re: Shuttle launch has been scrubbed
They are very cautious. The sensor that failed is 1 of 4 of which only 2 are needed. It would only come into play during an abort situation where they would burn long enough to deplete the hydrogen. If the hydrogen were depleted and the engine tried to keep running it would explode. So they want it to shut off when the fuel lever reaches a specified low level. They will not launch unless all 4 are operating normally. Had it failed after launch we probably would have never heard about it.