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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.
Some methods for unloading CMG angular momentum include the use of magnetic torques, reaction thrusters, and gravity gradient torque. For the space station, the gravity gradient torque approach is preferred [ citation needed ] because it requires no consumables or external hardware and because the gravity-gradient torque on the ISS can be very ...
There are also eight Monopropellant Rocket Engines (MRE-1), so called because they use only hydrazine as fuel. They are used for attitude control and momentum unloading of the reaction wheels. [2] [16] JWST has six reaction wheels for attitude control, spinning wheels that allow the orientation to be changed without using propellant to change ...
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.
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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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