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A small reaction wheel viewed in profile A momentum/reaction wheel comprising part of a high-accuracy Conical Earth Sensor to maintain a satellite's precise attitude. A reaction wheel (RW) is an electric motor attached to a flywheel, which, when its rotation speed is changed, causes a counter-rotation proportionately through conservation of angular momentum. [1]
Momentum wheels are generally suspended on magnetic bearings to avoid bearing friction and breakdown problems. [5] Spacecraft Reaction wheels often use mechanical ball bearings. To maintain orientation in three dimensional space a minimum of three reaction wheels must be used, [ 6 ] with additional units providing single failure protection.
CMGs differ from reaction wheels. The latter apply torque simply by changing rotor spin speed, but the former tilt the rotor's spin axis without necessarily changing its spin speed. CMGs are also far more power efficient. For a few hundred watts and about 100 kg of mass, large CMGs have produced thousands of newton meters of torque.
Spacecraft rotational operations, such as turning to point in a new direction, are usually performed by angular momentum storage devices such as reaction wheels or control moment gyroscopes. It is generally preferable to use these devices instead of traditional thrusters , as they are powered by renewable electricity instead of by propellant ...
Reaction control systems are capable of providing small amounts of thrust in any desired direction or combination of directions. An RCS is also capable of providing torque to allow control of rotation (roll, pitch, and yaw). [2] Reaction control systems often use combinations of large and small thrusters, to allow different levels of response.
The speed is sometimes stabilised to prevent unwanted torque reaction. The internal friction losses are minimised by design. The momentum wheel(s) on a spacecraft is used in conjunction with reaction wheels. A set of momentum wheels 'translates' applied torque into a programmed direction. A momentum wheel can be configured as a CW or CCW unit.
The outer iterations comprise two steps: Solve the momentum equation for a provisional velocity based on the velocity and pressure of the previous outer loop. Plug the new newly obtained velocity into the continuity equation to obtain a correction.
A space vehicle's flight is determined by application of Newton's second law of motion: =, where F is the vector sum of all forces exerted on the vehicle, m is its current mass, and a is the acceleration vector, the instantaneous rate of change of velocity (v), which in turn is the instantaneous rate of change of displacement.