Study On Trajectory Design And Guidance Algorithm For Combined-Power Reusable Launch Vehicles

As the new type of high specific impulse spacecraft,combined-power reusable launch vehicles are drawing comprehensive concerns in the aerospace field in recent years because they can effectively reduce the launch cost.However,the complexities of dynamic models and the difficulties of handling flight constraints bring new challenges to trajectory design and guidance research.Guidance technology has always been one of the key technologies in aircraft design.In this paper,the trajectory designs and guidance algorithms of the ascent and reentry phases are studied by taking a specific combined-power reusable launch vehicle as the research object.The main contents of this paper are as follow.The motion modeling and analysis of ascent and reentry phases for the combined-power reusable launch vehicle are carried out.The general motion model of the vehicle is derived by establishing the coordinate systems and coordinate transformation relationships.And the model is simplified according to the flight characteristics in different phases.What more,the aerodynamic and thrust models of the aircraft are analyzed and discussed emphatically.An ascent optimal trajectory design algorithm for the launch vehicle is proposed based on hp adaptive pseudospectral method.The dynamic pressure,overload and heat flow constraints are integrated into the aerodynamic bending moment constraint to reduce the design difficulty based on the analysis of the complex constraints in ascent flight of the vehicle.In view of the fact that thrust magnitude of the vehicle is highly coupled with state and control variables and difficulty is too deep to implement the optimal trajectory design by indirect methods,the hp adaptive pseudospectral method is proposed for trajectory optimization.The trajectory optimization problem with differential constraints is transformed into a non-linear programming problem with pure algebraic constraints based on the direct method.In order to improve the accuracy of interpolation,the strategy of adaptive adjustment node is put forward when interpolating.Taking fuel economy as the performance index,effectiveness and adaptability of the trajectory optimization method are verified under different specific flight missions.Two algorithms of real-time feedback tracking guidance based on nominal trajectories for ascent phase are presented.The first is the LQR-based nominal trajectory tracking guidance method.Because of high nonlinearity of the motion model,time-varying linear equation of state and control variables deviation is obtained by linearizing the nonlinear equation.According to the theory of linear quadratic finite-time state regulator,the optimal trajectory tracking is realized by designing real-time feedback.The second is the real-time tracking guidance method based on combined feedback linearization and fuzzy logic controller.Feedback linearization is adopted to adjust the throttle valve command to change vehicle's thrust to realize real-time tracking of flight altitude.Fuzzy logic controller is used to adjust angle of attack to suppress the deviation of velocity and flight-path angle,which achieves auxiliary tracking.The effectiveness and robustness of the two tracking guidance methods are examined under different simulations with model deviations.A segmented guidance method for reentry phase is proposed.According to the flight condition,the reentry phase is divided into initial descent stage and gliding stage.The conditions of shift point are designed.For the initial descent stage,an angle of attack profile and constant bank angle with simple initial state deviation feedback are used for guiding the aircraft.For the glide stage,the first is to construct reentry corridor on H-V plane.Then the nominal H-V profile is designed based on the corridor.The feedback linearization is used to design longitudinal height bias feedback tracking guidance law.The lateral motion in whole reentry phase is controlled by the sign of bank angle to make the vehicle's terminal heading satisfy the requirement.The model deviation conditions are set and the feasibility and robustness of reentry guidance are verified by numerous simulation.