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One of the concerns that comes up quite frequently is how strong the lunar exploration base needs to be, to support a layer of lunar soil.
Instead of piling lunar soil directly onto the habitat for thermal and radiation shielding, we plan to use a canopy over the habitat and put the regolith on that canopy. This provides access to the exterior of the lunar exploration base and its external equipment, and the voids beneath the canopy provide an unpressurized garage for robots and other equipment.
I chose beta cloth for the canopy because it has a solid weave and doesn't degrade in vacuum and ultraviolent light. NASA programs have several similar fabrics, so we still have some work to do before we can make a final selection for the fabric.
There are lots of ways to arrange the canopy. The image at the right shows a concept by Jim Nobles. Note the additional facilities -- an inflatable habitat for additional living and working space, and some external equipment. The canopy might be laid directly onto the lunar base as shown in the illustration, or it might be suspended on poles so that there is no contact with the equipment beneath it. If the canopy is laid directly onto the habitat, we will have to design a mechanism to jack it up out of the way if we need to get to the top of the module; however, this option means we can leave the weight of some of the canopy poles and frames off the first flight.
Six feet (about two meters) of regolith is sufficient to reduce radiation to the level we experience on the surface of the Earth. For a back-of-envelope look at the problem, let's use a regolith density 1.2 gm/cm3. The canopy would be supporting a load of about 0.24 kg (mass) per square centimeter; which adds up to about 0.6 psi in lunar gravity.
This is wimpy stuff, probably not the design driver for the strength of the canopy. To demonstrate that to yourself, try supporting a couple of sticks of butter (0.5 lb, covering about 1 square inch) standing end-on, on a piece of tissue paper. Try using a toothpick to keep the sticks of butter stacked vertically. [Use a toothpick to keep the sticks of butter stacked up; and do it when nobody is watching so they won't wonder about you. Kids, ask your mom first, and tell her it was Bob's idea.]
The real design driver for the strength of the canopy material might come from stress concentration at the poles; or maybe handling and packaging the canopy for launch; or supporting the weight of the robot; or most likely crew handling to erect the cover. I suspect that if the crew can put it up without wrecking it, the canopy will have no problem supporting the load.
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