Structural Challenges for Space Architecture

Engineering Habitats for the Moon and Mars

Several decades after the first space age, there is renewed interest in space exploration and specifically in future human habitation far beyond the Earth’s surface. NASA recently received funding with an ambitious target: to send a manned mission to Mars by the 2030s and allow for future human habitats and even cities. This is a challenging, multi-disciplinary problem that requires expertise from a wide variety of fields: aerospace engineering, environmental engineering, social science, urban planning, design, architecture – and especially structural engineering. Unlike structural engineering for the built environment on Earth, there are virtually zero rules of thumb or design precedents to draw on for construction on Mars or the Moon. There is exciting potential to shape this discussion with fundamental structural engineering principles and forward-looking material and fabrication strategies.

Much like their Earth-based counterparts, the requirements of future space habitat structures are defined by their ability to protect their occupants and provide usable space to live and work. On Mars, the environmental loads are more extreme and the settlements more confined and isolated. Due to the high cost of transporting resources from Earth, up to $2 million for a single brick, recent efforts have focused on using in-situ materials for long-term sustainability. Throughout human history, settlers have adapted their construction methods to the locally available resources: snow huts, adobe walls, thatch roofs, and bamboo structures are just a few examples. Martian soil may be next on this list.

Loading and Structural Considerations

Loading on the Earth, the Moon, and Mars.

Loading on the Earth, the Moon, and Mars.

Structural systems for space habitats must be designed for four main loading types: internal pressure, reduced gravity (one-sixth on the Moon and one-third on Mars as compared to Earth), thermo-elastic loads, and micrometeoroid impact.

Because of the lack of atmosphere on the Moon and Mars, a pressure differential of up to 2090 psf across the habitat enclosure is required to sustain Earth-level pressures inside. This results in outward pressures on the structure that are several orders of magnitude greater than conventional structural loads due to gravity and environmental loading on Earth. Therefore, the structure will be mainly subjected to tensile stresses instead of the compression induced in Earth-bound structures under gravity loading. In comparison, a tension structure on Earth, such as an air-inflated sports dome, typically withstands a net pressure of 1 psf (Herzog, 1976) and the pressure differential on an airplane may be between 1100 and 1400 psf. Furthermore, since the loss of pressure is catastrophic to human life, the structure must be designed with redundancy and safety measures against decompression disasters caused by accidental and natural impacts.

According to NASA research, it is possible to safely reduce the internal pressure to values that are lower than those typical on Earth. Minimum pressures of 1150 psf and 1100 psf are recommended for the Moon and Mars, respectively, for normal operations. However, these lower pressures require increasing the percentage of oxygen in the air from 21% to 32%. This higher oxygen concentration corresponds to the maximum non-metallic materials flammability certification level currently used in operational human space flight programs. These recommendations must be studied further before the development of requirements for surface habitats.

Thermo-elastic loading is related to the presence of the Sun that produces a thermal gradient of about 630 °F on the Moon (in 29 days) and 148 °F on Mars (in 24.6 hours). These gradients occur between the sunlit and the shadow-exposed parts of the structure, as well as between the internal and external face of the envelope. Regolith is loose, fractured soil or rock and is commonly available on the moon, Mars, and Earth. An external thick regolith layer on structures could be used to provide thermal mass to dampen temperature swings. This layer can also serve as shielding from solar and cosmic radiation fields. The thickness of the layer would be variable, with thicker construction in the directions with greater sun exposure.

Micrometeoroids are small space projectiles (up to 0.1 inches in diameter) that do not survive entry through Earth’s atmosphere but do reach the surface of the Moon and Mars at velocities up to 45 mps. Habitat structures must protect their interiors from penetration by these cosmic bullets. While it is impossible to predict this phenomenon deterministically, researchers have proposed that a habitat should resist projectile penetration with a probability of 99% over a mission time of 10 years. A commonly proposed strategy also involves utilizing the external regolith layer with a thickness of 3 to 6 feet to achieve this.

