Problem Grounds Space Shuttle

Once again, NASA has been forced to delay launching the space shuttle Atlantis because of a nagging problem with fuel sensors on the orbiter's external fuel tank. As Houston Public Radio's Jim Bell reports, this problem is a lot more complicated than you might think.

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The big fuel tank has four sensors that keep track of the liquid hydrogen fuel as it burns during liftoff to make sure the main engines shut off at precisely the right time, and something is causing two of the sensors to malfunction intermittently. NASA spokesman Kyle Herring says they're not unlike the gasoline gauge in your car.

"If you're sitting in your car and you just left the gas station, and you know without a doubt that you filled your tank up, but you get in and your fuel gauge is reading half a tank, you're saying 'Wait a minute. I know I filled this up so what's wrong?"

Herring says the sensors aren't the problem. The problem is in the electrical system the sensors are connected to, and it comes and goes. They have no idea where the problem is.

"There are these four sensors, but they're on a string that leads through cabling that has quick disconnect connectors, pins, wiring, avionics boxes, all of that, and what we want to do is try to trace it down to a finite point to determine what the problem is, and even if there is a problem."

Herring says even though they have four sensors, two of which are redundant backups, the launch is postponed while engineers troubleshoot the electrical system to find out what's wrong, if they can.  Right now, launch is set tentatively for January 2nd, if the sensor problem is resolved.  Jim Bell, Houston Public Radio News.