Aaron Snyder

Atomic-Oxygen Undercutting of Protected Polymers in Low Earth Orbit

Hydrocarbon-based polymers that are exposed to atomic oxygen in low Earth orbit are slowly oxidized into volatile gases, which results in their erosion. Atomic-oxygen protective coatings that are both durable to atomic oxygen and effective in protecting underlying polymers have been developed. However, scratches, pin window defects, polymer surface roughness, and protective coating layer configuration can result in erosion and potential failure of protected thin polymer films even though the coatings are themselves atomic-oxygen durable. Issues are presented that cause protective coatings to become ineffective in some cases yet effective in others because of the details of their specific application. Observed in-space examples of failed and successfully protected materials using identical protective thin films are discussed and analyzed. Ground laboratory atomic-oxygen testing was conducted and compared with water vapor transport analyses from a previous study of protective coatings on Kapton® (polyimide), which indicates that vapor-deposited aluminized films are not as protective as sputter-deposited silicon dioxide films because of a greater number of pin window defects. Computational modeling was conducted and indicates that atomic-oxygen atoms trapped between the front and back surface aluminized films cause accelerated undercutting damage.

Issues and Consequences of Atomic Oxygen Undercutting of Protected Polymers in Low Eerth Orbit

Hydrocarbon polymers that are exposed to atomic oxygen in low Earth orbit are slowly oxidized which results in recession of their surface. Atomic oxygen protective coatings have been developed which are both durable to atomic oxygen and effective in protecting underlying polymers. However, scratches, pin window defects, polymer surface roughness and protective coating layer configuration can result in erosion and potential failure of protected thin polymer films even though the coatings are themselves atomic oxygen durable. This paper will present issues that cause protective coatings to become ineffective in some cases yet effective in others due to the details of their specific application. Observed in-space examples of failed and successfully protected materials using identical protective thin films will be discussed and analyzed. Proposed approaches to prevent the failures that have been observed will also be presented.

Transport of Sputtered Carbon During Ground-Based Life Testing of Ion Thrusters

High voltage, high power electron bombardment ion thrusters needed for deep space missions will be required to be operated for long durations in space as well as during ground laboratory life testing. Carbon based ion optics are being considered for such thrusters. The sputter deposition of carbon and arc vaporized carbon flakes from long duration operation of ion thrusters can result in deposition on insulating surfaces, causing them to become conducting. Because the sticking coefficient is less than one, secondary deposition needs to be considered to assure that shorting of critical components does not occur. The sticking coefficient for sputtered carbon and arc vaporized carbon is measured as well as directional ejection distribution data for carbon that does not stick upon first impact.

Fast Three-Dimensional Method of Modeling Atomic Oxygen Undercutting of Protected Polymers

A method is presented to model atomic oxygen erosion of protected polymers in low Earth orbit (LEO). Undercutting of protected polymers by atomic oxygen occurs in LEO due to the presence of scratch, crack or pin-window defects in the protective coatings. As a means of providing a better understanding of undercutting processes, a fast method of modeling atomic-oxygen undercutting of protected polymers has been developed. Current simulation methods often rely on computationally expensive raytracing procedures to track the surface-to-surface movement of individual “atoms”. The method introduced in this paper replaces slow individual particle approaches by substituting a model that utilizes both a geometric configuration-factor technique, which governs the diffuse transport of atoms between surfaces, and an efficient telescoping series algorithm, which rapidly integrates the cumulative effects stemming from the numerous atomic oxygen events occurring at the surfaces of an undercut cavity. This new method facilitates the systematic study of three-dimensional undercutting by allowing rapid simulations to be made over a wide range of erosion parameters.

