Crts Reflectors

A design methodology for collapsible rib-tensioned surface (CRTS) reflectors is presented. Offset configurations are explicitly addressed. For a reflector with aperture of 10 m diameter and focal length of 7.8 m, a root-mean-square surface accuracy of about 2 mm is achievable; this result scales linearly with the diameter. The design of a 1.5 m diameter prototype is presented, and the accuracy and deployment behaviour of a physical model are measured experimentally. Figure 1: 1.5 m diameter model of offset CRTS reflector. INTRODUCTION A new concept for multi-purpose deployable membrane reflectors known as Collapsible Rib-Tensioned Surface (CRTS) reflector has been proposed by the European Space Agency [1]. These reflectors consist of three main parts. A central expandable hub, a number of thin-walled foldable ribs connected radially to the hub, and a precision shaped membrane that is supported and tensioned by the ribs. A photograph of a small-scale prototype is shown in Fig. 1. During deployment the hub is retracted to its largest radius configuration, so that the ribs can deploy the membrane without having to prestress it as well. After the membrane has been fully deployed, the hub is expanded, thus pushing the ribs radially outwards by a small amount. This has the effect of applying a state of prestress to the membrane, which sets it into its intended shape. The shape of the reflector is required to be as close as possible to a perfect paraboloid. The accuracy of this approximation is related to the deviation of the surface from the best-fit paraboloid, and is usually measured in terms of an overall root-mean-square error, δrms [2]. This value depends on both the number of ribs and the shape of the membrane between adjacent ribs. Non axi-symmetric configurations (offset) are of greatest interest for communication satellites. This paper presents a methodology for the design of offset CRTS reflectors to meet a set of specified requirements; symmetric reflectors are covered as a special case. After briefly describing the geometry of offset reflectors, a procedure is given for determining the prestress distribution, the shape and corresponding accuracy of a particular design. In particular, a method is presented for prestressing the membrane without applying outof-plane forces on the ribs, which is the major difficulty of offset reflector design. Then, the design of a 1.5 m diameter reflector is presented, and the surface accuracy and deployment behaviour of a physical prototype are measured experimentally. OFFSET REFLECTORS The standard way of defining the geometry of an offset reflector is to consider the intersection of a paraboloid