If a ship used reactions wheels and CMG,would it need to use RCS to desaturate?

[goldenpeach]

the tittle say it all!

If a ship used a system combining reaction wheel and CMG,using the reaction wheel to desaturate the CMG(or using the CMG to desaturate the reactions wheels),would it still need to use RCS to desaturate?

I'm sure that it'sthe case,but I don't see why.

Thank you for reading!

[OtherDalfite]

There's actually already a discussion about this topic at this link:

http://forum.kerbalspaceprogram.com/showthread.php/37945-Can-reaction-wheel-kill-a-certain-angular-momentum-of-ship-without-working-forever

Hope this answers your questions!

[andrewas]

Short answer - yes.

Longer answer - Conservation of angular momentum. If you have an external force applying a torque on the spacecraft, the CMGs or reaction wheels absorb the angular momentum and keep the craft stable. If the force is on-going, as with radiation pressure or gravity torque generally are, then your CMGs or reaction wheels will eventually saturate and you will need to dump the momentum using RCS or some other interaction with the outside universe. Depending on the mission, it may be possible to rotate the craft and reverse the external effect, in which case you can avoid saturating the CMGs in the first place. But if you need to keep a constant orientation than you need thrusters. Or you could do something clever, like pumping fuel around the craft to alter the effect of gravity torque, or moving solar panels to alter radiation pressure, but those are more complex solutions and additional points of failure, so they are not used in the real world to my knowledge.

[goldenpeach]

Thank you,both, for the answer!