NASA Scrubs Launch Due to Faulty Fuel-Tank Sensor

VUSE g-EE-k writes "NASA has scrubbed Wednesday's launch of Discovery due to a faulty fuel-tank sensor in the external fuel tank. They are going to begin the troubleshooting process. They have not released details as to how long this delay will last. The crew have begun to get off the shuttle. For more information, see the NASA TV site. Drudge Report has some initial coverage of the scrub." Reader adefa adds a link to NASA's Space Shuttle launch page with more info.

Comments

Re:Redundant system

[Aglassis]

They had a fuel low level sensor fail. This was some sort of instrumentation and control equipment or sensor fault. The possible causes could be that the actual sensor failed (which might require replacing the liquid fuel tank) or there was an instrumentation fault. Since they were using a test circuit to simulate a low level (since the tank is actually full), an instrumentation failure could be either a failure in the normal instrumentation circuitry or a failure in the test circuitry. Either of these two cases should be easy to fix.

As someone who has worked extensively on I&CE operation, maintenance, and repair on nuclear reactors, I fully understand why they scrubbed the launch. Redundancy is for faults in operation, not to compensate for damaged equipment prior to operation. From my experience, it is probably the test circuit that failed. Then the instrumentation circuitry. Then, in the most unlikely case, the sensor itself.

An astronaut on NASA TV explained that the there is a coincidence circuit if two low level alarms trigger that will cause an automatic engine turnoff. If this did not happen and the tank completely emptied, he said that it could cause major damage to the shuttle's main engines. I'm not sure exactly how, perhaps because either without liquid hydrogen, only the oxygen would flow through the engine and no chemical reaction would occur, cooling parts of the engine below their specifications? Or flow characteristics wouldn't be predictable?