Page:Advanced Automation for Space Missions.djvu/193

 are less rigidly constrained than when heavy bearing capability is required.

Metal fasteners must be strong to bear significant loads. In many cases they can be manufactured by powder metallurgical or casting techniques. Iron is a constituent of many types of metal fasteners, although titanium increasingly is coming into use in applications where strength must be balanced against light weight. In most applications where permanent bonding is required metal fasteners are replaceable by some form of welding or soldering. A major consideration here is whether the fabrication of welding rods and the process of welding is a more or less efficient use of available resources and energy than the fabrication and use of fasteners. For nonpermanent bonds there is not much choice except friction/pressure fittings and these run the risk of vacuum welding.

Both iron and titanium are in abundance on the Moon and each has received much attention as two extraterrestrial resources most likely to be investigated early for extraction and utilization. The manufacture of metal rivets from lunar or simulated lunar resources would be a worthwhile early materials processing experiment for an orbital laboratory. Space applications considerations include:


 * Zero-g - Metal fasteners may be lighter in weight because loads may be far less than on the ground.
 * Vacuum - Permanent bonds are largely unaffected by vacuum. Vacuum welding will promote tighter joining, a benefit in the case of permanent bonds but a definite hindrance if breakable or sliding bonds are desired. Very low vapor-pressure lubricants (e.g., graphite), surface poisoners, or careful choice of incompatible metals may help to eliminate this problem.
 * Radiation - Some metal fastener materials may become more brittle with time in the presence of ionizing radiation.

The fastening of rivets and bolts already has been automated in some terrestrial applications. Extending the techniques of automation to space, and including screws and nuts, clamps and pins, seems to present no special problems.

4F.3 Interlace Fasteners - Stitching

Interlace fastener stitching is a joining process by which pieces of material are interwoven through holes in the parts to be joined. The bond is primarily frictional if the joined pieces are not rigid,primarily tensional if they are rigid. On Earth, mostly fabrics are stitched, though items such as tennis racquets and sieves also require a type of stitching in their manufacture. Stitching material usually has physical properties and adhesive characteristics similar to those of the materials joined. Parts to be fastened must have a series of holes through which the interlace passes. These holes may be native to the material, as in a fabric, or specially drilled, as in wood or metal sheets. (Terrestrial stitching is applied to some processes not immediately obvious, such as the knitting together of thin plywood sheets to form a mold for fiberglass.) The primary space-related utility of interlace fasteners is expected to be in the manufacture of EVA pressure suits. Designs such as the Space Activity Suit (Annis and Webb, 1971) rely on tension instead of atmospheric pressure to counterbalance internal hydrostatic forces using corset-like interlaces to join special fabrics. Stitching materials may be organic or synthetic fibers, glass fibers, or even metals.

The space environment places a few constraints on possible stitching materials, as discussed below:


 * Zero-g - Except for holding parts in place during fastening, zero-g presents no special hardships as regards stitching. Indeed, one possible indirect advantage is apparent: The lack of gravity permits finer threads to be pulled from molten material than is possible on Earth, because of the absence of both the catenary effect and the necessity to support threads against their own weight in zero-g.
 * Vacuum - Vacuum poses two problems for stitching. First, it is nearly impossible to make an airtight interlace without sealant. Second, most interlace materials are hydrocarbon-based, hence are volatile and easily deteriorate in a vacuum. Fortunately, nonvolatile stitches made of metals or basalt glasses can be found, and there do exist sealants effective in closing small holes against the loss of atmosphere.
 * Radiation - The deterioration of interlacing materials caused by hard radiation is a serious problem for hydrocarbon-based stitches, but replacement of these by glass or metal substitutes may eliminate the problem. Radiation-proof coatings should be vigorously pursued as an important topic in space manufacturing research.

The availability of stitching materials is strongly constrained. Hard vacuum and radiation in space render hydrocarbon-based threads infeasible due to volatility and molecular deterioration, and hydrocarbons are also relatively rare in near-lunar space. On the other hand, glass and metal interlaces do not suffer from these problems and are easily accessible on the Moon.

Stitching most efficiently must be done by machines in most applications, and these processes are already largely perfected. Interlacing beam ends do not seem to present any special problems for automation. As for alternatives, gluing can replace stitching in some applications such as the joining of fabrics. Gluing has the advantage of airtightness but the common disadvantage of lesser strength. Tack welding can replace interlacing of metals in many jobs, but a penalty must be paid in higher energy consumption.