Domes are over-rated

Part of my series on common misconceptions in space journalism.

It is an unwritten rule of space journalism that any article about Moon or Mars bases needs to have a conceptual drawing of habitation domes. Little scintillating blisters of breathable air clustered between pointy antennas.

Look, I get it. Domes are cool. I’ve built several. And while I don’t regard myself as an expert on Mars urban planning, I believe domes are not a very good solution for building cities on Mars.

I’m going to motivate this post by describing constraints on “the mission”. There is a time and place for discussion of short term exploration missions with a few plucky astronauts, but as far as I’m concerned, the SpaceX Mars vision is the biggest, baddest vision for exploration and industrialization, and the obvious design reference for this blog.

The goal is to build a self-sufficient city on Mars. This will require enormous quantities of money, time, cargo, and people. The goal is to understand ways to reduce the requirements while increasing the probability of success. Given that human capacity to solve problems isn’t infinite, a good mission architecture allows the technical teams to focus on the core self-sufficiency problem rather than working around counterproductive constraints. As an example, one of the main weaknesses of the Lunar Gateway is that so much engineering effort is required to build the space station, which diverts attention and resources away from the more direct challenges of building landers and surface base hardware. Similarly for Mars – the last thing anyone needs is an architecture that requires undue design work and places hard limits on future growth.

For generations, engineers have internalized a hard mass constraint on Mars missions, resulting in scores of proposals that struggle to keep a handful of humans alive for a few years in a spaceship that weighs just dozens of tons. But while it may be *just* possible to do an Apollo-style human Mars landing with a few SLS-loads of stuff, building a city requires either non-existent self-replicating robots or much, much more cargo. Hence the significance of the SpaceX Starship, which plausibly can launch not tens, but millions of tonnes of cargo to Mars.

The Starship provides a mechanism to retire the Mars city mass constraint, just as Starlink can retire the capital constraint. What are some other important constraints?

Consider the raw material constraint. A self-sufficient city can be either open or closed, but total recycling of all materials with either huge reserve stocks or perfect efficiency, such as would be required for a slow interstellar spaceship, is much much more difficult than an open system. In an open system, an effectively infinite supply of natural raw materials can be used, and wasted, as needed. The Mars city solves this problem by building on the surface of a planet that is made of all the raw materials it could need.

The next constraint of concern is the labor constraint. Self sufficiency means different things to different people, but a sufficiently healthy and well-trained individual human can survive indefinitely in many places on Earth. Indeed, small groups of cooperating humans have been capable of survival since the dawn of technology, and even crossed oceans before the wide-scale adoption of metals.

That said, the surface of Mars is next level in terms of its sheer hostility to life. It’s a pitiless frozen vacuum. The Earth’s south pole in the middle of winter is closer to a beach in Hawaii than the nicest place on Mars on the nicest day of the year. So when we think of self-sufficiency on Mars, the traditional “pioneer with 20 acres and a mule” just won’t cut it. Like the bottom of the ocean or the stratosphere, human survival is possible only with advanced technology. Self sufficiency on Mars means the ability to build all the requisite advanced technology, and to do it faster than it wears out.

Instead of a small group of generalists, the Mars city will need teams of specialists to replicate every part of the modern industrial stack, from raw material extraction and processing through to advanced lithography, and everything in between. This process is enormously labor intensive, requiring on the order of 100 million humans on Earth. On Mars, SpaceX hopes to get by with “only” a million people and a lot of manufacturing automation. Even then, the high marginal cost of keeping an extra human alive changes almost everything we take for granted about how jobs are done here on Earth. For more in this vein, check out the relevant chapter in my book in Mars industrialization.

For the purposes of this blog, however, we need merely agree that humans building a self-sustaining city on Mars will be extremely busy. And so we come to the space constraint.

No matter what people are doing on Mars, the mission designers will do everything they can to make their jobs as easy as possible, to maximize productivity. The next most important constraint on productivity is space. On the domestic level, Marie Kondo can help us make the best of our inability to avoid accumulating worthless possessions. On an industrial scale, it turns out that manufacturing difficulty is exacerbated by not having enough space. Factories need room to move things, store things, and lay out assembly lines.

When cars were first built, factories were multilevel buildings in cities with elevators and total vertical integration. Today, most manufacturing plants are laid out over a single level in areas with lower land value, and cover millions of square feet.

So it must be on Mars. We cannot industrialize a new planet in pressurized trailer homes or tuna cans or subterranean tunnels. Nor can we operate efficient factories in space suits. The city will need unimaginably enormous climate controlled spaces to enable millions of people to work efficiently in a shirt sleeves environment.

