In addition, tests have not yet been deliberately conducted to the point of failure to determine actual engine operating margins. The Orbiter has also performed well. There is, however, one serious petential failure mode related to the disconnect valves between the Orbiter and the External Tank. The present design includes two 17-inch diameter valves, one controlling the oxygen flow, and the other the hydrogen flow from the tank to the Orbiter's three engines. Each of the disconnect valves has two flappers that close off the flow of the liquid hydrogen and oxygen when the External Tank separates from the Orbiter.
New designs are under study, incorporating modifications to prevent inadvertent valve closures.
Redesigned valves could be qualified, certified and available for use on the Shuttle's next flight. While the External tank has performed flawlessly during all Shuttle flights, one area of concern pertains to the indicators for the two valves which vent the liquid hydrogen and liquid oxygen. These valves can indicate they are closed when they might be partially open. This condition is potentially hazardous, since leaks of either gaseous oxygen or hydrogen prior to launch, or in flight, could lead to fires. This could, in turn lead to catastrophic failure of the External Tank.