Nonlinear Analysis of the Vega Launcher First Stage composite skirts with geometrical imperfections

Abstract

This paper presents the studies performed within CNES, French Space Agency, in order to verify the integrity of the front composite skirt of the P80, first stage of VEGA European launcher. Being subjected to high compressive fluxes, the P80 skirt with a monolithic definition is a buckling sensitive structure. Considering that the skirt has the specificities to be quite short, with high pre-buckling nonlinearities introduced at the boundary conditions, a complete verification based on linear and nonlinear FEM calculations of perfect and imperfect shell was necessary. The effects of different geometrical imperfections' shapes and amplitudes have been investigated in terms of buckling and fibre fracture. Considering the brittle behaviour of the composite, results show that the worst imperfection shape depends on the failure mode considered. Finally, results have been compared to a traditional sizing based on linear buckling analysis combined with experimental knockdown factor, showing that the second methodology is not appropriate for this kind of structure, because it is over conservative for instability and it doesn't address strength topics which are of primary importance.