Thruster Plume Research for Space Science and Exploration

Abstract

Many spacecraft rely on chemical propulsion systems for active attitude and orbit control or for landing operations on a planetary surface. With spacecraft and mission designers focused on performance parameters such as thrust and propellant efficiency, the typically undesired effects of thruster plume interaction with the immediate spacecraft environment are often overlooked. While rendezvous and docking maneuvers, or powered descent and ascent, present rather obvious impingement scenarios, plumes of chemical thrusters operated in space invariably also interact with adjacent spacecraft surfaces, as the ambient vacuum allows them to expand well upstream of the nozzle exit plane. Thruster plumes are thus a potential source of parasitic forces, moments, heat loads, and particularly of contamination and surface erosion. Plume contaminants may be gaseous, liquid, or solid and have been demonstrated to severely degrade functional surfaces on spacecraft, affecting power and thermal budgets, as well as scientific payloads and mission design. As mission science objectives and evolving scientific instrumentation put ever more challenging constraints on contamination control, potentially sensitive missions must thus conduct a careful plume contamination analysis already in the design phase. In this talk, we briefly state the recurring questions in plume impingement analysis and give an overview of the limited data available from both ground-based chamber tests and on-orbit flight experiments. Despite the maturity current hydrazine-based chemical propulsion systems have attained over the more than sixty years of space exploration and exploitation, many plume impingement mechanisms remain poorly understood. We address the challenges associated particularly with ground-based testing and numerical simulation of induced contamination from state-of-the-art hypergolic hydrazine thrusters. By example of a recent test campaign at DLR in support of European and US space missions, we demonstrate the importance of the concept of "contamination potential", and present a powerful technique to characterize this in situ. While hydrazine and its derivatives remain important propellants for in-space propulsion, special mission scenarios might make alternative propellants and novel production techniques gain greater interest. The talk will thus touch upon foreseeable trends and associated challenges in plume impingement analysis and assessment.