What is a Faulty O Ring?

Updated: 9/15/2023
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13y ago

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An O-Ring is a simple rubber seal used in hundreds of thousands of applications ranging from water-tight flashlights to automotive fuel injectors to kitchen faucets. Perhaps the most widely recognized O-ring application is the solid rocket booster on the Space Shuttle.

A Faulty (defective) O-Ring is one that fails to meet design criteria. These design criteria include visual imperfections including cracks or pits in the surface, excessive mold flash, and dimensions, high temperature stability, low temperature resiliency, compatibility with various chemicals, and resistance to flattening out (called compression set or compressive stress relaxation) and similar criteria. Visual imperfections are defined in documents like MIL-STD-413, AS871, AS708, and ISO 3601/3. Criteria for thermal, chemical, and mechanical properties are defined in individual specifications for each type of material.

Contrary to popular conception, the Space Shuttle Challenger disaster in 1986 was NOT caused by a fauty (defective) O-ring. The error in this tragedy was that the design criteria -- notably the low temperature functional requirement -- was not sufficient for the application. The O-ring was made from a fluorocarbon rubber material that doesn't bounce back to its original shape quickly when it is cold (in other words, it has poor low temperature resilience). Sadly, this was well known on several levels prior to launch, yet the decision to launch at a much lower temperature than had been previously attempted was made nonetheless.

In addition, the groove into which the O-ring was placed experienced unusual movement due to the pressure of the burning solid rocket fuel. At least two different seal manufacturers notified NASA before the FIRST Shuttle launch that the motion was too much to expect an O-ring to seal reliably. However, NASA did not make the recommended design changes. As a result, the movement of the joint coupled with the lack of low temperature resiliency of the seal allowed exhaust gases to escape past the O-ring. This caused hot gas to heat up the central fuel tank, causing it to fail and causing the rear mount of the solid rocket booster to separate from the vehicle.

While he did not "discover" the poor low temperature resiliency of this type of rubber material, Richard Feynman explains this phenomenon well at

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Q: What is a Faulty O Ring?
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