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With reaction wheels it is common to use actuators such as Thrusters and magnetorquers, to account for the saturation of the reaction wheels due to secular/non-cyclic disturbances. However, these disturbances are a result of spacecraft properties such as mass, area, attitude etc. So it is possible under certain conditions that the reaction wheels do not saturate (i.e never reach the maximum angular momentum storage capacity) and can be despun to release the angular momentum that they have build up due to these disturbances. And the torque that is acted on the spacecraft due to deceleration of the wheels (to release the angular momentum) and the changes in attitude and angular velocity (which are the result of the torque) can be counteracted by the typical control manoeuvre of acceleration, coasting and deceleration of the reaction wheels.

What I would like to know if this is a correct understanding or if there is something I have missed out? And also, have there been any missions that have flown with only reaction wheels on board without other actuators for momentum dumping?

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    $\begingroup$ I've up voted because I have a hunch there is an interesting question here, but I am not 100% sure I understand just what it is that you are asking is correct or not. Are you just asking if it is possible for a spacecraft to be in some situation where angular momentum unloading might not be necessary? $\endgroup$
    – uhoh
    Aug 10, 2018 at 12:38
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    $\begingroup$ My question is more towards what happens when the situation of momentum unloading is unnecessary with additional actuators. I assume in that case, the scenario of decelerating the reaction wheels to get rid of the momentum it has gained (although not to its upper limit). Which cause a torque to act on the system during the phase of deceleration, which results in a change in attitude and attitude rates of s/c (away from the reference state). And then the controller can perform a manoeuvre using the same reaction wheels to bring it back to the reference state! I hope I made it clear this time! $\endgroup$
    – srikarad
    Aug 10, 2018 at 17:38

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The short answer: yes, there are certain conditions that would allow desaturating reaction wheels without a secondary attitude control system (such as thrusters or magnetorquers), but those conditions are quite specific.

Condition 1: You don't have to have the spacecraft at a certain attitude all the time. There might be times when you do want it at a precise, specified attitude, for telecom, or to make science observations, etc., but there are also significant stretches of time when the spacecraft's attitude can be different, maybe greatly different. That duty cycle will probably be very roughly 50%, but could be 30-70 or even 20-80.

Condition 2: There is something the spacecraft can do that reverses the net disturbing torque. This could be an attitude change (which would, of course, involve transient spin-up and -down of the wheels), a change in geometry, such as: "feathering" a solar array previously face-on to the sun, and maybe moving another array so it is face-on to the sun; rotating an articulated radar antenna; etc. It is really rare that a spacecraft can exactly reverse the torque direction just by changing articulated components, so usually there will be some whole-spacecraft attitude change involved.

You have to be careful about accounting for all non-negligible sources of disturbing torques. In addition to solar light pressure you can have asymmetric radiation pressure from hot spacecraft parts, magnetic torques from planetary or interplanetary mag fields, leakage of such things as propellants, even outgassing, to name a few.

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    $\begingroup$ Great answer. I would also add the example of one of the early NASA mission which was planning on doing a spin-stablization (I forget the name of the spacecraft). In order to achieve a more stable attitude, the vehicle was designed with a beam in order to create a gravity gradient torque. Turned out the designers had their math slightly wrong, and the beam caused a spin around the second moment of inertia, which is unstable. Everything was going perfectly fine until, out of the blue, the craft rotated around its CG and started pointing to outer space. Moral: always have thrusters, they help. $\endgroup$
    – ChrisR
    Aug 11, 2018 at 19:17
  • $\begingroup$ Thanks for the answer! Makes sense! :) Also, would you happen to know any satellites with only reaction wheels on board? $\endgroup$
    – srikarad
    Aug 13, 2018 at 7:13
  • $\begingroup$ @srikarad No, not off the top of my head, but that doesn't mean there aren't any! $\endgroup$
    – Tom Spilker
    Aug 14, 2018 at 3:32

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