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In the game space engineers, the player can control the angular movements of a spacecraft with gyroscopes. Just to be clear, the purpose of this gyroscope is not to measure rotation, but to actually rotate the craft. Now, laying aside the game itself (It is full of oversimplifications, like the compounding effects of multiple gyroscopes and the fact that thrusters cannot apply rotations, only translations), is this possible in real life?

It sounds crazy at first, but the idea boils down to accelerating a large and heavy wheel, and the equal and opposite reaction is that the satellite begins to spin. If you had a wheel each for yaw, pitch, and roll, could you have full control of the satellite's orientation with a scheme like this, for a long period of time, without the help of thrusters, for a long period of time? Has it been done?

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  • $\begingroup$ The devices are called "reaction wheels" and are being used all the time. When they fail it's usually in the news because it endangers or ends the missions. $\endgroup$
    – CuriousOne
    Apr 13, 2016 at 22:02

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The term gyroscope is more often associated with sensors that can sense angular velocities, and these can be used in a control feedback system with actuators such as reaction wheels or control moment gyros that provide the application of torque to the body of the spacecraft and thus control attitude.

Thrusters indeed can provide translational force (delta-v) but if used in pairs can also provide torque for attitude control. Typically a spacecraft will use both thrusters and reaction wheels to steer the spacecraft to a rotational rate of zero, and if other sensors are available, to point in a specific direction. Thrusters using on-off or rather bang-bang control larger torque and acceleration, and when the spacecraft is within a smaller range, the reaction wheel control system will take over.

Reaction wheels are usually clustered as 4 along different axes to provide yaw pitch and roll torques. Only three are required, but the forth provides redundancy in the event any one of the other three fail. The geometry is such that any three can be used to provide three axes of orthogonal torque.

Reaction wheel or CMG attitude control systems are not without their limits. To obtain torque, the wheels must spin-up to high velocities creating a high level of angular momentum. At some point they saturate (reach their limit of speed) and require some means to dump their momentum. For low earth orbit satellites this can sometimes be done by using atmospheric drag or the Earth's magnetic field, but sometimes fuel must be consumed by the reaction control system (thrusters) to dump the angular momentum. Once momentum is lowered the wheels can be used again.

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  • $\begingroup$ I should add that reaction wheels and CMG's are the same in that they both store angular momentum, but they differ in how torque is extracted. The spin axes of reaction wheels are fixed relative to the spacecraft and torque is derived by the relative speeds at which the wheels are commanded to spin. CMG's in simple terms remain at a high spin rate and their axes are gimballed - so other actuators are actually pushing against the angular momentum. These differences make CMG's the choice where faster and larger torque reactions are required. $\endgroup$
    – docscience
    Apr 14, 2016 at 14:00

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