The key assumptions for the three types of missions are shown in Table I. Based on these assumptions, collector requirements are set out in Table II. Configuration Two possible types of concentrators were considered: lenses and reflectors. The latter were chosen considering specific mission needs, i.e. durability, low weight, practicality, etc. The question of whether to use an offset configuration or a symmetric configuration was not as easily answered. For optical performance, the symmetric configuration is preferable (as shown in a following section), while the offset configuration is better from the point of view of inertial moments, balance of mass and radiator performance. We decided on an offset configuration. Each collector is nearly circular (1.5 kWe, 4m; 10 kWe, 9m; 15 kWe, 11m) and consists of seven to 13 hexagonal units. Shape, size, number and arrangement of units were determined after consideration of the gap between units, H-II rocket/STS compatibility, and so on. Although individual units should have parabolic surfaces for optimal optical performance, in practice units are constructed of many spherical segments because of ease of fabrication and assembly and lower costs. Figure 1 shows examples of different collector configurations for the 10 kWe power level. Collector Efficiency and Error Analysis The required sun image size on the receiver and limits on collector error were obtained
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