Space Shuttle Challenger Disaster - O-ring Concerns

O-ring Concerns

Each of the two Space Shuttle Solid Rocket Boosters (SRBs) that comprised part of the Space Transportation System was constructed of six sections joined in three factory joints and three "field joints". The factory joints had asbestos-silica insulation applied over the joint, while the field joints—assembled in the Vehicle Assembly Building at Kennedy Space Center (KSC)—depended on two rubber O-rings, a primary and a secondary (backup), to seal them. (Since the destruction of Challenger, SRB field joints were using three O-rings.) The seals of all of the SRB joints were required to contain the hot high-pressure gases produced by the burning solid propellant inside, forcing it out the nozzle at the aft end of each rocket.

During the Space Shuttle design process, a McDonnell Douglas report in September 1971 discussed the safety record of solid rockets. While a safe abort was possible after most types of failures, one was especially dangerous, a burnthrough by hot gases of the rocket's casing. The report stated that "if burnthrough occurs adjacent to tank or orbiter, timely sensing may not be feasible and abort not possible", accurately foreshadowing the Challenger accident. Morton Thiokol was the contractor responsible for the construction and maintenance of the shuttle's SRBs. As originally designed by Thiokol, the O-ring joints in the SRBs were supposed to close more tightly due to forces generated at ignition. However, a 1977 test showed that when pressurized water was used to simulate the effects of booster combustion, the metal parts bent away from each other, opening a gap through which gases could leak. This phenomenon, known as "joint rotation," caused a momentary drop in air pressure. This made it possible for combustion gases to erode the O-rings. In the event of widespread erosion, an actual flame path could develop, causing the joint to burst—which would have destroyed the booster and the shuttle.

Engineers at the Marshall Space Flight Center wrote to the manager of the Solid Rocket Booster project, George Hardy, on several occasions suggesting that Thiokol's field joint design was unacceptable. For example, one engineer suggested that joint rotation would render the secondary O-ring useless. However, Hardy did not forward these memos to Thiokol, and the field joints were accepted for flight in 1980.

Evidence of serious O-ring erosion was present as early as the second space shuttle mission, STS-2, which was flown by Columbia. However, contrary to NASA regulations, the Marshall Center did not report this problem to senior management at NASA, but opted to keep the problem within their reporting channels with Thiokol. Even after the O-rings were redesignated as "Criticality 1"—meaning that their failure would result in the destruction of the Orbiter—no one at Marshall suggested that the shuttles be grounded until the flaw could be fixed.

By 1985, Marshall and Thiokol realized that they had a potentially catastrophic problem on their hands. They began the process of redesigning the joint with three inches (76 mm) of additional steel around the tang. This tang would grip the inner face of the joint and prevent it from rotating. However, they did not call for a halt to shuttle flights until the joints could be redesigned. Rather, they treated the problem as an acceptable flight risk. For example, Lawrence Mulloy, Marshall's manager for the SRB project since 1982, issued and waived launch constraints for six consecutive flights. Thiokol even went as far as to persuade NASA to declare the O-ring problem "closed". Donald Kutyna, a member of the Rogers Commission, later likened this situation to an airline permitting one of its planes to continue to fly despite evidence that one of its wings was about to fall off.