Re: Proposed sample return mission to Phobos

On Feb 19, 10:48 pm, "Alex Terrell" <alexterr...@xxxxxxxxx> wrote:
On 20 Feb, 05:18, "Michael Turner" <l...@xxxxxxx> wrote:



On Feb 19, 6:44 pm, John Schilling <schil...@xxxxxxxxxxxxx> wrote:

On 18 Feb 2007 01:33:07 -0800, "Michael Turner" <l...@xxxxxxx> wrote:

On Feb 17, 4:49 am, "Paul F. Dietz" <d...@xxxxxxx> wrote:
Michael Turner wrote:
"The added advantage of the microwave thruster is the potential
duality of the system. In 1985, Dr. T. Meek of the Los Alamos
National Laboratory (LANL) demonstrated the ability to extract water
from simulated lunar soil using 2.45 GHz microwaves [12]. Microwaves
readily couple to the water trapped in soils. According to Meek,
using conventional means to heat the soil would require 10,000 times
more energy than using the microwaves to selectively heat the water in
the soil.
First, I didn't write that, the authors of the paper did.
This is absurd. Microwaving lunar soil will not selectively
heat water.
No, but it will heat the water even as it heats other things in it.
Water is 10-20%, presumably, of Phobos. What's the fraction of other
compounds subject to heating? Is it significant?

Well, if water is 10-20%, the "fraction of other compounds subject to
heating" would be 80-90%. You, and Meek, were perhaps imagining there
was such a thing as a compound *not* subject to heating?

That's a fairly common misconception based on some common but completely
wrong descriptions of how microwave ovens work. The reality is, there's
no Magic Microwave Resonance that selectively heats one compound instead
of another, and if there were, the heat would just be conducted instantly
between the two anyway. *Everything* gets heated, to roughly the same
temperature, and you have to pay for all that heat even if you're only
interested in one part of the product.

I suppose some of my misconceptions about microwave heating derive
from the observation that, when I boil water in a coffee cup in a
microwave oven, it seems to me that the cup itself is not boiling hot
(except when I let it go too long), though the water definitely is.
By your reasoning about uniform heating, and this other peculiar
phenomenon I've never heard of, "instant conduction" of heat,
shouldn't the cup be "roughly the same temperature" as the water?

Think of a wet sponge. Conduction is almost instantaneous.

That said, "almost" is not "instantaneous". Shock heating of the water
elements could detach them from regolith before conduction cools them.
Under vacuum conditions, this will form vapour that can separate
before it reattaches.

Whilst this seems far fetched, I wouldn't presume to ridicule the work
of the Los Alamos laboratory.- Hide quoted text -

- Show quoted text -

It sounds like a lot depends on the granular structure of what you're
heating. (A wet sponge is, to a good first approximation, just a blob
of water, so the relatively uniform heating in that case hardly counts
for purposes of this discussion).

Maybe what we're talking about here is more like a chunk of frozen,
low-moisture clay.

It could be that faster is better -- zap the block really hard with
microwaves and it turns to rock powder and water vapor very quickly,
with the non-H20/nonmetallic components heating up hardly at all
because heat transfer from gases to solids is very poor.

It might turn out that slower is better. For example, the better use
of energy might be to just to dump rocks into box and keep shaking
them -- their surfaces will heat somewhat from impact with each other
and with the box sides, and sublimation from the dust abraded off in
the process gets the vapor out. This heats everything, ultimately,
but it doesn't have to heat everything above the freezing point of
water, much less its boiling point, so the net energy input might be
much lower when you consider the specific heat of H20 in all its
phases. Separation of vapor from rock dust might be done
centrifugally, with the H20 ducted into a much cooler receptacle where
it refreezes. The centrifuge might also capture ice crystals along
with dust particles, but separating those from each other is should be
relatively easy. Microwave at low intensity at all stages except the
ice trap could supplement the system, by helping to prevent
recrystallization from water vapor.

-michael turner

.

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