The pressure hull is the most basic building block for any Lunar Homestead (and any other type of settlement). No habitable space means no Homestead, which means all the rest of the gear doesn’t really matter for what we’re doing. The first project here at Lunar Homestead is to figure out how to make safe and comfortable pressure hulls for habitats out of locally available Lunar materials.
Construction of Lunar structures presents many challenges. We haven’t built anything on the Lunar surface yet, so we don’t know what we don’t know. The only way we’re going to resolve many of these questions is to just start building Homesteads. Our designs are going to have to be adaptable and robust so we can modify them based on new information and local conditions.
Lunar habitat pressure hull parameters
- Size and shape
The shape (or shapes) and size of our basic pressure hull is directly determined by the other parameters. Ideally, our pressure hull will be built from flat plates. Flat plates are easier to make, especially larger plates. A cylinder or dome shape will need curved sections, which are much harder to make. Square and rectangular pressure vessels are less efficient and require thicker plates to contain the pressure. The extra mass is a problem for spacecraft but not for surface habitats. The internal pressure of the habitat will be the primary driver for habitat shape. Habitat shapes and sizes also need to include the ability for the Homestead to expand.
- Atmospheric pressure
The primary mission of the pressure hull is to safely contain the desired pressure (go figure!). So we need to decide early on what atmospheric pressure we want. An atmosphere at sea-level would be the most comfortable but it will come at a cost. We’ll need more nitrogen for the atmosphere and more iron for the pressure hull. A curved hull may be mandatory. Lower pressures would reduce the engineering requirements but would be less comfortable for the Homesteaders, at least initially. Humans are very adaptable so it may be less of an issue if we don’t reduce the pressure too much.
- Ease of construction
Making a Homestead on Luna is going to be more difficult than doing it on Earth. We’ll need to make the process as simple and easy as possible. The shape and size of the habitat plays a large part in this. How we design, create, and assemble the components is also important. Joining techniques, such as welding, need to be determined. Remember, the hull needs to be pressure tight at all times. Easy connection of structural components is important. Modular and standardized components will help. We’ll also need to minimize the outside exposure of our Homesteaders and maximize the use of simple robotics. Defining and designing the types of construction equipment needed will be critical to the success of any Lunar Homestead.
A pressure hull protected by a shield wall will have its exterior exposed to hard vacuum. We need to be sure the hull behaves properly under this condition. The good news is that we don’t have to worry about wind, tornadoes, or other weather related shenanigans.
- Thermal conductivity
A properly shielded habitat (either with a shield wall and regolith or regolith alone) won’t be exposed to the extreme temperature shifts (-150 °C to 100 °C between night and day at the equator) that occur on the Lunar surface. But we’ll need to factor in how much heat the habitat will lose directly through the hull, both with and without a shield wall. This will determine how much we will have to actively heat and cool the habitat. We also want low thermal expansion properties for our habitats. Finally, we’ll need to factor in external temperature changes during the construction phase when the hull is exposed. We could have trouble if one side of the hull is hot and the other side is cold.
- Excavation and foundation
The Lunar regolith presents many challenges. Excavating it will be difficult due to the regolith’s locking nature, the dust generated, and the low gravity impacting the efficiency of construction equipment. But we’re going to have to figure it out for two reasons. First, excavating is how we’re going to get the resources we need to do everything else. Second, placing the Homestead below the Lunar surface will let us build taller habitats or use less effort covering the Homestead. We’ll also need to figure out how to secure the Homestead to the Lunar surface using footing pads, pilings, and foundations. We’ll need to learn how to prepare the regolith before placing foundations as well.
- Moonquake resistance
For a “dead” moon, Luna has quite a lot of activity. There are at least 4 types of moonquake that we know of: 1) deep moonquakes (>700 km below the surface) probably caused by tidal forces, 2) shallow quakes (20-30 km below the surface), 3) thermal quakes caused by the expansion of the crust when sunlight heats it up after 2 weeks of deep-freeze, and 4) vibrations caused by impacting meteoroids. Some of the shallow moonquakes clock in at 5.5 on the Richter scale and have lasted more than 10 minutes. That’s a significant amount of energy we need to design for. The last thing we need is for a habit pressure hull to fail during, or after, a moonquake.
