Reaction wheel

A reaction wheel is an actuator for position control of a satellite. It is usually composed of a motor, a flywheel, the electronic control unit and some small parts. The reaction wheel takes a torque to rotate the satellite on the same axis but in the opposite direction. The total angular momentum of the satellite system remains constant, as opposed to engines or solenoids that change the twist of the system. Figuratively speaking, that is, the angular momentum is only inserted between the housing and satellite reaction wheel back and forth ( conservation of angular momentum ).

Reaction wheels are not to be confused with spin wheels:

• A momentum wheel is always running at a high speed, thereby generating a stabilizing spin, ie, the response to disturbance torque is transverse to the rotational axis of the momentum wheel minimized. Therefore, there is typically only one momentum wheel (possibly two redundant ) per satellite. Momentum wheels are used for example in classical spin-stabilized GEO satellites.
• A reaction wheel is normally at a standstill and is only brought to the change in position of the satellite or to compensate for external disturbance torques to an appropriate speed. If after several position changes, the maximum speed is reached, the reaction wheels must drallentsättigt (stopped) are. Solenoids or attitude control nozzles bring this to an external torque which acts opposite to the moving down reaction wheels. This ensures that the satellite is in spite of the speed change its defined orientation in space and does not pass into retains unwanted wobbling motion. Thereafter, the reaction wheel is back for position changes available.

In space there two sets of reaction wheels are mainly used. In an axis is decoupled system each have a reaction wheel is used in any principal geometrical axis of the satellite, for a total of three. In order to achieve a redundancy, reaction wheels are increasingly installed in tetrahedral coordination in the recent years. This has the advantage that the system still remains fully functional in case of failure of a wheel. Disadvantage is the coupling of the individual wheels to one another, the rotation around a geometric axis of the satellite is always the change in the rotational speed of a plurality of reaction wheels result.

An example of the three-axis arrangement is the LAPAN TUBSAT and for the tetrahedral arrangement of the BIRD satellite.

Characteristics

A typical reaction wheel for a medium-sized commercial satellite has a diameter of 20 to 30 cm, a height of about 10 cm and a total mass of 5 to 10 kg. At a speed of about 5000 rpm it generates an angular momentum of 20 Nms. The upper limit for the torque is typically 0.2 to 0.5 Nm.

The angular momentum is related to the angular velocity and the moment of inertia:

Applies to the torque ( the angular acceleration):

The total angular momentum is conserved. A change in the angular momentum of the Reaktionsrads by varying causes an opposite rotation of the satellite around the rotation axis of the flywheel. The torque determines how quickly changes the rotation and a satellite can be tilted. Through minor changes of the rotation of the orientation of the satellite Reaktionsrads can thus be very accurately controlled.

• Aerospace Engineering
• Gyroscope