ALBERT

Description: Particle-based kinetic simulations of steady and unsteady hydrazine chemical rocket plumes are presented in a study of plume interactions with the ambient magnetosphere in geostationary Earth orbit (GEO). The hydrazine chemical rocket plume expands into a near-vacuum plasma environment, requiring the use of a combined direct simulation Monte Carlo/Particle-in-Cell methodology for the rarefied plasma conditions. Detailed total and differential cross sections are employed to characterize the charge exchange reactions between the neutral hydrazine plume mixture and the ambient hydrogen ions, and ion production is also modeled for photoionization processes. These ionization processes lead to an increase in local plasma density surrounding the spacecraft owing to a partial ionization of the relatively high-density hydrazine plume. Results from the steady plume simulations indicate that the formation of the hydrazine ion plume are driven by several competing mechanisms, including (i) local depletion and (ii) replenishing of ambient H + ions by charge exchange and thermal motion of 1 keV H + from the ambient reservoir, respectively, and (iii) photoionization processes. The self-consistent electrostatic field forces and the geostationary magnetic field have only a small influence on the dynamics of the ion plume. The unsteady plume simulations show a variation in neutral and ion plume dissipation times consistent with the variation in relative diffusion rates of the chemical species, with full H 2 dissipation (below the ambient number density levels) approximately 33 seconds after a two-second thruster burn.

Description: [1]  Detailed direct simulation Monte Carlo/Particle in Cell simulations involving the interaction of spacecraft thruster plumes with the rarefied ambient ionosphere are presented for steady thruster firings in Low Earth Orbit (LEO). A nominal mass flow rate is used to prescribe the rocket exit conditions of a neutral propellant species for use in the simulations. The charge exchange interactions of the steady plume with the rarefied ionosphere are modeled using a direct simulation Monte Carlo/Particle in Cell methodology, allowing for a detailed assessment of non-equilibrium collisional and plasma-related phenomena relevant for these conditions. Results are presented for both ram- and wake-flow configurations, in which the thrusters are firing into (ram) or in the direction of (wake) the free stream ionosphere flow in LEO. The influence of the Earth's magnetic field on the development of the ion plume is also examined for three different field strengths: two limiting cases in which B  → 0 and B  →  ∞ , and the LEO case in which B  = 0.5 Gs. The magnetic field is found to have a substantial impact on the resulting neutral and ion plumes, and the gyroscopic motion of the magnetized ions results in a broadening of the ion energy distribution functions. The magnetic field model also incorporates a cross-field diffusion mechanism which is shown to increase the current density sampled far from the thruster.

Description: This work employs in situ measurement data and constructive simulations to examine the underlying physical mechanisms that drive spacecraft plume interactions with the space environment in low Earth orbit. The study centers on observations ofthe enhanced flux of plasma generated during a maneuver of Space Shuttle Endeavour as part of the Sensor Test for Orion Relative Navigation Risk Mitigation (STORRM) experiment in May 2011. The Canary electrostatic analyzer (ESA) instrument mounted on the port-side truss of the International Space Station (ISS) indicated an elevated ion current during the shuttle maneuver. The apparent source of enhanced ion current is a result of interaction of the spacecraft thruster plume with the rarefied ambient ionosphere, which generates regions of relatively high-density plasma through charge exchange between the neutral plume and ambient ions. To reconstruct this event, unsteady simulation data were generated using a combined direct simulation Monte Carlo/Particle in Cell methodology, which employed detailed charge-exchange cross section data and a magnetic field model. The simulation provides local plasma characteristics at the ESA sensor location, and a sensor model is subsequently used to transform the local properties into a prediction of measured ion current. The predicted and observed total current are presented as a function of time over a 30 second period of pulsed thruster firings. A strong correlation is observed in the temporal characteristics of the simulated and measured total current, and good agreement is also achieved in the total current predicted by the model. These results support conclusions that: (1) the enhanced flux of plasma observed by the ESA instrument is associated with Space Shuttle thruster firings, and (2) the simulation model captures the essential features of the plume interactions based on the observation data.