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There are many
myths related to the Challenger disaster, but one fact remains: the explosion could be traced back to an o-ring that failed to seal in the gases it was responsible for.

O-Ring-Booster-Rocket-Endeavor.jpgActual O-Ring on the booster Rocket Endeavor


Background

The infamous Challenger disaster of January 28, 1986 can be traced to a failed seal at the aft field joint of the right solid rocket booster.  The purpose of the rocket boosters was to help propel the shuttle upward during the launch phase and then fall away from the shuttle during the ascent phase. Later study of the footage showed a puff of smoke from this area less than one second into flight, leading to its explosion 73 seconds later.

The disaster cost the lives of the seven astronauts on board — Gregory Jarvis, Judith Resnik, Dick Scobee, Ronald McNair, Mike J. Smith, Ellison Onizuka, and special guest Sharon Christa McAuliffe — and one NASA engineer named Elmer Thomas who died of a heart attack.  

Investigating Committee

President Ronald Reagan commissioned a committee to find the real cause of the disaster.  This committee included renowned physicist and Nobel laureate Richard Feynman and astronaut Sally Ride.  They uncovered a great deal of miscommunication between NASA and its subcontractors, inexcusable decisions on NASA’s part, and other failures within NASA’s own policies.  They also tracked down the physical cause of the failure.

Cause of Failure

The ultimate cause of the explosion was found to be an o-ring that did not possess the correct low-temperature performance needed.  The investigating commission stated in the Rogers Report that …

“…failure in the joint between the two lower segments of the right Solid Rocket Motor. The specific failure was the destruction of the seals that are intended to prevent hot gases from leaking through the joint during the propellant burn of the rocket motor.”

Due to the cold temperatures at the time of launch and the possible presence of frozen water in the solid rocket motor joints, the o-rings were too slow in returning to their correct shape.  

In addition, it is believed that the o-ring in question could not be pressure actuated enough to achieve complete sealing of the gap it was supposed to protect.  This could have been due in part to performance failure of sealing putty.

Feynman summarized this in incredibly simple terms:  because of the cold, the o-ring was simply too stiff to do its job.

The o-ring failure caused joint failure on the right solid rocket booster due to erosion and blow-by from extremely hot combustion gases.  This had a domino effect leading to the explosion of the Challenger.

Failure Blamed on Design

The conclusion of the committee was that the failure was caused by a design that was far too sensitive to the effects of “temperature, physical dimensions, the character of the materials, the effects of reusability, processing, and the reaction of the joint to dynamic loading.”

Conclusion

That is why it is so important to take care when specifying a sealing component for a system, whether it is a simple o-ring or a complex PTFE labyrinth seal.  Effects such as temperature, speed, dimensions, material properties, etc. should always be carefully considered.

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