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farmerben

magnetic alignment and orbital maneuver

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I'd like to know why this very simple idea is not used on satellites.  Three solenoid electromagnets powered by solar panels could give any satellite its own directionally controllable magnetic field.  The magnets on the satellite would interact with the Earth's magnetic field.  This would give useful orientation control, the entire satellite could slowly orient like a compass needle.  If the satellite starts to spin along an axis parallel to the Earth's magnetic field, this would be difficult to control, but I think through careful wobbling it would be possible to eventually cancel the spin.  The satellite could also gain some component of prograde or retrograde thrust, while usually changing its inclination at the same time, because it can accelerate with respect to the Earth's poles.  

I can see two drawbacks.  1) the effect may be very weak.  2) controlability is poor or non-existent in some directions from some positions.  

Despite these limitations, a triple electromagnet seems like a very useful thing to have.  Reaction wheels or thruster fuel are often the first things to fail on man-made earth satellites.  The electromagnets are much more long lived.  Electromagnets would allow many satellites to remain pointing in the proper direction much longer.  It would also give them a capability to deorbit themselves when their mission is over, without depending on moving parts or finite propellants.  

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Those seem like some very big drawbacks to me - "sometimes doesnt work".

And on another note, see my sig.

 

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Earth's magnetic field is a real mess and constantly changing. Annoying local anomalies and the fact that it doesn't coincide with earth's geometry is another drawback. Currents in the ionosphere and magnetosphere in the order of literally milliseconds would not make such an orientation thing for a low flying satellite really a realistic choice.

Also, i would expect (but don't know) that the ratio of weight needed for magnets, batteries, power generation to force exerted isn't really good, compared to chemical puffs every now and than. But i may be wrong on that one.

Edit: partly ninja'd by @p1t1o :-)

Edited by Green Baron

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38 minutes ago, farmerben said:

The magnets on the satellite would interact with the Earth's magnetic field.

It's still very weak.

Much better just to use gravity gradient...

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The advantage seems to be that electromagnets are among the longest lived components.  Presumably the power generation system, batteries, or computers would fail first.  If using primarily chemical thrusters, the fuel could be made to last longer.

Compared to a gravity gradient tether, the electromagnets are probably lighter, and offer deorbit capability.  

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Best guess is that it is more effective at LEO (which rarely need to bother with station keeping, if they need to move they need to move much further/faster than EM means allow) and less effective at GTO/GSO where stationkeeping is critical.

It makes a lot of sense for deorbiting: just using the magnetic field to produce current will begin deorbiting the satellite.

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42 minutes ago, farmerben said:

Compared to a gravity gradient tether, the electromagnets are probably lighter, and offer deorbit capability.   

Well, gravity gradient is all about the mass distribution, so just contruct your spacecraft accordingly. (and there's always an axis where it's present so you can point your spacecraft that way.)

Deorbiting is the one that might be of higher interest.

Though as UmbralRaptor said it's actually already used.

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Hmmm... This sounds a lot like magnetorquing. It's a somewhat common option for providing coarse stabilization for cubesats, but it is incapable of performing fine pointing. However, it does allow a cubesat to unload any reaction wheels it may have.

Edited by MaverickSawyer

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