Control Moment Gyroscope

Definition

A control moment gyroscope (CMG) is a type of reaction wheel that uses the conservation of angular momentum to control the orientation of a spacecraft or other vehicle. It consists of a spinning rotor mounted on gimbals that allow the orientation of the spin axis to be varied, generating a gyroscopic torque that can be used for attitude control.

5 Must Know Facts For Your Next Test

1. Control moment gyroscopes are used in spacecraft attitude control systems to precisely adjust the orientation of the spacecraft without using propellant.
2. The change in the orientation of the CMG's spin axis generates a gyroscopic torque that can be used to control the spacecraft's attitude.
3. CMGs are more efficient than traditional reaction wheels for large angular momentum changes, making them well-suited for controlling the orientation of large spacecraft.
4. The speed and direction of the CMG's rotor can be varied to produce the desired torque for attitude control, providing a high degree of precision and responsiveness.
5. CMGs are often used in combination with other attitude control systems, such as thrusters or momentum wheels, to provide a comprehensive solution for spacecraft orientation control.

Review Questions

• Explain how a control moment gyroscope generates a torque to control the orientation of a spacecraft.
  • A control moment gyroscope (CMG) generates a gyroscopic torque by changing the orientation of its spinning rotor. The rotor is mounted on gimbals, which allow the spin axis to be varied. As the spin axis is changed, the angular momentum of the rotor changes, resulting in a gyroscopic torque that can be used to adjust the spacecraft's attitude. This torque is applied to the spacecraft, causing it to rotate and change orientation without the use of propellant, making CMGs an efficient attitude control system for large spacecraft.
• Describe the advantages of using a control moment gyroscope over a traditional reaction wheel for spacecraft attitude control.
  • Compared to traditional reaction wheels, control moment gyroscopes (CMGs) offer several advantages for spacecraft attitude control. CMGs are more efficient for large angular momentum changes, as they can generate higher torques using less power. This makes them well-suited for controlling the orientation of large, heavy spacecraft. Additionally, the ability to vary the speed and direction of the CMG's rotor provides a high degree of precision and responsiveness in the attitude control system. CMGs also have the advantage of not requiring propellant to change the spacecraft's orientation, unlike thrusters. This allows for longer mission durations and more efficient use of onboard resources.
• Analyze the role of control moment gyroscopes in the context of the precession of a gyroscope, and explain how this phenomenon is leveraged for spacecraft attitude control.
  • The precession of a gyroscope, as described in section 11.4, is a fundamental principle that underpins the operation of control moment gyroscopes (CMGs) for spacecraft attitude control. When the orientation of a spinning gyroscope's axis is changed, the gyroscope will precess, or rotate, in a direction perpendicular to both the applied torque and the original spin axis. In a CMG, this precession is used to generate a gyroscopic torque that can be applied to the spacecraft, causing it to rotate and change orientation. By precisely controlling the speed and direction of the CMG's rotor, the resulting precession can be used to apply the desired torque for accurate and responsive attitude control, without the need for propellant-based systems. The ability to leverage the physics of gyroscopic precession is a key advantage of CMGs in spacecraft attitude control applications.

Related terms

Reaction Wheel: A reaction wheel is a type of momentum wheel used to control the orientation of a spacecraft by applying a torque as the wheel's angular momentum changes.

Angular Momentum: Angular momentum is the measure of an object's rotational motion, defined as the product of the object's moment of inertia and its angular velocity.

Gimbal: A gimbal is a device that allows the rotation of an object about a single axis, typically used to keep instruments like cameras or compasses steady and oriented properly.