Silver coated Teflon (SCT) has been used as a radiator material for spacecraft thermal control. In order to reduce the specular reflection, an attempt was made to roughen the heritage smooth SCT surface via sanding, leading to abraded surfaces. The objective of this study is to gain insight into the relative thermal performance degradation of smooth and abraded SCT radiator materials under identical exposure of concurrent UV irradiation and contaminant deposition. Contaminant molecules outgassed from representative spacecraft materials were deposited onto the smooth and abraded SCT samples with quartz crystal microbalances (QCMs) in close proximity to monitor real-time contaminant deposition. Thermal performance degradation is characterized by measuring solar absorptance () change on the SCT samples before and after contaminant film accumulation. Atomic force microscope (AFM) was used to examine the extent of surface roughness before and after contaminant deposition on smooth SCT samples. The preliminary findings indicate that less contamination accumulation was observed on SCT surfaces in comparison to the gold coated crystal surface of QCMs. In addition, the roughness of SCT surface appears to play a role in contributing a more pronounced change, suggesting the possibility of faster performance degradation of the abraded SCT materials in comparison to that of smooth SCT surfaces.
Contamination control plays an important role in sustaining spacecraft performance. One spacecraft degradation mechanism involves long-term on-orbit molecular outgassing from spacecraft materials. The outgassed molecules may accumulate on thermal control surfaces and/or optics, causing degradation. In this study, we performed outgassing measurements of multiple spacecraft materials, including adhesives, Nylon Velcro, and other assembly materials through a modified ASTM E595 test method. The modified ASTM E595 test had the source and receiver temperature remained at 125°C and 25°C, respectively, but with prolonged outgassing periods of two weeks. The condensable contaminants were analyzed by Fourier Transform Infrared Spectroscopy (FTIR) and Gas Chromatography/Mass Spectrometry (GC/MS) to determine their spectral transmission and chemical composition. The FTIR spectra showed several spacecraft materials, primarily adhesives and potting materials, exhibiting slight absorption from contaminants consisting of hydroxyl groups and carboxylic acids. To gain insight into molecular contaminant transport, simulations were conducted to characterize contaminant accumulation inside a hypothetical space system cavity. The simulation indicated that contaminant molecules bouncing inside the hypothetical payload cavity can lead to deposition on colder surfaces, even though large openings are available to provide venting pathways for escaping to space. The newly established molecular contaminant transport simulation capability holds the promise of providing quantitative guidance for future spacecraft and its venting design.
Polydimethylsiloxane damping fluids used for structural deployment mechanisms are not required to be low outgassing. During normal use, these damping fluids are typically encapsulated; however, an unintentional leak may occur which would cause an undesirable contamination at the leak point and form volatile condensable that could reach contamination-sensitive surfaces, degrading the performance of satellites. The collected volatile condensable material (CVCM) at 25 °C from ASTM E595 of a damping fluid, MeSi-300K, was < 0.10%, when the damping fluid was maintained at 125 °C for 24 hours under 10-6 Torr vacuum. MeSi-300K viscosity is 300,000 cSt, which indicates an average molecular weight (MW) of 204,000. This large MW polymer would contain about 2,756 dimethyl siloxane (DMS) units in the chain. These long chains are not expected to be volatile; however, during manufacture, linear chains and cyclic compounds of a smaller number of DMS units produced are volatile. Gas chromatography mass spectrometry (GC-MS) was used to identify the CVCM. Characterization of these materials revealed that the CVCM contained higher MW siloxanes, straight chain and cyclic, in the range of 682 to 1196 (9 to 16 DMS units), whereas CVCM from spacequalified, silicone-based materials have lower MW, 222 to 542 (3 to 7 DMS units). Consequently, contamination from MeSi-300K material would produce greater amounts of higher-MW siloxanes than space-qualified silicones. These higher-MW species would be harder to remove by evaporation and could remain on sensitive surfaces.
