Let’s put it all together and find out how much iron we’ll need to build one habitat pressure hull and how much regolith we’ll need to move to extract that iron. Remember, everything at this point is a VERY rough guestimate.
The pressure hull dimensions can be found on the Pressure Hull Shape, Thickness, and Size page.
- 4 large panels (Ceiling, floor, and 2 side walls)
- 6 meter x 10 meter x 4 centimeter = 600 cm x 1000 cm x 4 cm
- = 2,400,000 cm3 = 2.4 m3 of iron for each panel
- = 9.6 m3 for all 4 panels
- 4 corner plates
- 1.41 meter x 10 meter x 4 cm = 141 cm x 1000 cm x 4 cm
- = 564,000 cm3 = 0.564 m3 of iron for each panel
- = 2.256 m3 for all 4 panels
- 2 end caps
- 62 m2 x 4 cm = 620000 cm2 x 4 cm
- =2,480,000 cm3 = 2.48 m3 for each end cap
- 4.96 m3 for both end caps
- Total amount of iron needed to build 1 pressure hull
- 9.6 m3 + 2.256 m3 + 4.96 m3 = 16.816 m3 of iron
- Iron’s density is 7.87 g/cm3 (1). One cubic meter of iron weighs 7870 kilograms (which is 7.870 metric tons). Let’s round up and say we need 17 m3 of iron. We’re going to need 133,790 kilograms (133.79 metric tons) of iron to build one pressure hull. Good thing we aren’t launching this from Earth!
- I know I left out the floor in the middle and all the iron fittings. I’m just focused on the pressure hull itself right now. Interior floors and walls will probably be thinner than the hull and I have no way of knowing how much iron they will need at this point. I want to keep the pure guessing to a minimum.
Great. How much regolith do we need to process to get 17 cubic meters of iron to build our habitat? The Iron Project Outline page has all the relevant resource information.
- Using strictly free metallic iron. Found everywhere at a maximum of 1% by volume.
- Let’s say it’s 0.4% on average.
- 17 m3 ÷ 0.004 = 4,250 m3 of regolith processed.
- An American football field is 109.1 m long and 48.5 m wide = 5291.35 m2
- So if we process less than a football field’s worth of regolith to a depth of 1 meter we should have enough iron to build one habitat pressure hull.
- The average bulk density for the top 60 centimeters of regolith is 1.66 g/cm3 (2, pg 492). Lunar regolith bulk density varies widely depending on location, composition, and depth; making this estimate more of a vague guess at best. But let’s use it to calculate how much regolith we would need to move.
- There are 1,000,000 cubic centimeters in a cubic meter. So the average density of a cubic meter of Lunar regolith is 1,660 kg/m3.
- 1,660 kg/m3 x 4,250 m3 = 7,055,000 kilograms of Lunar regolith. That’s 7,055 metric tons of regolith.
- Of course, not all of this iron is pure. Some will contain varying amounts of nickel and other elements. We’ll need to figure out how that will affect the hull.
- Ilmenite. Most samples had <2% ilmenite but the Apollo 11 and 17 mare basalt samples are up to 15%-20% ilmenite by volume.
- Let’s err on the side of caution and call it 8% for the “average” mare basalt sample.
- Ilmenite is 36.81% Fe by weight (3). The other 63.19% is titanium and oxygen. We need 133,790 kilograms of iron. So we need 363,461 kilograms of ilmenite.
- Ilmenite has a density of 4.72 g/cm3 (4720 kg/m3)(3). We’ll need 77 m3 of ilmenite to get the iron required.
- Since we’re using an 8% concentration (by volume) of ilmenite in the regolith, we’ll need to move 962.5 m3 of regolith with a mass of 1,597,750 kg (1,597.75 metric tons).
- Olivine. Mare samples showed a range of Olivine abundance (0% to >60%).
- That’s such a huge range. I’m going to use 10% for the “average” mare basalt sample. If we’re lucky it will be higher.
- Olivine is 14.57% Fe by weight (4).
- We need 133,790 kilograms of iron. So we need 918,257 kilograms of olivine.
- Olivine has an average density of 3.32 g/cm3 (3320 kg/m3)(4). We’ll need 277 m3 of olivine to get the iron required.
