Chapter 9 - Orbit and Attitude Actuators

Abstract

This chapter is concerned with force and torque actuators. Force actuators may be used by orbit and attitude control systems. To generate a torque, the force vector must not pass through the spacecraft center of mass. Typical force actuators are mass ejection devices (propulsion devices or thrusters), as they can change the inertial S/C linear momentum. The principle of thermodynamic and electric propulsion is outlined pointing out their difference in terms of their specific impulse (the ratio in [s] between the delivered thrust [N] and the required propellant flow rate measured in weight/s [N/s]). Because a propulsion assembly usually consists of a redundant set of thrusters, the problem of optimally distributing the required force and torque vectors to the assembly elements is mandatory. The distribution law depends on the assembly geometry, in other terms, on the thrust application point and the direction of each device. A pseudoinverse law that accounts for nonzero minimum thrust is derived. As an alternative to the pseudoinverse law, a linear programming solution is also outlined, as it aims to minimize the propellant consumption. The second part of the chapter is devoted to pure torque actuators, namely to the momentum exchange actuators that transfer angular momentum between themselves and spacecraft. Also, in this case, redundant configurations are mandatory because of the progressive degradation of the rotating mass supports. By focusing on fixed-axis reaction wheels, pseudoinverse and minimax distribution laws are derived. The latter solution aims to minimize either the peak torque or the peak angular momentum, which is in favor of a longer lifetime. An important issue is the reaction wheel microvibration analysis: dynamic models and simple simulated results are provided. The last actuator type of the chapter is the magnetic torquer, which being capable of changing the inertial S/C angular momentum, can be used either to off-load the saturated reaction wheel capacity or to keep their momentum far from saturation. A brief initial section is devoted to recall reliability principles and their key equations.