What about domes?

A prefabricated dome assembled on Mars would certainly have more interior volume than a lander or short tunnel. On the other hand, domes have significant drawbacks that are underappreciated by their advocates, many of whom haven’t actually ever tried to build one!

Domes feature compound curvature, which complicates manufacturing. If assembled from triangular panels, junctions contain multiple intersecting acute angled parts, which makes sealing a nightmare. In fact, even residential dome houses are notoriously difficult to insulate and seal! A rectangular room has 6 faces and 12 edges, which can be framed, sealed, and painted in a day or two. A dome room has a new wall every few feet, all with weird triangular faces and angles, and enormously increased labor overhead.

It turns out that the main advantage of domes – no internal supports – becomes a major liability on Mars. While rigid geodesic domes on Earth are compressive structures, on Mars, a pressurized dome actually supports its own weight and then some. As a result, the structure is under tension and the dome is attempting to tear itself out of the ground. Since lifting force scales with area, while anchoring force scales with circumference, domes on Mars can’t be much wider than about 150 feet, and even then would require extensive foundation engineering.

Once a dome is built and the interior occupied, it can’t be extended. Allocation of space within the dome is zero sum, and much of the volume is occupied by weird wedge-shaped segments that are hard to use. Instead, more domes will be required, but since they don’t tessellate tunnels of some kind would be needed to connect to other structures. Each tunnel has to mate with curved walls, a rigid structure that must accept variable mechanical tolerances, be broad enough to enable large vehicles to pass, yet narrow enough to enable a bulkhead to be sealed in the event of an inevitable seal failure. Since it’s a rigid structure, it has to be structurally capable of enduring pressure cycling across areas with variable radii of curvature without fatigue, creep, or deflection mismatch.

Does this sound like an engineering nightmare? High tolerances, excessive weight, finicky foundations which are a single point of failure, major excavation, poor scaling, limited interior space, limited local production capability. At the end of the day, enormous effort will be expended to build a handful of rather limited structures with fundamental mechanical vulnerabilities, prohibitively high scaling costs, and no path to bigger future versions.

Is there a better alternative? I think so.

What we need is a method for pressurizing vast areas of the Martian surface with relatively little hassle, labor, and raw material. For a long time, I thought the key might be gigantic masonry vaults, but I’m increasingly convinced that tensile structures are inherently better due to much lower mass requirements. The same goes for digging tunnels, which is so labor intensive that almost no-one lives underground. In contrast, a thin, flexible tensile membrane supported by its own pressure seems to be a step in the right direction. But how would this work?

UV resistant polymers as as ETFE are routinely used for exterior cladding of structures that need transparent, curved, lightweight, waterproof barriers. Some of these structures are even pressure stabilized. A multilayer ETFE fabric incorporating Kevlar fibers is an ideal material for both high performance yacht sails and transparent pressurized structures on Mars. This material can be thermally or chemically welded in the field for ease of integration, modification, and repair.

To transmit pressure load into the ground, a pressurized ETFE membrane must be periodically anchored to the ground by steel cables that can be arbitrarily long, supporting ceilings high enough to permit cloud formation! Unlike a dome foundation, cable anchors can be pile driven from the surface with generic hardware and without trench excavation. Both ETFE and steel can be readily produced from local resources. Importantly, the cables are located in discrete areas and offload all the pressure, so that couplings to the surface at the perimeter don’t have to resist enormous pullout forces and can be done with a set of relatively simple buried steel membranes.

Conceptual sketch of section of tented area.

Within the pressurized structure, membrane tunnels, shelters, and bulkheads can be readily deployed to ensure any desired level of redundancy or compartmentalization. Tall structures can be supported from the ceiling in tension, reducing material requirements.

Despite its light weight and versatility, inflatable tensile structures have a long history and is not as exotic as it may sound. It is used for both Zodiac dinghies and inflatable mattresses! It has even been used to make a fully functional inflatable plane!

Such an approach has many advantages over rigid dome construction, but the most important one is that it crushes the space constraint. What is the per capita pressurized area requirement on Mars? In a future post I’ll estimate this more rigorously but I believe it’s on the order of 10,000 sqft. If a Mars base is doubling its population every launch window, then the 5000->10000 person increment requires the addition of about a thousand acres of enclosed area, in just two years. This is about 3000 standard prefabricated domes (roughly one per person!), or less than two square miles of additional membrane, with distributed concurrent anchor construction. Only one of these methods scales easily!

