scope:

Description of Driving Event:

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):

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).

B. Mechanical heating of or by particulate contamination impinging on a surface.

C. Compression and/or shock heating of one of the silastic o-rings in the shutoff assembly.

Testing at White Sands Test Facility was unable to duplicate these events in over 2200 attempts or confirm any of these ignition mechanisms.