this analysis (Austin, 1997). The 1/60 criterion has been exceeded on subsequent missions to accommodate Mir operations (Loftus, 1997).

The shuttle program has not established a guideline for the maximum acceptable risk that the orbiter windows or other systems will have to be repaired following a mission. The program has accepted that damage to the crew cabin windows will require that, on average, one window will have to be replaced after each flight.

Assessing Risks for Individual Shuttle Missions

Preflight meteoroid and orbital debris risk assessments for the space shuttle were first conducted in 1993 and are now conducted routinely prior to every shuttle mission. Figure 3–1 is a schematic diagram of the various steps involved in calculating the risk from meteoroids and orbital debris. These risk assessments are based upon an approximation of the altitude and attitude time lines predicted for a shuttle mission. The orbital debris environment model (ORDEM96) and the meteoroid model are combined with a model of the orbiter (BUMPER) to evaluate risks for each mission. (These models are described in detail in Chapter 4.)

For each orbiter mission, an initial risk assessment is presented at the cargo integration review (CIR), which typically takes place approximately 12 months before launch. This gives mission planners enough time to minimize the time the orbiter will spend in high-risk attitudes (Brekke, 1997). Specific risks evaluated for each flight profile include the probability of critical penetration, the probability of penetration of a radiator tube, and the probability of window replacement. When a risk assessment indicates that the risks of a proposed mission profile exceed accepted limits, changes are implemented iteratively until an acceptable level of risk is reached.

Refining Risk Assessments

Until 1995, models of the orbiter’s ability to survive the impacts of meteoroids and orbital debris incorporated extremely conservative failure criteria. For example, the pre-1995 criteria assumed that any penetration of the bottom side of the leading edge RCC elements of a wing, of the wings themselves, or of the wing elevons would be critical Considerable analyses by NASA and the orbiter manufacturer, Boeing North American Reusable Space Systems, however, have significantly improved the understanding of which penetrations could be critical (Hasselbeck et al., 1997).

NASA and Boeing North American first identified the RCC leading edge of a wing, the rest of the wing, and the elevons as the areas of the orbiter that appeared to pose the highest risks for critical failure. Next, detailed analyses and limited testing of the effects of impacts were conducted on these areas. The analyses examined the immediate effect of an impact (e.g., the hole and associated

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