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Tom Billings
The Lunar Base Research Team of the Oregon L-5 Society looked into the possibility of coating lava tube caves in some work they did for Lockheed in 1988-89. Remember that lunar lava tubes probably can be far larger than the (approximately) 25 meter diameter limit here on Earth. In addition, any non-metallic native coating material can be expected to crack as it cools. Measurements of collapse trenches believed to be associated with lunar lava tubes indicate diameters of several hundreds of meters. Given this situaution, they evolved several ways to use lunar lava tubes.
First, it would make sense just to use them as shelter from radiation, temperature changes, dust, etc. for rigid or inflatable habitats brought from Earth. This alone could mean large cost reductions by reducing the "emplacement costs" otherwise neccessary for such shelter requirements.
Second, it was noted that lunar glass fibers might be combined with native meteoritic iron-nickel particles to produce larger habitats from resources on the moon. Dr. Goldsworthy showed the possibility of making very strong glass fibers on the Moon during the 1980s. Native meteoritic metal could be refined by the "carbonyl" method, and the metal carbonyls used to produce a thin, tough, airtight metal layer on the inside of a habitat woven from lunar glass fibers. These habitats, inside lava tubes, would increase the available sheltered cubic volume greatly, and do it with cheap native materials.
Finally, if it is required to have an entire town inside one of these very large tubes, with the entire tube sealed, then the meteoritic nickel-iron could be used again. First use the carbonyl process to produce the pure metals in powdered form (micron-sized particles), as is done at Sudbury, Canada in producing much of the Earth's nickel supply. Then, chill the powder with Lunar oxygen. Now, put it through a modified (bucketless) massdriver some tens of meters long and shoot it at the walls of the lava tube at a 90-degree impact. With a velocity of about 2-3 kilometers/sec., the powder particles will splatter/self-forge to the wall of the tube, building up a layer of metal that seals all but the smallest cracks. To make sure that no leaks remain, we may now introduce gaseous carbonyls into the tube, and use Laser Chemical Vapor Deposition with a solar powered laser's beam to get a final continuous film producing an airtight seal.
The advantage to these methods is in limiting the amount of molten material that is handled in any bulk, especially in the open, especially around any humans. Either molten metal or molten rock/glass/lava are very corrosive. They require large amounts of energy to produce. In the case of molten aluminum, this means large amounts of electrical energy, which is already a major bottleneck in space operations. Carbonyls are non-corrosive liquids at room temperatures and are reduced to metal and CO at about 200°C at low pressures. The iron and nickel carbonyls require only carbon monoxide gas passed over the native material at 160°C to generate the carbonyl. In the vacuum of space they should be much safer and cheaper to handle.
This should allow significant cost reductions to lunar base activities fairly soon after the first outpost is in place, or with sufficient telerobotic preparation, at the first outpost itself.
Additional References in the Artemis Data Book
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