4 ms·

This is a fun dimensional analysis problem. I'm going to start with food energy. Leaving aside actual nutrition. 1000 people * 2000 kcal per day = 100000 watt

This is a fun dimensional analysis problem. I'm going to start with food energy. Leaving aside actual nutrition. 1000 people * 2000 kcal per day = 100000 watts carbohydrates (basic energy intake) 100000 watts / 60% = 160kw(raw vegetation) (40% food waste) 160kw / 5% = 3200kw(light) (conversion efficiency from light to food ~5%) 3200kw / 40% = 8000kw(e) (electrical energy to light for LEDs) 8000kw / 40% = 20MW(th) (thermal energy to electricity, carnot limit) So you need to have a 100% duty cycle 20MW(th) reactor just to supply the lighting for producing food energy, if you're trying to be self sufficient. You can do other, even more exciting problems for the other key resources. I'd guess that the water cycle and the entangled oxygen cycle would be interesting, as well. Also you can work out the positive energy coefficient of using the hydrogen split out of the water to power a sabatier reactor and produce methane. I can tell you already that the energy budget for this colony would be challenging in itself. Edit: added more specific units - part deux, corrected off-by-ten error (off-by-one-zero ?)

max76 8y ago
I don't think we should assume LEDs for lighting. High quality optical fiber can transfer sunlight with a degrading rate of 10%/km [0]. Light can be farmed on the surface from a larger surface area that it will be projected onto in the farm. Is the sunlight on mars incapable of growing plants? This process would remove the need of generating 2MW of electricity. [0] https://computer.howstuffworks.com/fiber-optic2.htm https://computer.howstuffworks.com/fiber-optic2.htm
- jaggederest 8y ago
  This came up last discussion. It's actually easier on mars to do completely artificial lighting than concentrate, since the concentrators need to be about 3-4 times larger in area than the farmland would be on Earth, and are thus extremely bulky. Unless you can produce optical quality mirrors in-situ, it's probably easier to ship LED arrays.
  - mLuby 8y ago
    Plus you don't have to clean the dust off your reflectors if you've just got a whole self-contained system underground.
  - lxmorj 8y ago
    With LEDs you can focus your output on photosynthesis-friendly wavelengths and collect in a wider spectrum, no?
- politician 8y ago
  Agreed; these guys [1] are developing concentrated solar for a lunar regolith melting application. And the high-quality fiber can be made in orbit [2]... [1] https://www.lpi.usra.edu/meetings/leagilewg2008/presentations/oct29pm/Nakamura4011.pdf https://www.lpi.usra.edu/meetings/leagilewg2008/presentation... [2] https://www.iss-casis.org/blog/taking-zblan-optical-fiber-production-to-space/ https://www.iss-casis.org/blog/taking-zblan-optical-fiber-pr...
  - gotocake 8y ago
    The Moon is a lot closer to the Sun than Mars, and you have to remember that EM radiation falls off with the square of the distance. Mars is unsuitable if you need to rely on solar power alone, especially for concentrating and piping light to grow crops.
- aunty_helen 8y ago
  Whilst using fiber optics to transport the light is a nifty idea, like you mentioned, you have to 'farm' the light on the surface. So that means bringing collectors, making them sturdy enough for the winds and then also going 'outside' to clean them. Might be easier to bring LEDs and something nuclear that also provides a lot of waste heat you can use. However, if you did user fiber you end up with a light source that's there about half of the time, but plants can grow better with more sunlight. I'm no expert on hydro/LED factory farming so I'll stop there but I'm sure anyone who has grown cannabis would be able to tell you of the advantages to a light source that's highly configurable.
philipkglass 8y ago
2000 kcal/day is about 100 watts. You should start with 100,000 watts of carbohydrates rather than 10,000. (I'm guessing that your eye skipped a zero and you carried the error forward; it's an easy mistake to make.)
- jaggederest 8y ago
  Absolutely correct, good catch! Points to one of the errors that dimensional analysis can't catch.
Tossrock 8y ago
The natural fit here is a naval nuclear reactor. For example, the A4W reactor on a Nimitz-class aircraft carrier puts out 550MW(th), and of course it has a continuous 100% duty cycle, and 20 years of operating time before requiring refueling. For slightly less overkill, an S6G reactor from a Los Angeles-class nuclear sub puts out 165MW(th), and is substantially smaller and lighter. Of course, this raises some thorny issues around potential launch failures spreading radioactive material over a large area. This is already a consideration even with the little RTGs that NASA launches for rovers.
- jaggederest 8y ago
  I'd expect that's about the size of the problem. Putting an aircraft carrier on mars, but with the carrier deck used to grow food, and the "propulsion" energy going to electrolyse and feed forward into the sabatier process. Give it 3-5 of the A4W reactors for "a year between transit windows" redundancy, and you're starting to get into the ballpark. Putting a pair of $13 billion dollar, 110,000 ton nuclear vessel on mars would "only" cost $110bln. A steal, really, when you think about it.
  - gotocake 8y ago
    It would cost a lot more, that’s just the cost of lift raw tonnage of the carriers themselves. You also have to lift the machinery to assemble and maintain them, spares of pretty much everything, and the cost of developing and lifting all of that. I think we’d all be looking at the “modest” billions inflate into enormous trillions before we knew it. Of course if it actually worked it would potentially be worth it.
- philipkglass 8y ago
  The natural fit here is a naval nuclear reactor. ... Of course, this raises some thorny issues around potential launch failures spreading radioactive material over a large area. This is already a consideration even with the little RTGs that NASA launches for rovers. Counter-intuitively, a nuclear reactor has lower radiotoxicity risks in case of an explosion during launch. Before a reactor reaches criticality for the first time, it doesn't contain any highly radioactive materials. It's only as radioactive as its uranium fuel. Uranium has a long half-life and corresponding low radioactivity. Plutonium 238, used in RTGs like that in the Mars Science Laboratory, has a half life of 87.7 years. Uranium 235 has a half life of 704 million years. Neglecting differing relative biological effectiveness of radiation for the moment, that means that the plutonium 238 in RTGs is about 8 million times more radioactive, kilogram-for-kilogram, than the uranium 235 in a reactor's fuel. The Mars Science Laboratory carried 4.2 kilograms of plutonium 238 (4.8 kg of PuO2): https://www.nasa.gov/pdf/532433main_FIN%20MSL%20Launch%20Nuclear%20Safety%20FS%202-9-11.pdf https://www.nasa.gov/pdf/532433main_FIN%20MSL%20Launch%20Nuc... It could have carried 33.7 million kilograms of U-235 for roughly the same radiological risk. Or, realistically, missions could launch with reactors designed for a few tens of kilowatts to megawatts and the maximum radiological risk would be much smaller than with RTGs. At least so long as the reactor attains first criticality after the risky launch phase.