Design Concepts and Fabrication Strategies

Since 1986, several types of structures have been proposed as concept settlements for both the Moon and Mars. As internal pressurization is the controlling load on the structural system, several inflatable architectural concepts have been explored over time. The first one from Archigram in 1966, with the Living Pod project, was a free-roving exploratory house inspired by the Lunar Modules that NASA was preparing for a moon landing. A few decades later, the architect Dante Bini developed design proposals in collaboration with Harrison Schmitt, the twelfth astronaut to set foot on the Moon in 1971 during the Apollo 17 mission. These projects are interesting because they are self-shaping, pressurized units. One of the proposals, Lunit, was essentially a kind of mechanical worm three meters (10 feet) in diameter. It would be transported and installed in a compact position and then its length would be extended telescopically using compressed liquid air stored in cylinders inside the unit.

NASA 3D printed Habitat Challenge. Project: Ouroboros. Team Digital Structures Research Group. Faculty Advisor Caitlin Mueller.

NASA 3D printed Habitat Challenge. Project: Ouroboros. Team Digital Structures Research Group. Faculty Advisor Caitlin Mueller.

To date, other inflatable solutions for Moon habitats have been explored by prominent architectural firms such as Foster & Partners and Andreas Vogler. The project conceptualized by Foster & Partners has an internal inflatable membrane covered by a shelter made of regolith which could be constructed using robotic fabrication processes (such as 3D printers). The state-of-the-art about rapid prototyping of building blocks is seen in research by the engineer Enrico Dini (Monolite Ltd.) who designed a 3D-printer, called D-shape technology. The technology has allowed for the construction of several prototype projects on Earth including housing, sculpture, military structures, and furniture. For the moon outpost (Foster & Partners in collaboration with Alta-Space), the 3D printer built a section through a regolith simulant.

The NASA Innovative Advanced Concepts Fellow, Neil Leach, is involved in a research project that aims to develop a robotic fabrication technology capable of printing structures on the Moon and Mars using lunar dust. The mechanical properties of the lunar regolith simulant (available at Orbitec of Madison, WI, USA, called JSC-1A) appear promising from a structural point of view, as the compression resistance is about 2900 psi and the Elastic modulus is equal to 341,000 psi. Density data for the regolith can be estimated from samples collected in space missions: the density of Moon regolith from the Apollo 15 mission data ranges from 84 pcf for the top foot, to 115 pcf at a depth of 2 feet. The powdered regolith with naturally occurring metallic oxides is mixed with chemical admixtures that react to form a type of concrete. One of the analyzed regolith mix design techniques is known as Contour Crafting, which is a digitally controlled construction process developed by Behrokh Khoshnevis that fabricates components directly from computer models. The material used is a form of rapid-hardening cement that gains sufficient strength to be self-supporting almost immediately after extrusion. At the moment, other 3D printing technologies are also being developed by private companies such as Made In Space and Redworks.

For the Mars regolith, indirect evaluations suggest densities from 75 to 100 pcf. In these respects, regolith is not very different from typical concrete aggregate used on Earth. Recent studies at Northwestern’s McCormick School of Engineering highlighted that a concrete mix design that includes Martian regolith exhibits characteristics similar to terrestrial concrete, as well as easy handling, fast curing, high strength, recyclability, and adaptability in a dry and cold environment. The mix uses molten sulfur which is abundant on Mars. The regolith can be properly proportioned to allow the optimization of both the coefficient of thermal expansion and the mechanical strength.

Andrea Vogler’s design, Moon Capital, is composed of domes, positioned over inflatable modules, which form a unique intelligent skin using a 3-meter (10-foot) thick layer of small-regolith sandbags. The weight of the regolith sandbags will provide protection from radiation and impact; however, it will not counterbalance the internal pressure of the entire structure. An innovative aspect of this project relies on the use of small swarm robots that will fill and mount the regolith sandbags on the smart skin. The potential application of swarm robotic systems is becoming very attractive because of their miniaturization and reduced costs, especially for areas with difficult or dangerous access. On Earth, drones have been used by University researchers at the Swiss Federal Institute of Technology (ETH), Zurich, to build a prototype rope bridge between two sets of scaffolding. While drones come to mind when picturing swarm robots, drones could only fly in indoor pressurized environments. Swarm robots operating on Mars or the Moon, exterior to the habitat, would be surface robots with collecting and hauling capabilities, similar to a robotic ant colony.

