Can reaction wheel kill a certain angular momentum of ship without working forever?

[Cesrate]

I mean if a ship is rotating in vacuum and without any other force can we stop its rotation only by running reaction wheel within a certain time? How it will be in real spaceflight cases?

[Awaras]

Well, the reaction wheels work by transfering angular momentum from the ship to the wheels themselves.

Basically, when they start spinning, the ship starts spinning the other way. If you stop the reaction wheels, that angular momentum will just be transferred back to the ship and you will be right back where you started...

The only way for the reaction wheels to absorb more angular momentum is to make them spin even faster, and there is a limit on how fast the wheels can spin without falling apart.

Once they are spinning as fast as they can, you need to find some other way to bleed off angular momentum, like RCS or something like that.

Edited June 27, 2013 by Awaras

[stupid_chris]

Yeah, IIRC that's how the ISS works. Once the reaction heels reach max speed, they stop them while counterbalancing with RCS, then they use it again.

[Awaras]

Yeah, IIRC that's how the ISS works. Once the reaction heels reach max speed, they stop them while counterbalancing with RCS, then they use it again.

Actually, I believe they use the Earth's magnetic field and gravitational anomalies for counterbalancing in some way.

[stupid_chris]

The ISS employs a total of four CMGs as primary actuating devices during normal flight mode operation. The objective of the CMG flight control system is to hold the space station at a fixed attitude relative to the surface of the Earth. In addition, it seeks a Torque Equilibrium Attitude (TEA), in which the combined torque contribution of gravity gradient, atmospheric drag, solar pressure, and geomagnetic interactions are minimized. In the presence of these continual environmental disturbances CMGs absorb momentum in an attempt to maintain the space station at a desired attitude. 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

Well, seems like you're right, the ISS use gravitational torque, my bad

[WhiteWeasel]

Is there a way to permanently discharge this momentum?

[Awaras]

Is there a way to permanently discharge this momentum?

Well, you could toss the still-spinning reaction wheel into space...

You need to transfer the momentum somewhere. It can be the reaction wheel, it can be the propellant expelled from RCS thrusters or it can be the Earth itself or some other planet through an interaction with it's magnetic or gravitational field, but you can't just make the momentum dissapear.

[Fuzzy Dunlop]

Well, you could toss the still-spinning reaction wheel into space...

Thats actually kind of how yo-yo despin mechanisms work

[drakesdoom]

Would it be possible to spin rotate the station so that the reaction wheels are facing the other way and can discharge their stored momentum into the opposite direction to counter ongoing momentum gain? This is ignoring functional problems with rotating the vessel

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[WhiteWeasel]

Would it be possible to spin rotate the station so that the reaction wheels are facing the other way and can discharge their stored momentum into the opposite direction to counter ongoing momentum gain? This is ignoring functional problems with rotating the vessel

.

I.E. You apply the two forces together and they cancel out. Right?

[Kryten]

It wouldn't be possible to end up with anything less than the original total momentum, at least without dumping some through e.g. RCS burns.

[K^2]

Total angular momentum is conserved. So you cannot change angular momentum of your ship without jettisoning something. So either reaction wheels keep spinning, or you apply RCS.

[Unit One]

Satellites in equatorial orbit can use an electromagnetic rod to orient themselves, like a compass. However, they may still need reaction wheels as a faster means of rotating, but the reaction wheels will eventually be relieved by the slower acting magnetic forces. However, most satellites simply carry fuel for RCS, they must carry fuel anyway for orbital boosting if they want to stay up for more than a year, and geostationary satellites must carry fuel to maintain their near-perfect circular orbit. Many many satellites end their missions due to running out of fuel or coolant, it's normal.

[TouhouTorpedo]

Many many satellites end their missions due to running out of fuel or coolant, it's normal.

The only stuff I've ever heard of that ends the mission because of running out of coolant are IR space telescopes. (which there aren't exactly many of) ... though I suppose its possible the nuclear russian radar satellites might have ended their missions for similar reasons. (the coolant thats leaked out of some of these is rather nasty debris). Running out of fuel though is generally how most GEO satellite missions end yes. Though they'll drag them out for as long as possible first, and its still not legally required, only recommended that satellites are sent to graveyard orbit and shut down. So quite a lot of operators just let their GEO fleet drift with minimal interaction, and just deal with some signal loss on the ground.

If you've ever thought your satellite TV has been getting progressively worse over the years, it probably isn't your imagination.

[drakesdoom]

What I was saying is that with a full reaction wheel array of 3 or more you can rotate in all directions. If there is a slow build up of reaction wheel speed to hold a certain heading that means there is also a predictable angular torque on the craft over time from some source. In my line of thinking you could use your full array of reaction wheels to reverse the position of the wheel fighting this drift and then discharge it's momentum and return to the correct orientation at an angle that coming to a stop would preload the most used wheel against expected use, i.e. it would be spinning the other way so slowing the wheel would hold your course. I think you might actually need 4 wheels to avoid one of them eventually being at max speed.

[K^2]

What I was saying is that with a full reaction wheel array of 3 or more you can rotate in all directions. If there is a slow build up of reaction wheel speed to hold a certain heading that means there is also a predictable angular torque on the craft over time from some source. In my line of thinking you could use your full array of reaction wheels to reverse the position of the wheel fighting this drift and then discharge it's momentum and return to the correct orientation at an angle that coming to a stop would preload the most used wheel against expected use, i.e. it would be spinning the other way so slowing the wheel would hold your course. I think you might actually need 4 wheels to avoid one of them eventually being at max speed.

