On this page I deal with the other parameters covered in the Habitat Pressure Hull Parameters page. Specifically, how well iron deals with vacuum, radiation, meteoroid impacts, thermal extremes, moonquakes, dust, vibration, reactivity, and structural loading. I also briefly touch on the weldability of iron.
The primary design for any Lunar Homestead is to have the habitat pressure hull separated from the regolith shielding by an unpressurized shelter. I’m calling this shelter a “shield wall”. The purpose of the shield wall is to protect the pressure hull from the regolith, which is extremely abrasive, and to allow easy access to the hull for routine maintenance and repair.
One of the results of using a shield wall is that the pressure hull is exposed to constant vacuum. Or near vacuum as the hull will probably contaminate the environment under the shield wall due to leakage. We need to make sure iron will be able to tolerate this environment.
The first worry is how fast iron will sublimate (when a solid changes directly into a vapor when heated) in a vacuum. According to Materials and Processes for Spacecraft and High Reliability Applications (2, pg 24) the sublimation rate of iron in a vacuum exposed to a specific temperature is:
- At 1050 °C iron will lose 10−1 cm/year (0.04 in/year)
- At 900 °C iron will lose 10−3 cm/year (0.0004 in/year)
- At 770 °C iron will lose 1000 Åa/year (3.937e-6 in/year)
The hull should never reach those temperatures once it is protected by the shield wall and the regolith covering. Those two components will block solar radiation and act as insulators. The temperature at the surface of the pressure hull should remain low and stable.
The second concern is the porosity of iron. I found a few vague references to the porosity of a metal being exacerbated by vacuum and causing it to leak gases. I couldn’t find anything specific to iron. There probably hasn’t been a whole lot of research into using iron in spacecraft construction. I’m going to put a pin in this as something to look into further.
Metal fatigue is also a concern. In this respect, vacuum may actually increase the fatigue life of iron. One paper states that increasing the vacuum causes an increase in fatigue life due to the exclusion of water vapor and oxygen (3, pg 3). The same paper states the “inorganics are not generally susceptible to the effects of space vacuum, at moderate temperatures” (3, pg 9). I’m not a materials engineer so we’ll have to do more research and consult with some experts when necessary.
Finally, contact (or cold) welding is an issue. On Earth, when an iron surface is exposed a thin layer of iron-oxide quickly forms. This oxide layer doesn’t form in a vacuum. The problem is that if two oxide-free iron surfaces are brought into contact they will instantly weld themselves together. The atoms of each surface combine to form a single metal. Cold welding can be a very powerful construction technique but it’s a huge pain if you want to avoid it. This isn’t an issue specific to iron (it will happen with any metal) but it’s something we need to be aware of.
Overall, it doesn’t seem like exposure to vacuum will be an issue for an iron pressure hull as long as it’s protected from temperature extremes.
Space is awash with radiation. Here on Earth our atmosphere and magnetic field shields us from most of it. Lunar Homesteaders will have neither of these protections. So their habitats will have to do the job.
Radiation is a complex and fascinating subject but I’m not going to go into detail here. It isn’t necessary because the primary radiation shield for Homesteads will be a thick layer of regolith, a shield wall, and perhaps a water container positioned over the habitat. Any radiation protection the habitat hull provides will be purely secondary.
The main types of radiation Homesteaders will need to deal with are:
- Thermal – Completely blocked by regolith and shield wall.
- Ultraviolet – Completely blocked by regolith and shield wall.
- Solar particle events
- Galactic cosmic rays
The first three types of radiation are emitted from our sun. Galactic cosmic rays are high-energy protons and heavy ions from sources outside our solar system. Right now I’m only going to worry about the last two.
Solar particle events (SPE) occur during a solar flare or a coronal mass ejection (a really big flare). An SPE mostly consists of highly accelerated protons but can include other types of nuclei. These particles move very fast and we usually only have less than 30 minutes of warning.
Galactic cosmic rays (GCR) consist of ionized particles from the entire periodic table moving at near the speed of light. The heavier elements, such as lead and iron, have a significant amount of energy which makes them very damaging and hard to stop. In fact, current spacecraft are not designed to stop CGRs. Our habitats will have to if we want to keep our Homesteaders safe.
One of the biggest problems is that an insufficiently thick shield could cause MORE radiation that in blocks. This is caused when an energetic radiation particle hits a shield particle, causing the shield particle to fragment. These secondary particles, still moving at high velocities, either hit other shield particles (causing more secondary particles) or enter the habitat.
Of the two, galactic cosmic rays are the hardest to stop. I found a paper that gave this simple formula for calculating how thick a shield needs to be to protect against CGRs (4, pg 2). I don’t know how accurate it is but it’s the only formula I’ve found so far.
(thickness)(density of material) ≈ 103 gm/cm2
Thickness ≈ 103 gm/cm2 ÷ 7.86 g/cm3
Thickness ≈ 127.23 cm (50.1 inches)
That’s a lot of iron! Our habitat hull is only 4 cm thick so it won’t provide any significant protection against galactic cosmic rays. Plugging in the average density of regolith 1.66 g/cm3 (5, pg 492) we get 602.41 cm. That’s a little over 6 meters (19.76 ft) of regolith needed to block out most galactic cosmic rays. Lunar Bases and Space Activities of the 21st Century states that 1.5-2.0m of regolith will block radiation to dosages encountered by terrestrial x-ray workers (6, pg 364). That seems kinda thin to me. I’m also pretty sure it doesn’t include a storm shelter. Some combination of regolith, shield wall, habitat hull, and water storage will be needed to protect our Homesteaders from solar and cosmic radiation.
The Lunarpedia page, Radiation Problem (lunarpedia.org/index.php?title=Radiation_Problem), goes into more detail.
