Minor Technical Issue Aboard Shuttle Discovery

IZ Reloaded writes "Space Shuttle Discovery has a problem with the pipeline for an auxiliary power unit that controls the shuttle's hydraulic steering and braking maneuvers. CNN reports that the pipleline is leaking 'fuel' at about six drops per hour." From the article: "The leak is more likely nitrogen, but there is no way of knowing that, so NASA is treating the problem as if the leak were fuel ... If it is fuel, the current rate is still 100,000 times slower than what would cause a fire ... Just in case, NASA will turn on the power unit with the leak early Sunday as part of its normal testing and then see if the leak rate changes. If it does, NASA may burn off the hydrazine and shut down the power unit before the shuttle returns to Earth to eliminate any fire hazard.'"

Comments

Re:Terminology

[NOLAChief]

There have been pipelines in space since the beginning of the use of liquid fueled rocket engines. Propellant has to get from the tanks to the engines somehow...

They mean it, there really is no way of knowing. They know there's a leak based on pressure readings. They know it's not an instrumentation issue because those pressure readings are redundant (i.e. if one sensor started trending down and it's backup didn't, then the sensor's bad). Based on those same pressure readings they know what the leak rate is (drops per hour was probably the guy's attempt at making it make sense to the layperson by analogy to a dripping faucet. Sadly that analogy seems to have fallen flat.) Since the fuel tank (hydrazine) is connected to the pressurant system (nitrogen), the entire system is at the same pressure, so since there is a leak, every pressure sensor monitoring the system is trending down.

(Time for my own bad analogy) Let's say you've got a Super Soaker with a pressure gauge in the water reservoir. You pump up the Super Soaker and put it in a box so that the only thing you can see is the pressure gauge. Now, somehow a hole forms in the reservoir. Because you can't see the reservoir, you don't know if it's your fuel (the water) or the pressurant (the air you pumped into the thing) that's leaking, but you know from the decreasing pressure reading that there's a leak present. That's essentially what's going on with Discovery. Hence, they're playing it safe and assuming the leak is fuel, when more likely it is the smaller nitrogen molecule that's escaping the system.