Satellite Reaction Wheels

This video uses reaction wheels developed by the University of Bristol and the CubeSat ESAT developed by Theia Space. Reaction wheels are a kind of flywheel used for the attitude control and stability of a spacecraft. By adding or removing energy from the flywheel, torque is applied to single axis of a spacecraft, causing it to react by rotating. The attitude is controlled by the change in speed. By maintaining file rotation, called momentum, a single axis of a spacecraft is stabilised, Their operation is based on Newton’s third law that you may all know, that for every action there is equal and opposite reaction. So if the reaction wheel is accelerated counterclockwise, it creates an internal torque on the spacecraft clockwise. This is the University of Bristol air bearing test bed with a simple reaction wheel. It has been instructed to move the system by a hundred and eighty degrees. It can be seen that the system passes 180 degrees and the reaction will compensate that by spinning in a different direction. Reaction wheels can provide high pointing accuracy and are very useful in cases when a spacecraft needs to be rotated by very small amounts. For example, they can keep the telescope pointing at a distant exoplanet. However reaction wheels can rotate a spacecraft only around the centre of mass. They are not capable of moving the spacecraft from one place to another. Usually three or more reaction wheels are used on a spacecraft to provide full 3-axis attitude control and stability, however, the ESAT has only one reaction wheel for demonstration. If it fails the mission can fail too. Another disadvantage of a reaction wheel is that over time, reaction wheels may build up enough stored momentum to exceed maximum speed of a wheel, called saturation, which will need to be cancelled. That’s why reaction wheel systems are supplemented with attitude control mechanism such as magnetorquers


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