Saturday, January 26, 2013

Aboveground Mars Houses

The design approach depicted herein is for permanent inhabited structures on Mars. This is not first mission stuff, it is for the follow-on voyages that plant permanent bases. There are at least a couple of necessary supporting technologies required to make this structure possible, technologies not currently in hand.

One is a concrete substitute made from local materials on Mars (“icecrete” or “pycrete” will not serve, since interior temperatures will necessarily be above freezing). The other is a window transparency material of overall characteristics similar to glass, but made locally on Mars. Neither currently exists.

Suspend disbelief for this discussion, and assume that the concrete and glass replacements have been perfected. What might serve as an aboveground habitation space, given the near-vacuum, the cold, and the radiation environment that we face on Mars? There just won’t be a cave to live in everywhere that you might want to go.

The structure must be pressurized, so that at least some surfaces must be round in order to be structurally efficient. It needs an upper surface that can serve as a radiation shield against both the steady drizzle of high-energy cosmic rays, and the erratic brief and intense blasts of lower-energy solar flare radiation.

Because this is a planetary surface, but with little atmosphere and no magnetic field, essentially half the interplanetary radiation fluxes are good design values. This is simply because half the sky from which the danger comes is blocked by the planet beneath your feet. For cosmic rays, the interplanetary flux varies with the solar cycle from about 24 to about 60 REM per year, so that would be 12 to 30 REM per year, as exposed on the surface outside.

The problem with cosmic rays is secondary particle showers from your shielding. It either needs to be thin, or else very thick. 20 cm of water will cut these exposures roughly in half (that’s down to about 6 to 15 REM per year on the surface of Mars). The applicable exposure limits for astronauts are currently 50 REM per year, and a career limit that varies between 100 and 400 REM accumulated, depending upon age and gender. The higher-level short-term limit (25 REM in 30 days) is not an issue here. No limits are available for children yet.

Clearly, the thin shield approach is inadequate for more-or-less permanent adult residents, because the career limits get reached in as little as 6 years, discounting completely the travel exposures incurred reaching Mars. However, such a shield would be quite adequate for solar flare exposures, and anything thicker is even better.

The other shielding approach is many meters of regolith, which essentially eliminates the exposures from both sources. It does have to be many meters thick, so that secondary showers also get absorbed in the shield. This is heavy, and could also do double-duty resisting vertically-oriented pressure blow-out forces. You could even bury ice-as-a-permafrost-layer in this structure, getting a lot of extra shielding benefit from the hydrogen in the water.

That last point (thick regolith layer as shield) leads directly to the notion of vertically-oriented cylindrical structures resisting internal pressure by tension in the hoop direction, and by ballast weight in the longitudinal (vertical) direction. The roof should have some overhang, so that these will essentially look like man-made mushrooms. See the figure at the end of this article.

The more-or-less flat roof “plate” must be capable of supporting the full deadweight regolith loading in the depressurized state. Pressurization can unload those stresses somewhat. This structure is supported by a central column, and by the surrounding pressure wall ring, which in turn is very likely to be a series of columns between which flat transparency panels are mounted. Such columns must be radially and circumferentially-restrained at both top and bottom, to provide the hoop stress resistance.

All of the column structures must resist heavy compression, and those in the ring must also resist bending, assuming the flat-facet approximation to a cylindrical shape applies, for ease of transparency panel construction. I also assume these panels are brittle like glass, so that the ring wall columns must be very stiff, stiffer by far than the transparencies. The transparencies themselves need to be fairly narrow to minimize panel bending due to pressurization, although they can be tall. There will be lots of columns in the ring wall.

The central column can be simple masonry, of local rock and the concrete substitute, with minimal reinforcement. It can be hollow, so as to provide closed architectural spaces in its interior. Such a structure is not a pressure vessel, so it cannot be used as a shelter in the event of a depressurization accident. I would recommend inflatables well-distributed within the building for that purpose, each with supplies and pressure suits stored within.

The ring wall columns must be shaped to achieve great bending stiffness indeed, and they must be capable of supporting great compressive load. Structural steel shapes seem ideally suited for this purpose, although it might be possible to achieve the same ends with a pre-stressed beam shape,  made of the concrete substitute. These would be an analog to the concrete highway bridge beams, stood on end, and loaded from the inside out.

This building is going to require large concrete-substitute foundations, in order to provide stability without movement upon pressurization. This is little different from construction practices here on Earth. One difference is the gas seal through the Martian regolith underneath the ring wall foundations. It might be prudent to lay down a buried layer of artificial “ice permafrost” well under the foundations, rising up to the bottoms of the ring wall foundations to effect a contact ice seal. This may or may not be necessary, depending upon how fast gases can percolate through Martian regolith. That requirement remains to be determined. But we can do this if required.

The roof can be a flat slab with an integral “waffle” grid of stiffening beams, much like a slab foundation on Earth. I would put the slab below and the beams above, to simplify the interface with the ring wall and the central column. The beams can extend tapered onto the overhang. Pockets in the roof slab,  and on the foundations, provide ring wall column restraint by simple socketing.

