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customers frequently have notified NASA Headquarters of a desire to change their scheduled launch dates. An example of how this kind of change can affect the manifest is the Westar satellite. Westar originally had been scheduled for STS 61-C in December 1985. Although no formal request was made, as it could have resulted in a penalty fee, it was indicated to NASA Headquarters that a slip to the March 1986 timeframe was desirable. Normally, NASA Headquarters tries to accommodate requests like this in order to maintain good customer relations. As a result, Westar was moved to STS 61-E, replacing GetAway Special (GAS) bridge assembly.

As a result of the departure of Westar from the STS 61-C manifest, HS-376 was moved from STS 51-L. The HS-376 is essentially a place-holder for one of the communications satellites returned to Earth on STS 51-A. The plan had been to refurbish one or both of these satellites, sell them to a new customer, and relaunch them on the Space Shuttle. Part of the incentive for a customer to buy these "used" spacecraft was a guaranteed early launch slot. Unfortunately, no customers were found during this time period. As a result, Headquarters delayed the HS-376 launch date. SPARTAN-Halley was moved from STS 61-D to fill the slot created on 51-L. STS 61-D, which included the SLS-1 payload, was canceled because of other factors. SPARTAN-Halley was designed to take observations of Halley's Comet and needed to fly in the January 1986 timeframe; therefore, the mission was scheduled for that month.

(3) Manifest Changes Due to Operational Constraints

An excellent example of the need to change the manifest because of operational constraints is reflected by the landing weight on STS 61-K. This mission, an Earth Observation Mission (EOM) Spacelab flight, first was planned for OV-102 because of the desire for a high-power, long-duration mission (i.e., only OV-102 has the capability to carry a fifth cryo set to provide extra power). As the payload definition matured, the payload configuration changed from only a long module to a long module plus a mission peculiar experiment support structure (MPESS). This new configuration resulted in a predicted end-of-mission landing weight of approximately 217 000 pounds, significantly above the maximum allowable landing weight of 214 000 pounds. Since OV-104 weighs several thousand pounds less than OV-102,  switching vehicles was a reasonable solution to this problem. As a result, STS 61-K was changed to OV-104, and STS 61-L was changed to OV-102. Because OV-102 is a heavier vehicle, its lift capability is also reduced; thus, the total payload weight for STS 61-L had to be decreased somewhat. By removing both the global positioning satellite (GPS) (moved to 71-A) and SPARTAN 2 (moved to 71-C), it was possible to add Syncom, which, because of another change, had been bumped from a previous flight.

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