Another interesting conceptual design that explores a temporary inflatable module on the Moon has been developed by MIT’s Department of Aeronautics and Astronautics and Brown University’s Department of Geological Sciences. The inflatable habitat will be folded and packaged into a manageable volume to fit on the Apollo Lunar Rover. To deploy the habitat, the astronauts will remove the habitat from its container and unfold it on a flat surface. The ribs will then be inflated, establishing the habitat structure. This ribbing consists of a frame of small-diameter inflatable tubes that, when inflated to high pressure, provide a rigid structure for the habitat. The structure can be used for protection on overnight missions while the astronauts remain in their space suits.

Habitat Organization and Current Projects

Previous and current space habitat design examples mirror the evolution of a spacecraft interior design that mostly follows activity functions. Typically, the organization of the interior layout follows the functional needs of the crew, such as working, hygiene, personal spaces, and preparing and eating food. Typical architectural tools for the interior organization of terrestrial buildings, such as bubble diagrams and adjacency matrices, could also be used to explore the relationships among the sizes, adjacencies, and approximate shapes of the spaces needed for various activities in space habitats.

Application of sphere packing as a form-finding strategy for inflatable Moon exploration habitats.

Application of sphere packing as a form-finding strategy for inflatable Moon exploration habitats.

A project underway by MIT’s Digital Structures research group is investigating this potential, developing a new sphere packing form-finding approach for conceptual space habitat design. The method aims to optimize the location of different functional systems and subsystems inside a space habitat. Spherical bubbles representing different functional programs can combine in 3D space by prioritizing the preferred connections and maintaining the requested volume. The sphere packing achieved through a dynamic relaxation algorithm allows for the combination of both bubble diagrams and adjacency matrices, allocating all activities and respecting all required linkages between functions and subsystems. The obtained functional diagram is also considered as a pressurized architectural space, made of spherical components, and evaluated, in terms of its structural performance, through finite element analysis tools. The individual spheres may be pressurized, and the encapsulating envelope can be pressurized, creating a layer of redundancy if one membrane is breached.

Redwood Forest City – Mars City DesignTM Competition 2017. Team: Valentina Sumini, Alpha Arsano, George Lordos, Meghan Maupin, Zoe N. Lallas, Sam Wald, Matthew Moraguez, John Stillman, Mark Tam, Alejandro Trujillo and Luis Fernando Herrera Arias. Faculty advisor: Caitlin Mueller.

Redwood Forest City – Mars City DesignTM Competition 2017. Team: Valentina Sumini, Alpha Arsano, George Lordos, Meghan Maupin, Zoe N. Lallas, Sam Wald, Matthew Moraguez, John Stillman, Mark Tam, Alejandro Trujillo and Luis Fernando Herrera Arias. Faculty advisor: Caitlin Mueller.

Designers and researchers are also working on proposals for human habitation at the urban scale, including the recent Mars City DesignTM competition that aims to develop concepts for future Martian cities. One winning proposal from MIT, called Redwood Forest, is located in an unusual circular depression where a network of bright, green, and water-rich pressurized habitats are proposed to nurture 10,000 people. The city will exist both above and below ground, mimicking the structure of trees. Within the root network, residents will have their private spaces protected from harsh radiation, meteoroid impact, and thermal environment. The root network will house most of the machines that process, store, and distribute resources vital to everyday life. The public spaces will exist above ground, in enclosed structures which filter daylight down to the root network. The main transportation network will be an underground thoroughfare modeled after rhizomes present in various plant species. The radiation protection will be enhanced by the inclusion of a layer in the shield consisting of a water reservoir. The regolith that was dug out for the initial root system will be used as a catalyst to start water production and extract other mineral resources for construction.

Going Forward

Designing a structure on an extraterrestrial surface includes numerous challenges, including the internal pressure, the dead loads/live loads under reduced gravity, the consideration of new failure modes such as those due to high-velocity micrometeoroid impacts, and the relationships between severe Lunar/Martian temperature cycles and structural and material fatigue. Also of concern is the structural sensitivity to temperature differentials between different sections of the same component, the very extreme thermal variations and possibility of embrittlement of metals, the out-gassing for exposed steels and other effects of high vacuum on steel, alloys, and advanced materials. In addition, the factors of safety and the reliability (and risk) must be major components for lunar structures, as they are for significant Earth structures.

