Challenger Case Material

Space Shuttle Challenger Case

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 added to 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 cause loss 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 consequently removed 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 some O-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 Thiokol remained 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, acknowledging that a problem of Criticality 1, 1R, 2, or 2R might occur.