Within the pressurized volume, people can build houses, schools, factories, farms, forests, or anything else they want. The space is versatile and flexible. For more in this vein, see my earlier post on Mars urban planning.

What are the potential drawbacks of such an approach? Thanks to great comments on Hacker News and Twitter, I have a good idea of what people are worried about.

Anchoring. Some have suggested anchoring to a second membrane on the ground, to avoid anchors and to isolate from Mars dirt (that may have toxic chemicals) and to avoid perfusion of gas through the ground. While enclosing membranes are great for bridges, corridors, and air handling, I think building directly on the dirt is the best approach. Provided the perimeter walls are deep enough, the risk of substantial leaks is low. All pressure structures leak, so the key is to ensure that there’s a ready supply of new gasses to make up the difference. As for perchlorates, they decompose in water at room temperatures. In other words, spraying a newly pressurized area with warm water should be adequate to neutralize the extremely thin layer of dust. Finally, farming on directly covered land won’t require moving millions of tonnes of dirt through an airlock.

Radiation. As explained in this blog post, I believe that unshielded radiation exposure on Mars is not one of the major problems to deal with. While it’s possible to build some kind of laminated inflatable structure with pockets of transparent water for shielding, in practice living and sleeping spaces will have modest shielding, and exposure in the “outdoors” will be part of life, just as excessive sun exposure on Earth can cause increased risk of cancer.

Mars’ atmosphere’s effective thickness as shielding is about 10 cm, compared to 10 m on Earth. But 100 m of 1 atm atmosphere under the pressure surface, condensed, would add another 10 cm of shielding! If it is decided that 50 cm of shielding is the minimal requirement, that can be met with 400 m ceiling heights. The only marginal increase in pressure structure cost is longer steel anchor cables.

Underground living. Quite a number of readers pointed out that natural caverns, canyons, and Boring Company-dug tunnels are options. While I agree these have their uses, they also have significant drawbacks. Underground spaces are incredibly labor and energy intensive to build – to the point that basically no private underground structures of any size are routinely constructed. Even natural caverns are location specific, difficult to expand, difficult to ensure structural stability, and lack natural light. I think TBMs will be used extensively to operate pressurized underground mines for minerals not abundant on the surface. But in general the easiest level to build is at ground level.

How does redundancy work? What happens if there’s a hole? Fiber-reinforced ETFE has really good ripstop properties, so a hole won’t suddenly spread over the whole area, leading to instant catastrophic failure. Still, the air will leak out stopped only by choked flow. If the leak rate is lower than the maximum replacement rate, no problems. The hole is found and patched. If the leak rate exceeds the maximum replacement rate, then the pressure will start to drop – which also reduces the leak rate. If the pressure drops below the level needed to tension the anchoring cables, then the whole ceiling will gradually deflate like a bouncy house at the end of a party. As the pressure drops, occupants will need to evacuate to “air shelters”, which would take the form of sealable corridors within the main structure. The membrane will gradually drape itself over whatever structures/former trees/non-spikey supports remain.

Additionally, the pressurized volume must be segmented by vertical bulkheads into separate compartments, so even total collapse (or toxic chemical leak) in one section doesn’t affect other sections. A collapsed membrane could still be repaired by robots or workers in space suits, and once the hole is patched, repressurized.

For particularly important areas, multiple membranes or layered membranes can be deployed to reduce the odds of a leak all the way through.

How much steel is required? Pressure at sea level on Earth is around 100 kPa, or 100 kN/m^2. On Mars, we can make do with lower pressure and enriched oxygen, reducing structural pressure loads, but the gravity is also lower so the total amount of rock needed to anchor is about the same. A 40 kPa atmosphere on Mars needs about 3-4 m of rock to completely react out the pressure load, though to be on the safe side the anchors would be driven somewhat deeper than this. According to The Engineering Toolbox, a 20 mm steel cable is adequate to lift 40 kN, implying an average steel fill fraction of 0.05% to anchor the roof, regardless of cable configuration. Span between anchors is determined by ground conditions and overall desired membrane tension, but I think 50 m is about the sweet spot. With a mass of 1.5 kg/m, the membrane could be flown as high as 6 km before the mass of the cable (to say nothing of its cost) was enough to hold down the membrane without an anchor.

Conclusion

So when one thinks of a city on Mars, don’t think of some quaint potato patch in a dome that’s too small for a game of tennis. Think instead of a gently puckered transparent plastic sky that stretches over the horizon in all directions, supported by a sparse forest of steel cables with endless open space between. A new savanna.