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

by on 9 June 2015in Physical Sciences No comment

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.

Related URLs:
http://hip.sagepub.com/content/20/4-5/371.abstract

Nucleate Pool Boiling Experiments (NPBX) on the International Space Station

by on 9 June 2015in Physical Sciences No comment

During the period of March–May 2011, a series of boiling experiments was carried out in the Boiling Experimental Facility (BXF) located in the Microgravity Science Glovebox (MSG) of the International Space Station (ISS). The BXF Facility was carried to ISS on Space Shuttle Mission STS–133 on February 24, 2011. Nucleate Pool Boiling Experiment (NPBX) was one of the two experiments housed in the BXF. Results of experiments on single bubble dynamics (e.g., inception and growth), multiple bubble dynamics (lateral merger and departure, if any), nucleate pool boiling heat transfer, and critical heat flux are described. In the experiments Perfluoro-n-hexane was used as the test liquid. The system pressure was varied from 51 to 243 kPa, pool temperature was varied from 30° to 59°C, and test surface temperature was varied from 40° to 80°C. The test surface was a polished aluminum disc (1 mm thick, 89.5 mm in diameter) heated from below with strain gage heaters. Five cylindrical cavities were formed on the surface with four cavities located at the corners of a square and one in the middle. During experiments the magnitude of mean gravity level normal to the heater surface varied from 1.2 × 10 − 7g e to 6 × 10 − 7g e . The results of the experiments show that a single bubble continues to grow to occupy the size of the chamber without departing from the heater surface. During lateral merger of bubbles, at high superheats a large bubble may lift off from the surface but continues to hover near the surface. Neighboring bubbles are continuously pulled into the large bubble. At low superheats bubbles at neighboring sites simply merge to yield a larger bubble. The larger bubble mostly locates in the middle of the heated surface and serves as a vapor sink. The latter mode continues to persist when boiling is occurring all over the heater surface. Heat fluxes for steady state nucleate boiling and critical heat fluxes are found to be much lower than those obtained under earth normal gravity conditions. The data are useful for calibration of results of numerical simulations. Any correlations that are developed for nucleate boiling heat transfer under microgravity condition must account for the existence of vapor escape path (sink) from the heater, size of the heater, and the size and geometry of the chamber.

Related URLs:
http://dx.doi.org/10.1007/s12217-012-9315-8

Droplet Combustion Experiments Aboard the International Space Station

by on 9 June 2015in Physical Sciences No comment

This paper summarizes the first results from isolated droplet combustion experiments performed on the International Space Station (ISS). The long durations of microgravity provided in the ISS enable the measurement of droplet and flame histories over an unprecedented range of conditions. The first experiments were with heptane and methanol as fuels, initial droplet droplet diameters between 1.5 and 5.0 m m, ambient oxygen mole fractions between 0.1 and 0.4, ambient pressures between 0.7 and 3.0 a t m and ambient environments containing oxygen and nitrogen diluted with both carbon dioxide and helium. The experiments show both radiative and diffusive extinction. For both fuels, the flames exhibited pre-extinction flame oscillations during radiative extinction with a frequency of approximately 1 H z. The results revealed that as the ambient oxygen mole fraction was reduced, the diffusive-extinction droplet diameter increased and the radiative-extinction droplet diameter decreased. In between these two limiting extinction conditions, quasi-steady combustion was observed. Another important measurement that is related to spacecraft fire safety is the limiting oxygen index (LOI), the oxygen concentration below which quasi-steady combustion cannot be supported. This is also the ambient oxygen mole fraction for which the radiative and diffusive extinction diameters become equal. For oxygen/nitrogen mixtures, the LOI is 0.12 and 0.15 for methanol and heptane, respectively. The LOI increases to approximately 0.14 (0.14 O 2/0.56 N 2/0.30 C O 2) and 0.17 (0.17 O 2/0.63 N 2/0.20 C O 2) for methanol and heptane, respectively, for ambient environments that simulated dispersing an inert-gas suppressant (carbon dioxide) into a nominally air (1.0 a t m) ambient environment. The LOI is approximately 0.14 and 0.15 for methanol and heptane, respectively, when helium is dispersed into air at 1 atm. The experiments also showed unique burning behavior for large heptane droplets. After the visible hot flame radiatively extinguished around a large heptane droplet, the droplet continued to burn with a cool flame. This phenomena was observed repeatably over a wide range of ambient conditions. These cool flames were invisible to the experiment imaging system but their behavior was inferred by the sustained quasi-steady burning after visible flame extinction. Verification of this new burning regime was established by both theoretical and numerical analysis of the experimental results. These innovative experiments have provided a wealth of new data for improving the understanding of droplet combustion and related aspects of fire safety, as well as offering important measurements that can be used to test sophisticated evolving computational models and theories of droplet combustion.

Related URLs:
http://dx.doi.org/10.1007/s12217-014-9372-2

Measurement of Smoke Particle Size under Low-Gravity Conditions

by on 9 June 2015in Physical Sciences No comment

Smoke detection experiments were conducted in the Microgravity Science Glovebox (MSG) on the International Space Station (ISS) during Expedition 15 in an experiment entitled Smoke Aerosol Measurement Experiment (SAME). The preliminary results from these experiments are presented. In order to simulate detection of a prefire overheated-material event, samples of five different materials were heated to temperatures below the ignition point. The smoke generation conditions were controlled to provide repeatable sample surface temperatures and air flow conditions. The smoke properties were measured using particulate aerosol diagnostics that measure different moments of the size distribution. These statistics were combined to determine the count mean diameter which can be used to describe the overall smoke distribution.

