Characterization of Spacecraft Thruster Plumes via Augmented View Factor Methods for Spacecraft Docking Interactions

Plumes of bipropellant spacecraft thrusters are typically complex hypervelocity rarefied flows in vacuum. In on-orbit use, thruster plumes are multiphase with dependence upon the proximity and orientation to the thruster nozzle exit. When simulating thruster firings, the fluid is treated as an aggregate of individual particles, or bundles. Thus, it is necessary to utilize Direct Simulation Monte Carlo (DSMC) methods to model the random interaction of colliding bundles in order to extract inter- and intra- plume energy and momentum transfer characteristics. However, the use of DSMC as a tool is slow, requiring extensive computational resources. In addition to the computational resources, there is also a significant burden associated with postprocessing and data interpretation. Furthermore, due to the computational load of what is simulated, several assumptions to reduce model complexity are made. When considering DSMC methods the following are typically forgone: (i) nucleation of solid particulate in the rarefied flow, (ii) effects of pulse mode thruster operation, (iii) thruster nozzle residue discharge and thermal barrier coating ablation, (iv) transients in static thruster firing, and (v) mechanical damage due to hypervelocity impacts (ejecta creation) on impingement surfaces.

As an alternative to DSMC analysis of rarefied flows, simplified methods in thruster plume-vehicle interactions which may occur during rendezvous, proximity operations and docking (RPOD) may be used with reduced computational burden. Such a simplified method may capitalize on the low-pressure rarified flow environment, enabling treatment of the flow as an ensemble of particles. As a result, an Augmented View Factor Method (AVFM) of the plume may be considered in which a view factor of an idealized thruster nozzle exit plane and a target geometry are calculated. Geometric dependency is then co-varied with a simplified characterization of number density (Nd [1/m3]), particle velocity relative to the nozzle exit in an axis-symmetric polar coordinate frame, and the temperature of the flow field. Such AVFMs give mission designers and engineers sufficient information to refine individual spacecraft design (e.g., thruster canting angles) and flush out specifics of mission concepts of operations (ConOps). Where necessary, more extensive DSMC tools may be then used at critical and/or edge cases which are rapidly identified by the AVFM methods.

The work showcased herein utilizes a AVFM tool developed which operates on MATLAB and utilizes easily updatable Stereo Lithography (.STL) files to act as the thruster nozzle exit and targets for plume analysis to facilitate development of NASA’s Deep Space Logistics (DSL) module or any orbital transfer vehicle (OTV) and RPOD at the Gateway Lunar Space Station. Several dependencies have been characterized to map specifics of a direct thruster plume, accounting for shadowing but not wake and reflections. The output from the AVFM tool enables the visualization of total pressure, exerted on facets of the .STL models loaded into the environment; by extension estimations of net forces and net torques on the .STL may be obtained. It is also possible to utilize mapped pressure fields as part of a two-way fluid-structure-interaction (FSI). A validation of the method against established DSMC tools, and several parametric analyses are presented for docking scenarios for upcoming Artemis Missions.