Joyce A. Dever

Environmental Exposure Conditions for Teflon® Fluorinated Ethylene Propylene on the Hubble Space Telescope

The outer layer of Teflon® fluorinated ethylene propylene (FEP) multi-layer insulation (MLI) on the Hubble Space Telescope (HST) was observed to be significantly cracked at the time of the Second HST Servicing Mission (SM2), 6.8 years after HST was launched into low Earth orbit (LEO). Comparatively minor embrittlement and cracking were also observed in the FEP materials retrieved from solar-facing surfaces on the HST at the time of the First Servicing Mission (3.6 years exposure). After SM2, a failure review board was convened to address the problem of degradation of MLI on the HST. In order for this board to determine possible degradation mechanisms, it was necessary to consider all environmental constituents to which the FEP MLI surfaces were exposed. Based on measurements and various models, the environmental exposure conditions for the FEP surfaces on the HST were estimated, including: the number and temperature ranges of thermal cycles; equivalent sun hours; fluence and absorbed radiation dose of x-rays, trapped protons and electrons and plasma electrons and protons; and atomic oxygen (AO) fluence. This paper presents the environmental exposure conditions for FEP on the HST, briefly describing the possible roles of the environmental factors in the observed FEP embrittlement and providing references to the published works which describe in detail testing and analysis related to FEP degradation on the HST.

Hubble Space Telescope Metallized Teflon ® FEP thermal Control Materials: On-Orbit Degradation and Post-Retrieval Analysis

During the Hubble Space Telescope (HST) second servicing mission (SM2), degradation of unsupported Teflon® FEP (fluorinated ethylene propylene), used as the outer layer of the multilayer insulation (MLI) blankets, was evident as large cracks on the telescope light shield. A sample of the degraded outer layer was retrieved during the mission and returned to Earth for ground testing and evaluation. The results of the Teflon® FEP sample evaluation and additional testing of pristine Teflon® FEP led the investigative team to theorize that the HST damage was caused by thermal cycling with deep-layer damage from electron and proton radiation which allowed the propagation of cracks along stress concentrations, and that the damage increased with the combined total dose of electrons, protons, ultraviolet and x-ray radiation along with thermal cycling. This paper discusses the testing and evaluation of the retrieved Teflon® FEP.

Characteristics of Elastomer Seals Exposed to Space Environments

Abstract A universal docking and berthing system is being developed by the National Aeronautics and Space Administration (NASA) to support all future space exploration missions to low-Earth orbit (LEO), to the Moon, and to Mars. The Low Impact Docking System (LIDS) is being designed to operate using a seal-on-seal configuration in numerous space environments, each having unique exposures to temperature, solar radiation, reactive elements, debris, and mission duration. As the LIDS seal is likely to be manufactured from an elastomeric material, performance evaluation of elastomers after exposure to atomic oxygen (AO) and ultraviolet radiation (UV) was conducted, of which the work presented herein was a part. Each of th e three candidate silicone elastomer compounds investigated, including Esterline ELA-SA-401, and Parker Hannifin S0383-70 and S0899-50, was characterized as a low outgassing compound, per ASTM E595, having percent total mass loss (TML) less than 1.0% and collected volatile condensable materials (CVCM) less than 0.1%. Each compound was compatible with the LIDS operating environment of –50 to 50 °C. The seal characteristics presented include compression set, elastomer-to-elastomer adhesion, and o-ring leakage rate. The ELA-SA-401 compound had the lowest variation in compression set with temperature. The S0383-70 compound exhibited the lowest compression set after exposure to AO and UV. The adhesion for all of the compounds was significantly reduced after exposure to AO and was further decreased after exposure to AO and UV. The leakage rates of o-ring specimens showed modest increases after exposure to AO. The leakage rates after exposure to AO and UV were increased by factors of up to 600 when compared to specimens in the as-received condition.

Effects of Radiation and Thermal Cycling on Teflon ® FEP:

Surfaces of the aluminized Teflon® FEP (fluorinated ethylene propylene) multilayer thermal insulation on the Hubble Space Telescope (HST) were found to be cracked and curled in some areas at the time of the second servicing mission (SM2) in February 1997, 6.8 years after HST was deployed in low Earth orbit (LEO). In an effort to understand what elements of the space environment might cause such damage, pristine second-surface aluminized Teflon® FEP was tested for durability to various types of radiation, to thermal cycling and to radiation followed by thermal cycling. Types of radiation included synchrotron vacuum ultraviolet and soft x-ray radiation, simulated solar flare x-ray radiation, electrons and protons. Thermal cycling was conducted in various temperature ranges to simulate HST orbital conditions for Teflon® FEP. Resultsoftensiletestingoftheexposedspecimensshowedthatexposuretohighfluencesofradiation caused degradation in tensile properties of FEP. However, exposure to radiation alone in exposures comparable to those experienced by HST did not produce reduction in ultimate tensile strength and elongation of Teflon® similar to that observed for HST-retrieved aluminized Teflon®. Synergism of radiation exposure and thermal cycling was evident in the results of three experiments: thermal cycling following electron and proton irradiation, thermal cycling following x-ray exposure, and additional thermal cycling of a sample retrieved from HST. However, irradiation and thermal cycling with comparable HST SM2 exposure conditions did not produce the degradation observed in the FEP material retrieved during HST SM2.

