Vaporizing Liquid Micro-resistojet experimentation

Abstract

The Delfi program at the Delft University of Technology aims to develop nano satellites between 1 and 10 kg to provide high level education and to develop a platform for novel space technologies. Miniaturized satellites require miniaturized propulsion systems to provide orbital adjustments for functions like station keeping, extending mission duration and precise formation flying. Current research into micro-propulsion systems with aims to improve performance and efficiency is focused on Micro-electromechanical systems (MEMS), solar thermal and Commercial off-the-shelf (COTS) resistojet thrusters. The COTS design and prototype by Versteeg was the first leak resistant micro-thruster produced and successful experiments were performed using hot and cold nitrogen as propellant. The usage of liquid water as propellant for a Vaporizing Liquid Micro-resistojet (VLM) has been suggested as it is easy to store, has a high storage density, decent performance and is relatively safe to use. However as of writing this thesis, experimental tests using liquid water as propellant have been unsuccessful. In this thesis thrust tests with nitrogen have been repeated and show similar results to the data obtained by Versteeg verifying results of both experiments. In order for liquid water tests to be performed in the absence of a liquid mass flow sensor, a syringe pump was used to expel a constant volumetric flow. The liquid mass flow has been calculated by calibrating the syringe pump combined with the measured chamber pressure. Using liquid water, it was found that the chamber pressure increased over the course of the thrust experiment due to the feed system being volume based instead of driven by pressure. Since the test bench is dependent on the center of gravity of the thruster, thermal expansion significantly effects the measured thrust. Using nitrogen this effect can accurately be corrected for, while for water the effect is more difficult to correct due to the unpredictability of the liquid water remaining between the last propellant valve and the thruster chamber. This led to a decrease in accuracy of the measured thrust using liquid water compared to nitrogen, where the error grew from ∼1.5% to ∼13%. Thrust tests with water have been successfully performed at a chamber pressure of 1.02 bar and temperature of 300 °C for 15 minutes producing 8.0-8.3 mN of thrust with a specific impulse of 94-100 seconds with an accuracy of 14%. Several areas of improvement have been listed for both thruster design and experimental setup.