Moon Miners' Manifesto

#100 November 1996

Settling into a Lava Tube

Peter Kokh

Many of our readers will be familiar with the classical Island II "Stanford Torus" space settlement design [Space Settlements: A Design Study, NASA SP-413, 1977]. Not counting multiple levels, this ring with an overall diameter of 1800 meters and a torus cross-section of 130 meters, has a circumference of 5.655 km or 3.5 miles and a usable surface area (lower slopes included) of about 50 acres.

With multiple levels, it was estimated some 10,000 people could occupy 106 acres. (Manhattan-like sardine packing; i.e., quite dense by modern urban standards of about 5,000 people per square mile = 640 acres.) That seems overdoing it especially since off-Earth settlements, wherever they are, will first and foremost be farming villages with lots of plants hosting very few people, not vice versa.

But thanks to the copious artwork that has accompanied the settlement design studies of the seventies, such a torus does give us an assist in conceptualizing a lava tube settlement. Cut it at one point and unroll it, and you have something comparable, if on the small end, to what we might someday see in lava tubes. The average lava tube is likely to be several times wider than the torus of the NASA study.

interior torus view, art by Pat Hill, IBID, p. 90

In practical fact, however, this scene gives us more of a goal to hold before us, than a model for feasible near-term reality. Sealing a lava tube to pressurize it may be easier said than done. If we succeed, filling the immense volume with the usual buffer gas of nitrogen imported from Earth in a 4:1 ratio with lunar oxygen may be budget-busting.

Near-term, pressurized ceiling clearances will have to be kept to a minimum. We will use lava tubes at first not to escape the vacuum, just to escape the deadly cosmic weather that normally comes with vacuum on the exposed surface.

The tube ceiling vault functions analogously to the Biblical firmament, protecting Lunans in their hidden valleys (lava tubes) from cosmic radiation, meteorites, temperature extremes, and the otherwise omnipresent dust. Even if the tube is not sealed and pressurized, it may be feasible to spray a high albedo coating on the upper walls and ceilings. (CaO lime, aluminum oxide, and titanium dioxide are all producible cheaply and in quantity, and are white). The trick is to make an anhydrous "whitewash." Unfortunately, bluing this inner "sky," perhaps with locally producible cobaltous aluminate, would be expensive.

Sunshine could be brought in down simple shafts or through optical cable bundles, to be turned on this sky-firmament, thus providing comfortable daylight-type ambient light. During nightspan, nuclear- or fuel-cell-powered lamps on the surface could use the same light transmission pathways. Possibly any whitewash material on the upper vault of the tube could have a phosphorescent component for a night span treat. Imagineering, it is called.

KEY: (a) sunshine access and defuser system; (b) white-washed "firmament" for best sunlight reflection; (c) "town deck" on tube-spanning beams; (d) assorted structures; (e) "yurt/ hogan" type home with translucent dome to flood interior with firmament-reflected sunshine; (f) monorail transit system; (g) lava tube floor left natural; (h) nature walks.

Instead of grading or even terracing the lava tube floor, it could be left natural with the town built on a spaceframe deck spanning the lava tube shoulder to shoulder. An overhead crane riding rails along the sides of this deck could be useful in constructing/erecting habitat structures. The use of stilt platforms is a possible alternative to the deck span (shoulder to shoulder beams).

Elevators to the surface can either be incorporated into "skyscrapers" reaching to the tube ceiling, or be built freestanding to provide great views of the town on the descent from or ascent to the surface.

Access to the settlement from the surface is vital. This can either be by freight and passenger elevator shafts or by a ramp road up the talus slope of a nearby natural entrance. We think the first option will bear the brunt of the traffic.

KEY: (a) sunshine access via suspended "daylux" defuser grid instead of coatings; (b) elevator shaft through "skyscraper"; (c) transit system on stiltway over tube floor.

The tubes are given to us dust-free. Thoughtful engineering of tube access systems will help keep them that way. For example, elevators could have their topside terminals opening not onto the dusty surface directly but onto a suspended platform/launchpad complex.

Appearances aside, a vital part of the settlement will be out on the surface and building material and component manufacturing out of "premined" regolith, the asset of the surface. Once a processing, manufacturing, or gas-scavenging operation is past the dust-using phase, further processing, manufacturing, assembly, or separation can be more safely and more economically done in the lee vacuum environment within the lava tube. Industrial siting decisions will take into account the degree of involvement of solar power and concentrated solar heating, of course. Operations that are electricity driven and not reliant on moon dust will be the first to move into the tube.

For the lunar architect and contractor, however, freedom from the need to be concerned with shielding is a considerable gain. Tube residences and other structures can have simple windows, and lots of them, through which to behold these netherworld landscapes. The shielded windows of in-surface structures which use mirrors and bent optical paths to thwart radiation, will be a cumbersome relic of pioneer beachhead days, still used where Lunans must live in the regolith blanket surface rather than in provident subsurface voids. Tube structure windows may be characteristically convex, curved in to the pressurized interior, so as to put the panes under compression. Glass and concrete are stronger under compression than under tension. Nor will in-tube windows need sacrificial panes.

