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From: eder@ssc-vax.UUCP (Dani Eder)
Newsgroups: net.space
Subject: Re: Light Sails
Message-ID: <464@ssc-vax.UUCP>
Date: Wed, 27-Feb-85 19:22:12 EST
Article-I.D.: ssc-vax.464
Posted: Wed Feb 27 19:22:12 1985
Date-Received: Fri, 1-Mar-85 08:01:13 EST
References: <595@mordor.UUCP> <1031@utastro.UUCP>
Organization: Boeing Aerospace Co., Seattle, WA
Lines: 46

> > From: John Heimann <jheimann@BBNCCY.ARPA>
> > 	I sat down and tried to sketch out just how fast a solar sail could
> > reasonably be expected to go, assuming a point source of illumination (the
> more helpful, if only by a little bit.  A more important point is that
> if you just hold up your sail against the sun you'll have problems unless
> your Area/mass ratio is quite large.  Below a critical limit you will simply
> reduce the sun's gravity by a constant fraction.  The angular momentum you
> posess from the Earth's orbit will carry you out to some distance from
> the sun into a new, and larger orbit, and there you will stay.  You need to 
> tilt your sail to gain angular momentum as you go.
> "Don't argue with a fool.      Ethan Vishniac
>  Borrow his money."            {charm,ut-sally,ut-ngp,noao}!utastro!ethan

     Congratulations! Ethan has rediscovered the 'lightness ratio'.  This is a performance measure for solar sails.  It is the ratio of light pressure to gravitational attraction for a given sail.  Since both fall off as inverse square of
distance, the figure is a constant for that sail.  Light pressure is
F=2P/c, where P is the light falling on the sail (watts), c is the speed of
light, and 2 is for a perfect reflector.  A typical real sail might be
1.8, meaning 80% reflected light.  The gravitational attraction is GMm/r^2,
where G is the gravitational constant, M is the mass of the Sun, m is
the mass of the sail, and r is the distance between them.  

     A lightness ratio of 1 means the sail can hang motionless, balanced
between gravity and light.  If the ratio is greater than 1, then on a
radial escape mission, at every point on the trajectory, net outward
force is (lightness ratio - 1)x gravity.  The final velocity is then
(L.R. - 1)^.5 x escape velocity.  Since escape velocity depends on 
where you start, there is no single answer.  For the more complicated
case of a spiral out mission, I don't know what the answer would be.

     As for what you make your solar sail out of, you use VERY thin
aluminum foil, preferably less than one micron thick.  Typical plans
call for vapor depositing the sail material in orbit, then somehow
getting it off the substrate.  Use graphite fibers to hang the aluminum
off of.  Spin the whole structure slowly, thus all the structure is
in tension, and tends to stay flat.  Don't get too near the Earth.
Below about 1000 km, air drag exceeds light pressure, and you fall
out of the sky very fast.

     Eric Drexler of L5 fame, and Robert L Forward, at Hughes Research
Laboratories (and science fiction writer) are two names you can look
up in abstracts for articles.  I've been a fan of lightsails (any non
rocket transportation, in fact) for quaite a while, so I can try to
answer any more questions you might have.

Dani Eder / ssc-vax!eder / Boeing / Advanced Space Transportation