Sorry I can't find a better link, but you don't really need a lava tube for settlement, it just makes it cheaper and easier. You're still going to need an inflatable habitat
The obvious problem with an inflatable habitat is that anything the size of dust is going to make at least one hole in it. Patching is likely to take up quite a bit of someone's time.
or similar (honestly, what else makes sense?) to sit in the tube.
Install two bulkheads some distance apart and pressurize the space in between to 75 kPa.
Installing bulkheads will work, whenever you can build them. For the larger lavatubes on the Moon, you would not want to lift the mass of a bulkhead, even an inflatable one. Even then, you must seal the rest of the lavatube, since lava fields are among the most porous rock formations we know of. Whenever basalt cools slowly, it cracks,....it cracks a *lot*, which is a major reason that *most* lavatubes collapse, giving us sinuous rilles on the Moon, and collapse trenches here on Earth.
There *are* ways to use insitu resources to seal lavatubes, and make bulkheads. There is native iron and nickel in most lunar regolith, from nickel/iron meteoroid impactors whose metal recondenses on the surface after vaporizing on impact. In some places, it's nearly 1 percent of the regolith. Gather it with a magnetic rake on a telerobot, then dump that into a reaction vessel, and blow carbon monoxide through it at about 160 C at 3-5 atmospheres pressure, and you will make it into iron pentacarbonyl and nickel tetracarbonyl. Distill these to separate them, then break down the carbonyls by lower pressure (.1 Atmosphere) and higher temperature (about 220 C) to get Iron and Nickel powders on a micron size level.
Use an electrostatic accelerator to throw the individual powder grains against the lavataube surface at a high enough speed they will splatter and stick to the surface. Build up a seal of the more common Iron component on the rock side of the seal, and then seal that off from water vapor in the future habitat atmosphere with a coating of Nickel, using the same technique as the Iron.
For the bulkheads, bring a mold with the continuous curvature of the bulkhead, but only a small part of it. Then coat it with enough Iron to hold the desired pressure, with margin. Then coat that with the same thin coating of Nickel as on the lavatube seal. Use a sub-millimeter thin coating on the mold that reacts with the Iron surface of the bulkhead against the mold to weaken its grip on the mold.
When that small section of the bulkhead is thick enough to hold the desired pressure in the habitat, slide the mold to the side, till it barely overlaps, then repeat the 2 coatings, making sure the 2 sections are welded to each other. Repeat this till you have constructed the entire bulkhead in place inside the lavatube.
This would give you a safe habitat, with bulkheads and a lavatube seal that do not need to be lifted from Earth. These techniques are especially useful when you look at lavatubes hundreds of meters in diameter, as may well exist on the Moon, because of its low gravity.
Till you can do something like this, use pressurized modules, either inflatable modules brought from Earth, or solid modules made from lunar glass-in-glass composites. That will let you get enough crew under the shelter of the lavatube to do the work of sealing a larger lavatube for a lunar community.
In the virtual world of Second Life our research team is modeling these processes in a 3-phase development, at the National Space Society Island, in the SciLands Archipelago of Second Life.
Some of our papers on this topic are at:
http://www.oregonl5.org/l5sr2002.html
Regards,
Tom Billings
