scope:

Description of Driving Event:

The LO2 and LH2 vent and relief valve position indicator switch tolerance allows the valve to indicate "closed" when it may be open up to 0.30 inch. This condition could allow undetected ullage gas leakage prior to launch. Hot GO2 may auto-ignite the ET thermal protection system (TPS) after SSME ignition. GH2 in flight can cause fire and loss of TPS which could result in a catastrophic situation. The vent/relief valve uses a position indicator switch that is highly accurate. However, no matter how accurate, no position indicator can accurately assure closure because leakage rates for qualification and acceptance are roughly 5E-6 lb/sec. Therefore, a redesign to eliminate the hazard has proven to be virtually impossible.