When considering a permanent settlement on another planet, one of the crucial aspects involves an evaluation of the total life cycle of the structure. That is, taking a system from conception through retirement and disposition or the recycling of the system and its components. Many factors affecting system life cannot be predicted due to the nature of the Lunar/Martian environment and inability to realistically assess the system before it is built and utilized. Therefore, even if the challenges in space exploration are very peculiar, the colonization of satellites and planets could teach us to be wiser in our consumption of natural resources, pushing us to pursue efficiency and sustainability here on Earth. The multidisciplinary methodology connected to space exploration research will be a wise starting point for optimizing the terrestrial consumption of natural resources for designing more sustainable architectures and improving ground logistics research.▪


For more on Mars regolith densities, see K. Seiferlin et al., Simulating Martian regolith in the laboratory, Planetary and Space Science, vol. 56, 2009-2025 (2008).

Comments posted to STRUCTURE website do not constitute endorsement by NCSEA, CASE, SEI, C3 Ink, or the Editorial Board.

15 Comments

  • Reply
    A
    December 28, 2017

    From the seventh paragraph: “Micrometeoroids are small space projectiles (up to 0.1 inches in diameter) that do not survive entry through Earth’s atmosphere but do reach the surface of the Moon and Mars at velocities up to 45 mph. ” Did you mean to write 45 miles per second?

    • Reply
      Valentina
      January 8, 2018

      Yes, thanks for the typo correction!

  • Reply
    Bob
    December 31, 2017

    Building our Lunar Home
    Creating a habitat on the Moon for a human colony has become easier and cheaper as our knowledge and technology have improved. The question is, which of the dozen or so potential building methods will be best? This month we consider some of the leading contenders.

    Proposals for structures for colonists to live and work inside have included empty rocket fuel tanks, inflatable habitats, glass domes, concrete shelters, tunnels and lava tubes. Which of these is chosen will depend on practicality, safety and – above all else – cost. Reusable launch vehicles are bringing down the expense of lofting hardware and supplies into low Earth orbit, but the cost of firing them from there to the Moon, slowing them into Moon orbit, and then bringing them down safely to the lunar surface is a major consideration.

    Current estimates put this cost at around $1.5–2 million – per kilo! So landing a metric tonne of ready-made habitat will set you back the wrong side of one and a half billion US dollars. And that’s without any people or supplies. The Apollo Lunar Landers weighed between 15 and 16.5 tonnes, which may help explain why the Apollo programme was cut short when its Cold War flag-planting mission was accomplished. So why not send up an inflatable habitat? That would be much lighter, right?

    Wrong. The Bigelow B330 expandable module weighs in at 20 tonnes, and there are three of them in the configuration (pictured) that Bigelow Aerospace suggest would make a suitable lunar habitation module. Not exactly the light’n’cheap option we are looking for.

    So, how about empty rocket tanks? They are an essential part of every launch and are then usually thrown away to burn up in Earth’s atmosphere. Surely it makes sense to utilise these ‘waste’ items that are already near low Earth orbit and repurpose them for lunar living accommodation? Well, maybe… maybe not. There is a plan to turn used fuel tanks into space habitats, but only as additional modules on the International Space Station. Considering all the talk is of de-commissioning the ISS in the next few years, this seems like a great idea that came too late. But could they be used for a habitat on the surface of the Moon instead?

    The Centaur upper stage is light enough at around 2.5 tonnes complete with its engine that could be used to fly it to the Moon, slow it to orbit and finally land it on the surface. Problem is, this tank is a stainless steel balloon made so thin to keep down weight, it has to be kept pressurized with nitrogen gas when not full of fuel, otherwise it would collapse under its own weight on Earth. How well it would cope with the rigorous conditions on the Moon’s surface, or the weight of 2+ metres depth of lunar regolith piled on top of it as protection, has maybe not been fully investigated. It would also need extensive modification to provide an airlock and infrastructure – work that could only be done once landed – so the jury’s still out on how practical this might be for a lunar habitat.

