Habitats and Shelters

Pressurized habitats are the foundation for the entire Homestead. No habitable space means no Homesteaders. There’s no point in doing this without Homesteaders.

The big thing I’m researching in this section is, of course, the SPORE concept. With SPORE, Homesteaders can create protected spaces underground. These spaces don’t have to be habitable (pressurized with a breathable atmosphere). They can be pressurized with other gas mixtures or left unpressurized. But the option to make each section habitable will be automatically built in.

One of the other ongoing research projects at Lunar Homestead is to determine the best material to use for building habitat pressure hulls. This is difficult because we’re restricted to using common Lunar materials. Lunar iron (not steel) and cast basalt are the two materials that I think are serious contenders. Both should be available in large quantities and are sufficiently strong. If iron doesn’t work out then we’ll look into using Lunar steel (I’m not a fan of using rare/expensive carbon for this). Sintered basalt powder is another option but I like it less because it’s structurally weaker than iron or basalt.

Let’s go over the various types of shelters and their possible uses.

Pressurized habitable space (PHS)

This is the most important type of shelter. This is where everyone is going to live. It’s also where we’re putting most of our equipment and infrastructure (see Rules). There are no Homesteads without pressurized habitable shelters.

PHSs are the hardest type of shelters to construct and maintain. They have to incorporate (or have access to) a full life support system (and back up system) to maintain their environment. The system has to include equipment to adjust the breathing gas pressure and composition, filter out toxins, and adjust temperature and humidity. We also have to include electrical, water, and waste distribution systems. A bunch of other capabilities have to be built in as well, depending on the function of the habitat (housing, ISRU, vehicle maintenance, etc.).

Many people insist on including windows in their surface PHS design. They claim that humans have to be able to see the outside with their own eyes or they’ll crack up. I think that’s bullshit. One of humanity’s superpowers is its adaptability. The initial group of Homesteaders might find it difficult to live underground but people that grow up in a Homestead will find it normal. Hopefully we’ll screen for people that can make the adjustment. Those that can’t will have a pretty rough time and probably won’t last. Since there’s no way back to Earth (at least initially) that means they’ll probably die. It’s harsh but that’s life on the frontier.

Why are windows bad? Mostly because of the increased radiation exposure but all the surface hazards apply. Windows are weak spots. And they are completely unnecessary.

One low tech solution would be to build large “periscopes” linking the top level to the surface. The dangerous radiation only moves in a straight line. These devices will bounce the light from the surface using two mirrors, thereby eliminating the radiation threat. Homesteaders can look at the surface while staying safely beneath many meters of regolith. The best part is they can be built using only Lunar glass and aluminum. At least I assume so. I’m not aware of anyone actually doing it yet. The second best part is that they won’t require any electricity to operate (although they may want to light up the landscape at night for a better view). The third best part is that they can provide natural light for the habitat during Dayspan.

Book 4 in this series, “Habitats and Shelter”, will go into much more detail on how we can build Lunar Homestead PHSs. This website also has more information.

On a side note, thermal management inside shelters is going to be difficult. Equipment, humans, and everything they do are going to generate substantial amounts of heat. This is going to be a challenge since sealed underground habitats are going to act like vacuum flasks and keep the heat in. We’re going to have to design thermal management systems to keep our shelters comfortable and use the “waste” heat for other processes. This is a pretty complex topic so I’ve decided to include it in the Waste section instead of here.

Pressurized non-habitable space (PNS)

There are lots of reasons why Homesteaders would create pressurized spaces that they can’t live in. SPORE is the first example. The SPORE work area is 100% oxygen pressurized to 48 kPa (6.96 PSI). Humans can survive in this atmosphere for a while but it wouldn’t be comfortable. And long-term exposure would be harmful. The LH website has pages with more details on this subject.

Another example would be specialized agriculture habitats. Certain plants might grow better in a high carbon dioxide/high pressure environment. The increased yield might make the additional effort worthwhile. Personally, I’m not sold that the increased risk of having a dangerous environment inside the Homestead is worth it. More research is needed.

Like the habitable sections, these spaces will need a full suite of equipment to maintain their specialized environment. We’ll also need to figure out ways to keep PNSs separate from PHSs. Obviously, we’ll need to have airlocks.

And speaking of airlocks. One of the tasks of LH is to figure out how to build and operate Homestead airlocks using only Lunar materials. As far as I know, this is a brand-new field of research. Current designs, such as the one used on the International Space Station, are pretty complicated and use a bunch of equipment that would not be easy to make on Luna. They also lose some of their breathing gas each time they open. Homesteads really can’t afford to let that happen.

