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4. External Factors • Late addition of Senator Garn (51-D) • Late addition of Congressman Nelson (61-C) • Late addition of PVTOS experiment (51-I) A detailed explanation of an example from each of these four categories of manifest changes to illustrate both the immediate impact of the change-cause as well as the downstream effects is given below. (1) Manifest Changes Due to Hardware Problems An example of how hardware problems can cause the manifest to change is reflected by the tracking and data relay satellite (TDRS) battery problem experience during the preparations for STS 51-E. TDRS-B and Telesat had been scheduled for a February 1985 launch on the 51-E mission, and the Orbiter and payloads were already at the launch pad when the battery problem was discovered. Because of the complexity of the problem, it was necessary to roll back from the pad and, in fact, the mission eventually was canceled. Syncom IV-3 and a long duration exposure facility (LDEF) retrieval were scheduled for the next flight, STS 51-D. In order to provide an early launch opportunity for Telesat, the 51-D cargo was changed to Syncom and Telesat, and the LDEF retrieval was deferred. This late change necessitated a minor slip in the 51-D launch date; however, the launch still occurred only 5 weeks after the changes were officially approved. Once the battery problem was resolved, it was necessary to schedule another launch opportunity for TDRS. As there were no available flights in the downstream manifest, numerous changes were needed in order to accommodate the new flight, STS 61-M, in July 1986. First, the launch dates for missions in the last half of 1986 had to be slipped. Insat, which had been assigned to STS 61-I, was moved to 61-M (although it was later moved back for other reasons). Finally, the LDEF retrieve activity was added to STS 61-I. Most of these changes are illustrated in figures 3 and 4, which are examples of how the manifest changed with time. The top line on each chart shows the last official manifest published before the start of 1985, the August 1984 baseline. The second line on the 1985 schedule shows the flights as they were actually flown. The second line on the 1986 schedule shows the flights planned for that year as they existed before the STS 51-L mission. These examples illustrate how a single event, such as a hardware problem, can cause the need for a number for changes to the manifest, extending many months into the future. (2) Manifest Changes Due to Customer Request Customer requests have caused numerous manifest changes instituted over the last several years. Because of development problems, financial difficulties, or changing market conditions, Space Shuttle -76-
