Characterization and Testing of a MEMS-Vaporizing Liquid Microthruster for Small Satellite Propulsion

Active propulsion is required in order to further develop the capabilities of small satellites like CubeSats. At TU Delft a micropropulsion system is being developed based on a Vaporizing Liquid Microthruster (VLM), using liquid water as propellant. This research presents the manufacturing, characterization and testing of an existing VLM-design. It focuses on the thruster component of the propulsion system, which is called the MEMS-VLM. The breadboard model consists of an inlet module, multiple heating chamber modules and a nozzle, integrally manufactured using MEMS-technology. The heating chamber modules consist of flow channels etched in silicon, of which the walls support silicon carbide heaters. These heaters are suspended in the center of the cross-section. The nozzle is a two-dimensional convergent-divergent duct. Characterization and testing of the MEMS-VLMis done in a bottom-up approach. First, the geometry and electrical properties of the flow channels and heaters are characterized, after which the performance of the heating chamber and the nozzle are independently studied by models and experiments. In order to facilitate manufacturing, minor changes are implemented to the design of the nozzle, electrical interface and fluidic interface. With these changes, successful manufacturing of theMEMS-VLM is achieved. Microscope images show significant but acceptable deviations between design dimensions and manufacturing results. Furthermore, resistance measurements of the heaters indicate partial mechanical failures in 20 out of 154 measured heaters. The thermal behavior of the heating chamber is studied by means of analytical models for conduction and radiation and a one-dimensional steady state model for convection. The conduction study shows that the chamber wall temperature is homogeneous, while the suspended heaters formhot spots. The convection model is used to study whether sufficient heat transfer can be expected to fully vaporize the propellant flow. It is found that at a chamber wall temperature as little as 10 K above the boiling point, full vaporization can be expected. An experimental setup is made to test the heating chamber, ultimately attempting to validate the design and the performance models. The ability to power the heaters with constant current and measure the mass flow, pressure and dissipated power is demonstrated. Furthermore, amethod to determine the local chamber temperature based on the resistance of the heaters is presented. However, current leaks and partial mechanical failures are detected in the heaters during tests with nitrogen gas, which compromise the ability to operate with liquid water. Hence, full validation of the heating chamber models could not be achieved. With respect to future designs it is recommended to change to metallic heaters, which are placed outside the chamber. The silicon chamber wall then serves as a heat exchanger. It is furthermore recommended to change the capping wafer material to glass, so that the boiling process can be studied optically. A second experimental campaign is done to determine the performance of the nozzle, which is described by the discharge factor CD, the Isp-efficiency ´I sp and the thrust coefficient CF . Nozzle performance tests are carried out in vacuum, using gaseous nitrogen as propellant. The pressure is adapted in order to match the Reynolds number in the nozzle throat to the designed operating conditions. At a measurement Reynolds numbers of Rem ¼ 1400 the performance quality factors are found to be CD ¼ 0.68§0.06 (discharge factor), ´I sp ¼ 0.3§0.05 (Isp-efficiency) and CF ¼ 0.33§0.06 (thrust coefficient). The poor performance is primarily explained by excessive friction and rarefaction losses in the divergent section of the nozzle and are expected to improve when increasing the Reynolds number and when decreasing the expansion ratio. Design validation of the MEMS-VLM could not be achieved, as on various aspects the design is rejected. Nonetheless, the experimental approach has been successful in revealing critical design recommendations and demonstrating the ability to test a microthruster, improving the understanding and advancing the development of micropropulsion at TU Delft.Aerospace EngineeringSpace Engineerin