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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 ...
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 slew of 3-axis stabilized spacecraft is typically in closed loop control with thrusters or electrically-powered reaction wheels maintaining or altering the craft's attitude based on sensor measurements. A typical example is a space telescope that should be turned to observe a new celestial object.
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.
Ithaco has produced items for the field of satellite control since 1962, such as Earth sensors, reaction/momentum wheels, magnetometers and magnetic torquers. [4] In addition to over 100 U.S. satellites, equipment made by Ithaco flew on Japanese, Canadian, French, German, Spanish, Swedish, and Argentinean spacecraft.
Several methods of pragmatic spacecraft propulsion have been developed, each having its own drawbacks and advantages. Most satellites have simple reliable chemical thrusters (often monopropellant rockets) or resistojet rockets for orbital station-keeping, while a few use momentum wheels for attitude control.