NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/sci/space/16269

Text File | 1992-11-23 | 3.0 KB | 59 lines

Newsgroups: sci.space
Path: sparky!uunet!utcsri!helios.physics.utoronto.ca!neufeld
From: [e-mail] (Christopher Neufeld)
Subject: Re: Solar sailing
Message-ID: <[e-mail]>
Sender: [e-mail] (News Administrator)
Organization: University of Toronto Physics/Astronomy/CITA
References: <[e-mail]>
Distribution: sci
Date: Mon, 23 Nov 1992 16:48:24 GMT
Lines: 46

In article <[e-mail]> [e-mail] (John Roberts) writes:
>
>Most proposed solar sails are reflective, not black. That's because the
>principles of conservation of momentum show that reflecting the beam of
>light can greatly increase the thrust of the sail. The best results
>are with a 100% reflective sail perpendicular to the beam of light (theta = 0);
>that doubles the thrust.
>
   Well, yes, for certain definitions of 'best results'. Unless your
sail+payload has a very low mean mass per unit area, though, the net
force on the sail is still going to be sunward. Since the solar pressure
falls off as the square of distance just the way gravity does, the effect
of setting the sail at normal incidence to the sun is the same (from a
mathematical point of view) as instantaneously reducing the mass of the
sun while maintaining the position and velocity of the spacecraft fixed
over that instant. The result is a new orbit which circumscribes the old
one and is tangent to it at the point where the sail was unfurled. This
new orbit, of course, is closed, like any bounded orbit in an inverse
square field.  Now, for some trajectories this is desirable, but for most
it probably will not do.
   The usual trick is to set the sail at an angle to the spacecraft-sun
line. Since the radial component of the thrust does no net work over one
complete orbit (to first order, it helps a bit to second order if the
sail is not at normal incidence), the sail should be set at an angle to
maximize the tangential thrust. This angle is about 35 degrees for a
perfectly reflecting, planar sail. The sail can be angled to thrust along
the velocity vector or against it, and the orbital energy of the sail can
be raised or lowered this way, allowing the sail to move sunward just as
easily as it moves outward.
   A higher generation sail might be designed with a collector area at
normal incidence to the sun but which focusses the light onto a small
secondary reflector which can be steered rapidly to new directions. The
sail can be smaller for a given thrust. There will be some weight and
design penalties on making the reflector (which has to handle a very high
power flux) and making a sail which can focus accurately onto the second
reflector.

>John Roberts
>[e-mail]


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 Christopher Neufeld....Just a graduate student  | Don't let your mind
 [e-mail]    Ad astra  | wander -- it's too little
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