Risk Management for the Tiles of the Space Shuttle

Abstract

The tiles of the space shuttle orbiter are critical to its safety at reentry, and their maintenance between flights is time-consuming. We performed a probabilistic risk analysis to identify the most risk-critical tiles and to set priorities in the management of the heat shield. The model is based on a multiple partition of the orbiter's surface. For the tiles in each zone, we used the following data: (1) the probability of debonding due either to debris hits or to a poor bond, (2) the probability of losing adjacent tiles once the first one is lost, (3) the probability of burn-through given the final size of the failure patch, and (4) the probability of failure of a critical subsystem under the skin of the orbiter if a burn-through occurs. A risk-criticality scale was designed based on the results of this model. It is currently used (along with temperature charts) to set priorities for the maintenance of the tiles. We found that 15 percent of the tiles account for about 85 percent of the risk and that some of the most critical tiles are not in the hottest areas of the orbiter's surface. We recommended that NASA inspect the bond of the most risk-critical tiles and reinforce the insulation of the external systems (external tank and solid rocket boosters) that could damage the high-risk tiles if it debonds at take-off. We computed that such improvements of the maintenance procedures could reduce the probability of shuttle accident attributable to tile failure by about 70 percent.