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Properties of Smoke from Overheated Spacecraft Materials in Low-Gravity

by on 9 June 2015in Physical Sciences No comment

Smoke particle size measurements were obtained under low-gravity conditions by overheating several materials typical of those found in spacecraft. The measurements included integral measurements of the smoke particles and physical sample of the particles for Transmission Electron Microscope analysis. The integral moments were combined to obtain geometric mean particle sizes and geometric standard deviations. These results are presented with the details of the instrument calibrations. The experimental results show that, for the materials tested, a substantial portion of the smoke particles are below 500 nm in diameter.

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Prediction of Atomic Oxygen Erosion Yield for Spacecraft Polymers

by on 9 June 2015in Physical Sciences No comment

The ability to predict the atomic oxygen erosion yield of polymers based on their chemistry and physical properties has been only partially successful because of a lack of reliable low-Earth-orbit erosion yield data. The retrieval of the polymer erosion and contamination experiment after 3.95 years in low Earth orbit as part of the Materials International Space Station Experiment 2 provided accurate measurements of the erosion yields of 38 polymers and pyrolytic graphite. The resulting erosion yield data was used to develop a predictive tool with a correlation coefficient of 0.895 and uncertainty of ±6.3×10-25 cm3/atom. The predictive tool uses the chemical structures and physical properties of polymers to predict in-space atomic oxygen erosion yield. A technique which uses the erosion yields of two materials is presented to allow prediction of the erosion yield of a composite material.

Related URLs:
http://dx.doi.org/10.2514/1.48849

Constrained Vapor Bubble Experiment for International Space Station: Earth's Gravity Results

by on 9 June 2015in Physical Sciences No comment

The constrained vapor bubble experiment schedule to fly aboard the International Space Station in the near future promised to give us new insight into the fundamental science of interfacial thermophysics. The evaporating meniscus formed at the corner of the vapor bubble is expected to behave in a significantly different manner in the microgravity environment as compared with the Earth’s gravity environment. Since the constrained vapor bubble can also behave as a micro heat pipe, it will additionally help in gaining a technical understanding of the performance of a micro heart pip in a space environment. Earth-based experiments have been conducted for the past two decades to gain a better knowledge of the rich phenomenon observed in the relatively simple constrained vapor bubble setup. Here, some recent Earth’s-gravity-environment-based data obtained on a 30-mm-long constrained vapor bubble have been presented. The data were fitted to a model, and a self-consistent value of the inside heat transfer coefficient was obtained. The external convective and radiative hear transfer coefficients were also determined. These ground-based experiment forma calibration against which the future data from space-based experiments will be compared.

Related URLs:
http://dx.doi.org/10.2514/1.47522

Fluid Merging Viscosity Measurement (FMVM) Experiment on the International Space Station

by on 9 June 2015in Physical Sciences No comment

The concept of using low gravity experimental data together with fluid dynamical numerical simulations for measuring the viscosity of highly viscous liquids was recently validated on the International Space Station (ISS). After testing the proof of concept for this method with parabolic flight experiments, an ISS experiment was proposed and later conducted onboard the ISS in July, 2004 and subsequently in May of 2005. In that experiment a series of two liquid drops were brought manually together until they touched and then were allowed to merge under the action of capillary forces alone. The merging process was recorded visually in order to measure the contact radius speed as the merging proceeded. Several liquids were tested and for each liquid several drop diameters were used. It has been shown that when the coefficient of surface tension for the liquid is known, the contact radius speed can then determine the coefficient of viscosity for that liquid. The viscosity is determined by fitting the experimental speed to theoretically calculated contact radius speed for the same experimental parameters. Experimental and numerical results will be presented in which the viscosity of different highly viscous liquids were determined, to a high degree of accuracy, using this technique.

Related URLs:
http://dx.doi.org/10.2514/6.2007-1151

Constrained Vapor Bubble Heat Pipe Experiment Aboard the International Space Station

by on 9 June 2015in Physical Sciences No comment

A constrained vapor bubble heat pipe experiment was run in the microgravity environment of the International Space Station. Here we present the initial results that demonstrate significant differences in the operation of the constrained vapor bubble heat pipe in the microgravity environment as compared to the Earth’s gravity. The temperature profile data along the heat pipe indicate that the heat pipe behavior is affected favorably by increased capillary flow and adversely by the absence of outside convective heat transfer as a heat loss mechanism. The reflectivity pattern viewed through the transparent quartz wall documented complex microflow patterns. Image data of the liquid profile in the grooves of the heat pipe indicate that the curvature gradient giving capillary flow is considerably different from that on Earth. Using experimental data for the temperature and meniscus profiles, a one-dimensional model gives the inside heat transfer coefficient, which was significantly higher in microgravity. An initial discussion of some of the data collected is presented.

Related URLs:
http://dx.doi.org/10.2514/1.T3792

Dynamics of interface pattern formation in 3D alloy solidification: first results from experiments in the DECLIC directional solidification insert on the International Space Station

by on 9 June 2015in Physical Sciences No comment

One of the critical microstructures in directional solidification of alloys is the cellular/dendritic pattern that governs the properties and reliability of the solidified material. Our quantitative understanding of the solid–liquid interface pattern has come mainly from experiments in thin samples where the microstructure selection is shown to occur during the dynamical growth process. A more realistic configuration is to examine the evolution of microstructure in three-dimensions, which is not possible terrestrially since convection effects dominate in bulk samples and prevent precise characterization of microstructure dynamics. Recently, experiments under low gravity conditions have been carried out jointly by NASA and CNES in the model transparent system succinonitrile–camphor on board the International Space Station using the Directional Solidification Insert in the DECLIC facility. After a brief description of the experimental setup and methods, the first results on dynamics of interface pattern formation obtained in microgravity will be presented and the information extracted from their quantitative analysis will be discussed.

Related URLs:
http://dx.doi.org/10.1007/s10853-011-5382-2