6.21 The ES group therefore developed a fixed relative orientation design. The sunlight and cargo cutouts in the shield would be lined up with the endcap window and other endcap cargo hatch, respectively. Since the hull orientation would be constrained, the shield could be shaped like the hull. This design is depicted in Figure 6.11. The form-fitted shield also had a mass advantage over the spherical model. Figure 6.12 graphs the masses of a formfitted shield and a spherical shield versus the spacing between hull and shield (constant except at the cutouts for the form-fitted design, and measured at the closest hull-shield distance for the spherical model). The ES group picked the fixed relative orientation, form-fitted shield option. The FT group, after considering how much space between hull and shield might be required for inspection and repair equipment, suggested a hull-shield spacing of 5 meters. The ES group accepted this as a minimum figure ,reserving the final de.cision until the issues of precession and nutation could be investigated (see Section VI.5). VI.4.5: The Sunlight Beam: To concentrate sunlight and bring it into the hull through the light beam cutout and the window, the ES group proposed a Cassegrain two-mirror arrangement as shown in Figure 6.13. A parabolic mirror would reflect a selected range of wavelengths to a focal point; a second mirror would reflect it as a concentrated beam along the spin axis into the cutout and window. Once within the colony, the beam would be spread onto the agricultural area by a set of mirrors. If there were optical problems in forming a parallel beam, one or more Fresnel lenses could be placed in the beam path to channel the light along the spin axis_. Plants absorb sunlight in wavelengths between 400 and 700 millimicrons for photosynthesis (6.3). This is also the human visible range (see Section V.2.6). Within this range, a light flux of 410 watts/m 2 saturates the photosynthetic mechanisms of plants (6.4). The ES group decided to use 400 watts/m 2 for illumination of the agricultural area. This was well within the 1100 watts/m2 maximum set in Section V.2.6. Thus, the total sunlight power on the agricultural area would be:
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