Modelling of Cryopumps for Space Electric Propulsion Usage

Abstract

Telecommunication satellites and also planetary exploration spacecraft use more and more electric propulsion systems for maneuvering in space. The main advantage of these propulsion systems is their low fuel consumption and with that higher v capabilities. Space systems and propulsion units have to fulfill extensive on-ground testing. As EP systems usually have low thrust and longer run times compared to chemical propulsion ground testing times are further extended. EP qualification has to be performed in dedicated vacuum chambers. The German Aerospace Center DLR operates the electric propulsion vacuum test facility STG-ET in Goettingen. EP testing requires powerful pumping systems for keeping pressure low enough. At STG-ET cryogenic pumps are used for these tasks, which use cryogenic pumps that freeze gases on cold surfaces inside the vacuum chamber. The detailed processes on cold pumping surfaces are very complex and depend on optical and mechanical properties of ice layers composed of several species. Furthermore, sputtering caused by exhaust plume ions modifies surface and deposits on a cold pump plate. This paper presents some basics and numerical modeling of the cryogenic processes during cooldown, operation, and recycling of the pumps. This may help for optimizing the usage of cryopumps for EP facility operation.