How will the hull interact with the vibrations and other effects caused by machinery and human activity? We’re going to need to figure it out. Especially since a lot of machinery will need to be inside the pressure hull to operate efficiently.
- Radiation protection
A properly shielded Homestead will have meters of regolith over all the habitable sections. This regolith, in addition to a shield wall, will provide the majority of the mass needed to protect our Homesteaders from solar and cosmic radiation. But the habitat pressure hull will contribute so we need to quantify how much. It might also be useful to design habitats that incorporate water storage at the top for that little bit of extra radiation protection.
- Meteoroid impact protection
Just like for radiation, the regolith and shield wall will provide most of the protection. But we need to know what the pressure hull can handle if those are breached (or if a shield wall is not included). Spalling is also a concern even if the hull itself isn’t ruptured.
Homestead habitats protected by a shield wall won’t have to be as durable as ones without. Regolith is extremely abrasive and direct contact can degrade the pressure hull. Humans are also pretty rough on equipment so we’ll need our habitats to be up to the task.
- Structural load bearing
While Luna only has 1/6th the gravity of Earth, we still need to figure out all the load bearing calculations. Especially if regolith is placed directly on the hull or if we place a significant amount of water shielding at the top.
- Reactivity to atmosphere
Our pressure hull has to contain a gas mix consisting of oxygen, water vapor (humidity), carbon dioxide, and all the other things humans and plants need and produce. These are great conditions for rust and other reactions. We’re also going to need to be sure our hull isn’t releasing (off-gassing) toxins into our Homestead living space. Fire safety is also a major concern for any habitat.
- Electrostatic and dust storms
There’s a neat phenomenon that occurs at the line between night and day. This line is called the terminator and no… not that kind of terminator. The terminator generates a significant electrostatic charge as it moves around the Lunar surface. Not only does this charge threaten equipment, it also raises a cloud of dust from the surface. It’s not a huge cloud but Lunar dust gets into everything and can quickly damage equipment (and humans). We’re going to have to factor this in.
- Easy maintenance
Everything made by humans requires maintenance if we want it to keep working properly. And we want out habitats to work properly ALL of the time. We’ll need to be able to access all parts of the hull and easily make repairs. Preferably without having to depressurize the hull. Easy inspection techniques are also critical.
People are going to be living in these habitats. Hopefully for a very long time. Homesteaders must enjoy their living spaces or they are going to have a hard time staying. Touch, smell, sound, and visual appeal (maybe taste?) are all critical to the success of Lunar Homesteading. Humans can put up with a lot if they need to but this is supposed to be a grand adventure, not a life sentence in an iron can.
A quick note about submarine hulls
Submarine hulls are often used as examples for analog spacecraft or space habitat hulls. While the all accomplish the same basic purpose (maintaining a habitable space in a hostile environment), they really are not interchangeable.
- Submarine hulls are subject to extreme external compressive forces (tens of atmospheres of pressure). To prevent catastrophic failure, submarine pressure hulls must be precisely manufactured from thick steel or titanium. Lunar homestead pressure hulls must contain, at most, 1 atmosphere of internal pressure.
- Submarine hulls are subject to a wide array of forces as they move through the water. Lunar habitats will mainly have to deal with vibrations from moonquakes.
- Submarine hulls are immersed in a very corrosive environment, sea water. If properly protected (under a shield wall covered by regolith) a Homestead pressure hull will only have to contend with vacuum on the outside and the warm, wet atmosphere inside.
Spacecraft pressure hulls are much more similar to aircraft than submarines. The pressure differences are much less, the pressure is internal, and mass is a significant issue. Fortunately, mass isn’t an issue for Lunar habitats.