Local defect study of membrane antennas and reflectors

One of the great enabling technologies for 21st Century space science missions will be gossamer spacecraft. Since resolution is proportional to diameter at the diffraction limit, larger antennas and optic apertures mean greater opportunities for increasing scientific knowledge. Due to finite launch vehicle capacity (launch mass and volume), these large (>12 m) apertures must also be ultra-low mass. This implies some sort of membrane/ inflatable structure. The current paper discusses the effects of local defects on reflector performance. Unlike classical glass optics for example, membrane apertures cannot be ground and polished to precision tolerances. The manufacturing process must account for minimum thresholds of surface smoothness and mechanical property irregularities. For example, sufficient numbers of small regions of thickness or Young´s modulus irregularities can lead to unacceptable surface error. This paper reports on analysis of such local defects on the surface precision of gossamer apertures. A nonlinear finite element code is used to model the effect of single and multiple defects in curved membranes. Two measures of performance are used. First, we compute the deviation of the local slope for a given defect geometry and property irregularity. Secondly, we compute the spatial influence function of the defect both on the neighboring uniform membrane, as well as on nearby like defects. Indications of manufacturing tolerances required to achieve minimum acceptable performance are discussed