There are two cylinder-end blow-out-load sealing problems to worry about: at the ring wall foundations, and at the wall-roof joint. If the roof, the foundation, and the ring wall can be built to the same effective stiffness, and the roof (with its regolith shield cover) is heavy enough, then sealing becomes nothing but interior caulking, something we have centuries of experience with, in ship-building.

I suggest double or even triple glazing in the transparency panels. These could be set separately into stepped pockets pre-manufactured into the columns, foundation, and roof panel, and then caulked there to “lock-in” while depressurized. They would be positively-pressed into place while pressurized. I would suggest graduated-step lower pressures in the spaces between layers, thus requiring that a means to control and monitor inter-panel pressures be built into every window bay. Double or triple glazing in this fashion provides both accident protection and good thermal insulation.

Inside the building, you have a large space with a dirt floor. Parts of this can be paved and used as desired, and parts of it can be farmed or gardened, as desired. With the roof, there is no direct exposure to the harsh UV component of Martian sunlight. Yet there is plenty of daytime illumination coming through the transparencies in the ring wall.

This is terrain-reflected sunlight, and it will be lower in UV content. The transparency material will also likely reduce the UV a little, as well. Some UV is required for plants to grow, so you do not want to completely eliminate it. You can adjust and augment the terrain-bounced sunlight with suitable reflective surfaces positioned and angled outside the building. Illumination and UV content can thus be easily tailored to each construction location.

Inside these buildings, you can easily run an Earth-normal atmosphere. Using local ice as a water source, Earth-type agriculture and living conditions become entirely practical inside these buildings. That eliminates the risks,  for pregnancy and birth, of exposure to a low-pressure oxygen atmosphere. It also reduces fire dangers considerably!

Smaller structures can be habitations and parkland recreation spaces, even small gardens. Larger ones can be agricultural food-growing spaces, which can include both plants and animals brought directly from Earth, and not modified in any way for an alien environment. We can just do what we already know how to do, as regards farming, and in our shirtsleeves, too!

The rooftops are perfect places to mount solar-thermal and solar-electric energy-gathering equipment. I suggest the water reclamation equipment be located outside these buildings in simple sheds in the near-vacuum conditions. That way, vacuum-flash distillation becomes trivially easy, and can be an integral part of the overall reclamation process. Recovered solids can be airlocked-in for use as fertilizers. (That total water reclamation process is another technology that is not yet in-hand.)

We do have some agricultural methods and food sources associated with both fresh- and salt-water aquaculture. That does not require buildings like this, and is not ultimately limited in size by the square-cube scaling effect, the way these mushroom buildings are. This is a self-pressurizing ice-covered pond approach, described in the article “Aquaculture Habitat Lake for Mars”, dated 3-18-12.

If you want to build the classic pressurized dome of science fiction, there are some structural considerations described in the article “Pressurizable Domed Habitat Structures”, dated 6-9-12. This approach does not address radiation shielding or thermal insulation, so I like the mushroom-building approach better. However, it might be useful for some purposes, not specified here.  We do not yet have the materials from which such a dome might be constructed.

Some structures might well be built from “icecrete” or “pycrete”, just not those exposed to above-freezing temperatures, or exposed to sublimation. For a discussion, see ““Icecrete”, a Substitute for Concrete as a Building Material on Other (Colder) Worlds”, dated 3-11-12.

Clearly, a supple spacesuit is necessary to carry out major construction activities on Mars. In my opinion, we will never “get there” with the standard “gas balloon” suits we have been using since the late 1950’s. I believe the answer lies with the mechanical counterpressure approach, pioneered by Dr. Paul Webb, who developed it from the “partial pressure suit” designs of the late 1940’s.

This design approach has been ignored for decades, and is currently held back by inappropriate design compression requirements, although funded small-scale by NASA at some academic institutions. See “Fundamental Design Criteria for Alternative Space Suit Approaches”, dated 1-21-11, for a discussion of design criteria that are actually appropriate, and which could be met successfully with technologies available ever since the late 1960’s.

I got the radiation data from NASA itself, and posted it in “Space Travel Radiation Risks”, dated 5-2-12.

I see lots of proposals for establishing bases on Mars that will require multiple vehicles landed at the same site. That will indeed be required to establish any realistic permanent bases. Multiple vehicles landed together raises not only the issues of precision-guided landings, but also the issues of collision risks, and of close-proximity rocket-blast damage effects. Those last two are among the several practical issues included in the discussions contained in “Mars Landing Options”, dated 12-31-12.

I do not think we are yet technologically ready to attempt long-term settlements on Mars. But we soon could be! Perhaps very soon. And, it is time now to think realistically about how we might go about building these settlements. That is the purpose of this article: to bring good ideas to that debate.

Update 1-28-13:  I forgot,  we'll need a good caulk/adhesive that can be applied in vacuum,  and works with concrete and glass.  We're also going to need heavy machinery (functional on Mars) that fulfills the roles of backhoes,  bulldozers,  and construction site cranes.  I don't see much problem obtaining this stuff,  but none of it is yet actually in-hand.  Like the concrete and glass substitutes,  we cannot build habitations like this,  until all those things are in-hand.  (Rebar we can ship from Earth until somebody builds a steel mill equivalent on Mars.) 

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