Doesn't work that way. If your reaction wheel is rotating a certain direction, you can't just flip the craft over and have the wheel spin in the opposite direction. Why? Because of something called gyroscopic effect. Lets label the reaction wheels X, Y, and Z to correspond with the X, Y, and Z axis of chosen coordinate system. Suppose, there is some torque about X, so your reaction wheel builds up some amount of angular velocity around X. You decide to use your Y wheel so that excess rotation is now in -X. Ok, you start spinning up the Y wheel, but instead, the craft starts to rotate around Z! Well, ok, it's not exactly what you wanted, but the craft is flipping over. Well, yes, but now the Y wheel isn't pointing along Y axis. It's rotating in the X/Y plane. So now as you keep spinning up the Y wheel, you are affecting rotation around X, and you don't want that. So you start compensating with the X wheel. It so happens that the compensation rotation is in opposite to direction the X wheel is rotating. So the X wheel is now spinning down, Y wheel is spinning up, and the craft is rotating around the Z axis. By the time the craft has rotated 90°, your X wheel has stopped, your Y wheel points along X axis, and it's spinning as fast as X wheel used to spin. You're right back where you started!

Of course, it all goes back to the concept of conservation of angular momentum. You can re-orient the craft, but the sum of angular momenta of all the reaction wheels will remain the same if the craft isn't spinning. So there is absolutely no way to get rid of the extra momentum.

Now, if asymmetry of the craft is the cause of the buildup, then this might not matter. You flip the craft over, and now the external torque changes sign and kills off angular momentum. But often enough this excess is due to position of solar panels and instrumentation that are critical to the mission, and you can't simply re-orient these and have everything working at peak efficiency. So you're kind of stuck with using RCS or similar to correct.

[steveman0]

Doesn't work that way. If your reaction wheel is rotating a certain direction, you can't just flip the craft over and have the wheel spin in the opposite direction. Why? Because of something called gyroscopic effect. Lets label the reaction wheels X, Y, and Z to correspond with the X, Y, and Z axis of chosen coordinate system. Suppose, there is some torque about X, so your reaction wheel builds up some amount of angular velocity around X. You decide to use your Y wheel so that excess rotation is now in -X. Ok, you start spinning up the Y wheel, but instead, the craft starts to rotate around Z! Well, ok, it's not exactly what you wanted, but the craft is flipping over. Well, yes, but now the Y wheel isn't pointing along Y axis. It's rotating in the X/Y plane. So now as you keep spinning up the Y wheel, you are affecting rotation around X, and you don't want that. So you start compensating with the X wheel. It so happens that the compensation rotation is in opposite to direction the X wheel is rotating. So the X wheel is now spinning down, Y wheel is spinning up, and the craft is rotating around the Z axis. By the time the craft has rotated 90°, your X wheel has stopped, your Y wheel points along X axis, and it's spinning as fast as X wheel used to spin. You're right back where you started!

Of course, it all goes back to the concept of conservation of angular momentum. You can re-orient the craft, but the sum of angular momenta of all the reaction wheels will remain the same if the craft isn't spinning. So there is absolutely no way to get rid of the extra momentum.

Now, if asymmetry of the craft is the cause of the buildup, then this might not matter. You flip the craft over, and now the external torque changes sign and kills off angular momentum. But often enough this excess is due to position of solar panels and instrumentation that are critical to the mission, and you can't simply re-orient these and have everything working at peak efficiency. So you're kind of stuck with using RCS or similar to correct.

To add to that I can point to a more visual explaination: the Mighty Cheese!

It's a great visualization of some awesome physics. It takes a lot of force to turn the spinning disc because of the collected angular momentum. To turn the spinning reaction wheels requires more effort on the part of the other reaction wheels such that you end up in a zero sum type of situation.

[K^2]

Well, in case of the "mighty cheese" all you need to know is the trick. You shouldn't be trying to flip it over. You should be trying to turn it to face the other way. That will force precession in an orthogonal direction, which will tilt the thing towards the ground.

[Yourself]

Well, in case of the "mighty cheese" all you need to know is the trick. You shouldn't be trying to flip it over. You should be trying to turn it to face the other way. That will force precession in an orthogonal direction, which will tilt the thing towards the ground.

Would have been fun to see a little skinny guy come up and just push the thing over that way.

[tchezick]

Since many real-life missions are limited by the amount of fuel they carry to perform station-keeping, it's pretty common to use magnetic torque rods to bleed off the momentum that the wheels build up. The wheels are like a bucket for momentum, the torque rods are like a tiny little drain. As long as you're not putting too much in the bucket.... it never overflows. They system operates by:

1.) Spacecraft checks reaction wheel speeds. Of note, many spacecraft have more than 3 wheels and they're not necessarily aligned to the body principal axes.

2.) Spacecraft calculates total stored momentum in each axis. Depending on the specific design, it decides to "unload" the momentum based on being above thresholds for a specific wheel or above the stored momentum in that axis.

3.) Spacecraft calculates the current to apply to each torque rod based on the momentum to unload, the local vector value of the earth's magnetic field, and the spacecraft orientation in the field.

IRL, your spacecraft accumulates momentum through silly things like solar radiation pressure and gravity gradients across the body of the spacecraft.

[TouhouTorpedo]

Interesting. Are you into AOCS systems tchezick? Wasn't expecting to see such a complete reply.

An interesting side note is that these Gravity Gradient torques are predictable and always approach to a stable point. (Aligning the highest Second Moment Inertia axis toward the planet) So they can themselves actually be a useful stabilising force, although alone aren't much good as they can't dampen, which magnetic torque rods are useful for doing.

I was developing a satellite AOCS system for an earth observation satellite some time ago, If I remember right the final solution was that for the whole of the eclipse side of the orbit the spacecraft held a 45 degree roll, bleeding off the excess momentum. Although its a very small torque it adds up over a longer period of time. Apparently the ISS does the same thing as well!