MEMS Micropropulsion

Design, Modeling and Control of Vaporizing Liquid Microthrusters

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In recent years, there has been an increase in the number of small multi-mission platforms such as CubeSats, in an attempt to reduce costs of space missions. CubeSats have been used for different purposes including Earth observation, research and technology demonstration.
However, a key technology that is still under development is the micropropulsion system that has the potential to significantly increase the capabilities of CubeSat missions. Micropropulsion has been recognized as one of the key development areas for the next generation of highly miniaturized spacecraft such as CubeSats and PocketQubes. It will extend the range of applications of this class of satellites to include missions that require, for example, orbital maneuvering or drag compensation.
An interesting option for CubeSats and PocketQubes is the Vaporizing Liquid Microthruster (VLM) which has received increasing attention due to its ability to provide high thrust levels with relatively low power consumption. The thruster uses the vapor generated in the vaporization of the propellant to produce thrust using a nozzle. The vaporization is usually done by applying power to resistive heaters that could be integrated into the device or externally attached to it. The nozzle is usually a convergent-divergent nozzle that can accelerate the propellant to supersonic velocities.
This thesis aims to develop modeling and control concepts for micropropulsion systems to allow the spacecraft to perform maneuvers of position and attitude control. The Vaporizing Liquid Microthruster has been selected due to its characteristics that suit the needs of very small spacecraft.
The first part of the research is dedicated to an in-depth literature study of the currently available micropropulsion systems. Those that are manufactured with silicon and MEMS (Micro Electro-Mechanical Systems) technologies have been analyzed and compared in terms of their thrust, specific impulse, and power. A classification in terms of complexity is introduced in an attempt to identify the suitability of the devices for the current trend towards simplifying architectures. The analysis of development levels of different types of micropropulsion systems revealed that although the actual thrusters are significantly developed, the interfacing and integration to other components of the system are still to be further developed.
The second part of the research focuses on the characterization and modeling of VLM systems. This is an extremely important step in the development of such systems since a proper model, i.e., one that sufficiently represents the dynamics of the system, is required during the design phase to help, for example, in designing controllers, and also during the operational phase to help reproducing the events happening when the satellite is in orbit. A comprehensive model has been developed using theoretical and empirical relations.
The third part of the research addresses the problem of controlling multiple redundant devices allowing failures to occur. This is very important to guarantee the successful operation of VLM systems with many thrusters while performing combined attitude-position maneuvers. A fuzzy control system was developed introducing an automatic rule generation algorithm that allows the fuzzy controller to solve control allocation problems.
Finally, the last part of the research investigates the possible applications of VLM systems. An example scenario is considered to analyze the performance required to execute different maneuvers and missions.
The key contributions of the work presented in this thesis are related to the modeling and control of Vaporizing Liquid Microthrusters. A comprehensive model of the complete system has been proposed and used to develop control algorithms for individual thrusters and for a set of thrusters. A fuzzy control system has been developed to solve the problem of controlling multiple devices with redundant outputs. Finally, an in-depth literature study and an analysis on the possible applications allowed to put VLM systems into perspective offering a glimpse into the future development of such systems.