Analysis of data from the first mission had already yielded new insights into the behaviour of fluids under microgravity conditions, led to significant findings for research into developing better spacecraft safety systems, and allowed investigators to develop a clearer understanding of the role of gravity in the production of protein crystals. ''The results from these experiments will provide important input into our future technology needs,'' USML-2 Mission Scientist Marcus Vlasse told journalists on the afternoon of 20 October 1995, as Columbia settled into orbit for the 18th time.
The mission itself would prove an enormous success, but the efforts involved in getting it off the ground would turn into a gargantuan task. On no fewer than six occasions would Commander Ken Bowersox and his crew prepare themselves for launch, only to be given the disappointing news of a delay as scrub after scrub plagued their mission. Originally scheduled for launch in late September, Columbia was hauled out to the pad on 28 August. So far, her processing flow had gone smoothly. That was about to change.
Already, that summer, a potentially hazardous problem had surfaced concerning erosion in the SRBs' O-ring seals following the STS-71 and STS-70 launches. A leak of hot gas through one of these rubbery seals had led to the loss of Challenger, and Endeavour's STS-69 mission - previously scheduled for 5 August 1995 - was postponed by almost a month until engineers reached a decision that the O-ring damage was within limits. "Depending on how the work goes, there could be as much as a week's threat [to STS-73],'' said KSC spokesman George Diller in mid-August. "That would be the worst case.''
Fortunately, all went well and Endeavour rocketed into orbit on 7 September, completing a spectacular 11-day mission; her boosters were subsequently checked and "all of the post-flight inspections show that they came out clear'', according to Lisa Malone of KSC. Columbia, it seemed, had a green light for her own liftoff on 28 September. However, shortly after technicians began loading her External Tank with 1.9 million litres of cryogenic propellants, the attempt was scrubbed when a sensor detected a hydrogen leak in one of the main engine fuel valves.
The valve was replaced and launch rescheduled for 5 October, but this second attempt to get Columbia off the ground also came to nothing because of fears of high winds, thunderstorms and lightning associated with Hurricane Opal. Weather forecasters expected the hurricane to make landfall near Pensacola on the evening of the 4th, which NASA feared could jeopardise the reloading of the External Tank with propellants and Columbia's launch window the following day. Another try on 6 October was called off when it turned out that fluid had mistakenly been drained from part of the Shuttle's nose-wheel steering hydraulics.
In any event, there was only a 30% chance of acceptable weather conditions, due to the prospect of gusting winds, rain and clouds associated with the hurricane. Columbia's next attempt on the 7th was scrubbed by Launch Director Jim Harrington at T — 20 seconds following an indication of a fault with one of two Master Events Controllers, which send commands to fire pyrotechnic devices to break the SRBs' hold-down bolts. These controllers are also responsible for separating the spent boosters and External Tank from the Shuttle at the correct instant during ascent.
"We did our best today and we sure hope to see you back here very soon and try all the way down to T— zero and a safe launch,'' Test Director John Guidi told a disappointed Bowersox as preparations to replace the problematic controller got underway. Since the devices were mounted in the spacecraft's aft compartment, engineers firstly had to drain the External Tank of its propellants and the next STS-73 launch attempt was rescheduled for 14 October. This later slipped again to the 15th when additional inspections of the main engines' oxidiser ducts had to be made.
These inspections were mandated following the discovery of a crack in a main engine oxidiser duct which was undergoing tests at NASA's Stennis Space Center in Mississippi. Ultrasonic checks were performed on welds on each of the engine's high-
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