Analysis of freely expanding chemical thruster plumes in a high-vacuum test facility

Abstract

Plume induced contamination is a matter of concern for various applications such as functional spacecraft surfaces, spacecraft instrumentation or spacecraft docking and landing sites. In order to estimate if contamination takes place in the aforementioned scenarios and if so, whether it has the potential to alter surface properties, one needs to characterize the thruster plume and its interaction with surfaces of interest under space-like conditions. This work intends to give an overview of the unique analysis capabilities at DLR’s high-vacuum plume test facility Göttingen for chemical thrusters (STG-CT), backed by exemplary measurement data which are still scarce in this matter. The cryogenic facility STG-CT is enclosed by a copper shroud cooled to 4.2K using liquid helium and provides vacuum conditions that enable free plume expansion when operating small chemical reaction control thrusters in pulse mode. We apply a range of experiments in order to investigate spatially resolved plume properties. Pitot probe (providing momentum flux), heat flux and force transmission measurements cover different plume parameters, while further techniques serve to characterize the chemical plume composition and other potential plume constituents such as particles or droplets. A mass spectrometer detects the gas phase composition at a distance of three meters from the nozzle exit. Additionally, cryogenic quartz-crystal microbalances (CQCMs) can be distributed across the test chamber to acquire mass flux per pulse and back up mass spectrometry with thermogravimetric analysis (TGA) data. Furthermore, a high-speed camera observes the nozzle exit plane, supplying information on size, velocity and trajectories of solid or liquid plume ejecta. Ultimately, material samples can be exposed to the freely expanding plume for later analysis of a change in chemical, mechanical and optical properties. All measurements can be conducted at various angles from plume centerline since we are working with a rotatable thruster setup. These data serve mission design to assess the contamination potential resulting from thruster firings and ultimately enable the implementation of reasonable mitigation strategies such as tilting of a thruster, pulse mode variation or shielding mechanisms.