Probabilistic Risk Assessment in Return-to-Flight Efforts

Speaker:
Luc Huyse
Southwest Research InstituteAbstract:Since the loss of the Columbia in February 2003 and the conclusion of the Columbia Accident Investigation Board, Southwest Research Institute has been involved in several return to flight efforts. This presentation summarizes some of the findings of two probabilistic risk assessments. The first risk assessment is related to fatigue cracking of the feedline flowiners that supply liquid hydrogen to the main engines of the Space Shuttle. The second risk analysis is related to the damage to the leading edge RCC panels and the Thermal Protection Shield due to foam debris impact. The first assessment was performed as part of an Independent Technical Assessment (ITA) for the NASA Engineering and Safety Center (NESC). The overall goal was to establish a flight rationale in light of a history of fatigue cracking due to flow induced vibrations in the feedline flowliners that supply liquid hydrogen to the space shuttle main engines. Prior deterministic analyses using worst-case assumptions predicted failure in a single flight. The current work formulated statistical models for dynamic loading and cryogenic fatigue crack growth properties, instead of using worst-case assumptions. New weight function solutions were developed to describe the crack "driving-force". Monte Carlo simulations showed that low flowliner probabilities of failure (POF = 0.001 to 0.0001) are achievable, provided pre-flight inspections for cracks are performed with adequate probability of detection (POD) -- specifically, 20/75 mils with 50%/99% POD. The second assessment was performed for NASA Johnson. NASA and SwRI engineers are developing and refining methodology to quantify the probability of damage to the thermal protection system. The primary debris sources during ascent are the insulating foam covering the external tank (ET), and the ice that can form on the ET before and during launch. Upon detachment, aerodynamic drag forces act to slow the speed of the debris, thereby increasing the relative velocity between the debris and the orbiter. Also during transport, lift forces act to disperse the debris about their idealized trajectories. Therefore, the farther downstream the debris travels before impact, the greater the potential impact kinetic energy and crossrange (dispersion). Damage depends on the concurrence of three events: debris release, debris impact, and impact kinetic energy exceeding panel capacity. The total probability of damage is obtained as the product of these three event probabilities summed over all release and impact locations. Current work with NASA Johnson focuses on removing some of the conservatism of the analysis and transforming the semi-probabilistic analysis into a full probabilistic analysis.

Location: Kaprielian Hall (KAP) - 203

Audiences: Everyone Is Invited

Contact: Evangeline Reyes