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Morton-Thiokol Inc. had engineered the space shuttle's solid rocket booster (SRB) based on the Air Force's Titan III design because of its reliability. The SRB's steel case was divided into segments that were joined and sealed by rubber O-rings. Although the Titan's O-rings had occasionally been eroded by hot gases, the erosion was not regarded as significant. A second, redundant O-ring was addedto each joint to act as back-up should the primary O-ring failed.
As early as 1977, a test of the SRB case showed an unexpected rotation of the joints which decompressed the O-rings making it more difficult for them to seal the joints. In 1980, a review committee concluded that safety was not jeopardized and the joints were classified as Criticality 1R, denoting that joint failure could causeloss of life or shuttle (the 1 in the rating); and that secondary O-rings provided redundancy (the R in the rating). During 1983, the SRBs were modified to use thinner walls, narrower nozzles, and more powerful fuel, which worsened the joint rotation. Tests showed that the rotation could be so large that a secondary O-ring could not seal a joint and provide redundancy. The R rating was consequentlyremoved from the joints' Criticality classification. Nevertheless, many NASA and Thiokol documents produced over the next three years continued to list the Criticality as 1R, and seemed to suggest that neither management thought that a secondary O-ring could really fail to seal a joint.
In a flight readiness review of March 1984, NASA's top managers discussed and accepted the idea that someO-ring erosion was 'acceptable' because the rings embodied a safety factor. The incidence of heat damage at the SRB joints was growing -- three of the five 1984 flights showed heat damage, eight of the nine 1985 flights, and the flight on January 12, 1986, just two weeks before Challenger. In spite of these signals, the management of the SRB project at Marshall Space Flight Centre and at Thiokolremained confident that the erosion was 'allowable' and an 'acceptable risk.' The April 1985 flight showed significant damage at one primary O-ring, with a substantial amount of hot gas blowing by this ring, which in turn eroded the secondary O-ring (Bell and Esch 1987). This led Lawrence Mulloy, SRB project manager at Marshall, to place a 'launch constraint' on all subsequent flights, acknowledgingthat a problem of Criticality 1, 1R, 2, or 2R might occur. However, Mulloy 'waived' the launch constraint for all subsequent flights up to the last one.
In early December 1985, Thiokol was told to 'close out' long outstanding problems, including O-ring erosion problems, and the relevant problem reports were subsequently marked as 'contractor closure received.' O-rings were no longer listed as alaunch constraint for the flight readiness review of January 15, 1986, and O-rings were not mentioned in the Flight Readiness Review documentation of the Challenger.
On the afternoon of January 27, 1986, the eve of the launch, the weather forecast predicted unusually cold weather for Florida, with temperatures at low -7° Celsius in the early hours of January 28. Thiokol engineers expressedconcern that at such cold temperatures, the O-rings would harden and not seal the joints against the hot ignition gases.
Two telephone conferences were held at three sites (Thiokol, Marshall SFC, and Kennedy Space Center) on the evening of January 27 to discuss whether the launch should be delayed. Thirty four engineers and managers participated in the second conference, where Thiokol engineers warnedthat at the forecast temperatures, the O-rings would seal more slowly than on the coldest launch to date, a January 1985 mission when the temperature was 12° Celsius, at which a primary O-ring was eroded so that it failed to seal, allowing hot gases to 'blow by' to the secondary ring. Although the secondary ring did seal the joint then, the engineers argued that a more extensive blow-by could...
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