Cover page of the Rogers Commission report
by Sara McCaslin Sara McCaslin 1 Comment

The O-Ring That Killed Seven Astronauts

I was eight years old when I saw the Challenger blow up during its launch. The infamous NASA Challenger explosion was carefully investigated and could be traced back to an O-ring that failed. In this blog post, we look at what went wrong and one of the lessons we can take away from it.

Challenger

The Challenger mission objective was to launch a second Tracking and Data Relay System (TDRS) satellite into orbit. This objective would enable longer periods of communication between space shuttles and eventually the International Space Station (ISS). Challenger would deploy the TDRS satellite mated to an Inertial Upper Stage (IUS). This stage would boost it from low Earth orbit to geostationary orbit. In addition, a space walk was planned along with a variety of science experiments. 

This article recounts the 1986 Challenger disaster, tracing it to an O-ring seal that failed in unusually cold launch temperatures, and summarizes how the Rogers Commission investigated and identified this root cause. It closes with an engineering takeaway for material selection: seals need to be tested and characterized across their full operating range, including low-temperature performance, since untested conditions are often what cause failures.

The Challenger Disaster

The infamous disaster on 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 propel the orbiter upward during launch and then fall away during ascent. A later study of the footage showed a puff of smoke from this area less than 1 second into the 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 known as the Rogers Commission to find the root cause of the disaster.  This committee included renowned physicist and Nobel laureate Richard Feynman and astronaut Sally Ride. The team identified the physical cause of the disaster within about one to two weeks of beginning its work, then delivered a comprehensive final report just over four months after the accident

The committee uncovered a great deal of miscommunication between NASA and its subcontractors, as well as inexcusable decisions on NASA’s part and other failures within NASA’s own policies.  They also tracked down the physical cause of the failure.

The Notorious O-Ring

The ultimate cause of the explosion was found to be an O-Ring that did not possess the correct low-temperature performance needed. In fact, just eight days after the commission was formed, Feynman famously demonstrated during a televised public hearing that the O-Ring material lost its resiliency when submerged in a glass of ice water. Due to the cold temperatures at launch and the possible presence of frozen water in the solid rocket motor joints, the O-rings were slow to return to their proper shape.  

Basically, temperatures were below the glass-transition temperature for the O-ring material, causing it to behave as a brittle material rather than a ductile one and this low temperature also caused it to compression set at that much lower temperature. This destroyed the ability of the o-ring seal to follow the joint gap when pressurized.

By Rogers Commission into the loss of the Space Shuttle Challenger – http://history.nasa.gov/rogersrep/v1ch4.htm, Public Domain, https://commons.wikimedia.org/w/index.php?curid=107789463

Here is a summary of what happened:

  1. The solid rock boosters (SRBs) were built in cylindrical segments joined at “field joints.” These field joints were sealed by two Viton FKM O-rings (primary and secondary) rather than a one-piece, welded casing. This was found to be a cost/reusability decision made under the pressure of a fixed-price contract.
  2. Under internal pressure at ignition, the steel joint segments in the SRBs flex and slightly separate. This was referred to as joint rotation, and it opened a gap. That gap required an O-ring to seal it by expanding quickly enough into it to maintain contact.
  3. The temperature at the O-ring that morning was 28°F. This was colder than the lowest temperature the shuttle had ever been tested at prior. Below its glass transition range, the fluorocarbon rubber used for the O-ring stiffened and was unable to spring back to close the gap in time.
  4. Because the primary O-ring was unable to seal the gap in time, hot combustion gas jetted past it. The O-rings were unable to seal the aft field joint on the right solid rocket booster. This caused pressurized hot gases and even flames to “blow by” the O-ring … And reach the adjacent external tank. That flame plume burned through the external tank strut, igniting the liquid propellant and breaking the vehicle apart barely a minute after liftoff.

What makes this disaster even more tragic is that test data had demonstrated a potentially fatal flaw in the O-rings, but NASA and the SRB manufacturer, Morton Thiokol, did not address it. The night before the launch, Thiokol engineers tried to raise the alarm. They were ignored.

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
  • Reaction of the joint to dynamic loading.

A Lesson for Modern Engineers

There are a host of lessons to learn from the Challenger disaster, but one in particular stands out to us at Advanced EMC. The choice of material for seals is critical. The O-ring was made of Viton FKM, and it remains a popular choice for seals and O-rings because it is a great material. It just was not the right choice for the Challenger field joints. 

Not all materials are equal. They have limits, operating envelopes, and reactivity with other chemicals, and pressure and temperature affect how they function. We cannot choose seal materials solely because basic properties like maximum operating temperature, chemical compatibility, and pressure rating seem to work for the design. But there is more to it than that. 

For example, consider a gasket rated for -20°F service that will only ever be tested and characterized at room temperature. This turns out to be the same blind-spot category that doomed the Challenger SRB joint. Compression set behavior, glass transition temperature, and low-temperature resilience deserve the same rigor as your upper-temperature and chemical-resistance data. Remember: the failure mode that gets you is almost always the condition nobody thought to test. 

Conclusion

If you need a dependable, reliable O-ring or backup ring (BUR), talk to one of the experts at Advanced EMD. Our engineers are ready to help you from the design phase to installation. Contact us today. 

