Contamination-sensitive space flight hardware is typically built in cleanroom facilities in order to protect the hardware
from particle contamination. Forest wildfires near the facilities greatly increase the number of particles and amount of
vapors in the ambient outside air. Reasonable questions arise as to whether typical cleanroom facilities can adequately
protect the hardware from these adverse environmental conditions.
On Monday September 6, 2010 (Labor Day Holiday), a large wildfire ignited near the Boulder, Colorado Campus of
Ball Aerospace. The fire was approximately 6 miles from the Boulder City limits. Smoke levels from the fire stayed
very high in Boulder for the majority of the week after the fire began. Cleanroom operations were halted temporarily on
contamination sensitive hardware, until particulate and non-volatile residue (NVR) sampling could be performed.
Immediate monitoring showed little, if any effect on the cleanroom facilities, so programs were allowed to resume work
while monitoring continued for several days and beyond in some cases. Little, if any, effect was ever noticed in the
monitoring performed.
Historical experience and previously published papers have shown that contamination sampling techniques influence the
cleanliness results of spaceflight hardware. Programs rely on this data to show that derived or contractual requirements
are met at delivery. Particle sampling using tape lifts and rinses was performed on the James Webb Space Telescope
(JWST) Primary Mirror Segment Assemblies (PMSAs) hardware. Sampling was performed on identical hardware with
both sampling techniques. The hardware was sampled at comparable stages of assembly which provided hardware with
similar levels of particulate contamination. Results from the two sampling techniques are compared. In one technique,
sampling was performed by rinsing (with a hand-squeeze bottle with low pressure) followed by a tape lift; the other
technique used a tape lift only. The relationship of particle size distribution, types of particles, level of particle
contamination, and particle removal rate by sampling technique are examined. Comparison of the particle sampling
results provides a basis for interpreting results depending on sampling techniques. Improving the contamination
engineer’s ability to interpret results is particularly useful when hardware configuration or surface finish dictate which
sampling technique can be used. When one can choose the sampling technique, the results of this study can provide
guidance on which technique is more appropriate depending on the circumstances. Results show that tape lifts remove
more particles than low pressure rinses; furthermore that tape lift only is better than the combined operation of a rinse
closely followed by a tape lift. Results also indicate that further work should be performed on different surface finishes,
rinsing techniques, and particulate contamination levels.
The Direct Simulation Monte Carlo (DSMC) Analysis Code (DAC), as released by NASA, is a general purpose, gas dynamic, transport analysis suite of codes. These codes have been acquired by Ball Aerospace & Technologies Corp. (BATC) through a software usage agreement and have been modified to do a more detailed analysis of contaminant molecular transport of spacecraft and spacecraft instruments over mission lifetimes, typically 5 to 7 years. This transport model takes advantage of the proven algorithms within DAC to handle complex surface geometries and time-dependant gas dynamics. Additions to the code include diffusion of contaminants through solid surfaces, temperature and coverage-dependant adsorption/desorption for the contaminants being modeled, and input data for molecular diameters, molecular weights, and diffusion parameters for the common contaminants found in spacecraft materials and coatings.
There has been a general awareness for several years that the IEST-STD-CC1246 standard particle distribution with a slope of -0.926 does not reasonably represent the contamination on optics that have not been recently cleaned. As a result, the CL (Cleanliness Level) nomenclature actually counters effective communication and modeling of particulate contamination scatter. An analysis method and communication standard centered on Percent Areal Coverage (PAC) and particle distribution slope is presented that improves the ability of Contamination Engineering and Stray Light Engineering to tackle ever more difficult instrument stray light requirements in the most cost-effective manner. Modeling the expected particle distributions for multiple contamination species improves accuracy and reduces costly overdesign.
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