NASA-LLIS-0008

Abstract

<strong>Description of Driving Event:</strong> <br>A flash fire occurred during a performance record test of the shuttle Extravehicular Mobility Unit (EMU). This fire occurred when a shutoff valve was opened, supplying 6,500 psi oxygen to the EMU. Ignition occurred and a very high temperature oxygen- rich aluminum fire ensued. The shutoff valve melted in the fire, relieving the system pressure and causing the metallic fire to extinguish. Residual fires were extinguished using carbon dioxide extinguishers. A finding of the mishap investigation board was that the most probable ignition site was within the regulator module in a flow restrictor. This flow restrictor consists of two drilled passages, which intersect in a "V" shape. Manufacturing practices allow some overdrill at the intersection of these two passages, which may have formed a stagnant volume at the base of the "V". This stagnant volume allows compression heating (adiabatic compression detonation) and/or shock heating, which ruptured and ignited a thin section of aluminum and spread to the other metallic components of the module in the oxygen enriched atmosphere. Other possible ignition mechanisms included (in priority order): <br>A. Compression and/or shock heating of contaminant(s) entrapped in the stagnant volume of the flow restrictor (all five other regulators disassembled and inspected showed some particulate debris within the system). <br>B. Mechanical heating of or by particulate contamination impinging on a surface. <br>C. Compression and/or shock heating of one of the silastic o-rings in the shutoff assembly. <br>Testing at White Sands Test Facility was unable to duplicate these events in over 2200 attempts or confirm any of these ignition mechanisms.