- Since we’re using a 10% concentration (by volume) of olivine in the regolith, we’ll need to move 2770 m3 of regolith with a mass of 4,598,200 kg (4,598.2 metric tons).
- Spinel. Up to 10% (by volume) of some mare basalt samples.
- I’m going to use 8% for the “average” mare basalt sample. If we’re lucky it will be higher. I’m also just going to focus on ulvöspinel (Fe2TiO4) and not on the other flavors.
- Ulvöspinel is 49.96% Fe by weight (5).
- We need 133,790 kilograms of iron. So we need 267,794 kilograms of ulvöspinel.
- Ulvöspinel has a density of 4.66 g/cm3 (4660 kg/m3)(5). We’ll need 58 m3 of ulvöspinel to get the iron required.
- Since we’re using an 8% concentration (by volume) of ulvöspinel in the regolith, we’ll need to move 725 m3 of regolith with a mass of 1,203,500 kg (1,203.5 metric tons).
- Pyroxene. Mare basalts are 40%-65% (5%-30% for a few samples) pyroxene.
- I’m not going to figure out how much pyroxene we need. Accumulating iron won’t be an issue by the time we have facilities to process pyroxene. Pyroxene will be the last iron oxide we tackle because it’s the most difficult to extract iron from.
- This all assumes that we take the raw Lunar regolith and separate out the free metallic iron and the iron oxide ores before refining. This is called beneficiation and I’ll be working on that in Phases 4 and 5.
These numbers will substantially decrease if we decide to go with a carbon steel alloy instead of pure iron. The naturally occurring nickel-iron alloys we’ll find will also change things. Steel is substantially stronger so the hull can be thinner. Of course our Homesteaders will probably need to import the carbon and would have to build or import the necessary equipment to make the alloy. But those are future Ben’s problems.
Just for giggles, let’s compare these numbers to how much regolith we’ll need to process to extract enough nitrogen for several types of atmospheres. Check out Atmospheric Pressure and Composition for more information.
Regolith needed to extract enough nitrogen for:
- One standard atmosphere
- 11,235.268 metric tons of regolith to fill 620 m3
- 74% oxygen/26% nitrogen at 34.40 kPa (Skylab mix)
- 1,270.008 metric tons of regolith to fill 620 m3
- 78% nitrogen/21% oxygen at 70.11kPa (One Standard Lunar Homestead Atmosphere)
- 7,764.26 metric tons of regolith to 620 m3
Those numbers don’t look too bad now! We might be able to pick up most (or at least a substantial fraction) of the nitrogen we need while also extracting iron.
- Free metallic iron = 7,055 metric tons
- One Standard Lunar Homestead Atmosphere (SLHA) = 7,764.26 metric tons
- That’s pretty similar. I swear I didn’t plan it!
- Break out the shovel! We’ve got a LOT of regolith to move!
- Actually, it’s not as bad as it seems. Let’s say we have 336 hours of uninterrupted sunlight (14 days) every 28 days to power our equipment. And let’s say we want to move 8,000 metric tons of regolith into our processor. We’d have to move 23.8 metric tons every hour. A cubic meter of regolith masses 1.66 metric tons (roughly). So we’re looking at a little over 14 cubic meters per hour (14.3 to be exactish). That doesn’t seem too far-fetched with the right equipment and set up.
- We could double our regolith gathering capacity if we had a nuclear power plant or sufficient energy storage. We might not have the energy to process the regolith but we could at least move it into position for the next sunlight cycle.
- Even 7 cubic meters per hour would be fine. Enough iron for a new pressure hull every two months would be pretty special!
- Besides, moving the regolith is only part of the challenge. We still need to be able to process 8,000 metric tons of regolith to get the iron out. And then we have to shape that iron into usable forms. And then we have to take those iron forms and build a pressure hull out of them. We’ve got a lot of work to do.
- MatWeb (www.matweb.com/search/DataSheet.aspx?MatGUID=654ca9c358264b5392d43315d8535b7d&ckck=1)
- Lunar Sourcebook (www.lpi.usra.edu/publications/books/lunar_sourcebook/)
- WebMineral (webmineral.com/data/Ilmenite.shtml)
- WebMineral (webmineral.com/data/Olivine.shtml)
- WebMineral (webmineral.com/data/Ulvospinel.shtml)