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Vacuum Ultraviolet Radiation and Atomic Oxygen
Durability Evaluation of HST Bi-Stem Boom Thermal Shield Materials
Bellows-type thermal shields were proposed for use on the Hubble Space
Telescope (HST) solar array bi-stem booms to reduce the thermal gradient-induced
jitter during orbital thermal cycling. Candidate thermal shield materials
included aluminized FEP Teflon with and without protective coatings for
durability to atomic oxygen (AO) and combined AO and ultraviolet (UV) radiation.
NASA Lewis (now Glenn) Research Center performed vacuum ultraviolet (VUV)
radiation and AO durability testing of candidate materials as part of an
overall program coordinated by NASA Goddard Space Flight Center (GSFC) to
evaluate the on-orbit durability of these thermal shield materials.
Coating adhesion problems were observed for samples having AO- and
the combined AO/UV-protective coatings which were attributed to exposure
to rapid thermal cycling used to simulate thermal cycling on orbit. Such
adhesion problems led to production of coating flakes from the material which
could pose a significant risk to HST optics if the coated materials were
used for the bi-stem boom thermal shields. No serious degradation was observed
for the uncoated aluminized Teflon as evaluated by optical microscopy, although
atomic force microscopy (AFM) revealed that an embrittled surface layer
would build up on the uncoated Teflon surface due to ultraviolet radiation
exposure. This embrittled layer was not completely removed by AO erosion.
Despite the formation of this embrittled layer, no cracks or particle flakes
were produced for the uncoated material upon exposure to VUV and AO.
Uncoated aluminized FEP Teflon was determined to be the most appropriate
thermal shield material and was used on the replacement solar arrays installed
during the December 1993 First HST Servicing Mission.
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Simulated Space Vacuum Ultraviolet (VUV) Exposure
Testing for Polymer Films
Vacuum ultraviolet (VUV) radiation of wavelengths between 115 and 200
nm produced by the sun in the space environment can cause degradation to
polymer films producing changes in optical, mechanical, and chemical properties.
These effects are particularly important for thin polymer films being considered
for ultra-lightweight space structures, because, for most polymers, VUV
radiation is absorbed in a thin surface layer. NASA Glenn Research Center
has developed facilities and methods for long-term ground testing of polymer
films to evaluate space environmental VUV radiation effects. VUV exposure
can also be used as part of sequential simulated space environmental exposures
to determine combined damaging effects. This paper will describe the effects
of VUV on polymer films and the necessity for ground testing. Testing practices
used at Glenn Research Center for VUV exposure testing will be described
including characterization of the VUV radiation source used, calibration
procedures traceable to the National Institute of Standards and Technology
(NIST), and testing techniques for VUV exposure of polymer surfaces.
International Test Program for Synergistic Atomic
Oxygen and VUV Exposure of Spacecraft Materials
Spacecraft in low Earth orbit (LEO) are subject to degradation in thermal
and optical performance of components and materials through interaction
with atomic oxygen and vacuum ultraviolet radiation which are predominant
in LEO. Due to the importance of LEO durability and performance to manufacturers
and users, an international test program for assessing the durability of
spacecraft materials and components was initiated. Initial tests consisted
of exposure of samples representing a variety of thermal control paints and
multiplayer insulation materials that have been used in space. Materials
donated from various international sources were tested alongside a material
whose performance is well known such as Teflon FEP or Kapton H for multiplayer
insulation, or Z-93-P for white thermal control paints. The optical, thermal
or mass loss data generated during the test was then provided to the participating
material supplier. Data was not published unless the participant donating
the material consented to the publication. This paper presents a description
of the types of test and facilities that have been used for the test program
as well as some examples of data that have been generated. The test program
is intended to give spacecraft builders and users a better understanding
of degradation processes and effects to enable improved prediction of spacecraft
performance.
Effects of Vacuum Ultraviolet Radiation on Thin
Polyimide Films
This paper describes the vacuum ultraviolet (VUV) radiation durability
screening testing of thin (12.7 to 25.4 µm) polyimide films proposed for
use on the Next Generation Space Telescope (NGST) sunshield. Materials
included in this screening test were Kapton®E, Kapton®HN,
Upilex®S, CP1, CP1 with vapor deposited aluminum (VDA) on its
back surface, and CP2 with a VDA coating on its back surface. Samples were
exposed to approximately 1000 equivalent sun hours (ESH) of VUV radiation
and examined for changes in solar absorptance, thermal emittance, ultimate
tensile strength, and elongation-to-failure. Changes in solar absorptance
were observed for some materials, and additionally, significant changes in
spectral reflectance were observed in the ultraviolet to visible wavelength
region for all polyimide materials tested. Changes in ultimate tensile strength
and elongation at failure were within the experimental uncertainty for all
samples. Longer exposures are needed to verify the observed trends and to
develop performance predictions for these materials on the NGST sunshield.
Steady Sate Vacuum Ultraviolet Exposure Facility
With Automated Calibration Capability
NASA Glenn Research Center at Lewis Field designed and developed a
steady state vacuum ultraviolet automated (SSVUVa) facility with in situ
VUV intensity calibration capability. The automated feature enables a
constant accelerated VUV radiation exposure over long periods of testing
without breaking vacuum. This test facility is designed to simultaneously
accommodate four isolated radiation exposure tests within the SSVUVa vacuum
chamber. Computer-control of the facility for long term continuous operation
also provides control and recording of thermocouple temperatures, periodic
recording of VUV lamp intensity, and monitoring of vacuum facility status.
This paper discusses the design and capabilities of the SSVUVa facility. |
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