A difficult road to space 237

of new-specification main engines, known as 'Block 1'. One of these had been test-flown by Discovery in July 1995, together with two 'old-style' engines, but on her STS-73 mission Columbia would become the first Shuttle to fly into space under the combined thrust of three of the new engines. Central to the system was the so-called Alternate High Pressure Oxidiser Turbopump.

''Completing the flight certification of the [new turbopump] is a major milestone,'' said main engine deputy manager Otto Goetz of NASA's Marshall Space Flight Center. ''The alternate turbopump is now ready for its first flight.'' Unlike the old-style turbopumps, the newer ones did not need to be removed and inspected after each mission; rather, they only required examination after 10 flights. Their housings were produced through a 'casting' process, which eliminated all but six of the 300 welds that existed in the old turbopump. Moreover, 'harder' ball-bearings, made from silicon nitride, improved reliability and reduced friction during the turbopumps' operations.

The second difference became apparent when the STS-73 crew headed out to the launch pad for the first time: they wore new suits. Since STS-28, each of Columbia's crews had worn David Clark Company partial-pressure ensembles; by 1990, however, work was already underway to develop a full-pressure garment. Known as the Advanced Crew Escape Suit (ACES), its production started in 1993 and the first flight units were delivered to NASA in May 1994. They offered a simple, lightweight and low-bulk suit that astronauts could put on and take off easily, quickly and without help from others.

After her return to Florida on 14 April 1995, Columbia was ensconced in OPF Bay 3 to begin processing for her next mission, STS-73, which was timetabled as a 16-day flight to carry the second United States Microgravity Laboratory (USML-2). In general, its focus was concentrated on the same general areas - fluid physics, materials science, biotechnology, combustion science and commercial space processing - as had been pioneered during the USML-1 mission in the summer of 1992, with many upgraded experiments flying a second time.

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