Space Shuttle Challenger CaseMorton-Thiokol Inc. had designed the Space Shuttle's solid rocket booster (SRB) based on the Air Force's Titan III design due to its reliability . The steel housing of the SRB was divided into segments connected and sealed with rubber O-rings. Although the Titan's O-rings were occasionally eroded by hot gases, the erosion was not considered significant. A second, redundant O-ring has been added to each seal to serve as a backup in the event the main O-ring fails. As early as 1977, a test of the SRB case showed unexpected seal rotation that decompressed the O-rings. making it more difficult for them to seal the joints. In 1980, a review board concluded that safety was not compromised and the seals were classified as 1R criticality, indicating that failure of the seals could result in loss of life or shuttles (the 1 in the note); and that the secondary O-rings provided redundancy (the R in the note). In 1983, SRBs were modified to use thinner walls, narrower nozzles, and more powerful fuel, which made seal rotation worse. Tests showed that rotation could be so great that a secondary O-ring could not seal a joint and provide redundancy. The R rating has therefore been removed from the joint criticality ranking. Nonetheless, numerous NASA and Thiokol documents produced over the next three years continued to classify criticality as 1R and seemed to suggest that neither management believed that a secondary O-ring could actually fail to seal a joint. . In a March 1984 flight readiness review, top NASA officials discussed and accepted the idea that some erosion of the O-rings was "acceptable" because the rings embodied a safety factor . The incidence of heat damage at SRB joints was increasing: three of five flights in 1984 had heat damage, eight of nine flights in 1985, and the flight on January 12, 1986, just two weeks before Challenger . Despite these signals, SRB project management at Marshall Space Flight Center and Thiokol remained convinced that erosion was “allowable” and constituted an “acceptable risk.” The April 1985 flight showed significant damage to a primary O-ring, with a significant amount of hot gas blown through this ring, which 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 1, 1R, 2 or 2R criticality issue could arise...