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A Little Too Optimistic

Alan Binder is too sanguine about the prospects for extracting lunar oxygen in the absence of ice at the poles:

"Itís totally irrelevant what form the hydrogen is inÖwhether itís solar wind implanted hydrogen or whether it is water. We just have to know what equipment to take. Thatís because you harvest solar wind hydrogen one way and you harvest the water ice another way. Itís still good news," Binder explained.

"In both cases, itís the hydrogen that is the valuable thing," Binder concluded, "because thereís plenty of oxygen around. We know that you can crack the rocks and get the metal and oxygen out."

Yes, we know that in theory we can do that, but as someone once said, in theory, theory and practice are the same, but in practice, they're different. Melting ice is a much easier chemical process than cracking silicates in the absence of water, particularly when you have to recycle the reagents. There have been many postulated methods for doing this, including hydroflouric acid leaching, and magma electrolysis, but neither of them have been demonstrated in a lunar environment, or even a simulated one. We were trying to get support for this kind of research at Rockwell back during the SEI scare, but it came to nought, and as far as I know there is still little research going on in this area aimed at useful demos (I hope that someone can point me to some activities that indicates I'm wrong).

But even if it is occurring, it's a much more difficult process, and to say that whether or not we find ice is "irrelevant" at least for the near term, is handwaving. The ice discovery was exciting because it clearly made things much easier than previous plans for lunar materials utilization, and it remains that way today.

[Wednesday evening update]

Well, here's a little encouraging news.

Posted by Rand Simberg at December 15, 2004 05:54 AM
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You don't extract oxygen from silicates, you extract it from ilmenite (FeTiO3). The process involves heating with hydrogen, reducing the iron to metal and converting its share of oxygen to water.

If you have some hydrogenous organic junk at the poles, I suspect just mixing it with ilmenite and cooking would make the reaction go. You'd get all sorts of other crud in the volatiles, but that's probably true of water from the poles anyway.

Posted by Paul Dietz at December 15, 2004 06:13 AM

You don't extract oxygen from silicates, you extract it from ilmenite (FeTiO3).

Perhaps, if you're only after oxygen (and iron and titanium...). But if you want silicon, and aluminum, it makes sense to go after the silicates and get the oxygen as well. And ilmenite reduction, while easier, is still harder than melting ice.

Posted by Rand Simberg at December 15, 2004 06:19 AM

What's the hydrogen in if it's not water?

Posted by Sam Dinkin at December 15, 2004 08:30 AM

Same thing as He3. It's entrained in the regolith--as protons deposited by the solar wind.

Posted by Rand Simberg at December 15, 2004 08:42 AM

Sam: hydrogen occurs in a wide variety of organic compounds. Maybe the lunar polar deposits are some kind of radiation-crosslinked organic tar (volatile precursors would be transported there in the gas phase, freeze out, then react slowly over time). Nonvolatiles like that could survive at the poles even if large impacts temporarily caused the moon to wobble and exposed the crater bottoms to direct sunlight.

It would be fun if we could make hydrocarbon fuels from the lunar polar deposits, visions of the LH2 proponents notwithstanding.

Posted by Paul Dietz at December 15, 2004 09:17 AM

It would be fun if we could make hydrocarbon fuels from the lunar polar deposits, visions of the LH2 proponents notwithstanding.

Indeed, that's my hope. I'd prefer a lox/methane infrastructure to a cryo one. It's much easier to deal with in terms of cooling requirements, and the bulk densities are much better. I'm assuming that we'll have to await asteroid mining to get the carbon, but if we can find sufficient organics on the moon, I'm all for it.

Posted by Rand Simberg at December 15, 2004 09:22 AM

I've heard somewhere that if the hydrogen detected at the Moon's south pole by Lunar Prospector is indeed water then there would be enough to fill a 6 mile x 6 mile x 60 ft. deep lake. While sure its great that there could be water to deal with once we get there, to me that doesn't sound like very much. How much of a energy resource is that to count on for any type of sustained settlement? What does one do when the resources run out?

Posted by Josh "Hefty" Reiter at December 15, 2004 10:18 AM

Josh: It's not an energy resource, it's a propellant precursor resource. The energy itself comes from elsewhere (solar, nuclear).

That's about 2 billion tons of water, btw. That's much, much more than the combined mass of all the rocket propellant ever used by humanity.

Posted by Paul Dietz at December 15, 2004 11:06 AM

Since Alumina is plentiful, solid or hybrid rockets make a lot of sense for the Moon. O2 could also be used as the reaction mass for a nuclear fission powered rocket. Given the small gravity well, high-thrust is not required nearly as much.

Posted by Gojira at December 15, 2004 11:15 AM

That South African process sounds like an extension of the FFC (Cambridge) process for refining titanium. In that process, solid titanium dioxide is reduced electrolytically as one of the electrodes in a bath of molten calcium oxide. I expect it would also reduce ilmenite to the metals, since iron is less electropositive than titanium (don't know about silicon; and some other metals such as magnesium would be vaporized at the temperature of the cell.)

Posted by Paul Dietz at December 15, 2004 06:31 PM

Rats! Now someone else has come up with the tar/oil organic crud theory for the hydrogen at the moon's poles.
I thought I was making an original suggestion when I thought of this a couple of years ago.

Posted by Mike Borgelt at December 16, 2004 12:22 AM

I guess if we only have a few thousand acre feet of water at the pole, we will have to recycle it. Rats.

Posted by Sam Dinkin at December 16, 2004 05:26 AM

XCOR lunar rocket-engine proposal

I heard about this at LOSCON where XCOR gave a presentation prior to an official NASA presentation about a liquid rocket engine using lunar derived oxidizer, LOX, and lunar derived fuel, aluminum. That's right, a liquid aluminum fueled rocket! I call it the foundary rocket.

The low melting temp alloy of aluminum is stored in a small heated insulated tank, and fed by helium gas pressure through heated lines into the combustion chamber. Wild, eh? But when you think about it no wackier than using cryogenic hydrogen for fuel.

Posted by Brad at December 16, 2004 10:46 AM

There have also been experiments with LOX/powdered aluminum and LOX/powdered magnesium monopropellants. (And, no, they don't explode when ignited.)

Posted by Paul Dietz at December 17, 2004 09:15 AM

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