    Any structure on the surface of the Moon will need to protect its inhabitants from the fierce heat of the sun by day and the deep chill of the lunar night. In addition cosmic radiation, solar flares and micro-meteoroid impacts are all life-threatening hazards for our colonists. So a thick layer (at least two metres deep) of regolith shielding will be needed and any structure will need to support the weight of this. Water can also be used to protect against some harmful particles but this precious commodity is unlikely to be used where a puncture in the building’s skin could lead to its loss.

    So what about creating a habitat underground? Lava tubes have been identified on the lunar surface and some theorise that these could be used to provide a protected environment. It is likely that most will be too vast to fill with air and seal against its loss, but a lava tube might accommodate one or more smaller thin-walled structures, protecting them from most of the surface hazards. But can a suitable lava tube can be found in the desired location? Most are likely to be in the lunar ‘seas’ or maria, which are sites of ancient lava flows. A colony will be built, initially at least, at one of the poles where there is a plentiful supply of water and continual sunlight.

    Alternatively, tunnels that are purpose-dug into the walls of craters and sides of mountains could prove more suitable. The snag is these require hefty rock boring machines that are typically 40-80 tonnes – too heavy to ship from Earth – and very costly to run in terms of power. Once a human colony is in place, such machinery could be built using local metals. Then tunnelling could be a very useful way to expand living space.

    Because weight is the major limiting factor, it may be far better to land a lightweight robotic rig that can utilise local resources to construct an initial living space before any people get there. This is the plan that several space agencies are working on to get their lunar habitats ready in advance of colonists. The European Space Agency has even commissioned British architects, Foster & Partners, to devise just such a programme which you can watch now, by clicking on the short video attached.

    The bottom line: ISRU or In Situ Resource Utilisation is now regarded as the most practical way to construct and maintain a lunar colony. All the materials needed to build a habitat can be derived from the lunar regolith. Sending robotic construction machinery to create the initial structure could be the most cost-effective way to get a permanent human settlement started.
    Previous blogs in this Moon colony series (gravity, atmosphere, water, power, farming and food) can be found here… http://www.timbuktu-publishing.co.uk/blog

  • Reply
    December 31, 2017

    Ice domes, constructed over entrances to caves or lava-tubes may offer simplicity and use of in-situ materials as well. Have explored many such lava-tubes here in Hawaii, and have imagined incorporating them into off-world habitats somehow (just seal the ends?). I believe ice, caves, and lava-tubes have been discovered on our moon and Mars as well. Ice could possibly be melted, sculpted and reshaped as needed, offering protection with some light penetration. Nudging asteroids into useful orbits for inter-world transport may also reduce the need to construct massive ships (ride the rock!). Aloha!
    Really wish I could go!

    • Reply
      Valentina
      January 8, 2018

      Thanks for your comment Russell. Water/ice is perfect also for shielding against cosmic radiation.

  • Reply
    George
    December 31, 2017

    Number one, the first thing you have to do is create a space station. A platform you can gather supplies personnel, and provide back up. Ask Napoleon, Hitler, or George Patton. And from all the designs and what we have built they are a joke. Building a quality station would teach us a lot about a habitat on a planet. Try the moon first. Yes, I have a better design. Who do I give it to. Private industry is doing so much more than the government, but the government wants to dictate our direction. So many wrongs.

  • Reply
    December 31, 2017

    We don’t need as much stuff to take with us than you think. Take tools and some and some habitat like taking you tent and build the cabin once there. First is water then food and that will be on our way. We will then have to be called Martians. Do we have to trash the place? All the junk we take or can we take back ? or can we use like plastic ? Along the way we could start on the moon and get stuff to take to mars and clean up that stuff too we could use the moon” stuff” there for the last 50 years but then we left clean. Or does man have to trash just to say we were there? Money Im all for it then if we can get with others then not shoot each other but use to go on. It will take less than you think. Imagine Structure made from Mars, and the no mess. Get the Koreans on board both side and China and while were at it lets go with solar and electric and hydrogen that needs a little help a car that will never need to go to a hydrogen place to fill up it will take it self that is possible way possible. Then we will have to work to gather oh my oops we need wars instead of good things possible for all.

  • Reply
    William
    December 31, 2017

    Am I understanding this right?
    “Because of the lack of atmosphere on the Moon and Mars”
    You’re saying that Mars, has no atmosphere?

