Thermal Propellant Gauging - Enhancing Spacecraft Propellant Management for the EDRS-C Satellite

Kurzfassung

During a geostationary satellite mission, one of the most crucial factors is the knowledge of the remaining propellant in the satellite’s tanks to ensure its maximum lifespan. Continuous station-keeping maneuvers (SKMs) are necessary to stay within their designated control box. Using principles of heat transfer and thermal dynamics, the Thermal Propellant Gauging Technique (TPGT) describes an alternative technical approach to measure the remaining propellant in comparison to the more conventional bookkeeping method. By applying controlled heat to a propellant tank using existing heater lines, monitoring and recording the resulting temperature changes with precise sensors, TPGT is not only almost independent of mechanical components but also a non-intrusive method ensuring that there is no contamination or isruption of the propellant. These properties make TPGT highly advantageous for a mission in space. Different thermal responses of fluids and gases constitute the fundamental physical concepts of TPGT. When heating the components of the tank, the rate of temperature hange varies depending on the thermal properties of these materials. The remaining liquid propellant has a higher thermal mass and conductivity than the ullage gas and therefore reacts differently to the heating process. Thermal gradients and temperature evolution are analyzed over time and the results are compared with thermal models of the tank. Considering the material properties and geometry of the tank, as well as the thermodynamic behavior of the propellant, the remaining amount of fuel can be determined. With decreasing propellant masses towards the end of the life of a satellite, TPGT demonstrates superior precision compared to the conventional bookkeeping method to keep track of used propellant. To put a theoretical method into context with real-life missions, the European Data Relay System (EDRS) serves as an illustrative case study. EDRS, alternatively known as SpaceDataHighway, is a network of geostationary satellites designed to provide fast real-time data transmission from Earth observation satellites in low Earth orbit to ground stations. The SpaceDataHighway is a public–private partnership between the European Space Agency (ESA) and Airbus, with the laser terminals developed by Tesat-Spacecom and the German Space Administration Deutsches Zentrum fur Luft- und Raumfahrt (DLR). Airbus, which owns and operates the system, provides commercial services for the SpaceDataHighway, while the execution of EDRS flight operations are conducted by DLR’s German Space Operations Center (GSOC) in Oberpfaffenhofen. Consequently, to prolong the lifespan of the EDRS mission and to ensure the continuous and reliable data relay, the TPGT method is used in combination with other proven techniques to manage the remaining propellant. This paper first gives an overview of the Thermal Propellant Gauging Technique within a physical context, followed by an implementation of the method using the EDRS-C mission as an example. In addition, it explores the different techniques for tracking the remaining propellant employed by the mission and compares the results between different phases of the mission. Concluding, the paper discusses potential approaches for automating this process as the satellite approaches the end of its operational life, positioning TPGT as the primary source of managing the remaining propellant.