This paper describes a MATLAB-based molecular transport model developed for modeling contamination of spacecraft
and optical instruments in space. The model adopts the Gebhart inverse-matrix theory for thermal radiation to analyze
mass (molecular) transfer due to direct and reflected flux processes by balancing the mass fluxes instead of heat fluxes
among surfaces with prescribed boundary conditions (contamination sticking fractions). The model can easily input view
factor results from current thermal tools as well as measured outgassing data from ASTM E 1559 tests or vacuum bake-outs
of flight components. Application examples of a geosynchronous satellite and an optical telescope are given to
demonstrate versatile applications of the developed model.
This paper describes two mathematical purge models, transient and steady-state, developed for investigation of purging
critical space systems with stringent humidity requirements. The developed single-cell purge model correlates well with
measured data from a purge-test engineering model. The validated purge models are being used to support various purging activities/plans associated with spacecraft/payload integration and test and spacecraft/launch vehicle integration. This paper also includes a dew-point analysis to address water-vapor condensation concern for purging critical space systems.
This paper describes a numerical model developed recently using MATLAB<sup>®</sup> for performing surface particle coverage
calculations. The model uses a multi-bin particle size distribution model with incorporation of Barengoltz's areal
density integration method and Raab's particle shape factor, a similar approach employed previously by Ma, Fong and
Lee at Lockheed Martin Space Systems Company (Sunnyvale). The developed model is a versatile and quick
turnaround tool and can easily account for variable particle size bins, variable shape factors or aspect ratios for various
size bins, and variable slopes (w.r.t. the IEST-STD-CC1246 slope) for different size bins. Model predictions compare
well with image analysis measurements of particle fallout data from various spacecraft cleanrooms and test
environments. Moreover, this study recommends using a standard equation to correlate particle area coverage with
IEST-STD-CC1246 levels (particles modeled as a cylinder with hemispherical ends) and applying a wide range of
conversion factors for accurately calculating particle area coverage for variable slopes for different particle size bins.
This paper examines outgassing kinetics in the air and purging flow environments, as well as including a comparison to full outgassing in vacuum. The experimental data and models reported in the literature on the material outgassing in those environments are examined and summarized. The models are used to examine evaporation of water and DC 200 silicone fluid and outgassing of representative spacecraft materials in the normal air, purging flow and vacuum environments. The models can be used to estimate the outgassing rates of materials under various conditions in those environments, by knowing the outgassing rate in one of those environments. The implication of study results to spacecraft contamination control is also addressed.
For contamination effects on thermal control surfaces, changes in solar absorptance are the effect noted. Emittance of the surface is not normally affected. The SIRTF (Space InfraRed Telescope Facility) and NGST (Next Generation Space Telescope) spacecraft will fly large low emissivity surfaces (e.g. aluminized Kapton shields and gold mirrors). During the orbital missions, these surfaces will not be exposed to the sun and will be at temperatures less than 150 K. Concern is that a thick molecular film, even water, will cause a change in emittance and results in affecting the thermal performance primarily controlled by emittance alone. Although an emphasis will be placed upon examining the effects on thermal performance for low emissivity surfaces, the effects on optical performance will also be examined because changes of the optical characteristics such as reflectance and scattering are of greater concern for the NGST mission.
This paper describes two modeling approaches developed at Lockheed Martin Missiles and Space Operations for analysis of the sputter erosion of spacecraft surfaces due to the use of Hall thrusters. The PIC-DSMC (Particle in Cell-Direct Simulation Monte Carlo) plume model developed at Massachusetts Institute of Technology was successfully modified to model the BPT-4000 thruster (4,5 kW, 350 V) plumes. In addition to modeling the complicated plume features using the PIC-DSMC codes, we also developed a semi-empirical plume model that requires less computational time for modeling the sputter erosion of spacecraft surfaces. The approach uses PLIMP (Plume Impingement) code as a ray- tracing tool to determine the plume distances from the exit to impinged objects (e.g. solar arrays), plume divergence angles, and impingement angles. Measured ion current flux and sputter rates were then used to examine the sputtering erosion for solar arrays on a representative geostationary spacecraft. This semi-empirical model allows one to perform a quick spacecraft-plume interaction investigation. Moreover, contamination deposition of eroded thruster products and sputtered spacecraft materials was examined.
General sticking coefficient models have been developed at LMMS for both unexcited molecules (no photochemical reaction effects) and photochemically excited molecules in a solar vacuum ultraviolet radiation environment. This paper describes applications of these models to spacecraft on-orbit contamination analysis. The first (non-photochemical) model was used for evaluation of internal contamination problem for an Earth-viewing instrument. The model examined potential molecular contamination to an instrument mirror from Chemglaze Z306 paint outgassing during the orbital flight. The model was correlated with available Chemglaze Z306 outgassing test data at a source temperature of 75 degrees Celsius and was then used to predict mirror deposition buildup under a relatively low source temperature of 30 degrees Celsius during 5-year mission flight. The second (photochemical) model correlated with measured photochemical contaminant deposition rates was used for examination of solar absorptance degradation of OSR (Optical Solar Reflector) radiators on a geosynchronous spacecraft in orbit for 10 years. The predicted degradation due to the photochemical reactions induced by solar vacuum ultraviolet radiation on spacecraft contaminants agrees well with flight data.
EOS AM-1 is the first in the series of the EOS spacecraft developed to advance the understanding of the biological and geophysical processes of the Earth's climate on a global basis. The fully integrated spacecraft is EOS AM-1 flight- phase contamination analysis has been performed to verify that the design of the spacecraft is compatible with limiting contamination to the level required for optical instruments and sensor, thermal control surfaces and solar array as well as to identify modifications if needed. This paper summarizes the approach and assumptions used in performing this contamination source and effects analysis for the EOS AM-1 spacecraft. Molecular and particulate contamination potential during the flight segment from launch through completion of orbital mission has been analyzed. Potential contamination sources examined include materials outgassing, instrument and spacecraft bus venting, spacecraft plume and other sources such as mechanisms, moisture absorption, and atomic oxygen. The modeling result have been used to confirm outgassing materials selection, to verify venting designs, and to develop bakeout requirements for components of the spacecraft bus and the instruments.
This paper discusses a comparison of molecular transport predictions between the Molecular Transfer Kinetics (MTK) model based on Gebhart's enclosure theory for thermal radiation and the Molecular Flux (MOLFLUX) model based on the ray-tracing concept. The analysis has shown that the two molecular transport models are equivalent if the 'infinite number of molecular reflections' condition is imposed on MOLFLUX calculations. The general multi-node molecular transport expressions for MTK and MOLFLUX are derived, followed by proving hue MTK/MOLFLUX equivalence through comparisons of the MTK and MOLFLUX solutions for a two-node problem and a special three-node problem. Finally, the MTK/MOLFLUX equivalence is demonstrated by numerical solutions for a 169-node satellite configuration using the two models.