Page:Advanced Automation for Space Missions.djvu/302

 magnesium alloys, and several more to forestall contamination between disparate batches, but three is the absolute minimum requirement.

Parts manufacturing. The construction Of a machine system as complex as a lunar SRS will require a great many individual parts which vary widely in mass, shape, function, and mode of assembly. If a complete parts list were available for the seed, then the manufacturing steps for each could be explicitly specified, precise throughput rates and materials requirements given, and closure demonstrated rigorously. Unfortunately, no such list is yet available so the team was forced to resort to the notion of the "typical part" to gain some insight into the performance which may be required of the casting robot.

Modern aircraft have about 105 parts and weigh up to about 100 tons, for an average of 1 kg/part(Grant, 1978). The average automobile has 3000-4500 parts depending on its size and make, so the typical part weights perhaps 0.5 kg (Souza, personal communication, 1980). A study performed for General Motors concluded that 90% of all automotive parts weigh 2 kg or less (Spalding, personal communication, 1980). A design study by the British Interplanetary Society of a very advanced extrasolar space probe assumed a figure of 9 kg per typical part (Grant, 1978). Conservatively estimating that the typical LMF part is only 0.1 kg, then a 100-ton seed is comprised of roughly a million parts.

If most components may be made of aluminum or magnesium then the density of the typical part may be taken as about 3000 kg/m3, so the characteristic size of the typical part is (0.1/3000)1/3 = 3.2 cm. This result is consistent with Souza's (personal communication, 1980) suggestion that the average automobile part could be characterized as "roughly cylindrical in shape, an inch in length and half an inch in diameter." The casting robot must be able to cast all 106 parts within a replication time T = 1 year. If the casting bay is only 1 m2 in horizontal extent, and only 10% of that area is available for useful molding, then each casting cycle can prepare molds for 0.1 m2 of parts. The characteristic area of the typical part is (0.1/3000)2/3 = 0.001 m2</SUP>, and dividing this into the available area gives 100 parts/casting cycle as the typical production rate for the robot. To produce 10<SUP>6</SUP> parts/year the casting robot must achieve a throughput rate or 10,000 cycles/year, or about 52 min/cycle. This in turn implies that the system must be able to carve or mold at an average rate of 30 sec/part. Since most parts should be simple in form or will have patterns available, this figure appears feasible. After the casting robot makes molds for the parts, the molds are filled with molten aluminum alloy. The metal hardens, the mold is broken, and the pieces are recycled back into plaster of Paris; the aluminum parts formed in the mold are conveyed to the laser machining and finishing station.

Very thin sheets of aluminum also are required in various applications, among them solar cell manufacture, production of microelectronic components, and solar furnace mirror surfaces. Extrusion, rolling, and direct casting were considered and rejected on grounds of lack of versatility and complexity. Vapor deposition, currently used in industry to apply coatings to surfaces and to prepare thin sheets of aluminum and other substances, was tentatively selected both because of its tremendous versatility (any curved surface may be coated) and because it is state-of-the-art technology. The major problems with the process in terrestrial applications are maintenance of the vacuum and high energy consumption, neither of which are factors on the lunar surface or in an orbital environment.

Plaster molds to be surfaced are passed to a laser honing station where they are finished to any desired accuracy, after which they move to the vapor deposition station and are coated with appropriate metals or nonmetals to the requisite thickness. The process is expected to proceed much as described by Miller and Smith (1979). The plaster mold is then removed and recycled, and the fabricated aluminum sheet is passed on to the electronic fabrication system or is sliced into wires by a fine cutting laser (Miller and Smith, 1979).

Mass throughput rates for this system appear adequate. Assuming that 10<SUP>4</SUP> m<SUP>2</SUP> of solar cells are needed for the original seed (Freitas, 1980) and that the casting bay is about 1 m<SUP>2</SUP> in area, then for T = 1 year the required deposition rate to produce 0.3 mm thick aluminum sheet is r<SUB>d</SUB> = (10<SUP>4</SUP> m<SUP>2</SUP> solar cells/year)(3&times;10<SUP>-4</SUP> m thick/sheet)(1 sheet/m<SUP>2</SUP>)(1 year/5.23&times;10<SUP>5</SUP> min)(10<SUP>6</SUP> um/m) = 5.7 um/mm. State-of-the-art deposition rates attained for aluminum commercially are about 50 um/min (Miller and Smith, 1979), nearly an order of magnitude higher than required. (The above throughput rate would also be equivalent to 1 m/sec of 0.3 mm aluminum wire production if cutting and wrapping can keep pace with deposition). Cycling time is about 52 min/sheet. Following Johnson and Holbrow (1977), a heat of vaporization of 10<SUP>7</SUP> J/kg for 10<SUP>4</SUP> solar cells each made of 0.3 mm Al of density 3000 kg/m<SUP>3</SUP> requires a continuous power draw of only 2.9 kW, which can be supplied by a small solar collector mirror 2 m in diameter.

A small number of LMF parts are expected to be made of cast basalt - fused as-found lunar soil perhaps with fluxing agent additives. Most parts will probably be aluminum because Al is an easily worked metal with high strength, low density (hence supporting structures need not be large), and relatively low melting point (hence is easily cast). The major advantages of basalt are its easy availability, its tolerance of machining, good compressive strength, and high density in some uses. Anticipated applications include machine support bases, furnace support walls, robot manipulator tools (to avoid vacuum welding), and other special parts where weight is not a problem. Because