From: neufeld@helios.physics.utoronto.ca (Christopher Neufeld)
Subject: Re: Unrealistic space scenes
Organization: University of Toronto Physics/Astronomy/CITA
Date: Tue, 15 Dec 1992 22:40:54 GMT

In article <1992Dec15.212616.24305@walter.bellcore.com> kenl@origami.cc.bellcore.com (Ken Lehner) writes:
>In article <BzBGqK.Cqy.1@cs.cmu.edu>, nickh@CS.CMU.EDU (Nick Haines) writes:
>|> In article <1gijgjEINN8jo@uni-erlangen.de> uhschreg@cip.informatik.uni-erlangen.de (Ulrich Schreglmann) writes:
>|> 
>|>    You also can only move into one direction using sails: away from the
>|>    sun. Sailing against the wind, like it's possible on the ocean isn't
>|>    possible.
>|> 
>|> Cobblers. To fall towards the sun you angle the sail so that the net
>|> force (which is perpendicular to the sail) reduces your orbital
>          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
>Isn't this only true in a medium such as air?  The perpendicular force is
>due to lift generated by the passage of air around the sail (or other such
>lifting body).  A sail in space could only provide force directly away
>from the sun, no matter what the angle of said sail.
>
   Nope, Nick is entirely right. I have some experience with this stuff,
too. I am on the sensors/electronics/actuation team for the Canadian
Solar Sail Project.
   The force is perpendicular to the surface of the sail because the sail
reflects light. It doesn't absorb it. The net force is parallel to the
vector difference of the incoming and outgoing light rays. You can show
that, for a reflecting planar sail, the optimal angle for actually
getting anywhere in the solar system is when the sail is inclined about
35 degrees to the sun-sail line. This maximizes your tangential thrust
contribution. Radial thrust is essentially useless for any sail designs
of the near future (designs in which the solar force is less than half
the gravitational force, so the sail stays inside the solar system).
Think of it this way: if the sail exerts a force smaller than half that
of gravity, and exactly radially, then that force falls off as 1/r^2
(since sunlight falls off like that) exactly as gravity does. From the
mathematics it's as if you had reduced the mass of the sun. So, you wind
up in a closed elliptical orbit. In one orbit you're right back where you
started, you haven't gained any energy at all. If you apply thrust
tangentially you can move out or in by increasing or decreasing your
total energy, so you can start in circular orbit around the earth and end
in circular orbit around mercury ... or maybe in solar polar orbit, or
around mars, or wherever.

   In case somebody asks where that 'half' came from inside the bracket,
that's because a sail launched from, say, the earth, is not at rest
relative to the sun. If you could reduce the force on it by a factor of
two it would have enough kinetic energy to get clean away.

>|> Anyone wanting to know more spend some time over on sci.space. I'm
>|> just passing through this newsgroup and I'm amazed at the amount of
>|> unscientific nonsense that is spouted in an authoritative manner here.
>   ^^^^^^^^^^^^^^^^^^^^^
>
>Ahem.

   No comment.


-- 
 Christopher Neufeld....Just a graduate student  | Entropy isn't what it
 neufeld@helios.physics.utoronto.ca    Ad astra  | used to be.
 utzoo.utoronto.ca!generic!cneufeld              |
 "Don't edit reality for the sake of simplicity" |
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