    • Reply
      Valentina
      January 8, 2018

      The atmospheric pressure on the Martian surface is on average only about one-hundredth to one-thousandth that on Earth at sea level. The Mars Atmosphere and Volatile Evolution mission (MAVEN) launched on November 18, 2013 aimed at exploring Mars upper atmosphere, ionosphere, and interactions with the Sun and solar wind.

  • Reply
    December 31, 2017

    Why is it that in every article I read on this subject the obvious and easiest method is never discussed? We can build massive tunnels from England to France under the ocean and tunnels through mountains and we can build tunnels under and through heavily populated areas but we cant build on on the moon or on mars? Weather you use small robots working like ants over an extended amount of time to tunnel into a hill side or a small boring machine that takes several passes to get the correct diameter or a larger machine for a single pass depends on the money and time allowed. Reinforce the inside to be air tight and a pressurized access port or just insert an inflatable. The tunnel will take care of the protection from wind, radiation, Temperature changes, and protection from projectiles. All of the protections the tunnel provides will significantly decrease the complexity and cost of the habitat you build inside it. I don’t have a degree in anything and I can come up with ideas, why haven’t educated people with PHD’s and access to billion dollar budgets made it happen yet?

  • Reply
    December 31, 2017

    A fascinating prognostication for formulation of effective protective living structures and their necessary support system components in environments such as Mars and the Lunar surface, which admittedly impose rather unique and severe mandates on any and all planning for human habitation thereupon. Reinvestment of the Lunar surface and the future establishment of a Mars habitat are givens, in my opinion; it is only a matter of time (and money). However, as for the closing statement [“The multidisciplinary methodology connected to space exploration research will be a wise starting point for optimizing the terrestrial consumption of natural resources for designing more sustainable architectures and improving ground logistics research”], I fear that if our explorations are driven by the usual dictates of American-style capitalism, the possibility of retroflexively applying those learned principles to our Earth is rather remote, unless we have at some point so devastated the world we originated on that it may become absolutely unlivable without drastic/draconian readjustments. Given human nature to overlook, delay, deny and procrastinate, any such attempts may be precondemned to utter failure even before such a crisis occurred. That would not necessarily be a bad thing, however, since were Homo sapiens to become extinct, ‘nature’ would hopefully have another stab at evolving a more suitably wise ‘highest sentient life form’ to replace us!

  • Reply
    Ari
    December 31, 2017

    Dig a hole, use the materials to build. Good, agreed. But, melt a small amount of the material to form dense ribs, like “rebar” inside your “concrete”. Hmm, tiny metal strands mixed in with the “concrete” like steel wool in pudding, would be synergistic, the concrete would protect the tiny strands of melted material and the tiny strands would give structural strength to the concrete. And, the strands would not need to be precisely positioned. Each such building block could be easily replaced. Layers of small building blocks with layers of membrane between the building blocks. Only need to figure out how to melt: solar reflector furnace? Catalytic reactions? Are there geothermal hotspots available?

  • Reply
    Eric
    December 31, 2017

    Early on, this article says, “Micrometeoroids are small space projectiles (up to 0.1 inches in diameter) that do not survive entry through Earth’s atmosphere but do reach the surface of the Moon and Mars at velocities up to 45 mph.”

    45 mph? OK, that has to be a typo. Isn’t the average velocity of impact estimated at about 10 or 15 km/s?

    http://adsabs.harvard.edu/full/1997LPI….28.1481V

    • Reply
      Valentina
      January 8, 2018

      Thanks for correcting the typo!

  • Reply
    Joel
    January 1, 2018

    I think the easiest habitation strategy for both Mars and Moon would be finding caves or lava tubes. Building just one wall to seal it off and taking advantage of the natural protection of a cave is pretty straightforward. Like many areas of earth, deep caves can provide pressurization and insulation from the outside temperatures. Even if you had to dig for additional room, it seems to me a better way than creating an entire structure.

    If we couldn’t find a good existing cave on Mars or Moon, we could also create our own using a projectile. We could drive a large structure directly into the ground or a hillside first as a completely unmanned mission. Then, after analyzing the hole and designing the structure around it, send in the building robots and humans.

Leave a Comment

Download this article