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. 

Spring Energized PTFE Seal
by Daniel Mays Daniel Mays No Comments

Backup Rings in High-Pressure Sealing Systems: Preventing Seal Extrusion and Extending Service Life

Backup rings in high-pressure sealing systems address face extrusion failure in both dynamic and static seals found in high-pressure systems. Extrusion in sealing occurs when pressurized fluid forces the seal material into the clearance gap between mating surfaces, compromising seal performance. However, backup rings are a simple, low-cost (but critical) solution for extending the service life and maintaining the performance of extreme sealing conditions.

The Challenge of Seal Extrusion in High-Pressure Applications

Seal extrusion happens when pressurized fluid forces part of a sealing element into the clearance gap between mating hardware surfaces. Under high pressure, the soft material deforms plastically and begins to “flow” into this gap, where it can be pinched, torn, or permanently deformed. The result from seal extrusion is nibbling damage along the edges, rapid loss of sealing capability, and, in severe cases, catastrophic leakage.

Extrusion not only shortens seal life but also accelerates equipment wear, drives unplanned downtime, and raises operating costs in hydraulic, pneumatic, and process systems. The causes of seal extrusion are typically high pressure differentials ( > 1500 psi) and/or significant clearance gaps, exacerbated by the use of elastomers or polymers that deform plastically under a load. Seal extrusion can be difficult to avoid under certain circumstances, but that is where backup rings come in.

Backup Rings in High-Pressure Sealing Systems: Purpose and Function

Backup rings in high-pressure sealing systems are annular support components that are installed next to a seal, such as an O-ring or a spring-energized seal. They act as an effective barrier to keep the seal from being forced into the clearance gap.

Backup rings come in different configurations, including single-ring, double-ring (for applications with bi-directional pressure), as well as split vs solid rings. Regardless of the configuration chosen, it is critical to achieve a precise fit because too loose undermines support, while too tight causes problems with assembly.

Material Considerations for Backup Rings in High-Pressure Sealing Systems

The three most commonly used materials for backup rings are PTFE, PEEK, and Nylon. However, other materials such as  UHMW-PE, filled PTFE blends for wear resistance, reinforced polymers for high PV limits, may be used.

PTFE

PTFE is an excellent option for backup rings with its extremely low coefficient of friction and extensive chemical compatibility. It works exceptionally well for applications requiring dynamic sealing or very low temperatures. Its primary limitations are the possibility of cold flow under sustained loads, so it might not always be suitable for extreme pressure conditions.

PEEK

PEEK is another good option for use as a backup ring with its high mechanical strength and excellent resistance to extrusion, as well as its thermal stability up to ~250°C. It also possesses exceptional resistance to extrusion. While it may be a more costly option compared to other polymers, it has found widespread application in industries such as aerospace, oil and gas, and high-performance hydraulics.

Nylon (PA)

Nylon works extremely well in moderate conditions with its strength, and it has a more economical price compared to PEEK and PTFE. However, it does have some critical limitations that including swelling and water absorption, both of which can heavily impact tolerances.

Design and Geometry Options

Solid backup rings provide the best extrusion resistance but can be challenging to install. Split rings can simplify the assembly process, but may allowed extrusion under extremely high loads, which essential defeats the purpose of having a backup ring. Another alternative is the use of spiral cut designs, which balance easier installation (without requiring excessive stretching) with maintaining good support for the seal. Contoured cuts such as scarf of step joints further reduce weak points found at splits.

For pressure direction, a single ring works when the load comes from one side, while double rings are necessary for bidirectional pressure. In every case, the trade-off is clear: easier installation often means slightly higher extrusion risk at the joint.

Engineering Considerations for Integration

Housing tolerances are critical for backup rings. Excessive clearance gaps increase the risk of extrusion, while precise fits provide reliable support. Single rings work well when pressure comes from one direction. However, double rings are required when it fluctuates or is bidirectional. Temperature adds another layer of complexity. Heat accelerates creep and changes dimensions through thermal expansion, which weakens long-term performance. Material compatibility is also important. Chemicals, lubricants, or swelling agents can reduce hardness and shorten service life. In failure analysis, extrusion often appears as edge nibbling or shearing. Compression set, on the other hand, leaves the ring permanently deformed. Recognizing the difference is key to preventing repeat issues.

Conclusion

Backup rings extend seal service life by preventing premature failure and protecting against extrusion. They reduce downtime and maintenance costs, which is especially valuable in high-value systems. Reliability and safety improve in critical applications where failures aren’t an option. They also make it possible to use softer elastomers for better sealing performance without increasing the risk of extrusion.

Investing in the right backup ring extends seal life, reduces failures, and ultimately saves money and downtime. Contact us at Advanced EMC to learn more about backup ring solutions.