I remember the designs for long storage life boosters for use in the asteroid belt and beyond
being worked on by NASA (Ames?) in the late 70s-early 80s. The ones I saw all had multiple
(5 or 7 or more) sheets of metalized mylar (not sure if it was double side plated or not,
also not sure what the metal was) mounted with a fair amount of space between them.
These racks of sheets were placed between the tankage and the engine chamber:
(____________) | | | | | <><
Tank--^ Reflectors Engine
...looking a bit like the radiator fins on the barrel of a Tommy gun,
but all fins having equal diameter.
The fuel of choice was typically methane liquid or slush.
Much easier to store than H2, less embrittlement risk, etc.
I seem to recall at least one design that used fluorine for the oxidizer.
IIRC, the standard orientation for minimum boiloff was with the engine bell
and reflector stack facing sunward.
There is a difference in using multiple, separated reflectors.
The result is easily computed to be superior to a single similar mirror layer on the tank,
especially because the layer that gets the lion's share of insolation has the hardest time
conducting to the tankage, and has an easy time radiating (as does the last sheet in the stack).
Yes, the layers would radiate to one another, reducing the effectiveness, but let's pluck some
figures from the air: say the first layer does a .5 job, and the others only do a .7 job each:
(.5)(.7)^6 is getting pretty good, around 5%. And I bet the numbers are better than that.
I'd think this would be covered in any good cryo engineering book with a chapter on dewar design.
Moral: Hypothesize some super reflecting material if you want, but look at what others have done, too.
"Robert J. Bradbury" wrote:
> 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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