Date: 20 Aug 92 17:54:54 GMT
From: Dani Eder <eder@hsvaic.boeing.com>
Subject: Electric Tethers
Newsgroups: sci.space

evert@CPSnet2.cps.edu (Mike Evert) writes:

>As I understand it, if a current is put into the tether, then that
>would cause the tether and spacecraft to gain kenetic energy and rise
>to a higher orbit.  The opposite will happen if current is drawn from
>the tether.  Would the acceleration always be in one direction and its
>reverse only?  I don't know if this would be in the direction of orbit
>or perpindicular to magnetic field.  Is it possible to use the tether
>for lateral motion?

The formula for the force developed in the tether is the same as
the force on any other current carrying wire in a magnetic field:

F = IL x B

Where all the components above are vectors, and the x stands for
cross product.  This means the force is proportional to the current
I, the length of the wire L and the magnetic field strength B
(in Newtons, Amperes, meters, and Teslas respectively).  The
direction of the force is perpendicular to the current direction
and the magnetic field.

The magnetic field is approximately a dipole tilted about 10 degrees
from due north-south, and you can mount the current carrying wire
in other orientations than straight up-and-down, so you have some
measure of control in thrust direction, but it is not a simple
thing to picture.

Note that the power consumed in an electric tether is mostly
I^2R resistance losses, plus the power to run the plasma contact
devices at the ends.  For a given number of watts of input power
for thrust generating, you can play with the wire diameter and
length to look for the least amount of weight and most thrust.
You can control current I, and length L.  Field B is a given.

It turns out generally that you want a wire in the km length
range, but not hundreds of km.  So if this is a propulsion system
attached to a really long tether, it may only cover a short segment
of the total length.

Another real world restriction to this propulsion system is that
it uses the ionosphere to close the current loop.  As you go
up in altitude, you have less ions to work with, so eventually
you can't keep the current flowing.  Also, the field strenth of
the Earth's magnetic field falls off like radius cubed, so that
also falls off with altitude.  Thus, this propulsion system is
limited to low and medium earth orbits.

Dani

--