Dynamic Load Identification Of Launch Vehicles By Time-Domain Piecewise Fitting

Due to the dual strict requirements for reliability and lightweight,the optimal design of aerospace structures needs to consider the mechanical environment fully.The most severe working conditions should be taken as the design basis.However,it is not easy to measure the external loads directly for the complex vibration system in engineering.It is a realistic choice to reversely calculate the loads acting on the structure by measuring the dynamic responses of the structure.This is what load identification does.The launch vehicle is subjected to complex and changeable external excitations during the flight,and the severe vibration of the fairing due to external excitations may affect the safety of the payload directly.To ensure the safe operation of satellites and other precision instruments,accurate external excitation information is required as the basis for the optimal design of the launch vehicle structures.Based on the remote measurement data of a certain type of launch vehicle,this thesis studies the inversion calculation of the external forces of the launch vehicle fairing.The orthogonal basis functions are used to approximate the time history curves of external force with the idea of function fitting.The requirement of the order of basis function is reduced and the length of Duhamel’s integral is decomposed through piecewise fitting,which realizes the long sample continuous force reconstruction with a much lower calculation amount.The adverse effects of the Runge phenomenon are overcome successfully by setting overlap of piecewise fitting,which leads higher accuracy of the time history curve approximation by Chebyshev orthogonal polynomials.The main work of this thesis includes:1)The advantages and disadvantages of the classic frequency-domain and time-domain load identification methods are summarized and compared.Aiming at the ill-posed problem in the inverse problem analysis,the solution of linear equations is taken as an example to illustrate the influence of the ill-condition matrix on the computational stability,and then the regularization method to deal with the ill-posed problem is introduced.2)The load identification method by modal shape normalized for measuring point degree of freedom,which has been applied to the engineering practice of launch vehicle load identification,is analyzed.And its extreme ill-posed characteristic is illustrated by numerical calculation examples.This thesis points out that it is no longer suitable for the need to further understand the mechanical environment of launch vehicles.There is a strong desire for a more suitable method.3)A time-domain load identification method based on function fitting is deduced.Then,a series of single-point excitation and multi-point excitation calculation examples show that this method has good calculation accuracy and anti-noise performance for smooth curves.And the advantages of this method for short sample impact load identification calculation are pointed out.In addition,the influence of the measuring position arrangement on the load identification is discussed by comparing the calculation results of different measurement schemes,and several principles of measuring position arrangement are briefly summarized.4)Aiming at the inversion calculation of the continuous excitation of the fairing during the flight of launch vehicle,a piecewise fitting calculation strategy is introduced,which reduces the requirement for the number of basis functions and decomposes the Duhamel’s integral.It reduces the overall calculation amount greatly.And the piecewise fitting with overlap overcomes the adverse effects of the Runge phenomenon in high-order polynomial interpolation successfully.Satisfying calculation results of long sample external force are obtained by Chebyshev orthogonal polynomial piecewise fitting.5)The 3D finite element model of the launch vehicle is simplified by the area equivalent method as a beam model.And the rigid-elastic decoupling is carried out by means of modal selection.The external forces of the launch vehicle fairing are calculated through the telemetry data by piecewise fitting.