The solar option for paving robots also has many degrees of design freedom, but for illustrative purposes a comparatively simple model was selected. The basic paving power module consists of a large, spherical polished aluminum mirror, constructed with easily manufactured small planar segments and affixed to a single-axis equatorial-drive turntable with a 90° sweep. This large dish is mounted on the north side of paving robots working in the lunar northern hemisphere. The robots travel east-west to maintain near-constant directional orientation at all times (except when beginning or completing a row of slabs). A planar rectangular mirror is mounted low in front of the dish, leaning forward at about 45° to direct the focus of the solar rays downward onto the carefully graded lunar surface. This second mirror may require three degrees of freedom for tracking and to permit it to project a proper square beam. Assuming accurate dish and plate mirror servo gearing, mirror positions are at all times accurately known. If the position of the robot vehicle is precisely fixed by the transponder network (see app. 5B), and an updated monthly lunar solar ephemeris is provided each robot by the seed central computer when work begins each lunar dawn, then the entire mirror pointing task can be fully automated and sun-tracking sensor apparatus eliminated. The basic optical geometry is shown in figure 5.35.
Main dish size is given by:
- D = 2(P/πk2aI cos q)1/2
where D is mirror diameter, k is the reflectivity of either of the two polished mirror surfaces (which may range up to 0.86 for aluminized glass, Weast, 1969), a is the coefficient of absorption of solar radiation for lunar basalt (taken as 0.93 for lunar albedo of 7%), I is solar insolation (1400 W/m2), and q is the angle between the mirror pointing axis and the Sun. In a worst case of q = 20° error, D = 5.4 m.
The planar mirror is roughly rectangular, long end pointing downward, of approximate dimensions 2m X 4m. The heat absorbed by this mirror is at most P(1 - k)/8 + I or 1710 W/m2, corresponding to a blackbody radiation temperature of 417 K which seems manageable. Mirrors should require resurfacing only rarely, since oxidation and meteorite pitting are not expected to be major problems.
The tentative design for the LMF paving robot is shown in figure 5.36. Each machine has a pair of dish and rectangular mirrors. Two small navigational receivers are at either end of the flatbed, permitting the onboard computer to calculate its rotational orientation with respect to the transponder network as well as its position, and a two-axis
