Development and qualification of deployable membranes for space applications

Deployment systems for innovative space applications such as solar sails require technology for a controlled and autonomous deployment in space. Before employing such technology for a dedicated mission, it is necessary to demonstrate its reliability with a Technology Readiness Level (TRL) of six or higher. On the example of the design implemented in the Gossamer-1 project of the German Aerospace Center (DLR), a stowing and deployment process for large deployable membranes mainly considered for solar sailing is analyzed and tested. It is based on a combination of zig-zag folding and coiling of triangular sail segments spanned between crossed booms. Possible membrane materials are evaluated and a deployment technique is explored through theoretical analysis and tests in order to verify their functionality for large membrane space systems. The requirements for membranes that are exposed to the space environment are studied and the materials are analyzed regarding their resistance against atomic oxygen, radiation and their thermal properties. The folding geometry and force progressions are described mathematically. Load introduction aspects, the stress-strain state and the billowing of the deployed membrane are analyzed with finite element models. The folding lines were examined with microscopes, and their impact on thermal behavior is shown by analytical analysis. The membrane and deployment mechanisms were manufactured and integrated in an ISO 8 clean room environment, and the deployment process was verified in an extensive test campaign. It ranged from component level to system level and included mechanical vibration, static acceleration, fast decompression, thermal vacuum and laboratory deployment tests. It is shown that state-of-the-art aluminum-coated polyimide foils are sufficient for a demonstration of deployment technology in an Low Earth Orbit (LEO) and that coating systems based on a combination of aluminum, silicon oxide and titanium oxide enhance the membrane properties for solar sails. The model of the deployment force progression under zero gravity shows a tendency that the loads are transferred along the cathetus of the sail segments. The finite element models show generally low stresses in the deployed membrane and interface forces on the order of several Newtons for a 25m2 membrane. The analysis of the folding lines reveals that coatings in this region are damaged, and that hot spots can occur due to multiple reflections. The verification testing showed the general suitability of the membrane and of the deployment strategy itself. Materials, mechanisms, and a stowing and deployment strategy are presented that enable the controlled and autonomous membrane deployment for space sails. While the analysis presented is applied on a sail with an edge length of about 5 m, it allows an analysis of other configurations as well. This is of particular interest because currently-considered solar sails are about one order of magnitude bigger. With the environmental tests conducted, the membrane-related aspects of the deployment technology are on TRL six for a 25m2 LEO deployment demonstrator. The deployment strategy is scalable and materials are available that can be used for bigger solar sails as well. With respect to membrane-related aspects there is nothing to prevent the development of full-scale solar sails.