The reaction wheels rotate the satellite only about the centre of mass.If the reaction wheels are not placed about the principal axes,Will the amount of torque produced be the same as when they are placed about the principal axes?

  • $\begingroup$ Related $\endgroup$
    – Chris
    Commented Aug 17, 2017 at 17:33

1 Answer 1


The torque will be the same - application of point torque to an object (in this case the point is the reaction wheel motor axis) results in the same torque exerted regardless of the point. But the moment of inertia won't be - because the layout of mass will differ. As the reaction wheels are not massles, they are a part of the system, and while mass won't change, its distribution - distances from center of mass - will differ. And as result, the cubesat will react differently than one with the RW axes through its center of mass; in particular with mass distributed broader (farther from center of mass) the same torque will result in lower angular acceleration.

  • $\begingroup$ SF Thank You for the answer. How significant is the placement for nano-satellites? Is it necessary to consider the change in effect if the reaction wheels are not placed about the principle axes? $\endgroup$
    – user97213
    Commented Aug 18, 2017 at 13:48
  • $\begingroup$ @user97213: Your attitude control system should be adaptive enough that it shouldn't matter at all, although if you botch the regulator parameters you may get oscillations where you would get a stable positioning otherwise. But the answer to that is "don't botch the parameters"; this can happen with central placement just as well, but if you blindly copy the parameters from one cubesat to another instead of considering the changed moment of inertia and recalculating them, you might encounter this problem. $\endgroup$
    – SF.
    Commented Aug 18, 2017 at 15:16

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