Z C Zheng , D Guo, Y J Zhang and Z C Hou 

DYNAMIC ANALYSIS OF LARGE-SCALE FLEXIBLE SYSTEMS 
FOR FREE-FREE SPACE STRUCTURES

Based upon the theory of modified constrained modal methods, 
in which the constrain can be set up on arbitrary points of 
the system or subsystems, a kind of simplified method of 
free-free structural modal analysis is presented in this paper. 
By using this method the structures with degrees of freedoms 
of rigid body motion can be constrained optionally. The 
structure becomes statically determinate system or statically 
indeterminate system. The modeling of finite element method 
can be verified or corrected by calculating results and 
experiments of statically constrained modes and dynamic normal 
modes of the constrained system. Therefore, the modal analysis 
of free-free system can be performed easily by using modal 
information from the experiment or calculating results. The 
traditional modal experimental method of hanging with flexible 
strings may be changed, and the effective experimental methods 
can be chose according to concrete structure and site. To 
verify the effectiveness of this method, the examples of a 
free-free rocket simplified as beam and cylindrical shell model, 
or divided into two substructures, and a modeling as an example 
of a strap-on rocket with four boosters are given. The presented 
method is a kind of practical method for modal analysis and 
suitable for large space structures, especially in the case of 
the assemblage of large flexible space structure systems, whose 
components can be tested in ground but the assemblage can not 
be opened in ground and only can be extended in space.