Optimization of Electrospray Thruster Configuration and Control Allocation for Spacecraft Attitude Control

The space economy is rapidly changing due to the establishment of small satellites, which provide an affordable access to space. Small satellites, built on the advances in miniaturization of many critical technologies, can be used for remote sensing, scientific measurements, and communications applications. Among the above-mentioned critical technologies, it is possible to find propulsion, which is being miniaturized into efficient and reliable systems. This introduces the possibility of using thrusters as attitude actuators, replacing, or complementing the currently available systems, such as reaction wheels or magnetorquers. Specifically, electric propulsion-based thrusters can play a prominent role in the feasibility of the mission due to their small dimensions, efficiency, and reliability. Electric thrusters like electrospray and Field Emission Electric Propulsion (FEEP) are able to provide extremely low torques to obtain a high pointing accuracy as proven for the LISA Pathfinder mission. In a combined use with reaction wheels, the thrusters could be employed for desaturation purposes as well, especially in cis-lunar or interplanetary missions, where magnetorquers could not be exploited. They could be also used for position control alone or in combination with a main thruster. In the latter case, it is possible to have a unified orbital and attitude control system with a unique propellant tank. If multiple tanks are present, the first to deplete will likely end the mission, making a single tank a more efficient configuration with reduced propellant margins. Controlling the attitude of a satellite is often a strict requirement that can lead to the success or to the failure of the mission. The optimal number of thrusters required to satisfy the needed pointing accuracy is difficult to establish since the problem is highly non-linear and subject to numerous disturbances. Electric propulsion can exert small torques on the satellite, hence, to obtain a sufficient control authority it might be necessary to use a large number of thrusters, which could be further increased if reliability is accounted for as well. In this scenario, a satellite is often an over-actuated system that needs a control allocation algorithm to activate the thrusters in an optimal way. The goal of this thesis is to provide an optimal thruster configuration for a selected satellite and for a specified mission and its various phases and pointing modes. The problem is posed as a multi-objective optimization, considering pointing error and fuel consumption as optimization objectives to be minimized. The aim is hence to find the number of thrusters, their position, and their orientation on the satellite that provide a high accuracy, a low fuel consumption or a compromise between the two factors. The methodology developed provides a concurrent optimization of the thruster configuration, solved through a stochastic optimization algorithm and control allocation, solved using convex optimization.