Columbia's scheduled 9 October launch date was postponed by nearly a month following a spillage of nitrogen tetroxide as technicians were loading the highly toxic oxidiser into the forward RCS unit. Five-and-a-half litres splashed onto the Shuttle's nose, requiring 379 fragile tiles to be painstakingly removed, cleaned and reapplied. A revised date was set for 4 November and seemed to be going well until two days before, when, during the loading of the oxygen tanks of the fuel cells that were to power most of the orbiter's electrical systems, technicians noticed that one tank was losing pressure. A change in the oxygen-loading procedure seemed to rectify the situation.
Apart from some minor concerns about poor weather, the 4 November attempt seemed to be proceeding normally until nine minutes before launch. At that point, the countdown was halted following an indication of lower-than-allowable pressures in the oxygen tanks of the fuel cells. The problem was resolved within a couple of minutes and the countdown resumed, but quickly encountered further problems. Five minutes before launch, the three APUs were switched on and quickly started showing higher-than-normal oil pressures.
The clock continued ticking to T —31 seconds - just before the point at which, normally, the Shuttle's onboard computers would take over command of the remainder of the countdown from the Ground Launch Sequencer (GLS). At this stage, the oxygen tank pressures registered as 'low' and the clock was stopped. Unable to override this command, the APUs were shut down and the clock was recycled to the T —9 minute point, then T —20 minutes, to enable NASA and Rockwell technicians to assess the situation. During their deliberations, however, the weather closed in, further jeopardising Columbia's chances of launching that day.
Eitherway, it was decided that the APU oil pressures were too high - 100 psi instead of the maximum allowable 60 psi - and the Mission Management Team made the decision to scrub the launch attempt. Subsequent analysis revealed that the APUs' oil filters had become clogged by pentaerythritol, a crystal formed when hydrazine penetrated their gearboxes, which had caused the rise in temperature. Both gearboxes were flushed, their filters replaced and the launch was rescheduled for 12 November. As the astronauts had also reported that their visibility was marginal, Columbia's cockpit windows were cleaned in time for the next attempt.
Eight days later, the men returned to the pad for a second try. There had already been minor problems during the loading of the oxygen tanks, which meant that Tank 3 had to be loaded and pressurised separately from the other two tanks. This resolved the issue. Another glitch cropped up late on 11 November when one of four Multiplexer-Demultiplexers (MDMs) - which provide instrumentation measurements, commands and data to the Shuttle's cockpit displays - failed. A spare MDM was fitted, but also turned out to be faulty, requiring another one to be flown from California in the early hours of the 12th.
The new MDM, interestingly, came from the second space-rated orbiter, Challenger, which at the time was undergoing a final checkout at Rockwell's Palmdale plant in readiness for transportation to KSC. It was the first of many occasions in which parts were 'cannibalised' from one spacecraft to enable another to fly. Challenger's first orbital voyage was tentatively booked for some time early in 1983; unlike Columbia she would not need to perform a series of flight tests and would fly an operational mission to deploy an important NASA communications satellite.
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