http://spacenews.com/experiment-designed-harness-magnetic-field-propulsion/
In 2010, NRL seems to think they should be able to achieve 5 km altitude
per day with only a 1 kilometer tether, that's orders of magnitude greater
than other forces on a satellite. The tether and power may not have to be
that long for a 5 or 10 year de-orbit.
https://www.nrl.navy.mil/media/news-releases/2010/nrls-tepce-spacecraft-undergoes-successful-deployment-test
Tether Electrondynamics Propulsion CubSat Experiment (TEPCE) made it to the
testing stage in 2010.
https://www.nrl.navy.mil/sed/programs/tepce
" Unique to TEPCE will be the electrodynamic devices for the propulsion
consisting of electron emission filaments, electron collection tapes,
electrically conducting tether and the tether deployment subsystem. The
experiment will also carry into space several plasma experiments, the iMesa
sensor developed by NRL code 7669 and dual impedence probes developed by
NRL, code 6755."
Only 3 cubesats for two experiments and the TEPCE experiment, if it works
http://space.skyrocket.de/doc_sdat/tepce.htm
Stay tuned in for results next year.
On Tue, Dec 19, 2017 at 11:34 AM, Henry Spencer <hspencer@xxxxxxxxxxxxx>
wrote:
On Mon, 18 Dec 2017, Craig Fink wrote:
Yes, there is a "magnetic thruster", that's how electric
motors/generators work, one mass pushing on another with a magnetic field.
In a vague, general sense, yes, it is possible for things to interact via
magnetic fields. But that *doesn't* mean you can build a "magnetic
thruster" in the sense of a simple compact package that can somehow produce
useful thrust on your satellite by interacting with Earth's magnetic field.
Satellite exchanging angular momentum with the Earth should create "drag
brakes" to deorbit the satellite.
Yes, but how to achieve that? Details matter.
A wire with a current flowing, placed in a magnetic field, experiences a
force (perpendicular to both current and field). The trouble is, to get a
sustained current flow, you need a *loop* of wire. And because the current
flows on opposite sides of the loop are in opposite directions, so are the
forces exerted. In a uniform magnetic field, the result is a *torque* on
the loop, but no net translational force. And on the scale of even a big
satellite, the Earth's magnetic field is awfully uniform.
...A 90 minute resonant circuit where the electrons are stored at one end
of the wire and then the other side of the wire might work.
Unfortunately, in realistic configurations the achievable currents are
negligible and so are the resulting forces.
Or, pumping electrons out one end of the wire, collecting them on the
other end?
Yes, if you close the current loop via the ambient plasma, then only one
side of the loop exerts force on the spacecraft. But this is not a compact
package tucked into a corner of your satellite -- to make this work
reasonably well, the wire needs to be *long*. So it's an electrodynamic
tether, an approach which has been mentioned before in this discussion. It
would work, but you have to deploy it, and you may need some non-trivial
gadgetry at the ends of the wire to make better contact with the plasma,
and it's easily cut by debris hits unless it's more than just a single
wire. It's a very promising approach, but the engineering is not trivial.
I guess it's a matter of scale for a small satellite, but in LEO a little
bit of extra drag could be all the difference between a few years to
deorbit and a few centuries.
Numbers matter. Getting *useful amounts* of extra drag -- enough to make
that difference -- this way isn't so easy.
Henry
