Exploring the Mystery of Satellites' Momentum Wheels

In summary, the conversation discusses the use of spinning momentum wheels on satellites to maintain constant angular velocity and how external forces, such as thrusters, can change the spin of the satellite. It is explained that the initial spin of the momentum wheel will cause the satellite to rotate, but this can be countered by using thrusters to bring the spin back to zero. It is also noted that there is no real difference between a momentum wheel and a reaction wheel, although the former has a bias spin while the latter works around a zero momentum.
  • #1
hamzaaaa
31
0
Dears,

I would like to know a basic thing, why does the Satellite doesn't rotate with the spinning wheel momentum wheel inside it?
 
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  • #2
Why should it? If there is a reaction wheel, the satellite has to maintain its spin, to account for friction. If the relative spin (wheel<->satellite) is constant, angular momentum conservation gives a constant angular velocity of the satellite (usually chosen to be 0).
 
  • #3
But for a geo satellite it has an angular velocity equal to Earth's rotation,right?
So the angular velocity is not zero but constant.
This angular rotation is provided by the wheel?
 
  • #4
That is an angular velocity as seen in the frame of earth, not for the satellite itself.
I don' think I understand what you really want to know.

Satellites do not need those wheels (but they can be useful).
 
  • #5
That is an angular velocity as seen in the frame of earth, not for the satellite itself.
I don' think I understand what you really want to know.

Satellites do not need those wheels (but they can be useful).
 
  • #6
What i want to know is that a satellite having a spinning momentum wheel on 1 axis with constant angular velocity doesn't cause rotation on the satellite itself? But if the wheel speed is increased or decreased it will cause satellite to rotate. Is it explainable with any equation?
 
  • #7
This is just conservation of angular momentum (in the frame of the satellite). With a frictionless reaction wheel (or a real wheel where friction is countered by a motor), there is no torque on the satellite, so it does not begin to spin.
 
  • #8
hamzaaaa said:
What i want to know is that a satellite having a spinning momentum wheel on 1 axis with constant angular velocity doesn't cause rotation on the satellite itself? But if the wheel speed is increased or decreased it will cause satellite to rotate. Is it explainable with any equation?

With conservation of momentum, you're right that the initial spin up of a momentum wheel will force the spacecraft body to rotate the opposite direction if that's all that happens. However, external actuators (such as thrusters) can also change the spacraft body's spin rate by changing the overall angular momentum of the spacecraft . You spin up the spacecraft body using thrusters, then spin up the momentum wheel to bring the spacecraft rotation back down to zero (although, in practice, the two actions will be virtually simultaneous).

So, in other words, it's not explainable by some equation, but by the physical actions taken to set up the initial conditions. There's no real difference between establishing the momentum wheel's spin and establishing the spin of a spin stablized satellite.

One additional note regarding the title/substance of the thread: There's a subtle difference between a momentum wheel and reaction wheel even if there's no physical difference between them. A momentum wheel has a bias spin that creates a large angular momentum vector. A reaction wheel works around a zero momentum and can spin either direction depending on the torques they're counteracting.
 
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1. What are momentum wheels and how do they work?

Momentum wheels are devices used on satellites to control their orientation and maintain stability. They work by utilizing Newton's Third Law of Motion, where a spinning wheel will produce an equal and opposite reaction when its rotation is changed. By adjusting the speed and direction of the wheels, the satellite can be kept in the desired position.

2. How do momentum wheels affect a satellite's orbit?

Momentum wheels do not directly affect a satellite's orbit. However, by controlling the satellite's orientation, they can help maintain the desired orbit and prevent any changes in trajectory due to external forces.

3. What are the challenges of using momentum wheels on satellites?

One major challenge is that momentum wheels can experience wear and tear over time, leading to a decrease in their accuracy. This can result in the satellite's orientation becoming unstable and affecting its ability to perform its intended tasks. Additionally, the wheels must be calibrated and controlled precisely to ensure the satellite's orientation is maintained accurately.

4. How do scientists monitor and maintain momentum wheels on satellites?

Scientists use various methods to monitor and maintain momentum wheels on satellites. This includes regular calibrations and adjustments, as well as remote monitoring through data transmitted from the satellite. If a momentum wheel malfunctions, it can often be fixed remotely by adjusting its speed and direction or by using backup wheels.

5. What is the future of momentum wheels in satellite technology?

Despite the challenges, momentum wheels continue to be an essential component of satellite technology. However, advancements in technology have led to the development of alternative methods for controlling a satellite's orientation, such as reaction wheels and control moment gyroscopes. These technologies may eventually replace momentum wheels, but for now, they continue to be a vital tool for exploring the mysteries of space.

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