Someone mentioned that to "tack" away from the sun, you just need an
assymetric magnetic field.  To move TOWARD the sun, you would need a field
that accellerates the particles in the solar wind.

Ed Uber


-----Original Message-----
From: Wirt Atmar [mailto:[log in to unmask]]
Sent: Friday, October 06, 2000 1:49 PM
To: [log in to unmask]
Subject: Re: Friday Trivia


Dave writes:

> Aw heck, I was going to write a dissertation on all the reasons why the
>  aircraft wing is not a valid analogy to a solar-sail spacecraft.  Bottom
>  line is that the wing still relies on motion through a fluid medium.  The
>  "keel" or centerboard that is missing from the solar-sail craft is
present
>  in an aircraft - it is the combined effects of all aircraft surfaces,
>  including the control surfaces, against the surrounding air.
>
>  In an aircraft, the thrust vector of the powerplant (or inertia, in the
case
>  of a glider) is the component analogous to the solar wind.
>
>  A solar-sail craft has no keel, and it's acceleration will be in the
>  direction of the solar wind only, gravitational effects of other
celestial
>  bodies notwithstanding.  No, you can't tack into the solar wind as long
as
>  you are flying in a vacuum.

Two things: (i) you don't to have a keel to have an asymmetric,
lift-producing, highly controllable wing (see:

     http://www.mucheswarbirds.com/N9MB.html

and (ii) space is not a vacuum, at least not in the presence of a solar
wind.
The solar wind presents a fluid flow virtually identical to an ocean current
or an atomspheric wind, albeit a little less dense. (BTW, the solar wind out
to the distance of at least Neptune remains a laminar, supersonic flow, so
it
actually should be very easy to navigate).

A sailboat's keel and rudder are providing you upright and horizontal
directional stability by virtue of anchoring the orientation of the upright
wing against the gravity well. But on the other hand, having that keel and
rudder stuck in a very much more vicious fluid also provides a great deal of
drag. A sailboat "falls" against the wind, not unlike a piece of paper
falling towards the earth. While gravity provides the accelerating force in
this latter example, you can get some sense of the lift forces involved in a
wing operating against a pure fluid flow in this scenario, with no other
extrinsic forces operating on it.

If the sailboat could sail in a pure wind, like the piece of paper, without
gravitational forces, the simplest form of sail (a hemispherical parachute)
could be used to capture that wind and push the sailboat along with it, so
long as you wanted to travel in the direction of the wind. But if you wanted
to travel against the wind, an asymmetrical lift-producing wing would be
necessary so that you could engage a tacking motion.

Wirt Atmar