It would not be the last potential show-stopper, however, for Columbia's launch was twice postponed, before she finally set off on her third attempt. To circumvent the possibility that Mission Specialist Steve Hawley - now with four spaceflights and around a dozen launch delays under his belt - was not to blame, he wore a brown paper bag over his head to go to the launch pad. The Hawley gremlin seemed to have struck again on 20 July, when the STS-93 countdown was halted seven seconds before launch, after a high level of hydrogen gas was detected in Columbia's aft compartment.
It was a particularly dangerous time, coming half-a-second before main engine ignition; if the halt had been called after the engines were alight, the result would have been an on-the-pad abort and probably a month-long delay in getting the Shuttle ready for another attempt. The cause of the 20 July problem seemed to be a hydrogen 'spike', which a sharp-eyed launch controller spotted briefly peaking at 640 parts per million - double the maximum-allowable 'safe' limit.
During the momentary crisis, the mood in the Launch Control Center at KSC was tense, as indicated by the voices on the communications loop. Sixteen seconds before launch, it seemed, one of two hazardous-gas-detection systems indicated the 640 ppm hydrogen concentration and even though the second device showed a more normal level of 110-115 ppm, launch controller Ozzie Fish radioed his colleague at the Ground Launch Sequencer (GLS) to manually stop the countdown. To the assembled spectators at KSC, listening to spokesman Bruce Buckingham's commentary over the loudspeaker, all was normal, however.
''T minus 15 seconds,'' announced Buckingham, then ''T minus 12 ... ten ... nine . . . ''
Inside the Launch Control Center, Fish urgently radioed: ''GLS, give cutoff!''
'' . . . eight, seven . . . '' continued Buckingham.
''Cutoff! Give cutoff!'' interjected NASA Test Director Doug Lyons.
''Cutoff is given,'' replied the controller at the GLS console.
''We have hydrogen in the aft [compartment],'' Fish reported, ''at 640 ppm.''
By now well past what would have been a 'normal' ignition of the main engines, Buckingham announced the disappointing news to the assembled crowds. Back in the Launch Control Center, the hydrogen concentration was already decreasing back to normal levels. Lyons polled his team, asking them if any emergency safing procedures were needed, such as evacuating the crew from Columbia, and was told that this would not be necessary. Within 10 seconds of the call for cutoff, the indication of high levels had dropped to 115 ppm. Engineers would later blame the problem on faulty instrumentation and flawed telemetry.
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