Challenger

NASA

Used with permission Challenger flew nine successful Space Shuttle missions.On January 28, 1986, the Challenger and its seven-member crew were lost 73 seconds after launch.

Why did the Challenger disaster happen?

The Challenger disaster occurred because hot gases created by the burning of solid rocket fuel escaped through a field joint. This exhaust succeeded in igniting the hydrogen fuel in the external tank. This resulted in the explosion that killed all seven on board the space shuttle Challenger.

Used with permission Although much of the publicity was concerned with the cold weather properties of the elastomer (rubber) O-ring sealants - the physicist Richard Feynmann showed how the resilience of the O-ring fell rapidly when placed into ice water - the failure was more complex in reality.

The failure was due to four separate events that followed one after the other. All had to occur for the failure to happen:

• Firstly, the zinc chromate putty that was used to provide a thermal barrier for the main O-ring seals had blowholes in it. These were created during leak testing and tended to channel the hot exhaust gases on to the primary O-ring and later on to the secondary O-ring.
• These high temperatures tended to erode the O-ring, the second event in the failure chain. This means that the presence of the blowholes will increase the erosion of the O-rings.
• The third event in the chain is the increase in gap created by the increase in pressure from the burning fuel. This increase in gap, which occurs very quickly, must be followed by the change in shape of the O-rings. If the O-ring becomes stiff then the change in shape will not occur quickly enough and will not provide a full seal.
• The little amount of the O-ring forming a seal was quickly eroded away by the hot gases. These gases were free to pass through the now ineffective joint and eventually to set off the explosion of the fuel in the external tank.

Used with permission The elastic behaviour of the elastomer in the O-ring is very different at -1^oC than it is at 24^oC. It will be much stiffer (5 times less responsive in returning to its original shape i.e. much slower), indicating that the risk of a failure occurring will be higher if the launch was to take place on a cold day, rather than a hot one. The fatal launch occurred when the outside air temperature was about 2^oC, about 9^oC colder than any previous space shuttle launch.

Redesign of the field joint was on the principle that all four stages of failure change would be examined, rather than just the well-publicised O-rings:

• Firstly, an adhesive joint replaced the putty, so that channelling through blowholes would be eliminated
• Secondly, the gap created by the increase in pressure was reduced by starting with an interference fit between the two sides of the field joint
• Thirdly, the O-rings would be at a fixed temperature of 24^oC, rather than kept subject to the vagaries of the external air temperature. This case used very little of the advanced techniques described earlier: the study of polymer science needed to understand the temperature effects on the time-dependent elastic properties of elastomers would be the least familiar concept to an old-style forensic engineer reincarnated from the early 20th century.

Further Reading

For more details about the Challenger Disaster visit the NASA website.

The report of the Rogers Commission set up by President Reagan.

The topic is also discussed in T839 Forensic Engineering.

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