O-Ring Failure in the Cold: The Challenger Lesson for Pump Seals

Cold makes an O-ring stiff and slow to spring back. Below its low-temperature rating, the sliding O-ring in a pusher seal can stick, hold the seal faces open, and the seal can leak. The best-known example of cold rubber failing happened in 1986, when cold FKM O-rings failed in a Space Shuttle booster joint. FKM (fluoroelastomer) is the same rubber family used in many pump seals.

Why does cold affect an O-ring?

Rubber stiffens and loses resilience as it cools. Resilience is its ability to spring back and keep pressing on the metal. Close to its glass transition, it cannot rebound after being squeezed.

What happened on Challenger?

The investigation found that the O-ring seals in the aft field joint of the right solid rocket booster failed. The faulty joint design was the cause. The cold launch made the rubber too slow to follow the joint as it opened, and hot gas got past. The O-rings were Viton, an FKM, and it was the coldest launch NASA had attempted.

Why does a pusher seal care?

Its secondary O-ring has to slide on the shaft or sleeve whenever the shaft moves or the faces wear. If the ring is too stiff to slide, it can hang up, the faces can open, and the seal can leak.

How cold can standard FKM go?

Typically about -15 to -26 C, depending on the grade. Low-temperature FKM grades go lower and keep FKM's chemical resistance. For colder duties or aggressive chemicals, low-temperature FFKM O-rings are rated well below standard FKM. Always check the compound's own rating, not the family name.

What is TR-10?

A standard lab test (ASTM D1329) that finds the temperature where frozen, stretched rubber starts to recover. It is a guide to a compound's low-temperature limit, not a full sealing test.

What to do about it

Buyers: put the lowest temperature, including cold starts, on the enquiry.

Fitters: if a seal leaks cold and stops when warm, check the O-ring grade.

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