Challenger Case for the Space Shuttle Morton-Thiokol Inc. had designed the Space Shuttle's solid rocket booster (SRB) based on the Air Force's Titan III design for its reliability. The steel case of the SRB was divided into segments joined and sealed by rubber O-rings. Although Titan's O-rings had occasionally been eroded by hot gases, the erosion was not considered significant. A second redundant O-ring has been added to each joint to serve as a backup in the event of primary O-ring failure. As early as 1977, a test of the SRB case showed unexpected rotation of the joints which decompressed the O-rings making it more difficult to seal the joints. In 1980, a review panel concluded that safety was not jeopardized and the joints were rated Criticality 1R, indicating that joint failure could result in loss of life or shuttle (the 1 in the rating); and that secondary O-rings provided redundancy (the R in the classification). During 1983, the SRBs were modified to use thinner walls, narrower nozzles, and more powerful fuel, which worsened 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 Joint Criticality classification. However, many NASA and Thiokol documents produced over the next three years continued to list Criticality as 1R and seemed to suggest that neither executive thought that a secondary O-ring could actually fail to seal a joint. In a March flight readiness review in 1984, top NASA leaders discussed and accepted the idea that some erosion of the O-rings was "acceptable" because the rings were a safety factor. The incidence of heat damage to SRB joints was growing: three of five flights in 1984 had heat damage, eight of nine flights in 1985, and the flight of January 12, 1986, just two weeks before Challenger. Despite these signs, SRB project management at the Marshall Space Flight Center and at Thiokol remained confident that erosion was "permissible" and posed an "acceptable risk." The April 1985 flight showed significant damage to a primary O-ring, with a significant amount of hot gas blown from this ring, which in turn eroded the secondary O-ring (Bell and Esch 1987). This led Lawrence Mulloy, SRB project manager at Marshall, to impose a "launch constraint" on all subsequent flights, recognizing that a criticality 1, 1R, 2 or 2R problem could occur.