Re: Recycling solar sails

From: Robert J. Bradbury (
Date: Fri Mar 23 2001 - 21:23:14 MST

Spike 'dissed my 'dissing using the solar sail as a reflector with:

> Suppose you have a solar sail that you wish to return to earth for
> another load of something. You want to bring a tank of liquid
> hydrogen. If you fold the sail and place it between the tank and
> the sun, much of the solar heat is reflected away, reducing
> the work needed to keep the hydrogen cold. The conductivity
> of the sail is immaterial if it isnt actually touching the tank, which
> it need not do. It could be supported by a spindly rod, a little
> like a lady's parasol. spike

Spike, I will grant you that you can reflect back "much" of the
visible light (99% perhaps). But I am moderately sure that
IR reflectance for most materials is much lower than visible
reflectance. (For example silver is the best reflector in V,
but I think gold becomes better in IR. But I think even gold
loses its effectiveness rapidly as you go into the mid->far IR.
There isn't any difference between sticking the solar sail
between the Sun and the H2 tank and mirroring the outside
of the tank. The solar sail will absorb IR and re-radiate
it as a black body thereby exposing the H2 tank to that
heat radiation. *If* you can coat the H2 tank with a several
hundred-to-thousand atom thickness dielectric that *can* reflect
VIS & IR, then the solar sail is irrelevant. If you can't
then the solar sail doesn't help the problem much. If all
you are going to reflect back is the visible, you might
as well simply mirror the tank itself. The coating
on the H2 tank is always going to be much thinner than
the thickness required for structural integrity, so its
really a point not worth considering (at least from a mass


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