MECHANICAL TESTING OF DEPLOYABLE THIN SHELL CFRP BOOMS IN IDEAL AND REALISTIC INTERFACES FOR THE SOLAR SAIL DEMONSTRATOR GOSSAMER-1

Kurzfassung

Gossamer space structures like solar sails, drag sails or solar arrays expand to very large dimensions while at the same time aiming for low masses, leading to a low areal density. In many cases these applications are designed using long lightweight booms, spanning out sails or other membranes. For launch and transfer however the booms need to be stowable and compact, while they are demanded to withstand the sail loads during deployment and in operation phase. In DLR´s Solar Sail demonstrator Gossamer-1, a 5 m x 5 m squared solar sail, four sail quadrants are spanned out by four thin shell CFRP booms. Facilitating a tip deployment, the booms are flattened and coiled each on a cylinder of a deployment unit for stowage, while the boom roots are fixed in an interface to the spacecraft bus in a cross like arrangement. Having its purpose in transferring occurring boom loads into the main structure, the interface needs to be flexible at the same time, mimicking the changing cross section of the boom, once it deploys and transforms from flat to its full cross sectional dimensions. Another interface connects the sails to the booms, while holding the boom´s cross section in a semi-deployed state. These found conditions are imperfect compared to using the full cross sectional shape of the boom with its full second moment of area and an ideally clamped root fixation. Simulating the boom and its interfaces of the Gossamer-1 demonstrator, several booms are tested in an advanced vertical boom test stand at the DLR Space Structures Lab @ Uni of DLR Braunschweig. Mechanical characteristics and properties of full scale booms with ideal boundary conditions and realistic interfaces are being investigated, enabling a direct comparison. Applying certain realistic load cases, like lateral bending, axial compression and combinations of both, thresholds and robustness as well as load carrying capabilities after buckling several times are quantified in practical tests. Thus characterizing the Gossamer-1 boom under ideal and realistic boundary conditions is realized, specifying the boom performance in reality and providing the base for a robust Gossamer Spacecraft design that facilitates thin shell CFRP booms. This paper gives a detailed insight on the test stand and setup, used interfaces and boundary conditions, testing procedures, and discusses the test results of both configurations on characteristic load-displacement curves and acquired values for buckling failure design.