meteoroid and orbital debris hazard; the results will have to include applicable ranges and associated confidence levels. This assessment would provide shuttle program managers with a complete picture of the potential risks for specific missions and would make it easier for NASA to determine which areas of the orbiter require better protection.

A valuable component of the survivability assessment would be an end-to-end sensitivity analysis to determine the impact of uncertainties and variabilities in parameters for each of the three components of the current risk assessment process: ORDEM96 (e.g., size distribution, ballistic coefficient, lifetime, atmospheric density profile, etc.), BUMPER (e.g., velocity effects, shape effects, density profile, etc.), and failure criteria (e.g., conservative estimates of damage effects, etc.). The results would be most useful if they included applicable ranges and associated confidence levels.

NASA may find the methodology used by the Department of Defense (DOD) for aircraft survivability studies (Ball, 1985) helpful. The DOD aircraft vulnerability process passes “shotlines” through aircraft to determine which components could be hit by various impactors at various velocities. The process takes into account shielding of critical components by less critical components that may not be necessary for continued flight. The process also allows the DOD to determine whether redundant components are adequately separated or if one impact could damage both redundant systems. “Damage modes and effects” analyses are conducted to determine whether critical components or subsystems could be rendered inoperable by various impactors at various velocities. “Failure modes and effects” analyses are used to determine which components and subsystems are critical to continued flight. In other words, the DOD uses a systematic process that determines the contributions of all components and subsystems to the vulnerability of the total system.

Finding. NASA has not conducted an end-to-end assessment of the survivability of the shuttle with respect to the meteoroid and orbital debris hazard. Similar analyses, however, have been conducted by the DOD to assess aircraft survivability.

Refining Risk Assessments

The in-depth analyses conducted by NASA and Boeing North American Reusable Space Systems to characterize the risk of critical penetration of the orbiter wings and elevons have provided mission planners with more complete information about potential risks to the orbiter. The analyses have also provided valuable input into decisions on whether to modify existing hardware to provide better protection from the impact of meteoroids and orbital debris.

Analyses like these have not yet been performed for other orbiter components or systems. For example, the risk from meteoroids or debris to redundant



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