On Thu, 20 Feb 2003, Michael Wallis wrote:
> > NASA looked at this for SEI, and concluded that supplying one lunar
> > mission a year with LOX/LH2 required a 400kW electrolysis plant running
> > continuously in LEO.
>
> They decided what? Are THEY daft? How big a settlement are they
> looking to supply? How much recycling are they expecting to do
> on-site?
No settlement; Apollo-style missions, although probably with bigger crews
and longer surface stays. And that's *fuel*, for TLI, LOI, and landing,
not resources for surface use. I don't know exactly how large a vehicle
they had in mind, but 100-200t of LOX/LH2 needed in LEO would be a
reasonable first guess based on Apollo (which used about 80t, but also had
30-40t of hypergolics on board, mostly for LOI and landing).
> This is ridiculous.
No, it's electrolysis. Electrolysis is *horrendously* energy-intensive.
At 100% efficiency, you'd need just under 16MJ/kg to decompose water...
and with electrolytic processes you're lucky if you get 50% efficiency.
(I don't have current industrial numbers for water electrolysis in
particular.) So perhaps 35MJ/kg, or about 10kW-hr/kg, if we're lucky.
400kW, operating continuously, would split 40kg/hr, or 350t/yr. If we
assume that "continously" actually meant "while in sunlight", which is
maybe 60% of the time in LEO, it comes down to about 210t. And we need to
budget some power for liquefying the products and for general process
overhead, so that is indeed in the right ballpark.
And of course, if what we want is hydrogen for things like nuclear
rockets, that 210t of LOX/LH2 contains only about 23t of LH2. Suddenly,
having twice the Isp no longer looks so good.
> And even if you needed that much, ship it
> to Luna as water and split it on the surface.
See above; that's what's required to *get you there*, not to operate on
the surface.
> > The practical way to ship hydrogen at room temperature is probably as a
> > metal hydride or related compound. There are hydrides which have slightly
> > more hydrogen per kilogram than water does, *and*, more important, will
> > decompose on mild heating.
>
> Now if they just had lots of oxygen too they'd be useful. 8-)
They'd be useful regardless. Oxygen is much easier to ship. If you're
averse to LOX for some reason, lithium perchlorate is 60% oxygen by weight
and decomposes on mild heating; that's what the Mir/ISS backup oxygen
generators use. Or you can use hydrogen peroxide; it's not as good an
oxygen source, unless you resort to electrolysis of the water, but the
water may be a useful byproduct. If nitrogen is more useful as a
byproduct, consider N2O or (ugh) N2O4. HNO3 will give oxygen, nitrogen,
and water, although not quite as easily.
Henry Spencer
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