Roger Boisjoly - O-ring Safety Concerns

O-ring Safety Concerns

Boisjoly wrote a memo in July 1985 to his superiors concerning the faulty design of the solid rocket boosters that, if left unaddressed, could lead to a catastrophic event during launch of a Space Shuttle. Such a catastrophic event did occur less than a year later resulting in the Space Shuttle Challenger disaster.

This memo followed his investigation of a solid rocket booster (SRB) from a shuttle flight in January 1985. During his investigation, he discovered that the first of a system of two O-rings had failed completely, and that some damage had been caused to the second O-ring.

The O-rings were two rubber rings that formed a seal between two sections of the SRBs. The sections of the boosters were joined using tang and clevis joints and the rings were intended to seal the joint, while allowing for the inevitable movement between the sections under flight conditions. By design, pressure from within the booster was to push a fillet of putty into the joint, forcing the O-ring into its seat. The system never functioned as designed. The rings were supposed to sit in a groove and seal the joint between the sections of the booster. It was found, however, that flight dynamics caused the joints in the SRB's to flex during launch, opening a gap through which rocket exhaust could escape. As the joints flexed, the rings would come out of their grooves and move to a new position in the joint, a process called extrusion. The extruded ring would form a seal in this new position, but during the time it took for the ring to shift, the joint was unsealed and hot gasses could escape, a process called blow-by. These hot gasses would cause damage to the rings until the seal was achieved.

Boisjoly's investigation showed that the amount of damage to the O-ring depended on the length of time it took for the ring to move out of its groove and make the seal, and that the amount of time depended on the temperature of the rings. Cold weather made the rubber hard and less flexible, meaning that extrusion took more time and more blow-by took place. He determined that if the O-rings were damaged enough they could fail.

If the second O-ring had failed, Boisjoly realized, the results would almost certainly have been catastrophic with the complete loss of the shuttle and crew seemingly the only outcome. His investigation found that the first O-ring failed because of the low temperatures on the night before the flight had compromised the flexibility of the O-ring, reducing its ability to form a seal. The temperature at launch had been only 10 °C (50 °F), the coldest on record (until January 28, 1986). The first rubber O-ring had formed a partial seal, but not a complete one, but the second O-ring had held.

Boisjoly sent a memo describing the problem to his managers, but was apparently ignored. Following several further memos, a task force was set up – including Boisjoly – to investigate the matter, but after a month Boisjoly realized that the task force had no power, no resources and no management support. In late 1985 Boisjoly advised his managers that if the problem was not fixed, there was a distinct chance that a shuttle mission would end in disaster. No action was taken.