Application of the NASA risk assessment tool to the evaluation of the Space Shuttle external tank re-welding process

The current Space Shuttle external tank design is called the super light weight tank (SLWT). A weight reduction of approximately 30% was achieved relative to the prior design called the light weight tank (LWT). The new NASA risk assessment tool, the quantitative risk assessment system (QRAS), was used to compare the risk of the two designs. The comparison includes consideration of the apparent reduction of the design safety factor for SLWT welds when a weld repair is required. The risk models for the structural failure accident scenario include five initiating events (IEs): (1) liquid oxygen (LO2) tank component failure; (2) liquid hydrogen (LH2) tank component failure; (3) LO2 tank weld failure; (4) LH2 tank weld failure; and (5) intertank failure. Although the risk results for the LH2 and LO2 tank welds for IEs 2 and 4 are higher for the SLWT vs. the LWT, the reverse is true for tank components IEs 1, 3 and 5. The SLWT has a slightly lower risk of structural failure. The impact of this difference is not significant to the total risk when the other six scenarios are also included.