Auslegung und Design eines Berückungssystems für ein kryogenes Tanksystem auf einem CompactSat Forschungssatelliten

Kurzfassung

In order to enable human space flight to the moon and beyond the use of high thrust chemical engines with propellant stored at cryogenic temperatures is a promising approach. A challenge when trying to store cryogenic propellant is the duration of such space missions, as undesired heating and the associated fluid boil-off can lead to considerable propellant losses. In order to close the knowledge gap between simulation and real behavior of cryogenic fluid in micro-g, the FROST mission currently planned at DLR will investigate the long-term storage capability of liquid nitrogen in a low earth orbit. Two subsystems are planned to pressurise the main storage tank on board. A helium pressurisation system and an autogenous system, which uses evaporated main fluid for tank pressurization. The development of concepts for the latter and an analysis of those concepts regarding mission critical requirements are the main tasks of this thesis. Additionally, the feasibility of the developed concept and possible advantages or challenges are discussed on the background of FROST. The autogenous pressurization unit (APU) is meant to be decoupled from any other subassemblies and is designed around a separate buffer tank to store pressurant. The APU is required to generate and store sufficient pressurant firstly, for storage tank- and experiment tank-pressurization, and secondly, for the operation of cold-gas thrusters once in orbit. Based on basic thermodynamic equation 4 approaches to an APU have been developed. A heater-based approach, a compressor-based approach, a combined-approach and an external approach. Based on these approaches multiple concepts are derived and necessary assumptions and equations defined. The concepts have been modelled analytically and a custom code written in Python is used to calculate relevant process values, such as temperatures, pressures, masses, energy requirements and possible pressurization cycles. Calculations have shown, that the pressurant inlet temperature has a significant influence on the amount of pressurant required for a specific pressure rise inside the target tank. Therefore, heater-based approaches overall achieved the best results regarding pressurisation cycles and maximum pressure evolution. According to the calculations, the Combined-Cryo-Concept has the highest energy efficiency regarding energy per cycle. It uses a combination of a micro pump, to pressurize the buffer tank, and an electric heater, to subsequently heat up the pressurant inside the buffer tank. Overall, autogenous systems that evaporate propellant onboard showed to be more complex compared to the common method of carrying an additional external pressure tank. For FROST the development of such an autogenous system may exceed the resources planned for this subsystem. Nevertheless, the development of reliable, efficient and lightweight autogenous pressurisation systems is a major task in the development of future space technologies, such as in-orbit fuel depots.