Page:Advanced Automation for Space Missions.djvu/301

 useful part (10% mold volume utilization). If the mean density of LMF parts (mostly aluminum) is taken as 3000 kg/m3, and the entire plaster mass is recycled once a day, then for a 100-ton seed the robot must have 2600 kg (0.91 m3) of plaster compound (gypsum, or calcium sulfate) on hand. To hydrate (set) this much plaster requires 483 kg of water, an amount of precious hydrogen already allowed for in LMF materials estimates presented in appendix 5E. Availability of sulfur is not a concern, since 2600 kg of plaster requires only 475 kg of S. Terrestrial plasters commonly have a small amount of strengthener added, but in the lunar application this substance should be designed to be recyclable or must be eliminated altogether.

Plaster casting is not the only way to make parts in a growing, self-replicating factory, but it is definitely one of the easiest both conceptually and in common industrial practice. Plaster methods are especially well suited for producing parts with hard-to-machine surfaces such as irregularly shaped exterior surfaces and in applications where a superior as-cast surface is important (Yankee, 1979). Plaster molded products commonly include aluminum match plates, cores and core boxes, miscellaneous parts for aircraft structures and engines, plumbing and automotive parts, household appliances, hand tools, toys, and ornaments. The technique is good for manufacturing parts requiring high dimensional accuracy with intricate details and thin walls (&gt;=0.5 mm). Castings of less than 0.45 kg and as massive as 11,350 kg have been made on Earth. Commercially, when compared to aluminum die casting, plaster mold casting is considered economical if 1000 parts or less are produced, although production runs up to 2000 parts may also be considered economical if the parts are especially complex.

Refractories. Refractories are materials which remain useful at very high temperatures, usually 1500-2300 K. They are employed primarily in kilns, blast furnaces, and related applications. In the lunar SRS refractories are needed as linings for drying kilns, roasting ovens, in the production of iron molds (to cast basalt parts) and iron parts, and also as material for special individual parts such as nozzles and tools which must operate at very high temperatures.

Refractories are usually, but not always, pure or mixtures of pure metal oxides. Tables in Campbell and Sherwood (1967) list the most important simple and complex refractory substances which LMF designers might choose. There are a few basic considerations, such as vapor pressure. For instance, although magnesia melts at 3070 K and has a useful operating temperature to about 2700 K in oxidizing atmospheres, it cannot be used in a vacuum at temperatures above about 1900 K because of volatization (Johnson, 1950). Similarly, zinc oxide volatizes above 2000 K and tin oxide sublimes excessively at 1780 K even in an atmosphere.

Refractory bodies are fabricated from pure oxides by powder pressing, ramming, extruding, or slip casting. The last of these is the simplest, but requires a very fine powder. This powder is normally prepared by ball milling. Steel mills and balls are used, and the iron is later separated by chemical means. For simplicity in LMF design, the iron alloy powder inevitably mixed with the milled product can be removed by magnetic separation.

High-alumina cements and refractories may be the best option for lunar manufacturing applications. Alumina is a maj.or product of the HF acid leach system in the chemical processing sector, and is capable of producing castable mortars and cements with high utility up to 2100 K (Kaiser, 1962; Robson, 1962). It will permit casting iron alloys, basalts, and low melting point metals such as Al and Mg. Unfortunately, it will not be possible to cast titanium alloys in this fashion, since in the liquid state Ti metal is very reactive and reduces all known refractories.

Alumina can be slip-cast from water suspensions. The oxide powder is first ball-milled as described above to 0.5-1.0 μm, then deflocculated by the addition of either acid (HCl) or base (NaOH), and finally the refractory body is developed by absorbing the liquid in a porous mold (plaster of Paris may be used with a base deflocculant). Gravity and hydrodynamic pressure of the flowing liquid produce a well compacted body of the suspended particles (Campbell and Sherwood, 1967). A fairly comprehensive review of alumina and alumina ceramics may be found in Gitzen (1966).

Metal alloys. A number of different metal alloys will be required for casting various parts and molds. Different alloys of iron may be chosen for the steel balls for ball milling, the basalt casting molds, and the individual part that might be comprised of steel or iron. Various aluminum alloys may be selected for parts, whereas pure metal is required for vapor deposition processes. Castable basalt may require fluxing but otherwise is a fairly straightforward melt.

Metallurgical duties are performed at the input terminus of the fabrication sector. Mobile chemical processing sector robot carriers dump measured quantities of metals and other substances into cold fabrication sector input hoppers (made of cast basalt and perhaps stored under a thin oxygen atmosphere to preclude vacuum welding). Mixing is accomplished by physical agitation, after which the contents are fed into a solar furnace to be melted. If net solar efficiencies are roughly the same as for the 5 kg capacity induction furnace (output 30 kg/hr) described in the MIT space manufacturing study (Miller and Smith, 1979), then about 30 kW of power are required which may be drawn most efficiently from a large collector dish roughly 6 m diam. There are at least three hopper/furnace subsystems required - a minimum of one each for iron, basalt, and aluminum alloys. Possibly another would be needed for