KEYWORDS: Molecules, Contamination, Monte Carlo methods, Particles, Systems modeling, Space telescopes, Telescopes, Argon, Optical components, Instrument modeling
We present a finite element model for the prediction of molecular contamination through narrow pathways in a hypothetical spaceborne instrument using the commercially available COMSOL Multiphysics software. The free molecular flow module of COMSOL uses the angular coefficient method as an alternative to particle based methods. In the angular coefficient method, the microscopic dynamical aspect of the material transport problem is reduced to a macroscopic problem by calculating emission and incident fluxes at each surface rather than the trajectories of individual molecules. The model was validated by comparing the simulated and experimentally measured pressure differential between two chambers separated by a mechanical test structure. The mechanical test structure was designed to exhibit narrow pathways with characteristic size that can be found on spaceborne optomechanical structures. It is shown that materials can slowly migrate through these pathways in a spaceborne instrument to cause noticeable performance degradation within a time scale of a few months. The model for material transport through the test structure was also verified using a stochastic method. To simulate water infiltration through narrow pathways of a hypothetical spaceborne instrument, nominal payload temperature profile was used in addition to setting empirical input parameters such as the desorption energy of water and the outgassing rate of water from multilayer insulator thermal blankets to the appropriate surfaces in the modeling domain. The rate of growth of ice films on low temperature optical components and how optical performance can be degraded over time are discussed in this paper.
This paper describes a novel approach for the suppression of contamination enhanced laser damage to optical
components by the use of fluorinated coatings that repel organic contaminates. In prior work we studied laser damage
thresholds induced by ppm levels of toluene under nanosecond 1.064 μm irradiation of fused silica optics. That work
showed that moderate vapor-phase concentrations (< 15%) of water and alcohols dramatically increased the laser
damage threshold. The data are consistent with the hypothesis that water and alcohols interact more favorably with the
hydroxylated silica surface thereby displacing toluene from the surface. In this work, preliminary results show that
fluorinated self assembled monolayer coatings can be used to accomplish the same effect. Optics coated with
fluorinated films have much higher survival rates compared with uncoated optics under the same conditions. In addition
to enhancing survival of laser optics, these coatings have implications for protecting spacecraft imaging optics from
organic contamination.
Concerns were raised for potential payload contamination inside payload faring (PLF) contributed from the
soot particles in the launch vehicle ignition plume. Soot particles, once ingested into PLF through vents, can
pose potential payload contamination risks due to their light absorbing characteristics. To gain insights into
the extent of soot particle contamination inside the PLF, analytical calculations and laboratory experiments
were performed using a PLF simulator to determine the rate of soot particle deposition onto surfaces.
The analysis assumed a non-venting setting as the worst case scenario, in which particles were trapped inside
the PLF simulator and allowed to deposit onto available surfaces. Soot particles were briefly introduced
inside a PLF mockup and after the soot generation source ceased, particle deposition rates were examined by
measuring the particle concentration decay as a function of time. Based on the experimentally determined
particle deposition rates and other parameters including the venting scenarios, the impact of soot particle
deposition for the full scale PLF and payload was evaluated. The effects of soot particles contamination were
also studied, and pronounced transmission degradation toward the UV region on a fused silica substrate was
observed.
We have characterized the thresholds for contamination laser induced damage (C-LID) process using toluene as a model
contaminant by varying oxygen and toluene concentrations. In the presence of 300 ppm toluene and nitrogen, the
damage threshold is (7.8 ± 1.9) × 103 laser pulses, in synthetic air the damage threshold is (18.0 ± 2.1) × 103 laser pulses.
We have found several high vapor pressure molecules that effectively inhibit the (C-LID) process and greatly extend the
lifetime of fused silica optics under high power laser irradiation. With the addition of ~4000 ppm of water, methanol or
ethanol, the lifetime exceeds 1 × 106 laser pulses with no damage observed. Possible mechanisms are discussed.
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