Space Environment Exposure of Polymer Films on the Materials International Space Station Experiment: Results from MISSE 1 and MISSE 2

A total of thirty-one samples were included in the National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) Polymer Film Thermal Control (PFTC) and Gossamer Materials experiments, which were exposed to the low Earth orbit environment for nearly 4 years on the exterior of the International Space Station (ISS) as part of the Materials International Space Station Experiment (MISSE 1 and MISSE 2). This paper describes objectives, materials, and characterizations for the MISSE 1 and MISSE 2 GRC PFTC and Gossamer Materials samples. Samples included films of polyimides, fluorinated polyimides, and Teflon® fluorinated ethylene propylene (FEP) with and without second-surface metalized layers and/or surface coatings. Films of polyphenylene benzobisoxazole (PBO) and a polyarylene ether benzimidazole (TOR-LM TM) were also included. Polymer film samples were examined post-flight for changes in mechanical and optical properties. The environment in which the samples were located was characterized through analysis of sapphire contamination witness samples and samples dedicated to atomic oxygen (AO) erosion measurements. Results of the analyses of the PFTC and Gossamer Materials experiments are discussed.

Mechanical Properties Degradation of Teflon ® FEP Returned From the Hubble Space Telescope

After 6.8 years on orbit, degradation has been observed in the mechanical properties of second-surface metalized Teflon® FEP (fluorinated ethylene propylene) used on the Hubble Space Telescope (HST) on the outer surface of the multi-layer insulation (MLI) blankets and on radiator surfaces. Cracking of FEP surfaces on HST was first observed upon close examination of samples with high solar exposure retrieved during the first servicing mission (SM1) conducted 3.6 years after HST was put into orbit. Astronaut observations and photographs from the second servicing mission (SM2), conducted after 6.8 years on orbit, revealed severe cracks in the FEP surfaces of the MLI on many locations around the telescope. This paper describes results of mechanical properties testing of FEP surfaces exposed for 3.6 years and 6.8 years to the space environment on HST. These tests include bend testing, tensile testing, and surface micro-hardness testing.

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.

Effects of Vacuum Ultraviolet Radiation on DC93-500 Silicone

Vacuum ultraviolet radiation is among the space environment elements that can be hazardous to DC93-500 silicone film, which has been proposed for use on spacecraft exterior surfaces. Investigations have been conducted to examine vacuum ultraviolet effects on DC93-500 film. Laboratory exposure tests were used to determine the effectiveness of various wavelength ranges in causing optical and mechanical degradation and to determine intensity-dependence of optical and mechanical properties degradation. Results indicated that wavelengths between 185 and 200 nm were significantly more effective in causing degradation than wavelengths between 140 and 185 nm. These findings were consistent with results of vacuum ultraviolet ellipsometric optical measurements, which provided data on depth of penetration in DC93-500 as a function of wavelength. Wavelengths between 185 and 200 nm penetrate to depths between 1 and 3 μm in DC93-500, depths where bulk degradation is likely, whereas the penetration of shorter wavelengths is much more shallow and more likely to result in only surface degradation. Results of exposures of DC93-500 film samples to vacuum ultraviolet radiation of intensities between 1.5 and 5.5 times the sun’s intensity indicated no intensity-dependence of optical and mechanical property degradation.

Effects of Vacuum Ultraviolet Radiation on Thin Polyimide Films

This paper describes the vacuum ultraviolet (VUV) radiation durability screening testing of thin (12.7–25.4 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 vapour deposited aluminium (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 at failure. Changes in the 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 of the polyimide materials tested. Changes in the 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.

Chapter 23 – Degradation of spacecraft materials

Publisher Summary This chapter provides descriptions of specific space environmental threats to exterior spacecraft materials. It summarizes space exposure studies and laboratory simulations of individual and combined space environmental threats. A significant emphasis is placed on effects of Earth orbit environments because the majority of space missions have been flown in Earth orbits, which have provided a significant amount of data on materials effects. Issues associated with interpreting materials degradation results are discussed and deficiencies of ground testing are identified. Recommendations are provided on reducing or preventing space environmental degradation through appropriate materials selection. The chapter concludes that spacecraft materials exposed to low Earth orbit environments have been found to undergo degradation or damage due to environmental threats including atomic oxygen, contamination, radiation, temperature effects and temperature cycling, and micrometeoroids and orbital debris. The degree to which the space environment degrades or damages materials depends upon the unique conditions of an individual spacecraft environment and the susceptibility of the material to being altered by these environmental exposures.