Modeling the plume contamination and emissions of an ammonia arcjet

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.