If Space Shuttle Columbia had been a sentient, thinking person, she would undoubtedly have been just as confused by the new numbering system as everyone else. However, following her record-breaking, 10-day STS-9 mission at the end of 1983, she was removed from active flight status for more than a year-and-a-half for major repairs and refurbishment. Unlike the Spacelab Only modifications, which were undertaken at KSC, these repairs would be done at the Shuttle's prime contractor, Rockwell International, at Palmdale in California, and would involve a complete overhaul and detailed structural inspection of the entire spacecraft.
The Shuttle flights planned for 1984 and most of 1985 would be performed by two other spacecraft - Challenger and Discovery - which, although outwardly identical to Columbia, were much lighter and lacked ejection seats. A fourth Shuttle, named Atlantis, would then join the fleet in the autumn of 1985, followed by the return of Columbia to active flight status to begin a full roster of ambitious missions in 1986. On 26 January 1984, meanwhile, Columbia was mounted on the Boeing 747 carrier aircraft for the cross-continental trip from Florida to California for her modification period.
After a stopover at Kelly Air Force Base in Texas, she arrived at Edwards on 30 January and was then transported overland to Palmdale. Subsequent modification periods would involve flying the Shuttles directly into Palmdale, but at this time Rockwell's plant was not equipped with a Mate/Demate Device to remove the spacecraft from the top of the Boeing. Removal from the carrier aircraft therefore had to take place at Edwards, which did have such a device, and Columbia was towed - again to the amazement of open-mouthed motorists - down the highway to what would be her home for the next 18 months.
Many have commented that the product of these modification periods, which have increased in frequency as the Shuttle grows older - and now take place every three or four years for each vehicle - is not so much an overhauled spacecraft, but a brand-new spacecraft! Already, by the time she flew to Palmdale, Columbia had changed a great deal since her maiden mission in April 1981. Parts of her ejection seats had been dismantled, the DFI pallet was more-or-less gone, sleeping bunks had been added, a new Ku-band communications antenna fitted in the payload bay, improved brakes and tyres installed and the payload bay floor strengthened.
Much of the data from Columbia during the four test flights also led engineers to the conclusion that they had over-designed the Shuttle. "It was too strong, too beefy," remembered Arnie Aldrich, "and what we could actually do was take about 1,800 lb out of the orbiter by redesign. That was very desirable because that would be directly related to payload. Both Columbia and Challenger were built to this heavier design, but Discovery, Atlantis and Endeavour weren't yet created. They could take advantage of this knowledge of areas where we could take some of the weight out.''
Out of each modification period would emerge a spacecraft which outwardly looked unchanged, but inside was brimming with new, state-of-the-art equipment. In addition to the structural inspections, Columbia also received a Heads-Up Display (HUD) for her pilots, lost her ejection seats altogether in exchange for lighter versions and was fitted with a myriad of sensors to monitor her performance during ascent, orbital flight and throughout re-entry. One device in particular was the Shuttle Infrared Leeside Temperature Sensing (SILTS) instrument, which took the form of a 50-cm cylindrical pod attached to the top of Columbia's tail fin.
It was intended that SILTS would acquire high-resolution infrared images of the upper surfaces of the Shuttle's port-side wing and fuselage during the high-temperature portion of re-entry. This was expected to highlight those parts of the spacecraft that experienced maximum amounts of heating during Columbia's fiery plunge back to Earth. A hemispherical dome at the forward-facing end of the cylinder contained two windows - one forward-facing, the other at an oblique angle - for the infrared camera. Throughout re-entry, a constant supply of room-temperature nitrogen gas would flow across the windows to protect them from the onslaught of atmospheric heating.
If the windows were not protected in this way, the infrared camera would only have been able to see the window itself, rather than what lay beyond. To accommodate SILTS, the pod and the top three metres of Columbia's tail fin were covered with black High Temperature Reusable Surface Insulation (HRSI) tiles, which would make her easily identifiable from a distance. It was intended that SILTS would be activated by Columbia's computers at an altitude of around 122 km - the point of 'entry interface' - when two plugs would be jettisoned from the windows.
For the next 20 minutes, the camera would monitor the port-side wing and fuselage, alternating between them every 11 seconds until the spacecraft had descended to 24.3 km and was through the worst of atmospheric heating. It would then be turned off. With the benefit of hindsight, it is a pity that SILTS did not remain operational through the remainder of Columbia's career, for it might have shed some light on the causes of the disaster on 1 February 2003 . . .
Other new devices included a brand-new nosecap for the Shuttle Entry Air Data System (SEADS), which consisted of pressure sensors to assess the spacecraft's aerodynamic performance at various stages in the high atmosphere. The new nosecap contained 14 tiny holes, through which the pressure of the outside airflow could be determined; this provided data about Columbia's attitude in relation to the airflow, but also allowed predictions of atmospheric density at different altitudes. To ensure that the 'holes' in the nosecap would not cause leaks and destroy the vehicle, the inside bulkhead 'behind' SEADS was covered with protective HRSI tiles.
The third device was the Shuttle Upper Atmosphere Mass Spectrometer (SUMS), which took air samples through a small hole on the underside of Columbia's nose, just between the nosecap and nosewheel well. These samples were then used to pinpoint the quantity and nature of gas species at different altitudes and the density of the atmosphere. With all of these devices fitted - and a thorough inspection that had involved detailed examinations of corrosion and wear-and-tear after six spaceflights - Columbia was finally flown back to KSC on 11 July 1985.
Her time at Palmdale was, according to astronaut Dave Leestma, much longer and more frustrating than anticipated. He had been named as a crew member for Columbia's next flight after STS-9 - along with Commander Bob Crippen, Pilot Jon McBride and Mission Specialists Sally Ride and Kathy Sullivan - but the delays in getting the spacecraft flight-ready meant that their mission was ultimately shifted onto Challenger. ''It wasn't going well,'' Leestma said of Columbia's modification period, ''and it was real hard to come back and be real upbeat when you know that your orbiter's not going to make your flight date.''
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