max76 8y ago

I don't think we should assume LEDs for lighting. High quality optical fiber can transfer sunlight with a degrading rate of 10%/km [0]. Light can be farmed on the surface from a larger surface area that it will be projected onto in the farm. Is the sunlight on mars incapable of growing plants? This process would remove the need of generating 2MW of electricity. [0] https://computer.howstuffworks.com/fiber-optic2.htm https://computer.howstuffworks.com/fiber-optic2.htm

jaggederest 8y ago
This came up last discussion. It's actually easier on mars to do completely artificial lighting than concentrate, since the concentrators need to be about 3-4 times larger in area than the farmland would be on Earth, and are thus extremely bulky. Unless you can produce optical quality mirrors in-situ, it's probably easier to ship LED arrays.
- mLuby 8y ago
  Plus you don't have to clean the dust off your reflectors if you've just got a whole self-contained system underground.
- lxmorj 8y ago
  With LEDs you can focus your output on photosynthesis-friendly wavelengths and collect in a wider spectrum, no?
politician 8y ago
Agreed; these guys [1] are developing concentrated solar for a lunar regolith melting application. And the high-quality fiber can be made in orbit [2]... [1] https://www.lpi.usra.edu/meetings/leagilewg2008/presentations/oct29pm/Nakamura4011.pdf https://www.lpi.usra.edu/meetings/leagilewg2008/presentation... [2] https://www.iss-casis.org/blog/taking-zblan-optical-fiber-production-to-space/ https://www.iss-casis.org/blog/taking-zblan-optical-fiber-pr...
- gotocake 8y ago
  The Moon is a lot closer to the Sun than Mars, and you have to remember that EM radiation falls off with the square of the distance. Mars is unsuitable if you need to rely on solar power alone, especially for concentrating and piping light to grow crops.
aunty_helen 8y ago
Whilst using fiber optics to transport the light is a nifty idea, like you mentioned, you have to 'farm' the light on the surface. So that means bringing collectors, making them sturdy enough for the winds and then also going 'outside' to clean them. Might be easier to bring LEDs and something nuclear that also provides a lot of waste heat you can use. However, if you did user fiber you end up with a light source that's there about half of the time, but plants can grow better with more sunlight. I'm no expert on hydro/LED factory farming so I'll stop there but I'm sure anyone who has grown cannabis would be able to tell you of the advantages to a light source that's highly configurable.

