America’s walk-on-eggshells return to manned spaceflight proved to be an anticlimactic disappointment yesterday, with the Space Shuttle Discovery’s launch being scrubbed due to faulty fuel sensors. (No, it was not the falling window cover that did it.) The sensors monitor propellant levels at the bottom of the external fuel tank during launch, signalling the need to shut off the enginges when they detect low levels of fuel. (Liquid hydrogen, in this case.) Engines normally shut off at a predetermined velocity in the launch process, but these fuel sensors act as an emergency backup to prevent the engines from continuing to burn without a continued supply of propellant, which would cause engine damage — a highly unlikely emergency situation, but a consideration important enought to warrant postponing the launch, it seems.
Next earliest possible opportunity for a launch will be Saturday afternoon, though that’s not looking likely. Update: Late next week at the earliest is now the current estimate. Countdown will have to restart at T-43 hours.