notes@ucbcad.UUCP (10/25/83)
#N:ucbesvax:8700008:000:704 ucbesvax!turner Oct 25 01:14:00 1983 I read somewhere (Omni?) about an order-of-magnitude improvement in solar sails. The idea is to perforate the sail to the extent that it's surface is 90% holes--but holes with diameters somewhat less than the average wavelength of incident sunlight. Less of the force from dissipating the kinetic energy of the photons striking the sail is taken up in moving the sail itself--leaving more to pull payload. The idea is either Freeman Dyson's, or some other megalomaniac's. :-) I suppose that the requisite material could be boosted up with the payload and then woven or pressed in orbital factories. Sounds like a very good idea. Anyone have more info? --- Michael Turner (ucbvax!ucbesvax.turner)
henry@utzoo.UUCP (Henry Spencer) (11/01/83)
Perforating a solar sail with holes smaller than the wavelength of
light not only greatly reduces the mass of the sail, it also greatly
reduces the air drag on the sail in low Earth orbit. (This would not
work in a viscous-flow regime, like Earth-surface pressure, but at
orbital altitude the individual air molecules are moving quite
independently of each other and it works fine.)
I heard about the idea in a talk by Robert Forward, but he may have
got it from somebody else.
You would definitely make the stuff in space, because it makes little
sense to apply the perforating technique unless your sail is as light
as possible to start with. The lightest known solid sails are Eric
Drexler's metal-foil sails: vacuum-deposited aluminum sheet about
30 nm thick. Sails made with this stuff must be manufactured in space,
because the stuff is too thin to be unfolded from a compact package in
a practical way. Drexler sails already have quite spiffy performance
(by solar-sail standards!), and a 90% reduction in mass would really
make them clip along, so if the practical details of perforating can
be worked out it would be great.
--
Henry Spencer @ U of Toronto Zoology
{allegra,ihnp4,linus,decvax}!utzoo!henrybane@umcp-cs.UUCP (11/01/83)
I first heard about the perforated solar sail idea at a talk by Robert
Forward at Constellation in September. I forget whether he claimed the
idea was his or not. The big win here is not that the sail gets lighter,
but rather that the sail will now let gas pass through it so it can be
used much closer to bodies with atmospheres; the holes are large enough
that gas molecules can get through, but smaller than the wavelengths of
most sunlight. This will allow neat tricks like geosynchronous satellites
away from the plane of the equator (the satellite is a solar sail thrusting
along the earth's axis).
There were many other strange propulsion ideas thrown out at that talk;
Forward had just finished preparing a report for the Air Force on the
future of propulsion for space. The one that had me gaping was not the
perforated sails, though. Forward mentioned the possibility of the existence
of a form of helium created at enormous energy cost which:
1) Would be a solid up to about 300 degrees C.
2) Could turn back into helium gas with an energy release of about
10 times the best possible chemical fuels.
Anybody out there know more about this stuff?
--
Arpa: bane.umcp-cs@CSNet-relay
Uucp:...{allegra,seismo}!umcp-cs!bane