This is the second post in a series on Construction in Space. You can read the first part, General Principles and Design Assessment Criteria, here.
In this post, I will present a possible architecture for Martian habitats which I believe has many strengths as a solution to this design challenge. The design I will present is modular, adaptable, and could be used on most planetary bodies in the solar system. For the purposes of this post I will discuss the case of a standalone habitat on Mars with an internal pressure of 50 kPa.
I would like to begin this post by restating the First Rule of Space Construction:
The fundamental structural load on any pressurized structure in space is the outwards force of the internal pressure.
The dominant structural load is the internal pressure of the structure, so the structure is designed first and foremost around containing that pressure. Unlike typical structural loads experienced on Earth, this pressure force is omnidirectional, so I will consider all three directions.
The two lateral directions are effectively identical. The vertical direction can be treated identically as well (like the example of a propane tank) or you can take advantage of some property of the surface or of gravity to use a different containment method in the vertical direction.
This realization leads to the first of two key aspects of my design: Pressure containment in the vertical direction can be achieved by using a mass of material on top of the structure, weighed down by gravity, which counteracts the vertical force of the internal pressure.
The second key aspect is how pressure is contained in the lateral directions. In most pressure vessels, internal pressure is contained by tensile stress in the outer wall. This creates a need for an outer wall which is strong, curved, and uniform. It also creates a multitude of failure points: Damage to any portion of the wall can cause complete structural failure of the entire enclosure. While all pressure vessels necessarily have curved walls, my design improves on this as follows.
Pressure is contained on a wall-by-wall, level-by-level basis. The load-bearing exterior of each wall is a curved, semicircular piece of material which is attached to a corresponding piece of material on the opposite wall by way of a number of steel cables strung between them. This pressure containment jacket is shown schematically in the diagram above.
This results in two main benefits, and numerous side benefits: The first is massive redundancy. The failure of a single cable, if it occurs, won’t cause structural failure. Neighboring cables will take up the slack, and the cable can be replaced. The second is that the footprint of each habitat can be square or rectangular with flat floors. This massively cuts down on wasted space and improves the usefulness of the enclosed volume.
Other benefits include the relative ease of inspection and regular maintenance (shirtsleeve environment, direct visual and tactile inspection possible), the fact that a single point of failure does not make the entire habitat useless, and the cost savings derived from mass production.
Second-Order Design Concerns
The First Rule of Space Construction states that the first-order concern for any pressurized habitat is how you contain the pressure. Having addressed this first-order concern, I will now address various second-order concerns. These are still important, but you can only have one first priority.
An important aspect of the structure that I haven’t yet discussed is the compressive structure. The compressive structure is a necessary part of the habitat and serves a number of different vital functions: It supports the lateral pressure trusses and walls, supports internal fittings (walls, floors, furniture, etc.), and also supports the counterweight overload.
The counterweight necessarily needs to weigh more than the theoretical equivalence value because you need that extra force to hold it in place in the structure and to make sure that there is good contact on a continuous basis between it and the rest of the structure. I believe that a counterweight overload of about 5% is sufficient for this function. The most important limiting factor is that the internal pressure never exceed the actual weight of the counterweight.
At this point I would like to describe an important design choice. The compressive structure can be designed to be able to withstand the full weight of the counterweight, even without the upwards pressure of the internal atmosphere (vacuum stable) or it could be designed only to support the loads it will experience with the habitat fully pressurized (pressure stabilized).
A vacuum stable design is safer, because in the event of an unplanned depressurization the structure will not collapse. By contrast, a pressure-stabilized design can use much less material because it doesn’t need to be nearly as strong, but requires a more complex construction process (partial construction of the counterweight, followed by incremental pressurization, followed by incremental construction of the counterweight, etc.).
While it is easy to imagine large gas reserves that boost the pressure by matching the gas outflow in the event of a depressurization, I believe it is safer and easier to build a vacuum-stable design. I will spec for this in future posts and discussions on this topic.
Something else I want to talk about is radiation. I may address my full thoughts on radiation generally in a later post, but in short I think it is a real concern worth discussing, although not one so serious that it can stand in the way of a robust settlement effort.