Hydrogen gas infrastructure
by Sara McCaslin Sara McCaslin No Comments

The Two Failure Modes That Define Hydrogen Sealing

Hydrogen infrastructure, including electrolyzers, refueling stations, fuel cells, storage and distribution, is rapidly scaling, and seal manufacturers are responding with dedicated hydrogen product lines. But hydrogen is not just “another gas to seal against.” Its molecular size and behavior create two distinct failure modes that do not appear with conventional gases, and a seal rated “hydrogen compatible” on a generic datasheet can still fail in service if the selection does not account for the specific duty cycle.

Failure Mode One: Permeation

Hydrogen has an extremely small molecular size. So small, in fact, that it can diffuse through elastomers far more readily than larger gas molecules; this is a diffusion problem rather than a chemical attack problem: the elastomer is simply not an effective barrier, as opposed to degrading. 

Permeation is a serious issue in hydrogen sealing solutions. It reduces system efficiency by causing hydrogen loss, poses serious safety risks in enclosed spaces, and can, over time, contribute to internal damage as the gas accumulates within the polymer matrix.

FKM generally exhibits lower hydrogen diffusivity than NBR or EPDM, though all common elastomers permeate to some degree. Keep in mind that this is a matter of degree, not absolute barrier vs. non-barrier. There is, however, a better polymer barrier material.

PTFE is virtually inert to hydrogen with markedly lower permeation than any common elastomer, which is why it has become the default choice as pressure and criticality increase

Failure Mode Two: Rapid Gas Decompression (RGD)

Rapid gas decompression (RGD) is the failure mode most specific to hydrogen sealing and the one most likely to be underappreciated by engineers accustomed to conventional gas sealing. In this failure mode, hydrogen molecules that have diffused into the elastomer during sustained high-pressure exposure need time to escape when pressure drops; if depressurization happens faster than the gas can diffuse back out, it becomes trapped and forms internal voids, blisters, or cracks as it expands within the material.

This is also an instance where the failure mode is not solely a chemical compatibility issue. A material can be chemically compatible with hydrogen yet still fail catastrophically under rapid depressurization (RGD) if the duty cycle involves fast pressure drops. This highlights the importance of understanding the kinetics of gas escape and the mechanical response of the seal material.

This is why RGD damage can range from subsurface blistering to structural rupture, especially in high-pressure hydrogen systems such as fueling stations, electrolyzer connections, and pipeline seals. Recognizing these failure modes is essential for preventing field failures during pressure cycling, which are common in applications like rapid fueling or frequent pressure adjustments.

Material Selection Is Not One-Size-Fits-All

Here are some suggested material selections.

EPDM (Ethylene Propylene Diene Monomer)

This is often the default for lower-demand applications, and specifically for alkaline electrolyzers, offering good general hydrogen compatibility and low-temperature flexibility at a reasonable cost. It exhibits excellent low-temperature flexibility, a good compression set, and resistance to alkaline media. However, it has a higher hydrogen gas permeation rate than some other options, namely fluoropolymers such as PTFE. It is primarily used with alkaline electrolyzers (AEL/AEM), water/coolant loops, low-pressure distribution, and low-temperature static seals.

FKM (Fluoroelastomer / Viton)

FKM is favored where chemical resistance matters more broadly (e.g., acidic environments in PEM electrolyzer balance-of-plant). However, it is incompatible with certain hydrogen-adjacent process chemistries, where EPDM may be required instead. It exhibits very low hydrogen permeation compared with synthetic rubbers and has excellent thermal stability across a wide temperature range. However, it becomes brittle at extremely low temperatures, which can lead to leakage during cold thermal cycling. Note that FKM and EPDM are not interchangeable substitutes for each other. It is often used with PEM fuel cells and PEM electrolyzers (oxygen-side/stack sealing), high-temperature gas processing, and high-pressure static connections.

HNBR (Hydrogenated Nitrile Butadiene Rubber )

This material is often specified for fueling-station dispenser seals where low-temperature performance (down to around -40°C) matters, since dispensing frequently involves pre-cooled hydrogen. It has excellent mechanical strength, wear resistance, and extrusion resistance. It is also highly resistant to RGD when compounded properly and retains its flexibility at low temperatures. However, it has a moderate hydrogen permeation rate, and its continuous-use temperature is capped at 300°F. HNBR is commonly used for high-pressure hydrogen valves, compressors, sour gas blends, and pipeline seals, as well as dynamic applications subject to rapid cycling

PTFE (Polytetrafluoroethylene)

PTFE is the default across nearly all pressure tiers once RGD resistance and minimal permeation become priorities. It is inert, low-permeation, and functional across an unusually wide temperature range, though it comes with its own design considerations. It is non-elastic in the sense that it has no inherent memory. It is also prone to creep under continuous pressure loads. Note that it is typically used in energized seal configurations rather than as a standalone elastomer substitute. . It is often used with spring-energized dynamic shaft/stem seals in high-pressure valves, cryogenic liquid hydrogen valves, compressor packing rings, and anti-extrusion back-up rings..

Material Selection and the Duty Cycle

Material selection must consider the actual duty cycle, including pressure cycling frequency, depressurization rates, and temperature swings, rather than relying solely on a generic ‘hydrogen compatible’ rating. For example, two systems operating at the same peak pressure may have vastly different RGD risks depending on their depressurization speed and cycling frequency, which directly impact seal longevity.

Conclusion

Hydrogen sealing involves two key failure modes, permeation and RGD, and understanding these helps engineers feel more equipped to select appropriate materials as infrastructure scales. If you need further assistance finding an effective, reliable sealing solution for your system, contact the sealing experts here at Advanced EMC.