O-ring - Challenger Disaster

Challenger Disaster

The failure of an O-ring seal was determined to be the cause of the Space Shuttle Challenger disaster on January 28, 1986. A contributing factor was cold weather prior to the launch. This was famously demonstrated on television by Caltech physics professor Richard Feynman, when he placed a small O-ring into ice-cold water, and subsequently showed its loss of pliability before an investigative committee.

The material of the failed O-ring was FKM which was specified by the shuttle motor contractor, Morton-Thiokol. FKM is not a good material for cold temperature applications. When an O-ring is cooled below its T_g (glass transition temperature), it loses its elasticity and becomes brittle. More importantly, when an O-ring is cooled near, but not beyond, its T_g, the cold O-ring, once compressed, will take longer than normal to return to its original shape. O-rings (and all other seals) work by creating positive pressure against a surface thereby preventing leaks. On the night before the launch, exceedingly low air temperatures were recorded. On account of this, NASA technicians performed an inspection. The ambient temperature was within launch parameters, and the launch sequence was allowed to proceed. However, the temperature of the rubber O-rings remained significantly lower than that of the surrounding air. During his investigation of the launch footage, Dr. Feynman observed a small out-gassing event from the Solid Rocket Booster (SRB) at the joint between two segments in the moments immediately preceding the disaster. This was blamed on a failed O-ring seal. The escaping high temperature gas impinged upon the external tank, and the entire vehicle was destroyed as a result.

The rubber industry has gone through its share of transformation after the accident. Many O-rings now come with batch and cure date coding, as in the medicine industry, to precisely track and control distribution. For aerospace and military/defense applications, O-rings are usually individually packaged and labeled with the material, cure date, and batch information. O-rings can, if needed, be recalled off the shelf. Furthermore, O-rings and other seals are routinely batch-tested for quality control by the manufacturers, and often undergo Q/A several more times by the distributor and ultimate end users.

As for the SRBs themselves, NASA and Morton-Thiokol redesigned them with a new joint design, which now incorporated three O-rings instead of two, with the joints themselves having onboard heaters which can be turned on when temperatures drop below 50 °F (10 °C). No O-ring issues have occurred since Challenger, and they did not play a role in the Space Shuttle Columbia disaster of 2003.