On Thu, 20 Feb 2003, Michael Wallis wrote:
> So they planned to launch water (likely on the shuttle) crack it in
> LEO and use it as fuel to get to the Moon?
It was suggested. In particular, if you're flying a bunch of shuttle
flights (to the same orbit) which are volume-limited for some reason,
topping up the mass with something well-behaved like water could be
essentially free. This suggestion has come up repeatedly over the years;
the SEI-related study is just the one I happened to know about.
They didn't *plan* to do this; they studied it briefly and concluded that
it was too much trouble to be useful.
I think one reason why the idea keeps coming up is that people don't have
a clear idea of the energy costs of electrolysis. If you really could
make all the fuel for the mission a day or two before launch, you could
dodge some of the annoying problems of in-space LH2 storage. *That* might
perhaps make it worthwhile. But of course, you can't, so you have to deal
with the storage issues *anyway*. That being the case, if you must use
LH2, just shipping it up from the ground is simpler.
> > ...So perhaps 35MJ/kg, or about 10kW-hr/kg, if we're lucky.
>
> I don't remember it being that hard in high school chem class, but
> that was 25+ years ago. Ok. It's hard to crack water.
In bulk. It's easy enough to demonstrate electrolysis with a couple of
batteries, but the production rate of that demonstration is very low --
you get gas measured in milliliters, which means you're cracking an amount
of water measured in milligrams. (Oxygen gas at room conditions is very
roughly a kilogram per cubic meter, i.e. a milligram per milliliter, and
that's most of the mass of the cracked water.)
Henry Spencer
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