Unpressurized enclosed space

This type of shelter will probably come in two flavors. One, pressure hulls created by SPORE can be left empty of life support equipment until they are needed. The Homestead might need the extracted resources but not the space. Or maybe they are building a tunnel to connect up with a nearby Homestead or resource. Or it’s a place to store stuff that isn’t affected by really low pressure or vacuum. Whatever the reason, they don’t want to maintain a pressurized environment so they let it reach equilibrium with its environment (pressure, temperature, etc.). The shelter is still protected from all the surface threats and could be pressurized when needed.

Two, certain specialized structures on the surface could be built to be sealed from the outside threats but still maintain a vacuum inside. I have no idea why such a thing would be built, but I’m not really focused on surface structures at the moment. The inside of the shelter would be protected from dust and solar heat. Radiation and impact protection would depend on how thick the exterior walls are and by the thickness of its radiation shielding, if it has any.

Unpressurized open space

This type of shelter is open to the Lunar atmosphere (which is really a vacuum right now). Which also means it’s exposed to Lunar regolith dust. I wouldn’t put anything complicated or valuable in this type of shelter. Not without a lot of dust mitigation tech.

At it’s most basic, this shelter is a simple lean-to made of thin film aluminum. Just something to block the sunlight. Something more substantial could be built that includes a layer of radiation protection on the roof. This protection could be regolith, regolith sandbags, water tanks, or anything else that has substantial mass. Radiation and thermal protection wouldn’t be 100% however unless the sides were also protected.

A lot of designs use this kind of shelter but I think it causes more problems than it solves. The dust is a real problem.

Shelter concepts

Let’s talk about some shelter concepts. I’m a proponent of creating underground habitats by tunneling (you may have noticed). However, there are several other types of shelters that have significant followings.

The most obvious habitats are the “rigid cylinders”. These are shelters built into the payload fairing of a rocket. Skylab would be a good example of a rigid cylinder habitat. Everything was built on Earth and launched as a finished unit. An O’Neill settlement is a different type of rigid cylinder as it is constructed on-site.

Rigid cylinder habitats have several things going for them.

  • They can be immediately used once they are properly situated.
  • They can be extensively tested on Earth before being launched.
  • All the equipment be built in and ready for immediate use.
  • They don’t require the development of Lunar ISRU tech.

There are also some significant disadvantages with rigid cylinder habitats.

  • Their size and volume are restricted. They are stuck with the dimensions of the payload fairing for the rocket they are launched on.
  • Because they are launched from Earth, mass is a critical issue. Everything will have to be as lightweight as possible. The habitat won’t have any radiation or impact shielding so it will have to be buried in regolith (with all the problems that entails).
  • The habitat will experience significant structural loads during launch and landing. It will have to be built to handle them.
  • It could be difficult to repair this type of habitat using local resources due to its high-tech construction.
  • This is the most expensive and least sustainable option because everything comes from Earth.

Inflatable shelters are probably the next most popular type of habitat. The hull of the shelter is a durable, air-tight laminate of many layers of fabric (usually involving Kevlar). The habitat is built on Earth and tightly packed into a shipping container. The container is delivered to the desired location and the shelter is unpacked. Breathing gas is then pumped into the habitat to inflate it to its final shape and size. It’s just like inflating an air mattress.

Inflatable shelters have all the advantages of rigid cylinders and more:

  • Much larger size and volume than rigid cylinders.
  • They can be immediately used once they are properly situated and inflated.
  • They can be extensively tested on Earth before being launched.
  • They don’t require the development of Lunar ISRU tech.
  • With the right engineering, you can make inflatable habitats into a wide variety of shapes. Anything other than a sphere will require supports and tethers to keep its shape.
  • Depending on the design, it may be possible to place the deflated habitat, pile regolith on top of it, and inflate it with the regolith shielding in place. There are still all kinds of problems with this but at least it makes things a little easier.
  • Inflatable structures in space are already in use. The Bigelow Expandable Activity Module was deployed on the International Space Station in 2016 and is still in use in 2020. Sure, it’s being used as a storage module but it’s functioning perfectly. Genesis I and II, both also built by Bigelow Aerospace, are also inflatable structures still in orbit (both are derelict now). Of course, there is a lot more development to do.

Inflatable shelters also have many of the disadvantages of rigid cylinders:

  • Everything comes from Earth. It really doesn’t help with self-reliance.
  • Mass is still critically important. There probably won’t be any built-in shielding. Although inflatable habitats can incorporate better shielding than metal hulls, Lunar habitats would probably rely on regolith to save mass.
  • There’s no way to repair inflatables using local resources.
  • The pressure hull is made of fabric, making it less durable than metal. The Lunar regolith is notorious for being wickedly sharp and abrasive. A lot of designs involve piling raw regolith on top of the habitat to provide shielding. Having regolith in direct contact with a fabric pressure hull makes me nervous.

3D printing of Lunar shelters also gets lots of attention. There have been contests requiring the 3D printing of habitats (on Earth, of course). It’s all very high-tech and futuristic. I’m not a fan.

