217 Risk to Space Shuttle Orbiter Windows from Particles in the Booster Separation Motor Plumes and from Foam Debris (PSAM-0178)

The Space Shuttle Orbiter forward windows are designed to withstand impacts from particulate emanating from various sources, including: aluminum oxide (Al₂O₃) particles in the plume of the Booster Separation Motor (BSM), foam fragments from the External Tank (ET), and micrometeoroids and orbital debris (MMOD). Thus far, none of these sources has caused a failure of a window to occur, but post-flight examinations of the windows have identified craters whose depths have exceeded the design margin of safety (MOS).

Therefore, a study was performed to determine which of these types of particles are most likely to have been responsible for the craters observed to date, and to estimate the probabilities for BSM and foam debris that the MOS will be exceeded during a typical flight. The probabilities of exceeding critical damage thresholds were calculated by integrating results from debris transport analyses and window impact damage testing into an event tree logic model. To populate the event tree, probability distributions were developed for fragment sizes, velocities, and impact angles, as well as for the amount of damage caused by fragments impacting with various sizes, velocities, and angles.

The results of the analysis indicate that the number of craters observed to have exceeded the design MOS, and their distribution over the various windows, is consistent with a combination of impacts from BSM particles and MMOD. BSM particles appear to be the principal contributor for impacts producing pit depths on the order of 0.001 inch or less, whereas MMOD becomes more important for pit depths in excess of 0.001 inch. The risk to the windows from foam fragments was found to be very small because of the small number of particles released from the ET during a typical flight, and because of the favorable orientation of the windows with respect to foam releases.

The two outermost windows have the smallest thickness and are the most sensitive to impacts from BSM particles. The results of this study supported a NASA decision to replace the outermost windows with thicker panes prior to the launch of STS-114.