The subsurface moonscapes within the lava tubes will be quite different from the surface ones, though sharing one all-important, all-infecting aspect: their barrenness and sterility. So tubers may share with topside moles the practice of placing plants in front of windows as a psychological filter.

Many architectures are possible. One simple tuber home plan would be a squat 2-story vertical cylinder section topped off by a convex-paned geodesic dome to let in the tube's ambient light. The design type might be called the Yurt or Hogan after the Mongolian and Navaho home shapes it resembles.

KEY: (a) 2-story vertical cylinder section, bedrooms on the lower level; (b) lunar translation of the geodesic dome for a high translucent ceiling vault over the family room and other common areas including a central garden atrium; glass panes are neither flat nor concave, but convex; (c) cable stays prevent internal pressure from literally "blowing off the roof"; (d) the residential deck of the townsite, leaving the tube floor ungraded.
NOTE: upscaled, the Yurt/Hogan design will make a fine church, synagog, or meditation chapel, with the simple use of stained glass convex panes in the roof dome. A dedicated shaft of directed sunshine on such a dome would surely help set the mood.

The early lava tube settlement will not be an assembly of individually pressurized buildings, but rather, like the in-surface burrowings, a maze of structures conjoined by pressurized walkways, streets, alleys, and parkways. In the netherspaces, thoroughfare cylinders can be generously paned with convex windows to flood their interiors with ambient reflected and diffused sunshine and views.

KEY: (1) cylinder section; (2) convex-glass panes to let in ambient reflected sunshine and views; (3) Yurt/Hogan style homes opening onto street via entrance tubes; (4); (5) pedestrian "sidewalks"; (6) rail-suspended goods delivery platform; (7) "cross-walks"; (8) landscaped, concrete-free garden strips; (9) dust-purged, conditioned regolith geoponic soils.

Along with solar access for reflection off coated upper tube surfaces, there can be some sunshine ports that direct intense pools of light downward, say on the convex-paned lunar geodesic domed park squares. Nothing is so soul-renewing as a visit to a pool of strong over-illumination, the feeling of being outdoors in the undiluted brilliance of the unmediated sun. Directed sunlight, minus the infrared removed by proper glass filters, will also be needed over agricultural areas.

You can see how construction and architecture in lava tube settlements differ from the other types of in-surface settlements we have discussed before. Initially, there will be a strong reliance on inflatable structures and inflatable-rigid hybrids. Here, in lee vacuum, with no need to cover them with shielding, no vulnerability to micrometeorite puncture or ultraviolet and flare and cosmic ray aging, inflatables will have their heyday. All the same, as the costs of new modes of lunar building materials and building components come down, and appropriate construction and erection methods are perfected, the bottom line money consideration will move settlement expansion in that direction.

An intermediate phase may involve the use of inflatable structures as slipforms for cold-casting (poured and sprayed lunar concrete) and arch/vault component placement.

As more generous endowments of nitrogen become financially feasible, larger domes over park space commons will make their entrance, affording a more generous "middoors" and the more obvious comfort of luxuriant flora and fauna, plants, and urban wildlife.

Meanwhile, in the lee vacuum but visible out the abundant windows of lavatube structures will be other extensions of the settlement: sculpture gardens and Japanese style rock landscaping. Electronic displays on the tube walls, even something reminiscent of drive-in theaters, or should we say through-the-windshield theaters? Backlit murals on glass can infuse the citizens with the dream of a Green Luna, not altogether out of reach. And I'm sure sooner or later we'll see some gross examples of tagging by artistically inclined youth without direction or access to approved ways of expression.

Nature walks can educate citizens on the fine points of lunar geology and variations in lava tube textures and formation.

The lava tube settlement will not be a solitary community. To provide around the clock manning of industrial and agricultural facilities owned in common, a string of three villages with staggered day/night lighting (the solar access ports can be shuttered, after all) will provide a succession of prime work time day shifts. A trio of villages can be separated by some distance along the inside of a lava tube, with intervening light baffle curtains (where convenient bends in the tube route do not offer the same benefit. Mass transit will unite them, and they can share 24-hour around-the-clock metropolitan facilities and amenities, including schools and parks and other investments that need to earn their peak full-time, or should we say all-time.

Settling the first lava tube should be part of a well-thought-out Outpost Conversion Strategy. An initial beachhead outpost is succeeded by a surface construction camp once a mature set of feasibility experiments leads to the production of on-site building materials. Proper site selection will have taken "graduation" to a nearby lava tube into account as an essential ingredient. Finally, after robot exploration and surveying of the proposed first site, will come the erection of lava tube village one, village two, a metro complex, and village three. Along with warehousing, farms, and industrial park sections -- a whole mini urban complex. MMM

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