Lubert J. Leger

Atomic oxygen testing with thermal atom systems - A critical evaluation

The use of thermal atom (kinetic energy near 0.04 eV) test methods as a materials selection and screening technique for LEO spacecraft is critically evaluated in this paper. The physics and chemistry of the thermal atom environments are shown to produce specific mass loss rates (mg/sq cm per min) and reaction efficiencies (Re) radically different from those produced in the LEO environment. A response surface study shows that specific mass loss rates change rapidly with plasma-asher parameters and seldom agree with flight data. FEP Teflon is shown to react by a different mechanism than Kapton, polyethylene, or graphite. The Re (Re = volume of material removed/oxygen atom) of Kapton, polyethylene, Mylar, Tedlar, FEP Teflon, and graphite measured in a flowing afterglow apparatus are 0.001 to 0.0001 those measured with high-energy atoms (kinetic energy 1.5 eV or greater) in beam systems or in LEO. The effect of sample temperature and atom impact energy on Re is discussed. A simple kinetic model describing the reaction of atomic oxygen with polymer surfaces is developed. Guidelines and recommendations for thermal atom testing and interpretation of test results are presented.

Vacuum ultraviolet radiation/atomic oxygen synergism in materials reactivity

Experimental results are presented which indicate that low fluxes of vacuum UV (VUV) radiation exert a pronounced influence on the atomic oxygen reactivity of such fluorocarbon and fluorocarbon spacecraft materials as the FEP Teflon and PCTFE that are under consideration for the Space Station Freedom. With simultaneous exposure to VUV fluxes comparable to those experienced in LEO, the reactivity of these materials becomes comparable to that of Kapton; VUV radiation has also been shown to increase the reactivity of Kapton with thermal-energy oxygen atoms. 8 refs.

EOIM-III Mass Spectrometry and Polymer Chemistry: STS 46, July-August 1992

The Evaluation of Oxygen Interactions with Materials III space-flight experiment was developed to obtain benchmark atomic-oxygen reactivity data and was conducted during Space Transportation System Mission 46. We present an overview of the flight experiment and the results of the Lyndon B. Johnson Space Center polymer chemistry and mass-spectrometer-carousel experiments. Mass-spectrometric measurements of gaseous products formed by O-atom reaction with 13C-labeled Kapton™ revealed CO, CO2, H2O, NO, and NCh. By operating the mass spectrometer to detect naturally occurring ionospheric species, we characterized the ambient ionosphere at various times during the flight experiment and detected the gaseous reaction products formed when ambient ions interacted with the 13C Kapton carousel sector. Direct comparison of the results of on-orbit O-atom exposures with those conducted in ground-based laboratory systems, which provide known O-atom fluences and translational energies, demonstrated the strong translational-energy dependence of O-atom reactions with a variety of polymers. A line-of-centers reactive scattering model was shown to provide a reasonably accurate description of the translational-energy dependence of polymer reactions with O atoms at high atom kinetic energies, and a Beckerle-Ceyer model provided an accurate description of O-atom reactivity over a three-order-of -magnitude range in translational energy and a four-order-of-magnitude range in reaction efficiency. Postflight studies of the polymer samples by x-ray photoelectron spectroscopy and infrared spectroscopy demonstrate that O-atom attack is confined to the near-surface region of the sample, that is, within 50 to 100 A of the surface.

Oxygen Interactions with Materials III— Mission and Induced Environments

The Evaluation of Oxygen Interactions with Materials III (EOIM-IH) flight experiment was developed to obtain benchmark atomic-oxygen-material reactivity data. The experiment was conducted during Space Shuttle mission 46, July 31 to August 7, 1992. Quantitative interpretation of the materials reactivity measurements requires a complete and accurate definition of the space environment exposure, including the thermal history of the payload, the solar ultraviolet exposure, the atomic-oxygen fluence, and any spacecraft outgassing and contamination effects. The thermal history of the payload was measured using 11 thermocouple sensors placed behind selected samples and on the EOIM-III payload structure. The solar ultraviolet exposure history of the EOIM-IH payload was determined by analysis of the as-flown orbit and vehicle attitude combined with daily average solar ultraviolet and vacuum ultraviolet fluxes. The atomic-oxygen fluence was assessed in three ways. First, the O-atom fluence was calculated using a program that incorporates the MSIS-86 atmospheric model, the as-flown Space Shuttle trajectory, and solar activity parameters. Second, it was estimated directly from Kapton film erosion. Third, ambient O-atom measurements were made using the quadrupole mass spectrometer on the EOIM-III payload. As of this writing, our best estimate of the O-atom fluence is (2.3 ± 0.3) X1020 atoms/cm2. Finally, results of postflight surface analysis of selected samples by x-ray photoelectron spectroscopy indicate low levels of molecular contamination on the payload surface.

