Solar sail is a promising propulsion concept that exploits solar pressure to navigate in space without the use of propellants, therefore enabling missions otherwise not attainable by traditional propulsion (i.e. electric or chemical propulsion). For instance, a synchronous solar sail with the Earth-Moon barycenter to be used as a long warning time of solar storms caused by Coronal Mass Ejections is the main objective of the Helianthus project, funded by the Italian Space Agency. This paper is aimed specifically at the presentation and description of the design of the structural subsystem for the solar sail of the Helianthus project. This subsystem is composed of four deployable ultralight booms, which deploy and keep the sail-membrane in tension. The booms have to withstand the axial load, generated by the tensioned membrane, which must be smaller than the critical load at buckling. At the same time, the booms need to have sufficient stiffness to prevent a large out-of-plane displacement of the membrane leading to reduction of the thrust. First, the geometry and the dimensions of the boom cross-section to optimize the stiffness is determined. Then, a structural numerical analysis on a full-scale model of a square solar sail (40 m x 40 m) with four supporting booms is performed. For such configuration, the sail tension is simulated in order to determine the axial load acting on the tip of each boom and the displacements due to the solar radiation pressure are evaluated. Simulations are carried out by finite element method using the software ABAQUS. Results are presented at both system and individual components level
