Design of a Level Sensing System for the Propellant Tank of a Microlauncher

Abstract: The present report aims to show the development process of a liquid level sensing system for the use within the propellant tanks of a launch vehicle. Whereas many recent launchers use discrete level sensing systems to indicate the amount of fuel on defined spots like fill-stop and engine cut-off, in this report a continuous capacitance based sensor probe is described. An accurate knowledge of the propellant level can serve as input for the throttling of the propellant valves to account for changes in the oxidizer-fuel ratio and therefore helps optimising the operation of the rocket engine. Furthermore, the more precise the amount of fuel is known, the less unused propellant mass will remain allowing a cost and mass optimisation for each mission. Additionally to the increased vehicle performance, the sensor designs described aim to have comparably low mass and cost. At the beginning of the report, an overview of different level measuring techniques is given before going into the special conditions and requirements regarding launch vehicles. Afterwards, the design and testing with RP-1 and LN2 of two different sensor probes using a capacitive measuring principle is described and compared to analytical calculations and numerical simulations using COMSOL Multiphysics. At the end, design suggestions for a flight probe and possible improvements for a higher reliability are given. All tested sensor designs show an accuracy of a few millimetres when tested within a settled, non-sloshing fluid. However, the theoretical models show a substantial deviation to the test data.