Stiffened Springback Reflectors

The recently developed Spring Back Reflector has low stiffness in the deployed configuration. A general method of stiffening this type of reflector without changing its folding properties is presented. The stiffening is achieved by adding a conical skirt around the edge of the dish; this skirt snaps elastically when the dish is folded. The paper concludes that it is possible to design spring back reflectors that are up to 80 times stiffer and with a deployed fundamental frequency up to 8 times higher than in the original design. INTRODUCTION The newly developed Spring Back Reflector [1], shown schematically in Fig. 1, consists of a thin-walled graphite mesh dish with an integral lattice of ribs and connecting elements. These components, together with a further stiffening edge beam along the rim, are made from triaxial plies of carbon-fibre reinforced plastic (CFRP). The whole structure is made as a single piece, without any expensive and potentially unreliable joints, and typically has a diameter of 6 m, thickness varying between 0.3 mm and 3.2 mm, and a total mass of around 20 kg. The folding concept is both simple and effective: since there are no joints or hinges, opposite edges of the reflector are pulled towards each other by about half of their original distance, and thus the reflector becomes folded elastically as shown in Fig. 1. It can then be stowed in the normally unused space in the nose cone of a rocket launcher. Once in orbit, the tie cables that hold the reflector in its packaged configuration are released, and the reflector deploys dynamically by releasing its stored elastic strain energy. Figure 1: Schematic views of spring-back reflector, folded and deployed. In order to be folded as described, Spring Back Reflectors need to have low stiffness but it is precisely their low stiffness that makes it difficult for them to achieve and retain a high shape accuracy. Furthermore, shape distortions that occur during the manufacturing of thin CFRP structures —of the order D/1000 in the present case—make it even more difficult to meet the stringent shape accuracy requirements imposed on communications antennas. This is a significant problem, and potentially a severe limitation on the applicability of this type of reflector. The surface accuracy of the reflector could be adjusted with mechanical devices, but this would defy the simplicity of the concept and reduce the reliability of the system. This paper proposes a modification of the original concept, based on the idea of adding a thin-walled stiffening element around the edge of the dish. This element significantly increases the overall stiffness of the dish in the deployed configuration, and yet its configuration is such that the stiffened dish can still be folded elastically. A detailed finite element study is presented of the effects of changing the various parameters of the stiffening element from which optimal configurations of a 1/10th scale model —i.e. about 0.5 m diameter— reflector are