Analysis Of Deviation Propagation For The Assembly Of Rocket Tank Bottom

The tank box composing of thin-walled structures is the main loadbearing component of the launch vehicle,the deviation in the assembly of each part is of great importance to the mechanical property of the whole structure.The thin-walled structural deviation of the rocket tank bottom shows the distribution of a field form in the space because of the large size,the weak stiffness and the non-uniform structure.And the distribution of deviation field presents strong randomness and unpredictability due to structural materials and technological parameters.The traditional description with random deviation of a few key control points in the assembly area is difficult to reflect the spatial distribution of structural deviation.At the same time,multi-source deviations such as initial deviation of parts,positioning of fixtures and welding distortion are randomly distributed because of the instability of the technological process,so the relationship of component deviations in the assembly is difficult to investigate.In the hierarchical assembly of the rocket tank bottom,the deviation of each point has an influence on the final assembly due to the deformation coordination constraint between the components.Therefore,it is necessary to establish a deviation propagation model for the large thin-wall structure,to reveal the deviation accumulation mechanism of the rocket tank bottom,which provides guidelines for the production technology and the tolerance design of parts.This paper studies the description method of the random deviation field of the large thin-walled structure,it also explores the influencing mechanism of multi-source deviations combined with the coupling effect of random factors in the deviation sources.Then the paper analyzes the deviation propagation of the hierarchical assembly for the large thin-walled structure of the carrier rocket fuel tank bottom.The main research contents are as follows:1)The description method of random deviation field of the large thin-walled structure based on cubic Bezier curvesConsidering the random characteristics of the large thin-walled structural deviation in the space,a description method to construct the random deviation field with cubic Bezier curves is proposed.Firstly,the characteristics of thin-walled structural deviation are analyzed in terms of the definition of the deviation field.Then the Bezier curve is used to describe the geometric surface of the standard component based on the shape parameters of the discrete surface.At last,the deviation vector of the discrete point set on the component surface is calculated by applying random statistical deviation to the key control points of curves,which is used to accurately describe the random deviation field of the large thin-walled structure.2)Deviation analysis of flexible assembly under the influence of multi-source deviationsBased on the N-2-1 fixture principle of large thin-walled structure in the assembly procedure,the linear relationship is established between the assembly deviation and the input deviation of parts,using the method of influence coefficient to solve the sensitivity matrix between assembly deviation and multi-source deviations,which includes initial deviation of the part,fixture positioning and welding distortion.Then taking the assembly of melon petal parts as an example,the initial manufacturing deviation of which is fitted with Bezier curves.The assembly deviation is described on the basis of the deviation sources under different normal distributions.Finally,the influence of various deviation sources with random distribution in the assembly are demonstrated.3)State space model of deviation propagation for large thin-walled structure during the assembly procedureBased on the multi-station assembly procedure of large thin-walled structures,the state space model is used to describe the component deviation propagation in the assembly.In terms of the multi-station assembly procedure of flexible parts,a state space model for multi-station deviation propagation of thin-walled structure is established,considering the coupling effects of various deviation sources such as manufacturing deviation,fixture positioning deviation,welding distortion,repositioning deviation and other random deviation.Then the tank bottom is taken as an example to describe the state equation of the state space model during the hierarchical assembly procedure.At last,the deviation propagation of rocket tank bottoms with different diameters is analyzed to provide guidance for the tolerance design.