Dynamic Analysis Of Launch Vehicles Based On Component Modal Synthesis Method

Launch vehicles are the basic tools for space exploration,and as well as an important symbol of the country’s overall strength.During the operation,on the one hand,launch vehicles are subjected to complex loads due to eccentric thrust and external air flow.On the other hand,launch vehicles are affected by severe vibration and noise generated by strong jets of the engine and aerodynamics.These unfavorable factors will not only cause fatigue damage to rocket,but also lead to failure of internal equipment,which greatly reduces the safety and reliability of the entire system.Therefore,it is necessary to analyze the dynamic behavior of launch vehicles.However,a launch vehicle may be assembled by many complex components,of which finite element model has hundreds of thousands of degrees of freedom.And the parameters of components are required to modify repeatedly for reanalysis.So traditional dynamic analysis methods for the entire model have low computational efficiency and high analysis cost.Therefore,it is of great theoretical and practical value to carry out research on the dynamic characteristic of launch vehicle based on model-reduced-order techniques.This paper mainly analyzes the dynamic behavior of the launch vehicle model based on modal synthesis method.Firstly,the formulations of the modal synthesis method are derived,and then the fixed interface modal synthesis method is extended to a special structure containing spring hinges.Second,based on MSC.Patran/Nastran,the parametric modeling of the launch vehicle was implemented.Then the modal analysis,transient response analysis and harmonic response analysis for finite element model of the launch vehicle were performed by using substructure method.Based on modal synthesis method,the random vibration problems of the launch vehicle is studied.Finally,by using the PyQt for secondary development,the flow of design for fixed interface modal synthesis method and free interface modal synthesis method are achieved.The specific work contents are:1)The basic flow of the substructure method is described,and the fixed interface method and the free interface method are deduced and compared in detail.The main mode,the restrained mode,the attached mode and the residual attachment mode appearing in the substructure method are introduced.Finally,the substructure method is extended to the special structure containing spring hinges,and the correctness of this method is verified by an example of a simple thin plate.2)Using Patran Command Language(PCL)to parametrically model the launch vehicle and the launch vehicle model is subjected to a dynamic substructure analysis.Firstly,the application of PCL is introduced.Then,a detailed three-dimensional fine finite element model is established based on the launch vehicle.And then,a total of 17 parameters of the structural model size and material properties of the launch vehicle model are used as design variables.The parametric modeling file was compiled through the PCL,which effectively reduced the modeling time and reanalysis cost.Finally,the substructure dynamic responses of the launch vehicle are analyzed by MSC.Nastran.The accuracy and efficiency of the substructure method are illustrated by comparing the computational results with that of the overall structure.3)Based on the substructure reduction model,pseudo excitation method is combined with the finite element software to perform random vibration analysis on the launch vehicle model.Firstly,the traditional algorithm CQC method for stationary random vibration is introduced,as well as the basic principle of pseudo excitation method.And the consistency of the two methods is verified theoretically.Secondly,the random vibration analysis of launch vehicle model is operated by combining pseudo excitation method with harmonic response analysis module in Nastran,which verified the accuracy and efficiency of this method.Finally,the substructure reduced-order model is applied to the method mentioned above,which further reduces the calculation time and reanalysis cost of the random vibration analysis.4)Secondary development of MSC.Patran/Nastran based on modal synthesis method.Using PyQt to design a more concise virtual operation platform,which calls MSC.Patran/Nastran to perform multiple dynamic analysis on the launch vehicle model with fixed interface modal synthesis method or free interface modal synthesis method.This perform not only makes up for the redundant and complicated operations in commercial software,but also is more convenient for users to familiarize themselves with the process of substructure analysis.