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Mir Solar Array Return Experiment:  Power Performance Measurements & Molecular Contamination Analysis Results

A solar array segment was recently removed from the Mir core module and returned for ground-based analysis. The segment, which is similar to the ones the Russians have provided for the FGB and Service Modules, was microscopically examined and disassembled by US and Russian science teams. Laboratory analyses have shown the segment to be heavily contaminated by an organic silicone coating, which was converted to an organic silicate film by reactions with atomic oxygen within the orbital flight environment. The source of the contaminant was a silicone polymer used by the Russians as an adhesive and bonding agent during segment construction. During its life cycle, the array experienced a reduction in power performance from ~12%, when it was new and first deployed, to ~5%, when it was taken out of service. However, current-voltage measurements of three contaminated cells and three pristine, Russian standard cells have shown that very little degradation in solar array performance was due to the silicate attributed to “thermal hot-spotting” or electrical arcing; orbital debris and micrometeoroid impacts; and possibly to the degradation of the solar cells and interconnects caused by radiation damage from high-energy protons and electrons.

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Atomic Oxygen Durability Testing of an International Space Station Solar Array Validation Coupon

An International Space Station solar array validation coupon was exposed in a directed atomic oxygen beam for space environment durability testing. At the NASA Glenn Research Center. Exposure to atomic oxygen and intermittent tensioning of the solar array were conducted to verify the solar array's durability to low Earth orbital atomic oxygen and to the docking threat of plume loading both of which are anticipated over its expected mission life of fifteen years. The validation coupon was mounted on a specially designed rotisserie. The rotisserie mounting enabled the solar and anti-solar facing side of the array to be exposed to directed atomic oxygen in a sweeping arrival process replicating space exposure. The rotisserie mounting also enabled tensioning, in order to examine the durability of the array and its hinge to simulated plume loads. Flash testing to verify electrical performance of the solar array was performed with a solar simulator before and after the exposure to atomic oxygen and tensile loading. Results of the flash testing indicated little or no degradation in the solar array's performance. Photographs were also taken of the array before and after the durability testing and are included along with along with comparisons and discussions in this report. The amount of atomic oxygen damage appeared minor with the exception of a very few isolated defects. There was also no indications that the simulated plume loadings had weakened or damaged the array, even though there was some erosion of Kapton due to atomic oxygen attack. Based on the results of this testing, it is apparent that the International Space Station's solar array should survive the low Earth orbital atomic oxygen environment and docking threats which are anticipated over its expected mission life.

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Combined Contamination and Space Environmental Effects on Solar Cells and Thermal Control Surfaces

For spacecraft in low Earth orbit (LEO), contamination can occur from thruster fuel, sputter contamination products, and from products of silicone degradation. This paper describes laboratory testing in which solar cell materials and thermal control surfaces were exposed to simulated spacecraft environmental effects including contamination, atomic oxygen, ultraviolet radiation and thermal cycling. The objective of these experiments was to determine how the interaction of the natural LEO environmental effects with contaminated spacecraft surfaces impacts the performance of these materials. Optical properties of samples were measured and solar cell performance data was obtained. In general, exposure to contamination by thruster fuel resulted in degradation of solar absorptance for fused silica and various thermal control surfaces and degradation of solar cell performance. Fused silica samples which were subsequently exposed to an atomic oxygen/vacuum ultraviolet radiation environment showed reversal of this degradation. These results imply that solar cells and thermal control surfaces which are susceptible to thruster fuel contamination and which also receive atomic oxygen exposure may not undergo significant performance degradation. Materials which were exposed to only vacuum ultraviolet radiation subsequent to contamination showed, slight additional degradation in solar absorptance.

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Indium Tin Oxide-Magnesium Fluoride Co-Deposited Films for Spacecraft Applications

Highly transparent coatings with a maximum sheet resistivity between 108 and 109 ohms/square are desired to prevent charging of solar arrays for low Earth polar orbit and geosynchronous orbit missions. Indium tin oxide (ITO) and magnesium fluoride (MgF2) were ion beam sputter co-deposited onto fused silica substrates and were evaluated for transmittance, sheet resistivity and the effects of simulated space environments including atomic oxygen (AO) and vacuum ultraviolet (VUV) radiation. Optical properties and sheet resistivity as a function of MgF2 content in the films will be presented. Films containing 8.4 wt.% MgF2 were found to be highly transparent and provided sheet resistivity in the required range. These films maintained a high transmittance upon exposure to AO and to VUV radiation, although exposure to AO in the presence of charged species and intense electromagnetic radiation cause significant degradation in film transmittance. Sheet resistivity of the as-fabricated films increased with time in ambient conditions. Vacuum heat treatment following film deposition caused a reduction in sheet resistivity. However, following heat resistivity values remained stable during storage in ambient conditions.

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Space Station Freedom Solar Array Blanket Coverlay Atomic Oxygen Durability Testing Results

The power system for the Space Station Freedom used a flexible solar array for photovoltaic power generation. Support for the solar cells and current carriers on the flexible array is provided by the solar array blanket. The main structural member of the array blanket is the coverlay (laminate), which is composed of Kapton, fiberglass scrim cloth and silicone adhesive. The anti-solar facing side of the laminate is protected from the atomic oxygen environment with a thin film coating of silicone dioxide. Coated Kapton and laminate samples were exposed to simulated atomic oxygen environments (plasma asher and directed beam) to determine whether the coated Kapton is durable and the degree to which the coating is damaged by the lamination process. Test results indicated that the mass loss relative to unprotected Kapton (relative reactivity) for the laminate was roughly a factor of 10 higher than for the coated Kapton possibly due in part to an increase in the number of scratches in the coating. This increase is probably due to handling during the lamination process. These results were not dependant on whether the exposure was performed in the plasma asher or the directed beam. Although atomic oxygen at thermal energies can produce results which are pessimistic indicators of in space durability, the data indicates that if surface scratching of the coating is limited and the coated Kapton is adherent to the underlying silicone, the laminate should survive for its desired lifetime of 15 years.
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Last Updated: 02/04/2014