philipkglass 8y ago

2000 kcal/day is about 100 watts. You should start with 100,000 watts of carbohydrates rather than 10,000. (I'm guessing that your eye skipped a zero and you carried the error forward; it's an easy mistake to make.)

jaggederest 8y ago
Absolutely correct, good catch! Points to one of the errors that dimensional analysis can't catch.

Tossrock 8y ago

The natural fit here is a naval nuclear reactor. For example, the A4W reactor on a Nimitz-class aircraft carrier puts out 550MW(th), and of course it has a continuous 100% duty cycle, and 20 years of operating time before requiring refueling. For slightly less overkill, an S6G reactor from a Los Angeles-class nuclear sub puts out 165MW(th), and is substantially smaller and lighter. Of course, this raises some thorny issues around potential launch failures spreading radioactive material over a large area. This is already a consideration even with the little RTGs that NASA launches for rovers.

jaggederest 8y ago
I'd expect that's about the size of the problem. Putting an aircraft carrier on mars, but with the carrier deck used to grow food, and the "propulsion" energy going to electrolyse and feed forward into the sabatier process. Give it 3-5 of the A4W reactors for "a year between transit windows" redundancy, and you're starting to get into the ballpark. Putting a pair of $13 billion dollar, 110,000 ton nuclear vessel on mars would "only" cost $110bln. A steal, really, when you think about it.
- gotocake 8y ago
  It would cost a lot more, that’s just the cost of lift raw tonnage of the carriers themselves. You also have to lift the machinery to assemble and maintain them, spares of pretty much everything, and the cost of developing and lifting all of that. I think we’d all be looking at the “modest” billions inflate into enormous trillions before we knew it. Of course if it actually worked it would potentially be worth it.
philipkglass 8y ago
The natural fit here is a naval nuclear reactor. ... Of course, this raises some thorny issues around potential launch failures spreading radioactive material over a large area. This is already a consideration even with the little RTGs that NASA launches for rovers. Counter-intuitively, a nuclear reactor has lower radiotoxicity risks in case of an explosion during launch. Before a reactor reaches criticality for the first time, it doesn't contain any highly radioactive materials. It's only as radioactive as its uranium fuel. Uranium has a long half-life and corresponding low radioactivity. Plutonium 238, used in RTGs like that in the Mars Science Laboratory, has a half life of 87.7 years. Uranium 235 has a half life of 704 million years. Neglecting differing relative biological effectiveness of radiation for the moment, that means that the plutonium 238 in RTGs is about 8 million times more radioactive, kilogram-for-kilogram, than the uranium 235 in a reactor's fuel. The Mars Science Laboratory carried 4.2 kilograms of plutonium 238 (4.8 kg of PuO2): https://www.nasa.gov/pdf/532433main_FIN%20MSL%20Launch%20Nuclear%20Safety%20FS%202-9-11.pdf https://www.nasa.gov/pdf/532433main_FIN%20MSL%20Launch%20Nuc... It could have carried 33.7 million kilograms of U-235 for roughly the same radiological risk. Or, realistically, missions could launch with reactors designed for a few tens of kilowatts to megawatts and the maximum radiological risk would be much smaller than with RTGs. At least so long as the reactor attains first criticality after the risky launch phase.