A Capillary-fed Evaporative Microthruster for Micro/Nano Satellites

Abstract

Small satellites offer a unique platform for short-term and low-cost communications and surveillance missions. They are essential for, scientific research, and technology demonstration due to their ability to provide rapid deployment and costeffective solutions. The motivation to develop these small satellites stems from the need to reduce launch and mission expenses while maintaining high performance and reliability. Despite their potential, small satellites face significant challenges, particularly in propulsion, due to their compact size, weight limitations, and restricted power availability. Addressing these challenges is crucial to unlocking their full potential and expanding their range of applications in the space industry. This thesis explores the development, fabrication, and experimental characterization, along with thermodynamic and fluid dynamic analysis, of a novel capillary-fed evaporative microthruster. Initially the first generations of the microthruster were developed using microelectromechanical systems (MEMS) based silicon devices. This MEMS-based vaporizing liquid microthruster utilizes microtextured silicon substrates for passive feeding of propellant (water) via capillary force and subsequent thin film evaporation through localized heating. The generated vapor flows through a converging nozzle to produce thrust. The first part of the thesis details the microfabrication techniques employed in creating various generations of silicon devices, their experimental characterization, and the performance estimation of the microthruster using a developed numerical model. The generation-1 silicon device exhibited undesired wicking of water to the exit of the nozzle, leading to unwanted ice formation at the nozzle exit. In the generation-2 and generation-3 silicon devices, a dedicated vapor flow path above the microtextured surface was introduced, which only partially alleviated the undesired wicking issue. This prompted a redesign and fabrication of an acrylic material-based macroscale device, which eliminated all the issues associated with microfabrication of silicon-based devices and undesired wicking. A numerical model is developed for estimating the performance of a MEMS based vaporizing liquid microthruster. It couples the evaporation characteristics at the liquidvapor interface with 3D nozzle flow dynamics. The evaporation phenomenon is captured using the kinetic theory of gases, while the nozzle flow is analyzed considering compressible-slip flow through the converging nozzle. The model is utilized for estimating the performance of a generation-2 device (with a dedicated vapor gap). The results show that the proposed silicon-based generation-2 microthruster can generate the thrust of ~ 60 µN and the ISP of ~67 seconds with a power input of ~ 3W. When compared to quasi-onedimensional isentropic values, the thrust and specific impulse efficiencies are determined to range between 12% - 40% and 55% - 92% for respectively, for power inputs ranging from 0.2W - 3W. The second part of the thesis provides the design, fabrication, and experimental characterization of a scaled-up microthruster. The microthruster consists of a reservoir, evaporation chamber, and a capillary wick. The capillary wick enables passive transport of the propellant (deionized water) from the reservoir to the evaporation chamber, where the water evaporates upon the application of electric heating. The generated vapor is expelled vi through a nozzle in the evaporation chamber to generate thrust. Pressure and temperature measurements in the evaporation chamber along with side-view imaging, facilitate realtime monitoring to understand the device behavior at different input power under vacuum conditions (~ 50 Pa) of the ambient. We present the relevant design parameters of the microthruster device to mitigate issues such as two-phase boiling, water ejection or ice formation, which are detrimental to the performance of vaporizing liquid microthrusters. We show that the evaporative microthruster can achieve thrust ranging from approximately 200 µN to 820 µN, with a specific impulse of ~ 100 seconds, for power input of 0 W - 3 W. The last part of this work focuses on the design, fabrication, and calibration of a torsional pendulum-type thrust stand. The calibration results reveal that the thrust stand can measure thrusts ranging from 30 µN to 340 µN. The thrust stand's measurement capability can be easily adjusted by changing the length of the cross-arm, making it suitable for a range of thrust measurements.