Bruce A. Banks

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.

Transport of Sputtered Carbon During Ground-Based Life Testing of Ion Thrusters

High voltage, high power electron bombardment ion thrusters needed for deep space missions will be required to be operated for long durations in space as well as during ground laboratory life testing. Carbon based ion optics are being considered for such thrusters. The sputter deposition of carbon and arc vaporized carbon flakes from long duration operation of ion thrusters can result in deposition on insulating surfaces, causing them to become conducting. Because the sticking coefficient is less than one, secondary deposition needs to be considered to assure that shorting of critical components does not occur. The sticking coefficient for sputtered carbon and arc vaporized carbon is measured as well as directional ejection distribution data for carbon that does not stick upon first impact.

Plasma and beam facility atomic oxygen erosion of a transition metal complex

Glassy residues of the complex bis(N,N′-disalicylidene-1,2-phenylenediamino)zirconium(IV), Zr(dsp)2, on glass slides were exposed to atomic oxygen in a plasma asher or an atomic beam facility for various amounts of lime in order to study the erosion process, determine the rate of erosion, and learn the chemical identity of the residue. The exposed films were characterized by weight loss, optical photography, profilometry, diffuse reflectance and total transmittance spectroscopy, scanning electron microscopy (SEM) with wavelength dispersive X-ray spectrometry (WDS), X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). Results indicate that these films erode much more slowly polyimide (Kapton™) film under identical conditions, that the erosion is very nonuniform, and that zirconium dioxide is the predominant product after extended exposure. This complex is currently being evaluated as a polymer additive.

Atomic Oxygen Protective Coatings

Atomic oxygen resident in low Earth orbit (LEO) impinges upon orbiting spacecraft such as Space Station Freedom (SSF) with sufficient flux to cause rapid oxidation and premature failure of organic spacecraft materials. Protective coatings consisting of metal oxides, fluoropolymerfilled metal oxides, and silicones can be used to minimize the reaction of atomic oxygen with organic materials. Such protective coatings are necessary for the long-term durability of polymeric films such as polyimide Kapton solar array blankets and other oxidizable materials. Defects in atomic oxygen protective coatings can enable atomic oxygen to react and oxidize the underlying polymeric material. The number and area of atomic oxygen defects is dependent upon surface irregularities, contamination during protective coating deposition, flexure or abrasion during materials processing, and micrometeoroid or debris impact in space. A combination of ground-based LEO simulation testing, in-space experiments, and Monte Carlo modeling have been utilized to forecast degradation modes of atomic oxygen protected materials exposed to sweeping atomic oxygen arrival conditions such as will occur on SSF.

LDEF spacecraft, ground laboratory, and computational modeling implications on Space Station Freedom's solar array materials and surfaces durability

The low Earth orbital (LEO) durability of Space Station Freedom (SSF) solar cell array materials and surfaces is evaluated using results from the Long Duration Exposure Facility (LDEF), ground laboratory simulation tests, and Monte Carlo modeling. These results indicate that thin-film SiO/sub x/ protective coatings are adequately durable to atomic oxygen, ultraviolet (UV) radiation, thermal cycling, and micrometeoroid or debris impact.<<ETX>>

NSTAR Extended Life Test Discharge Chamber Flake Analyses

The Extended Life Test (ELT) of the NASA Solar Electric Propulsion Technology Readiness (NSTAR) ion thruster was concluded after 30,352 hours of operation. The ELT was conducted using the Deep Space 1 (DS1) back-up flight engine, a 30 cm diameter xenon ion thruster. Post-test inspection of the ELT engine revealed numerous contaminant flakes distributed over the bottom of the cylindrical section of the anode within the discharge chamber (DC). Extensive analyses were conducted to determine the source of the particles, which is critical to the understanding of degradation mechanisms of long life ion thruster operation. Analyses included: optical microscopy (OM) and particle length histograms, field emission scanning electron microscopy (FESEM) combined with energy dispersive spectroscopy (EDS), and atomic oxygen plasma exposure tests. Analyses of the particles indicate that the majority of the DC flakes consist of a layered structure, typically with either two or three layers. The flakes comprising two layers were typically found to have a molybdenum-rich (Mo-rich) layer on one side and a carbon-rich (C-rich) layer on the other side. The flakes comprising three layers were found to be sandwich-like structures with Mo-rich exterior layers and a C-rich interior layer. The presence of the C-rich layers indicates that these particles were produced by sputter deposition build-up on a surface external to the discharge chamber from ion sputter erosion of the graphite target in the test chamber. This contaminant layer became thick enough that particles spalled off, and then were electro-statically attracted into the ion thruster interior, where they were coated with Mo from internal sputter erosion of the screen grid and cathode components. Atomic oxygen tests provided evidence that the DC chamber flakes are composed of a significant fraction of carbon. Particle size histograms further indicated that the source of the particles was spalling of carbon flakes from downstream surfaces. Analyses of flakes taken from the downstream surface of the accelerator grid provided additional supportive information. The production of the downstream carbon flakes, and hence the potential problems associated with the flake particles in the ELT ion thruster engine is a facility induced effect and would not occur in the space environment.