As you can see in the above map, Mars’s atmosphere provides meaningful shielding from radiation: Low altitudes experience about half as much as the tops of the tallest volcanoes. This shielding effect will be strongest in the horizontal direction and weakest in the vertical direction.
This structure provides an excellent complement to that. It provides tons of shielding in the vertical direction which should cut radiation from that direction down nearly to zero. The residual horizontal-direction radiation will be weaker than the vertical component, but additional external walls can be built to shield the habitat if desired.
Every rendering in this blog post shows the habitat as a multi-story building like a mid-rise apartment tower, rather than as a single-story building that spreads over a large area like a warehouse. I believe that given the broader constraints of this design that is what makes the most sense.
On Earth, it is cheaper to build a one-story building than a two-story building, and we typically build out before we build up. Land values on Mars will be low, and it’ll be a long time before any settlement reaches a size where, in the United States, we start building up.
The reason it makes sense to build up with every structure is related to the first rule of space construction: The additional compressive forces related to building up do not dominate habitat design in space in the same way they do on Earth. For a “regular” pressure vessel (propane tank style), the most efficient (but not necessarily the cheapest) shape is a sphere, which maximizes the ratio of internal volume to surface area and thus minimizes material usage.
Because pressure containment in the vertical direction is achieved by a counterweight whose mass need not increase with increasing height, the optimum dimensions are for the structure to be somewhat taller than wide. I will address the reasons for this in a more rigorous way in a math-heavy appendix.
I want to bring up two more things before I close.
The first is that the design as described does not address how corners would deal with pressure containment. It would not work for the outer jackets to meet at a point. There are a number of ways to deal with this. The best way depends on the specific function of the structure and I see no particular reason to go into detail at this time except to mention that one thing you can do is take advantage of the gap as a place to build in your windows or airlocks.
The second is leak prevention. This design is optimized for pressure retention, but pressure retention and leak prevention are not the same thing. This design features more joining points than others, and thus is somewhat more leak-prone. It will be desirable to cut down on leaks by introducing a sealant layer; this also ensures that pressure is applied to the outer walls in the correct places and the correct ways.
I will finish out this post by scoring my design against the six criteria I introduced in the previous post.
1. The structure sustains a pressurized atmosphere
In this post, I have repeatedly referred back to the First Rule of Space Construction. Pressure retention is a fundamental aspect of this design, and it is accomplished elegantly.
2. The structure provides protection from radiation
This structure will inherently create a large reduction in the amount of radiation experienced by its inhabitants and can be upgraded to provide nearly complete protection, to whatever level of radiation exists naturally in Martian materials.
3. The structure is failure resistant
The most important safeguard in this design is that tensile stress is local and not global. This enables redundancy: In a “regular” pressure vessel, multi-axial stresses are distributed across the entire structure, which means that the structure is only as strong as its weakest point. By contrast, this structure depends almost entirely on uni-axial stresses shared across independent members, and cuts out material pressure containment entirely in the vertical direction in favor of gravitational containment.
4. The structure is failure tolerant
The most important aspect of failure tolerance in this structure is the redundancy of the cables which hold in the pressure. Failure in any one cable will not cause structural failure, but rather a reallocation of stress to other cables. But it goes deeper than that. While the larger number of joints increases the likelihood of leaks, a sudden increase in the leak rate helps to serve as an indication that components are becoming deformed and need to be repaired. All of the main structural components are internal and can be inspected visually. Even in the case of a total depressurization the structure will not collapse: It is both pressure-stable and vacuum-stable.
5. The structure can be constructed in an affordable way
One of the most important strengths of this design is that it’s made primarily from simple, mass-produced components assembled in simple ways as well as bulk materials such as regolith, sand, or concrete. I believe that, compared to other habitat designs, this design will have both a lower cost of components and lower cost of construction. I will discuss specific construction methods in a later post in this series.
6. The structure is useful as such
The most important difference between this design and other designs is that internal space has uncurved floors and ceilings and square or rectangular footprints, which substantially increases the usefulness of its internal volume. It has all the otherwise desirable aspects of a building, including adaptability, habitable internal volume, ability to regulate internal temperatures, and size can be modified depending on need.
The next post in this series will look at the materials to be used in structures of this kind and what kind of dimensions and measurements this will result in.