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During orbit insertion, the satellite can start with a high tip-off rate which requires detumbling. From what I have read, detumbling is usually done using magnetorquers.

I was wondering if reactions wheels can be used for detumbling (reducing the attitude rates) rather than magnetorquers or if there are any satellites out there that have done this.

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    $\begingroup$ Yes, but then you'd have to sort out your reaction wheel rates afterwards (I've no idea what the technical term for that is... unloading? despinning?), and that will require some other means of attitude control whilst you do, right? So you may as well do it the easy way first, to save doing it twice later. (This is a bit of a wild guess, so I'm not going to file it as an answer) $\endgroup$ – Starfish Prime Dec 21 '19 at 12:34
  • $\begingroup$ I think you mean momentum unloading (desaturation). Yes, true. But can it be done with reaction wheels? I have this thought that the load might be too high and might damage the wheels, just a wild guess also. $\endgroup$ – John Dec 21 '19 at 15:46
  • $\begingroup$ @StarfishPrime: the technical term seems to be desaturation homepages.laas.fr/arzelier/publis/2013/Attitude-Allocation.pdf :-) $\endgroup$ – user34174 Dec 21 '19 at 17:28
  • $\begingroup$ Today I Learned ;-) Thanks. $\endgroup$ – Starfish Prime Dec 22 '19 at 22:08
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The language here tends to get in the way a bit.

Generally “reaction wheels” are used for pointing: you turn them, then stop turning them once the satellite is pointing as desired. They’re not meant to spin rapidly to soak up a bunch of angular momentum. If you just need to control pointing, a reaction wheel system isn’t going to be able to absorb the angular momentum of high rotation-rate tumbling.

“Momentum wheels” are generally built more massively and capable of higher rates so that they can soak up significant angular momentum. But they’re not common in small satellites (because they’re generally not small)

“Control moment gyroscopes” are the ultimate form, but are only seen on larger craft.

I think the general answer is that building a system to internalize the initial tumbling’s angular momentum just isn’t worth it: there’s no other need for that hardware, and you need the magnetorquers to offload angular momentum anyway, so use those.

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  • $\begingroup$ Ah I see! You mentioned that the reaction wheels stop turning when the satellite is pointing as desired. Isn't it suppose to continue turning in order to maintain the pointed attitude as there are perturbations and disturbances present? $\endgroup$ – John Dec 22 '19 at 0:47
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    $\begingroup$ For pointing, you start the wheel to start the craft moving, then stop the wheel and hence the craft when you get where you’re going. If there are external torques (which you work hard to null out), the craft will slowly start to rotate, and you can ramp the wheel (typically a momentum wheel) to cancel. But that can’t go on forever: you need another system to push the angular momentum off the craft and “unload” the wheel. $\endgroup$ – Bob Jacobsen Dec 22 '19 at 0:50
  • $\begingroup$ Nice to know I had the right idea. It hadn't occurred to me that there'd be various sizes of gyro devices... seems kinda obvious in retrospect. $\endgroup$ – Starfish Prime Dec 22 '19 at 22:09

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