  • 3D printing uses local resources as building materials. This is the primary draw. However, you can’t just scoop up raw regolith and start printing. At the very least you’re going to need to sort out the particle size you want. I think this advantage is mostly unproven at this time.
  • Lower mass requirement. All you need to bring from Earth are the robots to do the printing and the equipment to process the local resources. That’s actually quite a bit but it’s still probably less than shipping a fully equipped habitat. How we would go from raw regolith to a safe, durable pressure hull is also still largely unproven.
  • Printing can be automated. At least that’s the plan. Many designs require tele-operated robots to spend weeks (or months) printing the pressure hull of the habitat before people show up. Of course, these robots don’t actually exist yet and tele-operation has some problems. But hey, it looks cool.

Using 3D printing to make Lunar habitats also has some serious challenges we need to resolve.

  • It requires complex high-tech equipment from Earth. This is my biggest issue with 3D printing. The equipment has to be built and shipped from Earth. All the replacement parts have to come from Earth. Local production of 3D printing equipment will probably be impossible for most Homesteads.
  • It requires either a binding agent or a lot of energy to make the structure. The low energy option requires a binding agent to hold the particles together. This chemical will have to come from Earth because it requires high-tech manufacturing of chemicals rare on Luna. The non-binder option uses large amounts of energy to sinter (or melt) the material together. So now we have to design in significant solar thermal or electrical power sources.
  • Lack of materials testing. We can create regolith simulants and 3D print them on Earth. But that is completely different from doing it in the Lunar surface environment. We have no idea how strong or porous this material will be. The only way to find out is to actually do it on Luna.
  • Lower level of tech development. 3D printing is pretty new here on Earth. 3D printing on Luna is a completely new sub-field with very little usable data. There’s a lot we still need to learn.

OK, I’m probably being a little too hard on 3D habitat printing. I admit that I could be completely wrong and it could be the best technology for the job. It just seems like an unnecessarily high-tech solution. We’ll see how far LH can get with its low-tech research.

No discussion of Lunar shelters would be complete without mentioning lava tubes, craters, and rilles (long, narrow depressions).

Lava tubes are the Holy Grail for some people. Left behind by lava flowing underground, these caves could provide significant sheltered space. The thinking is that we can drop a settlement inside a lava tube and it will have instant protection from all the surface threats. And we wouldn’t have to do any digging.

This all sounds great except for a few problems. First, we haven’t actually seen a Lunar lava tube (because they exist underground). We have seen holes, called “skylights”, that could represent collapsed sections of lava tubes. Some rilles could also be collapsed lava tubes. We even have gravitometric observations that suggest the presence of lava tubes. But we won’t know until we actually send a robot into a lava tube. We’re still waiting on that particular mission.

Second, we have no idea how stable these things are. Sure, they may have existed for many millions of years. But that was before humans started living in them. Human presence and activity will change the atmosphere pressure and chemical composition. We’ll heat up the interior of the lava tubes. Our equipment could also cause significant local vibrations. All of this (and more) could degrade the walls of the lava tube, creating a hazardous situation. Collapsing a lava tube onto a settlement would be a Bad Thing.

Some people suggest we can seal off the ends of a tube and pressurize a section of it. Talk about changing the tube’s environment! Maybe this could be possible but we’ll need to answer a whole lot of questions first. And to do that we need to have people living on Luna. A Homestead near a confirmed (by robotic exploration) lava tube could do all kinds of useful research. And maybe get paid to do it.

Craters and rilles often come up because some designs involve building a dome, or other covering, over them to create a pressurized area. Of course, this dome is clear so we can use all that natural light to grow crops and play outdoors. A lot of the available artwork depicts domed over craters. It’s all very futuristic. And that’s where it will stay (in the future). There are so many problems (how do we keep the atmosphere from blowing the dome off the crater?) and unknowns (how fast will our atmosphere bleed through the surrounding regolith if we don’t seal it?) associated with this concept that it’s much closer to fantasy than science. It does look really cool though.

Lunaquakes

Also known as moonquakes. Or earthquakes on Luna. Whatever they are called, lunaquakes are a problem we need to understand and solve for.

Luna isn’t geologically active like the Earth, but it does experience quakes. Luna has shrunk more than 50 meters (150 feet) over the last few hundred million years as its interior cools [106]. This shrinkage causes the surface to wrinkle and break, forming thrust faults and releasing energy.

We’ve identified four different types of lunaquakes:

  • Deep quakes originating at least 700 km below the surface [107]. These are probably caused by tidal stress due to Earth’s gravity [107]. Generally mild and mostly harmless [107].
  • Impact vibrations caused by… meteorite impacts. Not much to worry about unless you happen to be in the impact zone.
  • Thermal expansion quakes. The Lunar crust gets really cold during Nightspan. It expands when sunlight starts to warm it up. These quakes are low energy and not anything to worry about.
  • Shallow quakes originating 20-30 km below the surface [107]. We’re not 100% sure what causes these quakes but the guess is that it’s slip faults and perhaps slumping of material off of large crater rims. These are the ones we need to prepare for.