Material interactions with the Low Earth Orbital (LEO) environment: Accurate reaction rate measurements

Interactions between spacecraft surfaces and atomic oxygen within the low earth orbital (LEO) environment have been observed and measured during Space Shuttle flights over the past 3 yr. The results of these experiments have demonstrated that interaction rates for many materials proposed for spacecraft applications are high and that protective coatings must be developed to enable long-lived operation of spacecraft structures in the LEO environment. A flight experiment discussed herein uses the Space Shuttle as an orbiting exposure laboratory to obtain accurate reaction rate measurements for materials typically used in spacecraft construction. An ion-neutral mass spectrometer, installed in the Orbiter cargo bay, will measure diurnal ambient oxygen densities while material samples are exposed at low altitude (222 km) to the orbital environment. From in situ atomic oxygen density information and postflight material recession measurements, accurate reaction rates can be derived to update the Space Station materials interaction data base. Additionally, gases evolved from a limited number of material surfaces subjected to direct oxygen impingement will be identified using the mass spectrometer. These measurements will aid in mechanistic definitions of chemical reactions which cause atom-surface interactions and in validating results of upcoming degradation studies conducted in ground-based neutral beam laboratories.

An overview of the evaluation of oxygen interaction with materials-third phase (EOIM-III) experiment - Space Shuttle Mission 46

The interaction of the atomic oxygen (AO) component of the low earth orbit (LEO) environment with spacecraft materials has been the subject of several flight experiments over the past 11 years. The effect of AO interactions with materials has been shown to be significant for long-lived spacecraft such as Space Station Freedom and has resulted in materials changes for externally exposed surfaces. The data obtained from previous flight experiments, augmented by limited ground-based evaluation, have been used to evaluate hardware performance and select materials. Questions pertaining to the accuracy of this data base remain, resulting from the use of long-term ambient density models to estimate the O-atom fluxes and fluences needed to calculate materials reactivity in short-term flight experiments. The EOIM-3 flight experiment was designed to produce benchmark AO reactivity data and was carried out during STS-46. Ambient density measurements were made with a quadrupole mass spectrometer which was calibrated for AO measurements in a unique ground-based test facility. The combination of these data with the predictions of ambient density models allows an assessment of the accuracy of measured reaction rates on a wide variety of materials, many of which had never been tested in LEO before. The mass spectrometer is also used to obtain a better definition of the local neutral and plasma environments resulting from interaction of the ambient atmosphere with various spacecraft surfaces. In addition, the EOIM-3 experiment was designed to produce information on the effects of temperature, mechanical stress, and solar exposure on the AO reactivity of a wide range of materials. An overview of the EOIM-3 methods and results are presented.

Evaluation of Oxygen Interactions with Materials 3: Mission and induced environments

The Evaluation of Oxygen Interactions with Materials 3 (EOIM-3) flight experiment was developed to obtain benchmark atomic oxygen/material reactivity data. The experiment was conducted during Space Shuttle mission 46 (STS-46), which flew July 31 to August 7, 1992. Quantitative interpretation of the materials reactivity measurements requires a complete and accurate definition of the space environment exposure, including the thermal history of the payload, the solar ultraviolet exposure, the atomic oxygen fluence, and any spacecraft outgassing contamination effects. The thermal history of the payload was measured using twelve thermocouple sensors placed behind selected samples and on the EOIM-3 payload structure. The solar ultraviolet exposure history of the EOIM-3 payload was determined by analysis of the as-flown orbit and vehicle attitude combined with daily average solar ultraviolet and vacuum ultraviolet (UV/VUV) fluxes. The atomic oxygen fluence was assessed in three different ways. First, the O-atom fluence was calculated using a program that incorporates the MSIS-86 atmospheric model, the as-flown Space Shuttle trajectory, and solar activity parameters. Second, the oxygen atom fluence was estimated directly from Kapton film erosion. Third, ambient oxygen atom measurements were made using the quadrupole mass spectrometer on the EOIM-3 payload. Our best estimate of the oxygen atom fluence as of this writing is 2.3 +/- 0.3 x 10(exp 20) atoms/sq cm. Finally, results of post-flight X-ray photoelectron spectroscopy (XPS) surface analyses of selected samples indicate low levels of contamination on the payload surface.

External induced contamination environment assessment for Space Station Freedom

An assessment of the Space Station Freedom performance as affected by the external induced contamination environment is in progress. The assessment procedure involves comparing the Space Station Freedom external contamination requirements, SSP 30426, Revision B (1991), with calculated molecular deposition, molecular column density, and other effects from potential sources of contamination. The current assessment comprises discussions of Space Shuttle proximity operations, Space Shuttle waste-water dumps (while docked to the Space Station), Space Station fluid and waste-gas venting, system gas leakage, external material outgassing, and a combined contamination assessment. This performance assessment indicates that Space Station Freedom contamination requirements are realistic and can be satisfied when all contamination sources are included.