(and a little of space too)Why do the reaction wheel need to desaturate sometime?

[goldenpeach]

I have heard that the reactions wheels need to desaturate sometime.

My question is,why does it need to do so?

If turning the wheel produce torque,why does it have to turn faster and faster to maintain the torque?

Thank you for reading,and thank you in advance for answering(if you do so ).

I hope you understood my question,if not,don't hesitate to ask for further information in this thread!

[dharak1]

I'm no pro but i think its because when it stops or maintains speed the energy goes back to the wheel not out anywhere else.

[Kerbin Dallas Multipass]

Acceleration/deceleration of the wheels produces torque.

Once the wheels are turning their momentum produces stability along their axis (which makes it harder to rotate around the other axes)

In an ideal world, with just one reaction wheel around the center of mass in an otherwise empty universe you could probably accelerate the wheel a bit and stop it to rotate the craft by an exact amount of degrees, minutes, seconds and come to a full stop with no loss.

In reality you will have some losses by friction and off center mass etc. Plus you want to rotate around all 3 axes freely.

In a probe, the wheels are mostly used to counterbalance other influences like solar wind or rotation/movement of the photovoltaic panels or other instruments. So, over a day in space they will pick up quite a few RPM just from making sure the craft just "sits there" and has one part (an antenna, or itself) pointed at the earth and another part at the sun (solar cells) and maybe a third one (a camera) pointed at something that looks photogenic.

In a podcast a space guy responsible for a scientific probe said they "unload" the wheels via RCS every day, using up 0.5 grams of fuel on average. Think it was a mercury (the planet) mission but I could be wrong.

Edited September 4, 2013 by Kerbin Dallas Multipass

[goldenpeach]

So,just to make sure I have understood you(by the way,thank you (both) for the answer ):

The torque is not produced by the fact that the reaction wheel turn but by the fact it accelerate/decelerate.

To keep constant torque,it have to accelerate constantly.

Reaction wheel will get to their maximum speed so they need to use RCS to provide torque so they can decelerate.

The reactions wheels are now ready to accelerate again!

Is that correct?

EDIT:does the control moment gyroscope(http://en.wikipedia.org/wiki/Control_moment_gyroscope) need to desaturate as well or,because the torque is given by turning a spinning wheel,it don't need to accelerate a wheel to provide torque t don,t need to desaturate?

[Kerbin Dallas Multipass]

Sounds correct to me.

Having all reaction wheels stand still might be a simplification, perhaps they want one or the other keep spinning a bit, they just would like to "unload" the RPM.

about your EDIT: the article has some info how they unload the CMG.

The CMGs may eventually saturate (absorbing momentum to the point where they can absorb no more), resulting in loss of effectiveness of the CMG array for control. Some kind of momentum management scheme (MMS) is necessary to allow the CMGs to hold a desired attitude and at the same time prevent CMG saturation. Since the CMGs are momentum-exchange devices, external control torques must be used to desaturate the CMGs, that is, bring the momentum back to nominal value. Some methods for unloading CMG momentum include the use of magnetic torques, reaction thrusters, and gravity gradient torque. For the space station, the gravity gradient torque approach is preferred because it requires no consumables or external hardware and because the gravity-gradient torque on the ISS can be very high.

They could as well redirect the tachion beam to the warp core using a nanoflux radiation so jerody la forge can see the radyon emissions with his visor... technobabble to me.

[KOCOUR]

If you look into the 1st newton law it says: If body rest motionless or if it moves constant speed there is no force applied.¨Second newtons law says that if you aply force body will accelerate constantly. From this you can see that if the reaction wheel would just rotate in constant speed it would not excert any force to the rocket or probe, and so it wouldnt actually do anything. Thats why the wheel need to accelerate.

http://teachertech.rice.edu/Participants/louviere/Newton/law1.html

http://teachertech.rice.edu/Participants/louviere/Newton/law2.html

[goldenpeach]

Thank you so much for the answers!

Now I understand why the reactions wheels need to desaturate!

PS:Why does the CMGs have to desaturate?

I mean,you don't need to constantly accelerate the wheel since the CMG work by turn a spinning wheel,since turning a wheel need force(force that is used to porive the torque),it don't need to accelerate the wheel,right?

If it don't need to accelerate the wheel,what cause the CMG to saturate?

Thank you again for the answers!

[Ralathon]

Thank you so much for the answers!

Now I understand why the reactions wheels need to desaturate!

PS:Why does the CMGs have to desaturate?

I mean,you don't need to constantly accelerate the wheel since the CMG work by turn a spinning wheel,since turning a wheel need force(force that is used to porive the torque),it don't need to accelerate the wheel,right?

If it don't need to accelerate the wheel,what cause the CMG to saturate?

Thank you again for the answers!

This is a little harder to explain. It has to do with singularities in the system.

Lets say you have a spaceship. It has 3 dimensions: x,y and z. You have a CMG you can rotate around each of those axis. Say you want to rotate along the x axis. To do so you need to rotate the X CMG. The problem is that the axis of rotation for the wheel starts to line up with one of the CMG's of the other axis. At a 90 degree turn it will exactly match up and suddenly you can no longer easily rotate along that axis (since the X flywheel will work against you).

So they need to dodge these singularities by desaturating the CMGs.

[goldenpeach]

Thank you!

Effectively it was harder to understand!

PS:does desaturating is really needed with the CMGs or it to be able to rotate more easily?

I mean:if they don't desaturate,what happen?

PS2:thank you for the answer!

[Jackissimus]

Thank you!

Effectively it was harder to understand!

PS:does desaturating is really needed with the CMGs or it to be able to rotate more easily?

I mean:if they don't desaturate,what happen?

PS2:thank you for the answer!

You would lose attitude control of your spacecraft. I believe that's the famous gimbal lock problem that you had to avoid on apollo spacecraft for example.

http://en.wikipedia.org/wiki/Gimbal_lock

[goldenpeach]

ok,thank you all for the answers!

[cantorobriar]

Another thing to account for is, if the wheels keep spinning faster, they will generate heat, and because they're made of metal, that metal can overheat and melt, and when it melts, IT WON'T WORK! Humanity has no device that does not erode. Even gently touching something with a feather causes a very small amount of damage to it, yet that amount of damage in unmeasurable (unless you use an uninvented, very small measuring tool).