Predict-Correct Guidance Algorithm For Mars Aerocapture

Braking and becoming an orbiter are the key action for Mars exploration mission.How to use lower propellants and achieve safe and fast capture is a problem that must be solved in future large-scale Mars exploration missions.The aerocapture method takes full advantage of the deceleration effect of the Martian atmosphere,and the propellant requirement and flight time are reduced.It is hopeful to solve the above problems.An efficient,accurate,and robust guidance algorithm is the key to implementing aerocapture.In this dissertation,the future Mars sampling return and manned landing on Mars missions are used as application scenario.The aerocapture guidance strategies based on lift control,continuous drag control and discrete drag control are researched,and the corresponding analytical predict-correct guidance algorithms are proposed.Finally,simulation analysis and verification are made to support these algorithms.Through the appropriate assumptions and approximations,a 3DOF dynamics model was established to analyze the dynamic characteristics of the probe during the aerocapture process and the derivation of the analytical guidance algorithm.In order to fully reflect the influence of atmospheric density disturbance on the aerocapture guidance algorithm,a time-varying atmospheric density error model is constructed based on the statistical characteristics of the measured data.The motion modes and flight characteristics for aerocapture are mastered by analyzing the control variables of the lift and drag control modes and the features of the aerocapture corridor.In the research of aerocapture guidance algorithm based on lift control,the aerocapture process is divided into three stages:initial entry,equilibrium gliding and flight out.The corresponding guidance strategies are proposed for each stage of motion characteristics.In the initial entry stage,the control variable remains constant due to severe control saturation;the equilibrium gliding state is maintained by feedback control in the equilibrium gliding stage;maintain a fixed rate of change in altitude during the flight out phase.Subsequently,the influence of switching speed for guidance precision is analyzed.The influence of switching velocity on guidance precision is analyzed.A velocity threshold adaptive switching method is proposed.The flight path angle zero crossing is used as the switching point to achieve robust guidance and control under high dynamic conditions.In the research of aerocapture guidance algorithm based on continuous drag control,the basic motion characteristics of the probe under continuous drag control mode are analyzed,and the corresponding fixed relationship between flight path angle and flight velocity is found.A piecewise linear function approximation is proposed.The analytical expression between the flight path angle and the flight speed is established,and realized the analytical prediction of the end flight state,and solved the analytical predict-correct guidance problem under the continuous drag control mode.The effectiveness of the proposed algorithm is verified by comparison with algorithms such as numerical predict-correct guidance and drag tracking guidance.In the research of aerocapture guidance algorithm based on discrete drag control,the dynamic equation is transformed into the classical Yaroshevskii's equation by state transformation and approximation.The Poincare-Lindstedt method is used to solve the equation,the zero-order,first-order and second-order analytical solutions are derived under the constant ballistic coefficient.Based on this,the analytical form of segmented discrete ballistic coefficients is derived,and the analytical predict-correct guidance law of segmental variable ballistic coefficients is proposed.In order to solve the problem that the state variables in the engineering application are unmeasurable or the measurement accuracy is poor,the design idea of the adaptive open parachute algorithm of the Mars lander is borrowed,and a predict-correct guidance method based on the.drag measurement data is proposed.Firstly,the Monte Carlo simulation is used to determine the corresponding relationship between the fixed moment drag and the velocity increment.Then,the dynamic adjustment of the switching time is performed according to the measured value of the drag acceleration.The algorithm has the advantages of simple hardware facilities and small computational burden.