Study On Mechanical Behaviors Of Lunar Soil Simulant And Footpad Dynamic Response In Soft Landing

The Moon, the satellite of the earth, is full of abundant mineral resources. Lunar exploration is of great significance for human development. In China, lunar exploration program has been being vigorously carried out in terms of three-stage exploration objectives, including "orbiting","landing" and "returning". At present, the first stage has been successfully completed and the second is being considered. It is the safe and stable lunar soft landing of the lander that is one of the major tasks in second stage and the primary requirement of the third-stage objective. The footpad, as the landing buffer mechanism would contact with the lunar surface first, as well as the interaction force and relative motion being generated. After that the buffer would be used to absorb the energy for safe and stable lunar landing. In addition to the interface behavior between lunar soil and footpad in landing process, the mechanical behavior of lunar soil subjected to impact loading, would be the critical factors for landing buffer.The static, dynamic and interface behavior of lunar soil stimulant TJ-1were studied by means of particle characteristics statistics and laboratory tests, and then the major mechanical and interface properties were obtained. The vertical impact and horizontal dragging test equipment were developed based on the various stages of the landing impact process. A large number of model tests were also investigated. According to the test results and soil mechanics theory, the vertical impact and horizontal dragging models were proposed, which were combined together using trajectory theory to describe the mathematical and mechanical interaction model between footpad and lunar soil stimulant in impact process. The details are shown as:1) The shape parameters of lunar soil stimulant particles were studied and their shape indices were quantitatively analyzed on the basis of stereomicroscope scanning of the particles, digital image processing software and computer aided program. The results are shown that the particles of lunar soil stimulant have the same shape as the ones of real lunar soils. Besides the multi-angular, serrated, and grooved structure, there is apparent honeycomb structure in particle holes of lunar soil stimulant. Compared to sand particles, its particle surface is rougher, longer and narrower.2) A series of mechanical parameters, associated with lunar soil stimulant such as compression modulus and index, elastic modulus, shear strength indices and so on, were obtained by laboratory tests. In addition, its shear and compression wave velocity of were measured by piezoelectric ceramic bending element. At last, its small strain shear modulus and poisson’s ratio were calculated. 3) The dynamic properties including dynamic strength, dynamic shear modulus, damping ratio and so on were obtained under various conditions of cyclic triaxial tests. The backbone curve and damping ratio calculation models were proposed, which provide the theoretical basis and calculation parameters for simulating the dynamic process.4) The systematic direct shear tests associated with smooth contact surface of lunar soil stimulant and footpad materials, were carried out to get the shear stress-displacement curves of contact surface. The various effects of factors such as relative density, normal stress and shear rate, on contact surface deformation and strength parameters were studied and a contact model of lunar soil stimulant and footpad materials were proposed based on different densities.5) According to impact stage characters and influencing factor in soft landing, a vertical impact test equipment were developed and lots of model tests were investigated in terms of impact mass, impact velocity and dense state of lunar soil stimulant. Besides, based on the laboratory and numerical simulation results, the vertical impact model proposed by NASA was revised. Considering the progressive development of shear failure zone in impact process, a simplified vertical impact model of footpad related to two freedom dynamic differential equation was also set up. Finally, the determination of model parameters associated with the mechanical behavior of lunar soil stimulant was proposed.6) A horizontal dragging test equipment was established to simulate the footpad horizontal slip process and a large number of model tests were investigated in terms of drag rate, footpad attitude, piercing depth, dense state of simulated lunar soil and so on. In addition, based on the results, a mechanical model to analyze the horizontal slip process of footpad was proposed by means of slip line theory and reciprocal theorem of work. By comparing the test results, a method to measure the model properties was finally put forward.7) The oblique impact model of soft landing proposed by NASA was introduced and a simple discussion was made in the thesis. At last, combining the vertical impact model with the horizontal dragging model proposed in the thesis based on trajectory theory, a mechanical model was obtained to describe the interaction between footpad and lunar soil stimulant in landing impact process.