Minor Technical Issue Aboard Shuttle Discovery 98
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.'"
Re:STS-9 APU Fire (Score:3, Informative)
I would say that it is less worrying for the astronauts, and more worrying for the engineers on the ground. The astronauts know that a fire has occurred before and that it wasn't deadly (though the circumstances are different). Mission control knows a fire has occurred before and doesn't want to take the chance again!
On a side note, the two APU fires (I miswrote in my previous post--there were two!) were minor issues for STS-9 compared to the 2 failed GPC's and failed IMU that almost killed the astronauts.
Explosive bolts (Score:3, Informative)
Re:Terminology (Score:5, Informative)
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.
Once again, the engineers don't have a clue! (Score:3, Informative)
As for the risk of explosion, well that's poorly written, but if you don't have enough to cause a pressure increase of damaging magnitude, well then there's nothing to worry about. In this case they have a lot less. That suggests to me they looked it over and decided it must be leaking in a way that there's no place for it to accumulate.
Furthermore, according to NASA [nasa.gov], they don't actually use pure hydrazine in the shuttle RCS jets. They use nitrogen tetraxide and monomethyl hydrazine (add on a carbon atom), which are hypergolics. They're more stable but they react spontaneously in each others presence.
Of course, the shuttle engineers don't have a clue about any of this. They like playing dice with their co-workers lives.