Reaction control wheels are used in spacecraft to control orientation in space or to correct small trajectory errors. Reaction wheels are basically just flywheels, and are normally aligned along three axes perpendicular to each other, so that each may absorb the angular momentum component about its axis.

During the flight of the spacecraft, conditions may arise where the reaction control wheels are spinning as fast as they can, and can no longer be used to absorb spacecraft momentum. This condition is called momentum saturation, and requires the spacecraft to perform a maneuver called a momentum dump or an angular momentum desaturation (AMD). This maneuver just means that the spacecraft uses its thrusters to cancel out the momentum in the reaction wheels. Force is applied to the wheels to slow them, and the rotation induced in the spacecraft is canceled out with the thrusters.

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