In article ,
Uncle Al wrote:
Despite a recent article in New Scientist, a solar sail does not break
the laws of physics.

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.

If the sail material has the same radiative properties on both sides,
there is no net counter-thrust at all -- the sail will emit equally from
*both* sides. In any case, for a good reflector, this is quite a small
effect compared to the thrust of the reflected light.

Guys, this stuff is not new. The basic physics have been understood quite
well for decades. Do a bit of research before sounding off, please. See,
for example, Appendix A of Jerome Wright's "Space Sailing".

Solar wind will mostly stick rather than
bounce, incrementally increasing sail mass.

Solar-wind atoms will stick *momentarily*, and then wander off again.
There may be a very slight initial increase in sail mass, but it will
reach equilibrium quickly. And in any case, the solar wind carries
several orders of magnitude less momentum than solar light pressure,
so its effects are slight.

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.

Sure it does. This stuff is well understood and is routinely allowed for
in precision tracking of deep-space missions. Moreover, unbalanced light
pressure is the biggest source of attitude disturbance for GSO comsats.
There is very little that isn't known about the subject by now. The major
unknowns of solar sails are the engineering hassles of deploying and
controlling large areas of very lightweight material.

If the sail is metallic, there will be god's own Hell of field
interactions.

Hardly. Almost all current sail designs are simply aluminized plastic...
quite like materials routinely used in the construction of spacecraft,
including some quite large ones.

Space is rich with large scale electromagnetic this and that.

Very feeble this and that.

If you think solar flares are tough on power lines...

Solar flares, by themselves, wouldn't do anything at all to power lines.
The trouble comes not from the flares themselves (more precisely, from
the associated particle events), but from the way they punch and pummel
Earth's magnetosphere. Outside the magnetosphere, nothing happens.

There have been deep-space spacecraft with very long wire antennas, for
low-frequency radio astronomy. They've had no trouble.

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.

You seem to be under the impression that solar sails are like parachutes.
They're not; they can't be, because light doesn't act like air. It won't
keep a flexible canopy inflated. Sails have to be *rigid*, either from
centrifugal force or from structural members. The structural mass is
annoying but not prohibitive. Again, please do some research before
sounding off; this problem has been explored in considerable depth and
reasonable solutions do exist.

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.

10mg/cm^2 is 100g/m^2, which may sound light to you, but is a load of lead
bricks by solar-sail standards. JPL's late-1970s design for a solar-sail
Halley-rendezvous mission had a total mass (film, reflector, backside
emitter, joints) of 3.2-3.5g/m^2. They thought 1g/m^2 was a reasonable
near-term lower limit. Highly advanced sail materials could reach
0.1g/m^2 or perhaps a bit less, although not soon.

The thinnest capacitor aluminum foil
is 0.0015 inch for an areal density of 10.3 mg/cm^2.

Massive, horrible, hideous. That's 38um thick. The JPL Halley design was
2um of Kapton, topped by 100nm of aluminum, with 12.5nm of chromium on the
back. 2-3um Kapton is commercially available.

0.0005"
aluminized Mylar is commercial, but it doesn't like getting warm or
irradiated and it is *fragile.*

Sails will be fragile. But they won't be exposed to much stress.

Hey... aluminum metal doesn't tolerate heat either.

It doesn't get particularly hot.

...Use of Parylene-C ultrathin membrane as in ornithopters
would bust even NASA's budget.

Hardly. Parylene N was considered for the Halley mission, but its space
compatibility wasn't certain then (not sure if it is now), and it wasn't
available in large sheets.

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

Bare Kapton is quite vulnerable in low Earth orbit. Sails cannot operate
in low Earth orbit anyway, because of air drag. Atomic-oxygen erosion is
insignificant long before drag falls low enough for practical sail
operation.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |