Modal analysis of monorail rocket sled and its cabin connections

Abstract

Structural stiffness is an important dynamic parameter for the rocket sled. In this paper, both modal test and finite element modal analysis are carried out on the certain monorail sled and its two different cabin connection kinds which are bolt and thread. By comparing the natural frequencies and modal shapes obtained by test and simulation methods respectively, it is found that the stiffness of bolted cabins is consistent with the simulation results, while the threaded cabins performs a severe stiffness degradation due to the thread clearance, which also causes the natural frequencies of the completed sled to be reduced and the modal shapes to be inconsistent with the simulation results. The finite element model of threaded connections is modified by giving anisotropic material properties to the connected elements, and the simulation deviation is reduced. According to the analysis results of two different kinds of cabin connections, threaded cabins on the final assembled rocket sled are strengthened by welding. The modal test results of the reinforced threaded cabin connection indicate that negative influence of the threaded connections is eliminated and the natural frequency and vibration mode are very close to the simulation results.