Pressurized habitats

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Pressurized Habitats

Pressurized habitats are a means of creating living space with a significantly higher internal pressure than that of the ambient air. Since the average atmospheric pressure on Mars is far too low for animal life to survive, pressurized habitats will be a necessity for any Martian settlement.

Structural Types

Essentially, pressurized habitats are sealed containers that hold in air. This requires the habitat walls to be able to withstand considerable stress due to air pressure differentials. Internal forces in the walls are used to create counterpressure to hold the air in, which in turn creates mechanical stress in the wall. Also, the thermal environment of Mars is extreme, causing additional stresses due to daily heating and cooling in any wall that is not thermally isolated from the Martian environment.

Internal forces in the wall can be tensile, compressive, or shearing, depending on its design, and configurations combining two or more layouts can be beneficial as well.

Examples of designs using tensile forces to retain pressure are Thin-walled pressure vessels such as gas storage tanks and most manned spacecraft.

Examples of designs using compressive forces to retain pressure are brick domes and concrete cisterns.

Examples of designs using shearing forces to retain pressure are primarily found in large windows in pressurized buildings such as Biosphere II.

Combinations of forces can also be used in composite structures. The simplest combination, suitable to any pressure wall, is the double wall. Another suitable combination is the earth-sheltered pressure vessel, in which the compressive force of an external structure reinforces an interior pressure vessel under tensile stress.

Internal Environment

In addition to maintaining a suitable internal pressure, a pressurized habitat must maintain a livable range of environmental conditions. This creates additional requirements for the structure of the habitat beyond simply being able to retain pressure.

The habitat must be resistant to large temperature swings driven by the Martian environment, both thermally and structurally. The habitat must be insulated to prevent excessive cooling. It must also accommodate an air conditioning system able to circulate air within the habitat for cooling and treatment.

The habitat must accommodate the crew along with all of their equipment and supplies. To do this, the habitat needs an internal structure. Loads within the habitat will be neither static nor uniform.

Structural Fatigue

Any variation in the distribution of loads within a pressurized habitat will change the distribution of stresses in that structure, and can concentrate stresses so that some places are subjected to several times the average structural load. The crew moving around in the habitat will subject it to light shocks over the course of a day. In addition, the extreme thermal environment of the Martian surface will subject the habitat to a slow, moderate amplitude load cycle of expansion and contraction due to heating and cooling over the course of each day. Just as bending a small wire back and forth will create material fatigue in the wire, these repeatedly cycling loads will create fatigue in the structure of the habitat.

Many organic materials, such as wood, are able to withstand so many repeated load cycles that their working lifetime is effectively infinite. However, their fatigue strengths tend to be very low compared to metals. (A material’s fatigue strength is the stress above which it will eventually fail, given enough load cycles.) Some non-metals, such as concrete, fiberglass, and most epoxies, have no remaining fatigue strength after an effectively infinite number of load cycles, no matter how small the load. Their lifetimes can be extended by making them thicker, reducing their loads, and/or incorporating them into composite structures to reduce stress. A pressurized habitat constructed of concrete or brick, for example, would need walls quite thick simply to avoid cracking under thermal stresses within only a few years.

Metal alloys, such as steel and aluminum alloys, tend to have much greater fatigue strength than non-metals, with fatigue strengths up to half of their ultimate strength. Thus, they are suitable for structures that are subject to very high stresses. This does not make metal structures inherently longer lasting than other structures– metal structures fail, too – but it does mean that they can have thinner walls and lighter weight. These are desirable qualities for any structure at a Martian settlement, whether shipped from Earth or built on site. Steel reinforcing bars incorporated into a concrete, fiberglass, or plastic wall can reduce its required thickness by at least half. Metal reinforcements and other metal components should be incorporated into any pressurized habitat that is intended to last longer than a few years. This includes greenhouses, crew quarters, etc.

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External Links

Wikipedia Article: Cabin Pressurization