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Gydrodynes, aka control moment gyroscopes, are spinning wheels that are torqued at their axis (a motor pushes the axis to make it a different direction) for the purposes of creating counter-torque that rotates the whole spacecraft. They're a way of attitude adjustment without expending any fuel. Instead, they spend electric energy. They're also good for fine attitude control, (though reaction wheels are even finer.)

Do they need to be at the center of mass or can they be located anywhere without losing any efficiency? (rotation per electric energy expended.)

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The closer they are to the center the better. However that is often compromised. On ISS the flywheels don't get moved every time a ship docks. I don't think they have ever been moved, even as modules have been added. (I know they got used to death during the construction phase and had to be replaced.) I would think on a ship it is something the pilot gets use to.

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    $\begingroup$ Do you have a citation for the ISS construction phase assertion? $\endgroup$ – Jerard Puckett Aug 25 '16 at 2:28
  • $\begingroup$ Here is an article from the time. It doesn't say it was thought that the bearings failed as a result from over use. I read that elsewhere but I'm sorry I can't remember else. I bet it wouldn't be hard to find. flightglobal.com/news/articles/… $\endgroup$ – Johnny Robinson Aug 27 '16 at 17:36
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Reaction wheels and the like provide a pure couple or moment. As such it does not matter where they act -- the craft will still rotate about its CG.

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    $\begingroup$ Can you go into more detail to show that efficiency remains the same in all cases? $\endgroup$ – Nathan Tuggy Aug 25 '16 at 20:37

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