Overview

Improving performance while reducing fabrication costs of primary structures in modern space launcher systems is crucial for the sustainable development of Space industry. Every kilogram sent to space in modern launchers (Ariane 5, Atlas V, Falcon 9, Proton-M, Soyuz-2, etc.) costs from 5 k$ (LEO) to 20 k$ (GEO).

Despite the carbon fibres are 3-4 times lighter than aluminium, they must be laminated at different angles within a resin matrix to compensate their weak transverse properties and obtain usable parts. Unfortunately, the fibres in these angle-ply laminates must be 3-4 times underloaded to prevent matrix cracks, which effectively cancels any significant weight reduction. In consequence, most of the primary structures of modern launchers still rely on metal fabrication concepts. For example, the metallic isogrid cryo-tanks of SpaceX´s Falcon or NASA´s SLS launchers are first machined from flat or curved aluminium panels and then stir welded into cylinders.

Besides angle-ply laminates, the state-of-the-art primary structures for space launch systems rely on isogrid or anisogrid design concepts. These lattices of rib stiffeners are the most weight-efficient choice for load-bearing applications since the ribs are loaded axially, where the material properties are most favourable.

The carbon fibre anisogrids are generally fabricated by fibre placement or filament winding, like the interstages or payload adaptors of Proton-M. By contrast to angle-ply lamination, these techniques truly take advantage of the outstanding strength and stiffness of unidirectional carbon fibre composites . However, they suffer material built-up at the nodal positions, where two or more ribs running in different directions cross each other one on top of another. The significant deviation from the optimal fibre/resin proportion at these points reduces the effective strength and performance.

In addition, despite hollow ribs are preferred to solid ribs, because their high specific stiffness minimizes buckling, the available fabrication methods for metallic or composite lattices are only intended for solid ribs.