Meteoroid impact protection
Again, the regolith blanket and shield wall will be the primary defense against high-energy impactors. Honestly, I started researching this section and my eyes glazed over. I’m not a materials engineer and I don’t play one on TV.
I’m not going to blow a gasket trying to figure out impact protection right now for four reasons:
- The odds of a Homestead taking a substantial hit are really low. One source has 68% of the meteoroid impact mass hitting Luna consisting of micrometeoroids massing 5.5×10-5 to 7.0×10-8 g (8, pg 269). That’s really small.
- The regolith blanket should absorb all the more common, smaller impacts.
- Over-engineering the habitat to withstand a substantial impact seems like a waste of time and resources given how rare one would be.
- Simply stopping the impactor isn’t enough. We’ll need to factor in spalling, hull deformation, and metal fatigue. That’s all beyond my capabilities right now.
Moving on! I’m going to make the assumption that 4 cm of solid iron will be adequate protection when paired with a shield wall and regolith blanket.
Check out Meteor Hazards (lunarpedia.org/index.php?title=Meteor_Hazards) at Lunarpedia for more information.
I’m also not going to worry about thermal conductivity right now. When I start working on the life support system I’ll need to know how much heat is leaking out of the habitat. A properly shielded habitat shouldn’t be letting outside heat in. Once the shield wall is in place, our habitat should be thermally stable. We may have some problems with metal expansion during the construction phase if the hull is exposed to direct sunlight. The solution may be to construct the shield wall first THEN build the hull inside it.
Just for shiggles, Extraterrestrial Materials Processing and Construction states that regolith is excellent thermal insulation (1×10-4 w/cm deg K) and 10cm of regolith will dampen out the diurnal heat pulse (7, chap X,M). Our 2 (or 6) meters should be more than enough.
I’ll need a structural engineer and/or an architect to figure this out exactly. It seems to me that quake resistance is more a function of design than the building material. Sure, the material is important. But you created the design BASED on the material, already knowing its limitations. Besides, steel is used in many earthquake resistant designs so Lunar iron should be an adequate material. Honestly, I just don’t know at this point but I’m not considering my lack of knowledge a deal breaker.
Lunar dust is a huge complication for building and living on Luna. It’s very fine and get into everything. It’s very abrasive and can easily damage equipment and people. It holds an electrostatic or electrochemical charge which causes it to stick to everything AND creates a moving cloud of dust following the terminator.
Lunar dust won’t be as much of an issue to the habitat hull once it’s properly protected. The shield wall will be directly in contact with the regolith blanket, not the habitat. And the habitat will be protected from the surface environment by the shield wall and regolith blanket. Lunar regolith is also a good electrical resistor so electrostatic build-up shouldn’t be a problem (7, chap X,M). The biggest challenge will be to keep dust from entering due to surface operations.
To learn more, check out this video on Cosmoquest (cosmoquest.org/x/365daysofastronomy/2016/01/20/jan-20th-responsive-environments-with-dream2/)
Figuring out how to deal with vibrations caused by humans and machinery probably falls under the expertise of an architect or structural engineer. Either way it’s beyond my capability. It probably doesn’t matter at this point anyway. A properly designed habitat made out of iron should be able to handle vibrations we generate.
Reactivity to atmosphere
We’re probably going to want to coat the interior surfaces of the pressure hull with some sort of protective film. Why?
- The iron hull WILL react to the oxygen and humidity inside. That means rust. Nobody wants to live inside a rusty can.
- Preventing rust will extend the life span of the hull.
- Personalizing the interior color of the habitat could improve moral and make the Homestead more livable.
Maybe we’ll use some form of titanium oxide paint. That would make everything white which would be pretty neat. Other elements could add different colors. I haven’t come across any research on this yet so it’s on my to-do list. Eventually.
Structural load bearing
I think it’s pretty clear I’ll eventually need to consult with some structural engineers and architects. The 1/6th Earth gravity will help quite a bit. Plus the fact that a habitat with a shield wall will have no external loads placed on it (the shield wall supports the regolith blanket). That changes if we start stacking hulls or placing the water supply at the top. Anyway, it’s not a show stopper.
I did a brief Internet scan on welding elemental iron. There wasn’t much there but that’s not surprising because elemental iron isn’t used for much on Earth anymore. Steel is way better and cheaper. The few things I did find made it sound like it shouldn’t be too difficult to weld iron plates together though. One said it was “suitable for all types of welding” (1). Sounds like a necessary experiment!
I know, I didn’t do the most in-depth research possible on these topics. I honestly don’t feel like it is necessary at this point. Nothing came up as a possible deal-breaker while I was writing this page. Sure, all this stuff will have to be hammered out at some point. And I will definitely need to talk with some specialists. But there is no point hanging things up here when I can move on to Phase 3 and start building stuff!
- Special Steel Sections Limited (www.specialsteelsections.com/en/pure-iron/)
- Materials and Processes for Spacecraft and High Reliability Applications (www.springer.com/cda/content/document/cda_downloaddocument/9783319233611-c2.pdf)
- Effect of Vacuum on Materials (ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690026573.pdf)
- Shielding Astronauts from Cosmic Rays (www.dartmouth.edu/~sshepherd/research/Shielding/docs/Parker.pdf)
- Lunar Sourcebook (www.lpi.usra.edu/publications/books/lunar_sourcebook/)
- Lunar Bases and Space Activities of the 21st Century (www.lpi.usra.edu/publications/books/lunar_bases)
- Extraterrestrial Materials Processing and Construction (ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19830003839.pdf)
- Lunar Stratigraphy and Sedimentology (www.lpi.usra.edu/publications/books/lunar_stratigraphy/)