The Hubble Space Telescope (HST) was launched on April 25, 1990, into low Earth orbit as the first mission of NASA’s Great Observatories program. The HST was designed to be serviced on orbit to upgrade scientific capabilities. In December 1993, during the first servicing mission, the original power-generating solar arrays (SA-I) were replaced with a second set of arrays (SA-II). In March of 2002, after 8.25 years of space exposure, the second set of arrays were replaced with a third set during the fourth servicing mission, and the second set of arrays were brought back to Earth.
The majority of the HST is covered with thermal control materials, which passively control temperatures during orbit. These materials utilize back-surface-metallized Teflon FEP (i.e., DuPont, fluorinated ethylene propylene)as a space-facing layer because of the material’s excellent optical properties (low solar absorptance and high thermal emittance). A section of the retrieved SA-II solar array drive arm (SADA) multilayer insulation (MLI) blanket was provided to the NASA Glenn Research Center so that environmental durability analyses of the top layer of silver-Teflon FEP (Ag-FEP, DuPont) could be conducted.
HST photographed in December 1999 during the third servicing mission. Left: HST with SA-II. Right: Closeup of an MLI-covered SADA.
The MLI was wrapped completelyaround the SADA and, therefore, had solar- and anti-solar-facing surfaces (see the preceding photograph). The circular configuration ofthe insulation, along with the long-term space exposure, provided a unique opportunity to study solar radiation 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.
A retrieved SA-II SADA MLI sample was provided by the European Space Agency (ESA) for this study. The following photograph shows the 36.9- by 9.1-cm sample. The sample’s top layer was approximately 10 mil (0.25 mm) thick, consisting of a space-exposed 5-mil (127-μm) Teflon FEP (DuPont) layer coated on the backside with vapor-deposited-Ag and INCONEL (1500-Å-thick Ag layer and 275-Å-thick INCONEL layer). The FEP/Ag/INCONEL layer was adhered with 40-μm-thick 966 acrylic adhesive to a fiberglass cloth impregnated with polytetrafluoroethylene (PTFE). The MLI has 16 layers of double-sided aluminized-Kapton (50 μm thick) separated by Dacron (Invista, Inc.) netting and a bottom layer of PTFE-impregnated fiberglass cloth. All analyses were obtained from the top layer of insulation (FEP/Ag/INCONEL/adhesive/scrim), referred to as Ag-FEP. For this study, 0° was defined as the direct solar-facing surface (indicated as a line in the following photograph) and 180° was the anti-solar-facing surface, with 90° and 270° being solar-grazing surfaces.
The HST SADA MLI sample (the line indicates the solar-facing position, 0°).
The solar-facing surface of the SADA was found to be extremely embrittled and contained numerous through-the-thickness cracks. Tensile testing indicated that the solar-facing surface lost 60 percent of its mechanical strength and 90 percent of its elasticity, whereas the anti-solar-facing surface had ductility similar to that of pristine FEP. The solar absorptance α 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 that of pristine Ag-FEP (0.074). The following figures are polar plots of elongation at failure and of solar absorptance versus solar angle.
Polar plots of the HST SADA Ag-FEP. Left: Percent elongation at failure versus solar angle. Right: Solar absorptance versus solar angle.
A thermal model was developed to determine temperature versus solar angle for the insulation. Plotting elongation at failure versus temperature indicated a trend of increasing embrittlement for decreasing temperatures below -50 °C, appearing to correspond to a cold transition temperature, but this effect was not nearly as deteriorative as the solar-facing higher temperature exposures. These results indicate a very strong dependence of embrittlement on solar exposure, and temperature appears to play an important role.
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. However, 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. These results provide valuable information on space environmental degradation of Ag-FEP, particularly with respect to solar radiation and temperature effects on embrittlement.
Find out more about the research of Glenn’s Electro-Physics Branch: http://www.grc.nasa.gov/WWW/epbranch/ephome.htm
Glenn contacts:
Kim K. de Groh, 216-433-2297, Fax: 216-433-2221, Kim.K.deGroh@nasa.gov
Joyce A. Dever, 216-433-6294, Fax: 216-433-2221, Joyce.A.Dever@nasa.gov
Dr. Aaron Snyder, 419-621-3388, Fax: 419-621-3298, Aaron.Snyder-1@nasa.gov
Authors:
Kim K. de Groh, Joyce A. Dever, Dr. Aaron Snyder, Sharon Kaminski, Catherine E. McCarthy, Allison L. Rapoport, and Rochelle N. Rucker
Headquarters program office:
Science Mission Directorate
Programs/projects:
Hubble Space Telescope, International Space Station, Earth Observing Satellites, Crew Exploration Vehicle
Last updated: December 14, 2007
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