rapid trajectory planning technology.Firstly,the gliding equation is proven to be differentially flat by utilizing the dynamic pressure and positions as flat outputs.Then,the original trajectory planning problem is reformulated in flat output space.The dimension of the optimization problem is significantly decreased.The Problem discretization is achieved using pseudospectral method,based on which an initial guess method is innovatively proposed.The efficiency of the proposed algorithm is remarkably improved.A trajectory tracking control law is designed using feedforward linearization method based on differential flatness property,which ensures the robustness of the proposed guidance strategy.Simulation tests show that proposed trajectory planning algorithm is effective in reducing computational time while the generated trajectory is more flexible.The proposed guidance method provides a satisfactory solution in terms of precision and robustness.4.Considering that only limited autonomy flight capability can be achieved by the referencetrajectory-based guidance scheme,an advanced TAEM guidance approach is proposed based on second-order sliding mode principle.A new sliding surface is constructed on the basis of guidance-oriented mathematical model in altitude domain.By integrating the dynamic pressure profiles shaping method,the dynamic pressure during flight is well controlled ensuring a safety TAEM flight.In addition,focusing on the problem of bank limitations induced by potential actuator faults,the trajectory is reshaped and guidance commands are regenerated considering input saturation.The simulation results demonstrate that the guidance precision and autonomy flight capability are both improved.This dissertation is focused on the exploration of new TAEM guidance strategies for the next-generation RLV's safely atmospheric return to Earth.The new methods in the field of rapid trajectory planning and advanced guidance and control are imported and improved,allowing for the dissertation's significance in theoretical.Besides,the flight capability of an unpowered RLV during TAEM phase is with enough consideration.This makes the dissertation has something of significance for further study on overall design of the next-generation RLV and advanced guidance and control system design as well.