Composite grid vs. composite sandwich: a comparison based on payload shroud requirements

The use of sandwich construction, which is arguably the most structurally efficient structure, gives large weight savings over metallic structures. The substitution of Advanced Grid Stiffened (AGS) structures is not targeted at reducing structural weight, but at reducing manufacturing cost and the acoustic transmission characteristics of the payload shroud. To better quantify the advantages or disadvantages between the structural types a design comparison is made based on a composite sandwich shroud, currently in production, and an AGS composite shroud. These shrouds are in the 4-m class. The analysis includes: stiffness (deflection limits), material failure, stability, and natural frequency response. These analyses were done using a combination of finite element modeling (FEM), and grid optimization programs. AGS structures are characterized by a lattice of rigid, interconnected ribs, which proves to be a highly efficient design. AGS structures also offer other inherent advantages such as high impact resistance. When damaged, delamination and crack propagation tend to remain isolated to a cell. They also do not suffer from moisture infiltration and the associated problems such as face sheet to core delamination, corrosion, and increased weight. One of the most valuable assets of AGS structures is their high potential for automation. Several AGS structures have been successfully manufactured at the Air Force's Phillip Laboratories using automated techniques [5]. These techniques have been verified by flight qualification and flight demonstration on the Combined Experiments Program (CEP) Sub-Orbital launch.