Composition measurements in a freely expanding green propellant thruster plume

Kurzfassung

Spacecraft contamination due to reaction control thruster plume impingement is a subject of concern since the early days of space exploration, especially for long-term and manned missions, but also for spacecraft bearing sensitive instrumentation. Exhaust plume constituents of chemical propulsion systems are known to affect the properties and performance of functional spacecraft materials. In particular, they may alter the thermo-optical properties of thermal control surfaces, and mechanically or chemically erode protective or functional coatings. While the pertinent data gathered in space and on ground is sparse and hardly generalizable for the common hydrazine-based mono- and bipropellant propulsion systems, it is practically non-existent for either of the many alternative propellant / thruster combinations developed since about the mid 1990's in response to the imminent threat of a hydrazine ban under the European Regulation No 1907/2006 ("REACH"). This absence of data makes reliable plume contamination predictions during the design phase impossible, and potentially hinders the adoption of the novel propellant/thruster combinations for contamination sensitive missions. ESA has thus initiated a research activity in which the impact of plume impingement from a particular green propellant/thruster combination on properties of exposed representative spacecraft materials is evaluated in order to derive applicable input parameters for industry-standard numerical prediction software. This paper presents the results of experiments dedicated to determining the composition of the freely expanding plume from a green propellant thruster. Actual contamination prediction for spacecraft is inherently complex, as it depends not only on the source of contamination (here a reaction control thruster), but also on the properties of the impinged surface and the environment is subjected to. In this activity we study the plume of a 20N bipropellant propene/nitrous oxide reaction control thruster supplied by Dawn Aerospace. We seek to assess the contamination potential of the thruster by experimentally studying the thruster plume expansion, chemical composition and non-gaseous effluents. It is well known that the absence of an appreciable atmosphere in space allows for a wide expansion of the thruster plume, and it is necessary to replicate that high-vacuum environment in a ground test to obtain a distribution of the gaseous plume species. Measurements are thus conducted in the DLR high-vacuum plume test facility Göttingen for chemical thrusters (STG-CT), using a mass spectrometer to record the gas phase composition at three meters distance from the nozzle axis, at various angles from the plume axis. STG-CT's test section is almost completely enclosed by copper walls cooled to 4.2K using liquid helium, thus providing the pumping capacity required to maintain high vacuum during pulsed operation of the thruster. Non-gaseous effluents, i.e. droplets and particulates, are recorded with high-speed cameras, providing information on density, velocity and trajectories. The data experimentally obtained for the plume composition and expansion from an additively manufactured propene/nitrous oxide green propellant thruster is novel and unique, and provides engineers not only with contamination-relevant information to this particular thruster, but also gives an indication of the potential contamination behavior of green propellant thrusters that use carbon-based fuels. The impact of plume contamination on the properties of relevant surface coatings and materials is studied in a second phase of the activity.