In making structures by vacuum vapor fabrication the goal is to create a uniform deposit of metal alloy with good mechanical properties. This should be accomplished with minimal equipment, labor, metal consumption, and environmental degradation. While a number of critical experiments must be performed, presently available information suggests that these goals may be attainable.
In physical vapor deposition of metals, most alloy systems show a fall density, fine grained microstructure at a substrate temperature 0.3 times the melting point of the metal (ref. 32). As substrate temperature is increased, the grains become coarser, the yield strength decreases, and ductility increases. Because these properties correspond to those of rolled and annealed sheet, vapor deposited metals have been termed "a true engineering material (ref. 33). Despite the fairly consistent behavior shown by many metals, experiments must be performed with the specific alloys that are of structural interest for space applications.
Given a metal deposit of adequate yield strength and ductility, uniformity becomes of concern. Irregularities in the substrate are replicated in the final metal surface; the problem is to ensure that non-uniform metal buildup (across steps and grooves, for instance) leaves metal with structural strength in the zone underlying the irregularity. This is aided by increased substratic temperature (to encourage migration of surface atoms), by a nearly perpendicular atomic flux (to discharge self-shadowing), and by use of an initially smooth substrate. Adequate uniformity seems possible with the above controls; if needed, however, there are several promising means of eliminating defects part way through metal buildup.
The equipment used in vacuum vapor fabrication can be very lightweight. It handles sunlight, thermal radiation, rarefied vapor, and an aluminum feed rod; forces on it are virtually nonexistent. The greatest mass in the system appears to be the solar furnace mirror area, which is directly proportional to energy consumption. This consumption is, in turn, driven by the efficiency of energy use (thermal radiation to heat of vaporization), efficiency of vapor use (aluminum vaporized to aluminum reaching substrate), and by the total quantity of aluminum deposited.
Ignoring efficiency factors, for a heat of vaporization of l.l X 10^4 J/g, a colony mass of 300 kt, a solar constant of 1.4 kW/m^2, and a fabrication time of I yr, the ideal mirror area is 7.4 X 10^4 m^2. An average flux deviation from perpendicular of 20 degrees probably represents adequate collimation; with proper evaporator design the inefficiency of vapor use should be less than 2.5 (unused vapor is condensed and recycled); even a poor energy efficiency should keep the total inefficiency below a factor of 10. Allowing a full factor of 10, the mirror area is 7.4 X 10^5 m^2 . At 100 g/m^2, this is 0.74 kt.
The remainder of the system includes refractory metal foil boxes for the actual solar furnace evaporation units, plastic film hoods to intercept scattered metal atoms, and a carefully made balloon in the shape of the desired structure. Including these masses, the total system is very likely less than 1.5 kt; if the fabrication time were extended over several years this mass would be less.
Because colony structures have rotational symmetry, the solar furnace evaporation units can cover different areas as the colony rotates beneath their beams. With proper arrangernent, complex shapes and structures can be created, and the direct human labor required for fabrication is very small.