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Old July 3rd 03, 08:10 PM
Uncle Al
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Default Solar sailing DOESN"T break laws of physics'

"Geoffrey A. Landis" wrote:

Despite a recent article in New Scientist, a solar sail does not break
the laws of physics.

[snip]

Actually, it does as proposed. The sail will come into thermal
equilibrium with the radiation field and emit photons from its other
side, counter-thrusting. Solar wind will mostly stick rather than
bounce, incrementally increasing sail mass. The claimed efficencies
will be, surprise!, *much* higher than anything obtained. More
studies will be needed. Damn! It didn't scale linearly like it was
supposed to.

If the sail is a dielectric mirror, I don't know how it could be
fabricated to perform given engineering, cost, and weight constraints
including dense folding for launch. (This includes 3M multimicrolayer
plastic supermirrors). Dielectric sails will accumulate patches of
static charge from the solar wind. Judging from meter-scale
satellites, a square kilometer of arcing HV capacitor plate will be...
memorable. Hard UV will disintegrate organics.

If the sail is metallic, there will be god's own Hell of field
interactions. Space is rich with large scale electromagnetic this and
that. If you think solar flares are tough on power lines (EMP pulse),
imagine what they will do to a square kilometer of conductor. The
thing might buck like a bronco. Mechanical control lines must be
maximally thin or their mass is punitive. I don't care if you use
Kevlar - a klick of thin will stretch like taffy and suffer a low
speed of tensile conductance. There will be no real time control.

Uncle Al says, "Baikonur sleighride!"

Lastly, the payload or at least the control pod must be on the same
side as the sun since the solar sail can only bulge away from the
light. Any attempt at a building a rigid framework will negate the
payload. If the focus washes across the pod, hasta la vista baby.
Non-imaging caustics will also incinerate the pod.

I get 10^5 cm/km and 10^10 cm^2/km^2. At 10 mg/cm^2 the solar sail
alone weighs 100 metric tonnes. The thinnest capacitor aluminum foil
is 0.0015 inch for an areal density of 10.3 mg/cm^2. 0.0005"
aluminized Mylar is commercial, but it doesn't like getting warm or
irradiated and it is *fragile.* Hey... aluminum metal doesn't
tolerate heat either. 5-micron aluminized Kapton is about the best of
all worlds - unless you have to pay for a km^2 of the stuff as a
throwaway. Use of Parylene-C ultrathin membrane as in ornithopters
would bust even NASA's budget.

If you push optimistic numbers,
http://solarsails.jpl.nasa.gov/intro...struction.html
you get a bowl of moldy farina.

Oh yeah... even chemically tough Kapton in orbit gets chewed -
especially its reflectance,
http://setas-www.larc.nasa.gov/esem/..._append_b.html

--
Uncle Al
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