DEM-FEM-MBD Coupling Algorithm For Spacecraft Landing Process Analysis And Applications

The impact response of lunar lander and reentry capsule during landing process is the key to the success of landing mission.Sampling return and manned landing put forward higher requirements for impact response.The impact load should be lower than the astronaut's bearing capacity,and ensure the safety of equipment and instruments.The safe landing of spacecraft is an important guarantee for lunar exploration and sampling return.During the landing process,the interaction between spacecraft and satellite soil(soil,lunar soil,et al.)involves both the discontinuous characteristics of satellite soil and the continuous characteristics of spacecraft structure.The problem of multi-media coupling brings difficulties to the numerical simulation.Therefore,a numerical method for analysis spacecraft landing process is proposed.To analyze the interaction between the spacecraft and the satellite soil during the landing process,a discrete element model(DEM)of the satellite soil is established by using the spherical element with the properties of bonding and breaking.The finite element model(FEM)of the spacecraft including lander and reentry capsule is established using the beam element and the shell element,and the dynamic characteristics are analyzed.The discrete element-finite element coupling algorithm(DEM-FEM)of the landing process of the reentry capsule is developed and verified by the cone impact experiment.The finite element-multibody dynamic coupling model(FEM-MBD)of the lander is established.The discrete element finite element multibody dynamics coupling algorithm(DEM-FEM-MBD)is developed to study the landing process of the lander.In addition,the coordinated time-step method is established for the asynchronous time step problem of the multi-media coupling algorithm.The main research contents are as follows:(1)DEM-FEM coupling algorithm for spacecraft soil interactionAiming at the interaction between the spacecraft and the satellite soil,a DEM-FEM coupling algorithm is developed.The motion equation of particle discrete element,the contact judgment and calculation method between particles and structure,and the calculation method of equivalent nodal force between particles and shell element and beam element are introduced.The bond-break model of satellite soil is established by DEM,which is used to simulate the crushing process of the soil under impact.The impact experiment of conical object is designed and completed to verify the DEM-FEM coupling algorithm.(2)DEM-FEM coupling analysis of landing of reentry capsuleBased on the DEM-FEM coupling model,the landing process of the reentry capsule is analyzed.Firstly,the shell element is used to establish the FEM of the reentry capsule.The maximum acceleration and rebound velocity calculated by DEM-FEM coupling are compared with the results obtained by LS-DYNA simulation,which verifies the correctness of the calculation model and the rationality of the DEM-FEM coupling method.Then,the stress distribution of the bottom of the reentry capsule during landing process and the dynamic characteristics of the granular medium under impact are analyzed.Because the reentry capsule has to pass through the "black barrier area" before entering the atmosphere,the final landing attitude cannot be predetermined.Therefore,the dynamic characteristics of the reentry capsule under different landing attitudes are analyzed.Considering that the rebound of the reentry capsule threatens the safety of the experiment equipment and astronauts,the influencing factors of rebound and the methods to avoid rebound are studied in detail,and puts forward an effective scheme for its safe landing.(3)Coordinated time-step method for multi-media coupling algorithmTo solve the problem of asynchronous time-step in multi-media coupling algorithm,a coordinated time-step method is established.The contact force of the structure in each discrete element time step is obtained by extrapolation method,and the equivalent load is obtained by averaging method,so as to update the position of the structure.Then the impact model,free fall model and electric pendulum model are established to verify the correctness of the above method.Finally,the main factors affecting the efficiency of the multi-media coupling algorithm are discussed.The results show that the number of elements and the time step of the discrete element simulation have a great influence on the wall clock time of the calculation process.(4)DEM-FEM-MBD coupling analysis of landing process of landerThe DEM-FEM-MBD coupling algorithm is developed to solve the problem of the interaction between deformable body and granular medium.The FEM-MBD coupling model of the structure is established in the mixed coordinate system,and the analysis of the structural displacement and deformation coupling problem is realized.The correctness of the above algorithm is verified by the tire rolling model.Based on the above methods,the FEM-MBD coupling model of the lander is established by using shell element and beam element,and the secondary buffer device of the lander is designed to study the stress distribution of the foot pad and the dynamic characteristics of lunar soil during the landing process.The results show that the lunar soil can effectively absorb the mechanical energy of the lander,and the lunar soil can absorb more energy in the inclined landing mode.Finally,the research of DEM-FEM-MBD coupling algorithm and spacecraft landing process is summarized,and the main problems of follow-up research are discussed.