Reentry Guidance For Lunar Return Capsules And Pinpoint Soft Landing Guidance For Lunar Landers

The moon is the nearest celestial body to the earth.The exploration of the moon lays the foundation of the exploitation of the lunar resources,promotes the progress of science and technology,boosts the economic devlopement,and produces an experience base necessary to successfully explore the deep space.Additionally,the lunar exploration mission also reflects the level of a contry's comprehensive national strength,which further produces great political influence.Therefore,world-wide countries have had great focus on the technology of lunar exploration.Reentry guidance technique of capsues and lunar pinpoint soft landing guidance technique are two of the key techniques of lunar exploration.At the altitude of 120 km,the velocity of a lunar renturn capsule is as high as about 11 km/s,which implies that the capsule and the crew have to suffer much higher aerodynamic load than that of the low earth orbit(LEO)return,leading to a much narrower reentry corridor.Further more,in order to always land the capsule on the domain of one country or avoid bad weather conditions,the flight range of the vehicle has to cover a very wide range of distance.Hence,the reentry guidance method must be able to steer the vehicle along both direct reentry trajectories and skip trajectories.All of these issues chanllenge the design of the reentry guidance system.In addition,the lunar exploration is not limited to sample return mission any more.In the future,the permanent moon base will be established to build future lunar colonies with the development of the aerospace technology.To meet the requirement of building future lunar colonies,pinpoint soft landing is especially important.Besides,terrain hazards may result in failure to land vehicles on the lunar surface,so the capability to detect and avoid surface hazards is desireable.Advances in sensor capability allow for accurate detection of these surface features.The trajectory guidance must allow for dynamic retargeting to avoid the hazards detected by the sensing systems.The trajectory shape and other constraints must take into account for allowing the sensors to sense the landing area.So guidance and control systems that meet the demanding performance are necessary to be studied.In this dissertation,reentry guidance technique of lunar return capsules and lunar pinpoint soft landing guidance technique of landing vehicles are investigated.Firstly,by employing the HP-Radau psudospectral method and fuzzy theory,an algorithm for multi-object lunar retrun trajectory optimiazation is designed,and then the characteristics of the lunar return trajectories are analysed.Different nonlinear member functions are tested and how the memeber functions affect the optimal results are studied.Further,a backstepping method based trajectory tracking algrothm is propsed to track the resultant optimal trajectory.For the numerical predictor-corrector guidance technique of reentry capsules,different load relief strategies are investigated in various reentry scenarios(reentry from a LEO after an aeroassisted orbit transfer and reentry from the moon-earth transfer orbit directly),and the problem that bank reversals of the numerical predictor-corrector guidance algorithm for the long range lunar return mission are too many is solved.Additionally,multi-constrained suboptimal powered descent guidance for lunar pinpoint soft landing is presented through combining the zero-effort-miss / zero-effort-velocity(ZEM/ZEV)algorithm and the model predictive static programming(MPSP)method.Next,the ZEM/ZEV algorithm is further extended,and an integrated guidance and control system of landing vehicles is developed with the help of the backstepping scheme.The main achievements are as follows:1)A multi-object optimization algorithm for lunar return trajectory optimization is developed by employing fuzzy theory and the HP-Radau pseudospectral method,and the main characteristic of member functions that affects the optimization results is given,which provides a theoretical reference for multi-object trajectory optimization.HP-Radau psudospectral method is used to discretize the trajectory optimization problem and the fuzzy theory is employed to deal with the problem that the multiple objects are conflicting and incommensurable,making the results of the multi-object trajectory optimizatiom problem more desirable.Different member functions are constructed and the corresponding optimization results are analysed,according to which we conclude that the main factor that impacts on the results of the fuzzy optimization problem is the concavity and convexity of the member functions.2)A novel lunar return trajectory tracking algorithm is proposed by employing a backstepping technique,which can be a new tool for lunar return trajectory tracking.Weighted reference trajectories of altitude and downrange are tracked.Additionally,a disturbance observer is designed to evaluate the disturbance during the reentry,which improves the trakcing accuracy.For the problem that the derivative of the virtual control is hard to obtain,a second order filter is used to deal with it.The simulation results show that the proposed algorithm is more accurate and robuster than conventional mthods(e.g.LQR).3)Load-relief schemes for the guidance of the two different lunar return reentry scenarios(reentry from a LEO after an aeroassisted orbit transfer and reentry from the moon-earth transfer orbit directly)are suggested and the problem that bank reversals of the numerical predictor-corrector guidance algorithm for the long range lunar return mission are too many is solved,advancing the performance of the guidance algorithm.For capsules return from a LEO,a load-relief strategy embedded in the guidance algorithm through elaborately designing a bank-angle profile in each guidance circle makes load-relief an inherent characteristic rather than additional logic of the algorithm,which enables the guidance algorithm to have greater load-relief capability and stronger robustness.For the moon-earth transfer orbit reentry mission,to relieve the aerodynamic load,bank angles with large magnitude are flown in the Kepler phase,and drag acceleration is fed back in the final phase to compemsate for the commanded bank angles.In additation,through searching for the energy point of the only one bank reversal in the skip phase,the bank reversals are effectively reduced.4)For the lunar pinpoint soft landing guidance scheme,firstly,the ZEM/ZEV algorithm is improved to avoid landing vehicles crashing the ground.Secondly,a concept of virtual control is introduced for both the continuity of the guidance commands and the enforcement of the thrust vector constraint at the terminal point.Finally,taking the trajectory generated by the improved ZEM/ZEV algorithm as the initial guess history of the MPSP method,and the virtual control history as its control history,a multi-constrained fuel suboptimal powered descent guidance algorithm is developed.To meet the final constraints in the integrated system design,the ZEM/ZEV algorithm is extended further,and then the extended ZEM/ZEV algorithm is employed to get the virtual control of the backstepping mehod.Lastly,the real control is obtained by using a backstepping approach.The simulation results show that the integrated guidance and control system performs more accurately and robustly,and balances the two subsystems better.