Development and Test of Deployable Ultra-Light Weight CFRP Boom for a Solar Sail

Abstract

Solar sail technology holds the promise of significantly enhancing the interplanetary infrastructure for low-cost space exploration missions in the new millennium, by exploiting the freely available space resource of solar radiation pressure for primary propulsion. By making use of this innovative means of low-thrust propulsion, extended missions in our solar system which require a Dv of several tens of kilometers per second would become possible. For missions with such high propulsion energies, solar sails could either be enhancing or even enabling compared to the more traditional means of space propulsion. Although the basic idea behind solar sailing appears simple, challenging engineering problems have to be solved. Since the propulsion efficiency depends on the overall spacecraft mass to solar sail area, technological solutions for in-orbit deployable, ultra-lightweight sail surfaces are required. The main technical challenges are to fabricate sails using extremely thin films and deployable lightweight booms, to package the sails and booms into a small volume, to deploy these lightweight structures successfully in space, and to control and navigate these large structures. The paper summarizes the main results of the development of the deployable lightweight CFRP booms and the ground tests. Significant progress in light-weight deployable structures, using advanced materials and processing methods, opens new perspectives concerning the realization of a solar sail mission in orbit. Carbon Fiber Reinforced Plastics (CFRP) have matured within the last decades so that they are now potential candidates to be utilized for solar sail structures. A visible milestone toward this goal is the development, analysis, manufacturing, and successful test of four CFRP booms each with a length of 14 m and a unit weight of 100g/m. These deployable CFRP booms, developed by the DLR Institute of Structural Mechanics in Brunswick/Germany, combine high strength and stiffness with extremely low density and can be stored within a very tight volume. In order to avoid thermal bending caused by asymmetric radiation, the laminate stacking sequence is arranged to have thermal neutrality in longitudinal direction. In storage configuration they are pressed flat together and can be coiled around a hub. After uncoiling in space, they obtain their full capabilities by self-expanding to their tubular shape. As a first major milestone in terms of technology demonstration a 20m x 20m breadboard model was developed, manufactured and tested in 1999. It demonstrates the feasibility of a fully deployable lightweight solar sail structure in a simulated zero-g environment under ambient environmental conditions. For the joint DLR-ESA technology development effort a square sail with diagonal booms supporting four triangular sail segments was chosen as the baseline configuration. The sail structure is composed of three major elements: the booms, the sail film segments and a central deployment module. It was shown that deployable ultra-light weight booms and extremely thin sail film materials can be handled and used to manufacture large solar sail structures. In a ground demonstration, within the joint DLR-ESA effort to pre-develop solar sail technology, the functionality of the deployment concept and associated mechanisms was demonstrated. During on-ground deployment the booms are highly endangered to collapse under their own weight. Therefore, a special method for gravity compensation was developed by DLR to simulate zero-g conditions. Helium-filled balloons are used to support the deployed booms. Due to the change of deployed boom mass during the unwinding process, the balloons must be designed to allow for an adjustable lift. To resolve this problem, the lift of the helium balloons can be adjusted actively by using remotely controlled pumps draining water from small tanks below the balloons.