Synergies between Space Research and Space Operations— Examples from the International Space Station

Primary objectives for the International Space Station (ISS) in support of the Vision for Space Exploration include conducting research to counteract the harmful effects of space on human health, test new space technologies, and learn to operate long-duration space missions. In pursuit of these objectives, NASA is interested in closer cooperation between the ISS operational community, scientists, and engineers. To develop the exploration vehicles for missions to the moon and Mars, NASA must test materials, foods, and medicines to ensure their performance in the space environment. These results will enable important decisions on the materials to be used for future space vehicles. Another critical factor for the success on future missions beyond Earth orbit is the capability for repairs of equipment. On the ISS, the practice of crewmembers performing repairs in microgravity will increase our understanding of the repair processes in space; when these capabilities are needed during future space exploration missions, we will have the knowledge and experience to perform them. The ISS is a unique and irreplaceable training ground for building the operational knowledge required to safely conduct future exploration missions, and the growing links within the science, engineering and operations communities are reinforcing the value of that training. Current interactions between the communities that support the ISS have already produced many synergies that are significantly accelerating NASAs advancement towards future exploration missions in support of the Vision.

Simulated Solar Flare X-Ray and Thermal Cycling Durability Evaluation of Hubble Space Telescope Thermal Control Candidate Replacement Materials

During the Hubble Space Telescope (HST) second servicing mission (SM2), astronauts noticed that the multi-layer insulation (MLI) covering the telescope was damaged. Large pieces of the outer layer of MLI (aluminized Teflon® fluorinated ethylene propylene (Al-FEP)) were cracked in several locations around the telescope. A piece of curled-up Al-FEP was retrieved by the astronauts and was found to be severely embrittled, as witnessed by ground testing.

Mir Solar-Array Return Experiment: Power Performance Measurements and Molecular Contamination Analysis Results

A solar array segment was recently removed from the Mir core module and returned for ground-based analysis. The segment, which is similar to the ones the Russians have provided for the FGB and Service Modules, was microscopically examined and disassembled by US and Russian science teams. Laboratory analyses have shown the segment to be heavily contaminated by an organic silicone coating, which was converted to an organic silicate film by reactions with atomic oxygen within the orbital flight environment. The source of the contaminant was a silicone polymer used by the Russians as an adhesive and bonding agent during segment construction. During its life cycle, the array experienced a reduction in power performance from ~ 12%, when it was new and first deployed, to -5%, when it was taken out of service. However, current-voltage measurements of three contaminated cells and three pristine, Russian standard cells have shown that very little degradation in solar array performance was due to the silicate contaminants on the solar cell surfaces. The primary sources of performance degradation is attributed to thermal hot-spotting or electrical arcing; orbital debris and micrometeoroid impacts; and possibly to the degradation of the solar ceils and interconnects caused by radiation damage from highenergy protons and electrons.

Optical and Scanning Electron Microscopy of the Materials International Space Station Experiment (MISSE) Spacecraft Silicone Experiment

Under a microscope, atomic oxygen (AO) exposed silicone surfaces are crazed and seen as “islands” separated by numerous crack lines, much analogous to mud-tile cracks. This research compared the degree of AO degradation of silicones by analyzing microscopic images of samples exposed to low Earth orbit (LEO) AO as part of the Spacecraft Silicone Experiment. The experiment consisted of eight silicone samples exposed to different AO fluence levels (ranged from 1.46 to 8.43 × 1021 atoms/cm2) during two Materials International Space Station Experiment (MISSE) missions. Image analysis software was used to analyze optical microscopic images. The fraction of sample surface area occupied by crack lines was obtained and used to characterize silicone degradation and the resulted loss of specular transmittance. SEM images from the eight samples exposed to different AO fluences suggest a sequence of surface shrinkage, stress, and crack, followed by re-distribution of stress and shrinking rate on the sample surface. Energy dispersive spectra (EDS) indicated that after long AO exposure, silicone samples will eventually have a SiO2 surface layer with some trapped CO and CO2.