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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.
The CMGs will eventually saturate (accumulating angular momentum to the point where they can accumulate no more), resulting in loss of effectiveness of the CMG array for control. Some kind of angular momentum management scheme (MMS) is necessary to allow the CMGs to hold a desired attitude and at the same time prevent CMG saturation.
However, momentum storage devices have a limited capacity, and that capacity soon becomes saturated when they are required to absorb spacecraft disturbance torques caused by (gravity gradient, solar wind, and aerodynamic drag); when in other words they reach their momentum storage limit. Once saturation is reached, momentum storage devices ...
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.
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.
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Scale analysis is an effective shortcut for obtaining approximate solutions to equations often too complicated to solve exactly. The object of scale analysis is to use the basic principles of convective heat transfer to produce order-of-magnitude estimates for the quantities of interest.