Experimental Study and Test Preparation for New Generation MEMS Vaporising Liquid Microthrusters

Abstract

With the price of launching satellites remaining high, many companies and universities are turning towards smaller satellites as more affordable options. This requires the miniaturisation of their different components, including the propulsion subsystem. At TU Delft, Vaporising Liquid Microthrusters (VLM) are being developed, with the latest iteration being a new generation Micro-Electromechanical System (MEMS) design. In this work, the updated design of this microthruster is remanufactured and prepared for future operational validation, heat transfer and instabilities studies. This is done by simplifying the manufacturing process, creating a testing interface, and performing measurements and extensive experimental characterisation of the thruster and test set-up. The newly-created interface allows for the study of heat transfer efficiency and convection heat transfer coefficient (HTC) from the silicon wall to the propellant, as well as multiple ways to study the instabilities in the system. One interface + thruster assembly was shown to survive to at least 180 °C temperature and 6-7 bar relative pressure. On the manufacturing side, the fabrication process was streamlined by reducing the number of steps by 13%, most significantly by the use of a soft photolithography mask instead of a hard silicon dioxide one. The deviations in horizontal dimensions are in the same order of magnitude as the ones obtained by the previous student working on this project, and lower than other previous attempts at the Faculty of Aerospace Engineering. The etching process encountered some difficulties, leading to inconsistencies in the feature depth between wafers. Across wafers, etching rate deviations as large as 8% were observed. Within the final production wafer, the range of deviation from the design values on throat depth was from +3.5% to −16.5%, showing a measurable higher etching rate at the edges of the wafer than in the centre. Recommendations were given on which features’ depths are most important to be measured, in order to increase the confidence in the predicted values. With these results, future researchers are ready to use the produced thrusters and testing interfaces. It is recommended to continue with the experimental studies, especially by measuring the heat transfer efficiency, thrust levels, HTC, and by studying any instabilities that could appear.