Had the STS-93 Chandra-deployment mission flown, as planned, in the winter of 1998, Columbia's modification period should have been completed in time to enable her to stage a high-priority repair of the Hubble Space Telescope in June 2000, featuring no fewer than six spacewalks - a new Shuttle record. It never happened. Delays in getting Chandra and its IUS booster ready pushed STS-93 into mid-1999; then an electrical short four seconds after liftoff, which knocked out two critical main engine controllers, leaving them running on the backups, forced a lengthy series of inspections of Columbia's 375 km of wiring.
Not only that, but Columbia also suffered a small leak of liquid hydrogen from her right-hand main engine, causing all three to shut down four seconds early. The leak was minor, but significant enough for the main engines to consume liquid oxygen at a higher-than-normal rate to maintain thrust during the remainder of ascent, resulting in the engines shutting down earlier than expected when the oxygen tank ran dry. This, in effect, left the STS-93 crew in an orbit 11 km lower than intended. Fortunately, the mission otherwise ran perfectly.
Detailed analysis after Columbia's landing traced the cause of the leak to several impact-damaged coolant tubes lining the interior of the affected main engine nozzle. This damage, engineers later concluded after extensive borescope inspections, was probably caused by a 2.5-cm-long steel-alloy pin which came loose and hurtled through the combustion chamber, holing three of the 1,080 coolant tubes. Indeed, photographs taken 15 seconds after STS-93's liftoff revealed an unusual bright 'streak' coming from the suspect engine's bell, strongly indicative of leaking hydrogen.
''It appears pretty conclusively that we were impacted by some type of debris,'' said Shuttle planning manager Bill Gerstenmeier in early August 1999. ''What you see is evidence of an impact that scraped away some of the metal and thinned it out enough that the tube could no longer hold pressure. The tubes ruptured, creating the three holes.'' The question of whether NASA's decision to fly older engines on STS-93 - part of efforts to shave weight off Columbia to house Chandra - contributed to the problem was not overlooked. In fact, the leaking engine had been used 18 times since 1983.
''We very carefully review these engines prior to flight,'' said Gerstenmeier. ''These were perfectly acceptable to use and more middle-of-the-road even than some of the other engines we have flown.''
The working-loose of the pin was a one-off incident and had no immediate implications for the fleet. However, two failed engine controllers proved much more serious and far-reaching. By mid-August, NASA gave the controllers themselves a clean bill of health and focused its attention on damaged wiring insulation beneath Columbia's payload bay floor. ''From looking at [the wiring] with the electron microscope,'' said spokesman James Hartsfield, ''the indentations in the wire were a tell-tale sign that it was really an impact type of damage,'' adding that the damage could have developed at any point since the orbiter had been built.
Worse, if it had occurred during routine maintenance, the other orbiters might be similarly affected. Endeavour, Discovery and Atlantis, which were partway through processing flows for their own missions later in 1999, were inspected and instances of damaged wiring were repaired or replaced. Steps were also taken to install flexible plastic tubing over some wiring, as well as smoothing and coating rough edges nearby and fitting protective shields. Columbia, meanwhile, had her wiring inspections added on to her already-scheduled modification period at the Palmdale plant in California.
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