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Boeing has been developing predictive models to determine the exposure of a spacecraft surface to atomic oxygen (AO). The primary AO model is used to determine the AO flux (atom/cm2-s) and fluence (atom/cm2) to flat surfaces. The model includes the effects of thermal motion of ambient AO atoms and co-rotation of the atmosphere in addition to the ambient atmospheric density and the velocity of the spacecraft. The model treats noninterfering surfaces as arbitrary, but definite, orientations with respect to the spacecraft motion (ref 1).
In addition to the primary model, the AO microenvironments model was developed to account for interference, or shadowing, by the three-dimensional structure of a spacecraft. This model also accounts for specular and diffuse reflectance from surfaces exposed to either primary or secondary impacts, and accounts for the potential of individual atoms to recombine on, or react with, the impacted surface (ref 1).
The following sections are from the Analysis of Materials Flown on the Long Duration Exposure Facility: Summary of Results of the Materials Special Investigation Group, NASA CR, 1995 (ref 2).
Atomic Oxygen Fluence Versus Incidence Angle
Molecules in a gas in thermal equilibrium have a Maxwellian speed distribution characteristic of their temperature. At 1000K, a typical LEO temperature, the average molecular speed of atomic oxygen is 1.15 km/sec compared to an average speed of a spacecraft relative to the atmosphere of 7.24 km/sec at 400-km altitude in an easterly orbit. Thermal molecular motion effects atomic-oxygen flux on a surface at high incidence angles.
The atomic oxygen calculation developed by Boeing takes into account the effect of thermal molecular motion. When thermal molecular velocity is considered, at the temperatures encountered, the calculations show that surfaces parallel to the ram direction receive approximately 4% of the ram flux. On LDEF, surfaces out to angles of about 105 deg from ram experienced atomic oxygen fluxes which caused measurable changes on specific materials. For incident angles less than approximately 87.5 deg, predicted atomic oxygen fluxes with or without the inclusion of thermal velocity are nearly equal.
Atomic Oxygen Fluence Versus Time
Atomic oxygen flux was not constant during the mission. The flux rate to the surfaces varies by about two orders of magnitude from solar minimum to solar maximum conditions. Decreasing solar activity caused atomic oxygen flux to decrease during the first 3 years of flight. This decrease in solar activity was sufficient to overcome the countering influence of the slight altitude decrease on the flux during this time period. Thereafter, the combination of increasing solar activity and decreasing altitude caused the atomic oxygen flux to increase rapidly. Because the flight began near solar minimum conditions and ended essentially at solar maximum, coupled with the decrease in altitude over the mission, the majority of the oxygen exposure occurred rather late in the mission. About 57% of the atomic oxygen exposure accumulated during the last 6 months of the LDEF mission. The last year of the flight accounted for roughly 75% of the total exposure.
In addition, because the vehicle was pitched so that the space end was forward of vertical by 8 deg, trays on the space end of the vehicle received more atomic oxygen than did trays on the Earth end of the vehicle.
Spacecraft Design Considerations
Some considerations for spacecraft design relative to atomic oxygen were presented by Dr. Ann Whitaker during the LDEF Materials Results for Spacecraft Applications Conference, Huntsville, AL, October 27-28, 1992. These relate to analyses of Experiment AO171, and are presented below.
Data from Experiment AO171 indicated that long term AO erosion of carbon composites can be predicted from carbon reactivity, with reactivity defined as a change in material thickness per AO fluence. Glass fiber composites tend to become self-protecting and would thus perform well in an AO environment. In addition, AO171 data indicated that thermalk control tapes worked well in protecting the underlying composite from AO attack. Data on the AO171 polymers, coupled with short-term shuttle polymer data indicated that the unfilled "pure" polymers react linearly with AO such that long-term AO erosion can be predicted from short-term shuttle data. AO171 glass ceramics underwent a densification accompanied by a decrease in film thickness of less than a few hundred angstroms as a result of the space exposure. The role of AO in this densification process is not clearly understood. Data on the AO171 paints indicated that the AO erosion process is nonlinear. However, thermal/optical property data, in which emissivity values increased slightly while solar absorptivity values generally decreased slightly, indicates that the paints would last longer than previously predicted from short-term shuttle data. AO interactions with AO171 metals clearly showed a nonlinear relationship which is strongly dependent on temperature, stress, and material microstructure (ref 3).
1. Bourassa, R.J., and H.G. Pippin, Model of Spacecraft Atomic Oxygen and Solar Exposure Microenvironments, LDEF Materials Results for Spacecraft Applications Conference, October 1992.
2. Pippin, H.G., Analysis of Materials Flown on the Long Duration Exposure Facility: Summary of Results of the Materials Special Investigation Group, NASA CR, 1995.
3. Whitaker, A.F., and R.R. Kamenetsky, Atomic Oxygen Erosion Considerations for Spacecraft Materials Selection, LDEF Materials Results for Spacecraft Applications Conference, October 1992.