The Apollo seismic network (each mission placed a Passive Seismic Experiment package at their location) registered 28 shallow quakes between 1972 and 1977 [107]. Several of these were up to 5.5 on the Richter scale [107]. These are considered “moderate” and powerful enough to damage poorly constructed buildings [108]. That’s not the real problem though.

The kicker is that shallow quakes can last more than 10 minutes [107]. On Earth, even the biggest quakes stop after less than 2 minutes [107]. More than 10 minutes of shaking, even if it’s not very powerful, can put a lot of stress on structures. It can cause fatigue that weakens the building materials and can create air leaks at seams and seals. It’s also not going to be very pleasant to experience.

Homesteads are going to have to be lunaquake resistant. The first step is figuring out where shallow lunaquakes are most prevalent. We’ve got some limited data on this but the Apollo seismic network consisted of only 4 instruments, all located in one part of Luna. We’re going to need a global network distributed over the entire surface to get the data we need. A good start would be to avoid building Homesteads near identified fault lines.

The second step is figuring out when shallow lunaquakes occur. The Apollo data shows that more shallow lunaquakes occur when Luna is at its furthest point from the Earth (apogee) [109]. It’s not a guarantee but Homesteaders can be a little more alert during that part of Luna’s orbit.

The third, and most important, step is designing the habitats and equipment to withstand magnitude 6.5 quakes (building in a buffer because there’s a lot we don’t know) lasting 20 minutes (adding another buffer). Not just once every couple of years (or decades), but on a monthly basis.

Lunaquake resistant engineering is beyond the scope of this book but it’s something we’re going to have to research and incorporate into every design (not just habitat designs). One bit of good news is that underground structures are typically less affected by quakes. I’m not a civil engineer or a geologist but from the few articles I’ve read it seems that surface structures are more affected because of the layers of soil they sit on. Structures tied into the bedrock (like skyscrapers) are better protected. Structures inside the bedrock are even more protected. It’s unclear how Homestead habitats embedded in the mega-regolith will fare but it’s a safe guess that they will be more protected than any surface structure as they will move with the material surrounding them.

Environment suits and clothing

I’ve also included environment suits and clothing under the shelter category. Both provide the user with protection from their environment so it seemed like the place for them. I haven’t done any research on them so I don’t have much to add here.

Environment suits will be an issue for Homesteaders. Lunar surface suits and vacuum suits (for use in the SPORE work area) are made from high-tech materials and equipment that Homesteads will be hard pressed to manufacture. I really don’t like the idea of Lunar settlers being forced to import something so critical to their survival. We’re just going to have to be cleverer and more creative.

One solution is to figure out how to build a vac suit using local resources. Once we have a functioning vac suit, we can add layers over it to protect against the Lunar surface environment. I don’t have a clue how we can pull this off but that’s the fun of research, right?

Another solution is to skip the suit altogether and design an EVA pod. This could like the Discovery pods from 2001: A Space Odyssey, only with tracks. Or it could look completely different. I don’t know. The point is that we encase the Homesteader in a metal shell instead of a fabric suit. Homesteads will have access to metal (at least iron) so this might be a more sustainable option.  Or maybe we can build an Iron Man suit using Lunar iron. How cool would that be?

As for clothing, that’s a whole other line of research. Again, we don’t want to rely on Earth to keep our Homesteaders from running around naked. Unless that’s what they want to do. Hey, I don’t judge. Clothing also includes footwear. I wouldn’t want to walk around barefoot on cold metal (or basalt) floors.

We’re going to have to research the whole enchilada.

  • What fabrics can we grow in Lunar Homesteads?
  • How are we going to grow them?
  • How are we going to process these resources into fabrics and thread?
  • How are we going to color these fabrics?
  • What articles of clothing need to be made?
  • How are we going to make clothing?
  • How are we going to wash and dry our clothing?
  • How can Homesteaders personalize their clothing?
  • How are we going to maintain and repair clothing?
  • What are possible uses for unusable clothing?
  • How are we going to recycle clothing? Or use it as feedstock for another process?

Obviously, we’ve got a lot of work ahead of us figuring all of this out.

Resources

106) https://www.nasa.gov/press-release/goddard/2019/moonquakes

107) https://science.nasa.gov/science-news/science-at-nasa/2006/15mar_moonquakes

108) https://en.wikipedia.org/wiki/Richter_magnitude_scale

109) https://www.sciencefriday.com/educational-resources/future-astronauts-moonquakes/

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