7
$\begingroup$

Given the interest in using lunar water for propellant manufacture, how do you calculate the production rate for electrolytic breakdown and liquefaction of water into cryogens? A corollary question is how do you calculate power needed?

$\endgroup$
2

1 Answer 1

6
$\begingroup$

I believe electrolyzing would take at least 286,000 joules per mole of water. See this Wikipedia article. A mole of water would give ~16 grams of oxygen and ~2 grams of hydrogen.

So to make a million grams of propellent we'd need at least (1,000,000/18)*286,000 joules. That's about ~15,888,888,889 joules for a tonne of propellent.

According to this NASA article, the ISS solar array wings have a specific power of 32 watts per kilogram. The Dawn space craft's solar array has a specific power of 80 watts per kilogram. They believe in the near future we'll have arrays with a specific power of 175 watts per kilogram. Being an optimist, I'll use 175 watts/kilogram as the specific power of our power source.

To make arithmetic easier let's consider a 1000 kg power source. At 175 watts per kilogram, that would be a 175,000 watt power source. It cranks out 175,000 joules per second. Recall we need about 16 billion joules to make a tonne of propellent. 15,888,888,889 joules/175,000 watts = 90,793 seconds which is about 1 day.

So it'd take a 1 tonne power source at least 1 day to make a tonne of propellent (please check my arithmetic, I sometimes make mistakes).

If there are minable lunar volatiles, they would likely be in the permanently shadowed regions on the floors of the polar craters. These regions are already very cold -- around 30 to 40 Kelvin. I believe that'd be cold enough to store oxygen with little boil off. But hydrogen needs to be cooled to about 20 K.

Once at a Earth Moon Lagrange 1 or 2 (EML1 or 2), the moon and earth are a good distance from a depot. The sun is the major heat source we need to worry about. A conical Multi Layer Insulation (MLI) shield can block the sun and leave a depot open to space which is about 3 Kelvin. Hydrogen boil off can be used for station keeping. This ULA paper argues that we can have cryogenic propellent depots with acceptable boil off rates.

In summary, we'd need a massive power source to produce lunar propellent at a good rate. In my opinion this is one of the biggest obstacles to lunar ISRU propellent. But I don't think it's a show stopper.

$\endgroup$
3
  • $\begingroup$ This was very helpful. I am wondering if the power source in your example is a tonne, what do you think would be the mass of the electrolysis and liquefaction hardware for this same level of production? $\endgroup$ Commented Jul 11, 2014 at 17:21
  • $\begingroup$ Hard to say. And besides the hardware you mention, fractional distillation would be needed: In addition to water ice, the lunar volatiles will also likely contain carbon dioxide ice, ammonia ice, formaldehyde, and other volatiles. You would need shovels and hauling trucks capable of operating in PSRs where no solar energy is available. Quite a complex undertaking! I've invested some time and effort looking at it but I'm still hesitant to venture estimates mass requirements on a lunar propellent mine. $\endgroup$
    – HopDavid
    Commented Jul 11, 2014 at 17:50
  • $\begingroup$ I will give a link to Spudis and Lavoie's estimate for what it'd take to start a propellent lunar mine: spudislunarresources.com/Papers/Affordable_Lunar_Base.pdf A caveat: Spudis is a lunar zealot so it's possible his estimates are optimistic. $\endgroup$
    – HopDavid
    Commented Jul 11, 2014 at 18:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.