Degradation of Hubble Space Telescope Aluminized-Teflon Bi-Stem Thermal Shields

A section of retrieved Hubble Space Telescope (HST) bi-stem thermal shields (BSTS), which experienced 8.25 years of space exposure, was analyzed for space environmental durability. The shields were comprised of 2 mil (0.051 mm) aluminized-Teflon® fluorinated ethylene propylene (Al-FEP) rings fused together into a circular bellows shape. As the circular thermal shields had solar, anti-solar and solar-grazing surfaces and were exposed to the space environment for a long duration, it provided a unique opportunity to study solar effects on the environmental degradation of Al-FEP, a commonly used spacecraft thermal control material. Therefore, the objective of this research was to characterize the degradation of retrieved HST BSTS Al-FEP with particular emphasis on solar effects. Data obtained included tensile properties, density (as-retrieved and after 200 3C heating), solar absorptance, and surface morphology and chemistry. The solar-facing surfaces of the thermal shields were found to be extremely embrittled and contained numerous through-thickness cracks. Tensile testing verified that near solar-facing surfaces lost their mechanical strength and elasticity, whereas the anti-solar-facing surfaces maintained their ductility. The density of the as-retrieved BSTS insulation was similar to pristine FEP. Heating at 200 3C resulted in significant increases in density for the solar-facing BSTS indicating chain scission damage, consistent with the loss of mechanical strength and elongation. The solar absorptance of the solar-grazing and the anti-solar-facing surfaces were found to be similar to pristine BSTS, whereas the solar-facing surfaces were found to have significantly increased solar absorptance. Both solar- and anti-solar-facing surfaces were microscopically textured from sweeping atomic oxygen erosion with the solar-facing surface appearing to have a more pronounced texture in spite of being exposed to a lower atomic oxygen fluence indicating a possible solar/atomic oxygen synergistic effect. These results provide valuable information on space environmental degradation of Al-FEP, particularly with respect to solar radiation effects on embrittlement.

Solar Effects on Tensile and Optical Properties of Hubble Space Telescope Silver-Teflon Insulation

A section of the retrieved Hubble Space Telescope (HST) solar array drive arm (SADA) multilayer insulation (MLI), which experienced 8.25 years of space exposure, was analyzed for environmental durability of the top layer of silver-Teflon (DuPont) fluorinated ethylene propylene (Ag-FEP). Because the SADA MLI had solar and anti-solar facing surfaces and was exposed to the space environment for a long duration, it provided a unique opportunity to study solar effects on the environmental degradation of Ag-FEP, a commonly used spacecraft thermal control material. Data obtained included tensile properties, solar absorptance, surface morphology and chemistry. The solar facing surface was found to be extremely embrittled and contained numerous through-thickness cracks. Tensile testing indicated that the solar facing surface lost 60% of its mechanical strength and 90% of its elasticity while the anti-solar facing surface had ductility similar to pristine FEP. The solar absorptance of both the solar facing surface (0.155 ± 0.032) and the anti-solar facing surface (0.208 ± 0.012) were found to be greater than pristine Ag-FEP (0.074). Solar facing and anti-solar facing surfaces were microscopically textured, and locations of isolated contamination were present on the anti-solar surface resulting in increased localized texturing. Yet, the overall texture was significantly more pronounced on the solar facing surface indicating a synergistic effect of combined solar exposure and increased heating with atomic oxygen erosion. The results indicate a very strong dependence of degradation, particularly embrittlement, upon solar exposure with orbital thermal cycling having a significant effect.

Modeling of Transmittance Degradation Caused by Optical Surface Contamination by Atomic Oxygen Reaction with Adsorbed Silicones

A numerical procedure is presented to calculate transmittance degradation caused by contaminant films on spacecraft surfaces produced through the interaction of orbital atomic oxygen (AO) with volatile silicones and hydrocarbons from spacecraft components. In the model, contaminant accretion is dependent on the adsorption of species, depletion reactions due to gas-surface collisions, desorption, and surface reactions between AO and silicon producing SiOx (where x is near 2). A detailed description of the procedure used to calculate the constituents of the contaminant layer is presented, including the equations that govern the evolution of fractional coverage by specie type. As an illustrative example of film growth, calculation results using a prototype code that calculates the evolution of surface coverage by specie type is presented and discussed. An example of the transmittance degradation caused by surface interaction of AO with deposited contaminant is presented for the case of exponentially decaying contaminant flux. These examples are performed using hypothetical values for the process parameters.

Fast Three-Dimensional Modeling of Atomic-Oxygen Undercutting of Protected Polymers

A method is presented to model atomic-oxygen erosion of protected polymers in low Earth orbit. Undercutting of protected polymers by atomic oxygen can occur as a result of the presence of scratch, crack, or pin window defects in the protective coatings. As a means of providing a better understanding of undercutting processes, a fast method of modeling atomic-oxygen undercutting of protected polymers has been developed. Current simulation methods often rely on computationally expensive ray-tracing procedures to track the surface-to-surface movement of individual “atoms.” To reduce the burden of time-consuming calculations, the method introduced replaces computationally demanding individual particle simulations by substituting a model that utilizes both a geometric configuration-factor technique, which collectively governs the diffuse transport of atoms between surfaces, and an efficient algorithm, which rapidly computes the cumulative effects stemming from the series of atomic-oxygen collisions at the surfaces of an undercut cavity. This new method facilitates the systematic study of three-dimensional undercutting by allowing rapid simulations to be made over a wide range of erosion parameters.