An original strategy for the design of large-scale composite sandwich structures is developed. Such structures are typical of launcher and spacecraft structures, but the methodology remains applicable to any thin-walled sandwich structure (such as aircraft fuselage panels or watercraft hulls). The method hinges on multi-step strategies devised for variable-stiffness laminates. Indeed, laminate optimization problems corresponds to large combinatorial design spaces and their efficient approximate solutions often combine continuous relaxation and metaheuristics. Typically, the first step consists of a gradientbased optimization that handles the large number of variables with a continuous representation of the composite mechanical behavior. In our work, this continuous representation is adapted to the specific features of laminated sandwich composites. In particular, we modeled the dependency of the material transverse shear stiffness on the F.-