Re: Efficiency of electrodynamic tether reboost

Alex Terrell schrieb:

Does anyone know how efficient this would be?

I suppose the measure is (electrical energy supplied) / (orbital energy
increase)

It is basically an electric motor, so the efficiency could be pretty
high. Of course there are some factors that reduce efficiency.

First of all, you need to connect the conductor to the surrounding
plasma to create a closed circuit. Some schemes involve active systems
such as electron emitters, which of course consume energy. But other
systems use just a very long section of blank wire. AFAIK this has not
been tested yet.

Then there is a tradeoff between efficiency and time. When the tether
is not perfectly perpendicular to the magnetic field lines, you lose
some efficiency. But if you pulse the current only when the efficiency
is at a maximum, it will take you forever to change your orbit.
Probably you will need to compromise and have a current when the
misalignment is less than 30 degrees or something. On the other hand,
if your power comes from solar cells there is no point in not using it
even if the misalignment is big.

For a rotating tether, the situation is I suppose complicated further.
If it rotates every 30 minutes or so, should the input current be
varied (as well as reveresed).
You probably would not want to vary it. Just use everything you can get
from the solar cells. That should simplify the electronics. Since
changing your orbit will take a long time compared to chemical
thrusters, any adjustments that have to be made can be done by just
switching the current on and off at appropriate times. Some kind of
pulse width modulation.

But you probably need to reverse the current, otherwise you will not be
able to create thrust for half of the time.

If this is the case, what's the best
storage mechanism for electrical power in space? Flywheels? Ultracaps?
Li-ion batteries?

Li-ion batteries, probably. There are rumors that a company called
eestor has made a capacitor with a storage capacity (Wh/kg) comparable
to Li-ion batteries. But they have not shown a working prototype yet,
so it might turn out to be a scam.

Probably you could get away with not using energy storage at all. A
typical tether reboost facility will be in an elliptical orbit, so the
time spent in the shadow of the earth is much less than 50%.

So now to answer your question: if you design for maximum efficiency,
you could get pretty close to 100%. Maybe 90% is a realistic number.
But when you have no storage capacity and your power supply is "use it
or lose it" like solar in earth orbit or an RTG in jupiter orbit, your
overall efficiency will go down to maybe 40-50%.

These numbers are just educated guesses based on my limited
understanding of tether dynamics. So don't shoot me if they are off by
an order of magnitude :-)

.

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