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Say I'm designing a probe or satellite. To control its attitude, I can add some gyros, or I can put thrusters on each corner, or I can add both and pick one in flight according to the situation. How do I decide which to use? How does it depend on the design lifetime, mass, expected frequency of attitude changes, and orbit of the probe?

It'd be great if you could support your explanation with an example or two.

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2 Answers 2

For the record, there's actually a third possibility, Magnetorquers. Here's why you would use one vs the other.

Magnetorquers - Inexpensive, low maintenance, but don't work in all situations. Used by LEO spacecraft typically, and small in size. They work by pushing off of a magnetic field.

Reaction Wheels (Or Gyros) - The trick here is that you can't take out a momentum change using only reaction wheels. You either need to stop the spacecraft from moving, or else offload the momentum with another form of propulsion. These work great if you can reasonable expect to have the spacecraft with the same angular momentum eventually. They tend to break down over time, but can last for many years. The reason they can't is because basically they exchange momentum from the spacecraft with momentum in the wheel. You can only make a permanent change if you are pushing against something external, or pushing something towards or away from you, Reaction Wheels just change the momentum until the wheel stops spinning. They can change the orientation, just not the momentum. See this video for a good demonstration.

Thrusters - These work all the time, but use fuel. Thus, you are limited to how much you can use them based on the fuel load vs. the lifetime of the mission. Also their exhaust can be dangerous to your satellite, if it carries something sensitive - that is the case of the Hubble Space Telescope, where thrusters could contaminate the optics.

Bottom line is, you use what you can, and will cause the minimal impact to your spacecraft.

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Why can a reaction wheel not make a permanent change in attitude? –  User58220 Mar 24 at 22:08
    
Where the impulse will come from? –  oakad Mar 25 at 0:37
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Impulse is defined as the change in linear momentum of a body, and is not relevant to a change in angular position... –  User58220 Mar 25 at 3:09
    
Reaction Wheel's can't make permanent changes because they don't cause an external force on the spacecraft. –  PearsonArtPhoto Mar 27 at 15:50
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For a slightly connected situation, Google on "motorcycle angular momentum" to see how motorcycle jumpers in a jump (effectively VeryLEO) use throttle/brakes to change their angular orientation in the air, using the bike wheels as reaction wheels. –  User58220 Mar 29 at 19:45

Further to the answer of @PearsonArtPhoto, it is possible to use a reaction wheel to make a permanent change in attitude. Consider a non-rotating satellite carrying a non-rotating gyro. If a motor is used to spin up the gyro, Newton's Third Law requires a reverse torque be exerted on the satellite, and it will start to rotate in the opposite direction. After a suitable period of time, the gyro is braked to a stop, at which point conservation of angular momentum dictates that the satellite and gyro are both non-rotating again, with the satellite pointing in the desired, new, direction.

If the satellite experiences any outside torque, then the reaction wheel can be used to store the new angular momentum, keeping the attitude of the satellite fixed. In this case, the gyro must be kept rotating constantly, until the angular momentum can be "dumped", or some opposite outside torque comes along...

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Could you clarify a bit please? When "the gyro is braked to a stop" the momentum from it is transferred back to the braking body, so you're essentially back at where you started before the gyro was spun (momentum conservation). Reaction wheels can store limited translational momentum since their spin rate is limited and they can't spin indefinitely, so I have to agree with Pearson that their change in attitude is not permanent either. Am I missing the point you're trying to make? How can you "dump" angular momentum stored in reaction wheels? –  TildalWave Mar 27 at 13:16
    
Try this: stand up, turn around, and sit down again. Do you snap back to your original position? You temporarily changes your angular velocity, transferring angular momentum to the earth. When you stop turning, AM is re-arranged as it was, but the change in angular position is permanent... –  User58220 Mar 27 at 15:26
    
Only that there's no Earth to sit on in space ... then what? –  TildalWave Mar 27 at 15:29
    
The earth was your reaction wheel; in space, use what you can. An astronaut floating in space could twiddle his thumbs While twiddling, he would rotate (slowly!) forward (or back, depending on the direction of twiddle) in pitch; when he stopped twiddling, he would stop rotating. He wouldn't snap back to his pre-twiddle attitude... –  User58220 Mar 27 at 16:31
    
That last analogy is not really helping, but let's pretend that twiddling thumbs can serve as a momentum storage device, then yes, he would stop. That's the same point I was making. You don't seem to fully appreciate what "attitude" means in flight dynamics. It's about the object's orientation in an inertial frame of reference. That "he would stop rotating" is his initial attitude. So yes, you would in fact revert back to where you started as far as attitude is concerned. Therefore, no permanent change in attitude. –  TildalWave Mar 27 at 16:58

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