Modeling the plume contamination and emissions of an ammonia arcjet

Abstract

The direct simulation Monte Carlo method is employed to compute cold flows of ammonia for a large arcjet that is to be tested in space in the upcoming ESEX flight experiment. The nozzle flow computation indicates that the flow is almost in thermal equilibrium at the nozzle exit. A very large and expensive computation of the back flow region of the actual spacecraft geometry is performed to provide predictions of mass fluxes that will be measured in flight by quartz crystal micro-balances. It is indicated that contamination of the spacecraft occurs even in regions lying behind a plume shield. A further computation is performed to simulate the interaction of the arcjet plume with the ambient atmosphere. The high impact energy is offset by the very low atmospheric density at the spacecraft operational altitude of 833 km. Nevertheless, it is indicated that ammonia chemistry occurs and the primary products are NH, NH2) and OH. These species radiate strongly in the ultra-violet. The estimated emission intensities of the molecules are similar to those measured previously in situ by a lower velocity reentry experiment. An estimate is also made of the intensity of emission from ammonia. In all cases, it is concluded for the cold flow that none of the emissions will be detectable by the ground based observation facility that is part of the space experiment. Introduction Arcjets represent a mature electric propulsion technology that is replacing chemical propulsion engines for orbit maintenance procedures on many satellites. The U. S. Air Force has developed the Electric Propulsion Space Experiment (ESEX) to study several of the effects of using a very high power ammonia arcjet on a spacecraft. The ESEX experiment is part of the ARGOS flight that will orbit the Earth at an altitude of about 833 km. The flight experiment, due for launch in the winter of 1998, includes diagnostic measurements to analyze exhaust plume contamination and radiative heating effects as well as arcjet performance. Remote 1 Associate Professor. Mechanical and Aerospace Engineering. Senior Member AIAA. 2 Graduate Research Assistant. Mechanical and Aerospace Engineering. 3 Post-doctoral Research Associate. Mechanical and Aerospace Engineering. 4 Research Professor. Chemistry. Member AIAA 5 Research Scientist. Deceased. optical measurements are also planned from a ground station at Maui. Several arcjet operational modes will be investigated including the cold flow case where the arc is not ignited. The goal of this paper is to conduct numerical simulations of the cold flow of the arcjet. The primary motivation for these computations is to provide estimates of the contamination on the spacecraft by ammonia that will be measured by quartz crystal micro-balances. Hence, the focus of these computations will be the back flow region behind the thruster exit plane. A secondary focus of the study is to investigate the interaction of the ambient atmosphere with the ammonia jet exhausting from the thruster. In particular, the goal is to estimate the radiative emission generated under these conditions. The layout of the paper is as follows. First, the arcjet thruster and diagnostics of ESEX are described. Next, the computational approach is discussed. Due to the relatively low densities involved, the direct simulation Monte Carlo method (DSMC)2 is employed. Discussion of results is divided into separate sections concerning the back flow contamination studies and the radiative emission calculations.