Question

Discuss the following actuators by describing: 1) how they work, 2) how they are useful to attitude control, and 3) their control performance (ability to provide pointing) relative to each other. reaction wheel, attitude control thruster.

Answer 1

Reaction wheel Actuator

1) They work by equipping the spacecraft with an electric motor attached to flywheel which, when its rotation speed is changed, causes the spacecraft to counter-rotate proportionately through conservation of angular momentum. Reaction wheels can rotate a spacecraft around its centre of mass but can't shift the moving spacecraft from one place to another.

2) It is sometimes operated as momentum wheel, by operating it at a constant rotation speed, in order to spread a satellite with a large amount of stored angular momentum. By doing so alters the spacecraft's rotational dynamics so that disturbance torque perpendicular to one axis of the satellite do not result in spacecraft angular motion about the same axis as the disturbance torque; instead, they result in angular motion of that spacecraft axis about a perpendicular axis. This has the effect of tending to stabilize that spacecraft axis to point in a nearly-fixed direction, allowing for a less-complicated attitude control system.

3) These Actuators provides a high pointing accuracy and are particularly useful when the spacecraft must be rotated by very small amounts.

Attitude Control Thruster

1) It consists of a combustion chamber and an expansion nozzle. Both fuel and oxidizer are introduced into to the combustion chamber, and they are ignited, and then the exhaust flows in the nozzle accelerating and exchanging linear momentum, producing thrust. To produce torque, they are grouped in a RCS, a d they are commanded coupled: two thrusters , installed at a certain distance from each other and firing in opposite direction, are activated simultaneously, producing a defined torque. If thrusters are throttable, amount of thrust can be regulated. If not throttable , thrust cannot be regulated.

2) They are used to control attitude by generating lower thrust levels than by the higher thrust levels as lower thrust levels provides high precision.

3) Control performance depends upon the type of thruster used. If thruster is not throttable then their performance will be lower than the throttable thruster. Other than throttable and not throttable, there are other types of thrusters such as Ion Thruster, Hall effect Thruster, Electrohydrodynamic thruster, Pulsed inductive thruster etc., their performances varies depending on how they work and constructed.