# [amsat-bb] BY70-1

Phil Karn karn at ka9q.net
Wed Mar 1 22:21:03 UTC 2017

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On 2/27/17 10:37, Howie DeFelice wrote:
> Thanks Phil, that explanation cleared up many questions. I had been
> thinking about ways to use the earths magnetic field to raise perigee by
> storing energy in capacitors and pulsing something along the lines of a
> magnetorquer with a high current pulse at the right time and vector. The
> problem, I think, is that I would need to generate the thrust at apogee
> where the earths magnetic field is weakest.

If you're going to use the earth's magnetic field to change orbit, you
want a linear force, not a torque as you'd use for attitude control.

A straight conductor will experience a Lorentz force perpendicular to
both the current flow and the ambient magnetic field. Problem is, how do
you close the circuit to keep the current flowing? If you run a second
conductor through the same magnetic field, it will experience an equal
and opposite Lorentz force. If it's physically separated from the first
conductor (i.e. forming a loop), the result will be a net torque, not a
net linear force.

So you need one of two things. Option #1 is a gradient in the magnetic
field. If your "forward" conductor is in a stronger part of the field,
it will experience a greater Lorentz force for a given current than the
"return" conductor in a weaker part of the field. You'll then get a net
linear force, along with plenty of torque.

The other possibility is to use something not attached to your
spacecraft as the return conductor so you won't feel the opposing
Lorentz force. But what conductor is there in space??

LEO is actually within the ionosphere, and that's your return path. This
is in fact how tethers have been used so far to either add or remove
energy from a satellite orbit.

Some years ago N6NKF and I looked at how much linear force you could get
from a practical magnetorquer coil. (I think it was when Oscar 13 was
decaying, so that was 20 years ago!) I don't remember the exact result
but I do remember it being negligible.

Here's a wild idea I just had. I wonder if it would be practical to
build an ion thruster for use in the ionosphere that uses the ambient
ions as your reaction mass? Think of it as vaguely analogous to the
Bussard Ramjet in science fiction. You'd collect both protons and
electrons and accelerate the protons rearward and the electrons forward
to maintain charge balance. The protons, being much more massive, would
result in a net forward thrust. You could also see this as analogous to
an airplane, only the "propellers" are electric fields that work on an
ambient atmosphere that is ionized.

Phil

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