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Proceedings of the Eighth International Conference on Probabilistic Safety Assessment & Management (PSAM)

Michael G. Stamatelatos
Michael G. Stamatelatos
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Harold S. Blackman
Harold S. Blackman
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ASME Press
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Meteoroids and orbital debris (M/OD) pose a serious threat to spacecraft and crew. For years, NASA has evaluated M/OD risk for the Space Shuttle and the International Space Station (ISS) using the BUMPER II software code. However, BUMPER II provides only a point estimate of risk with no uncertainty bounds or consideration of uncertainty effects.

The uncertainty in BUMPER II risk predictions is thought to be significant. Uncertainties in environment models, ballistic limit equations, and failure criteria are all believed to be significant contributors to this uncertainty. Results from BUMPER II risk predictions are currently used in NASA's probabilistic risk assessments (PRAs) for both the Space Shuttle and ISS. Including uncertainty bounds with BUMPER II risk predictions is necessary to provide a more meaningful picture of the M/OD risk and to provide needed input to PRAs and decision makers.

Under the sponsorship of NASA Headquarters, work is ongoing by the University of Denver Research Institute (DRI) and NASA Headquarters to develop a tool, called Sampling, which can be used as a preprocessor to BUMPER II to allow a Monte Carlo type analysis to be performed using BUMPER II as the engine for the analysis.

The first version of the preprocessor was used to perform an exploratory study using the Space Shuttle. The study was designed to answer, at least in part, the following questions:

1. How do we characterize input M/OD uncertainties?

2. How do we quantify the effects of these uncertainties on the results?

3. Are the uncertainties on the results significant?

4. How do we prioritize the uncertainty contributions?

This version of Sampling can include the effects of the uncertainty from the following input sources:

1. Density of the impacting particle

2. Flux intensity

3. Ballistic limit equations (BLEs)

4. Failure criteria

5. Velocity distribution

The exploratory study examines the viability of the approach of using BUMPER II as the engine for Monte Carlo style assessments to quantify uncertainties in M/OD risk predictions. The results from the study indicate that M/OD uncertainties are significant and that including effects of uncertainties may result in significantly different mean and median results as compared to the baseline point estimates currently used. The effect of uncertainty in density appears to be quite significant.

More work is needed to fully identify key inputs for uncertainty evaluation. Some potentially key inputs such as uncertainty due to the shape and orientation of an impacting particle have not been included in this study. Further work to define robust approaches for characterizing uncertainty distributions for the key inputs is needed to improve the fidelity of the results.

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