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. 

by Daniel Mays Daniel Mays 1 Comment

Seals in Space: The Unsung Heroes of Rocketry

The quest for the cosmos is a testament to human ingenuity and determination. As we propel ourselves into the vast expanse of space, the intricate dance of technology and nature becomes ever more critical. One such marvel of engineering, often overlooked, is the role of seals in rocket engines and spacecraft. These small yet mighty components are vital for ensuring the success and safety of space missions.

The Silent Guardians of Rocket Propulsion

Rocket propulsion systems rely on the flawless separation of propellants until the moment of ignition. This separation is crucial, especially when dealing with hypergolic bi-propellants like RP-1 (highly refined kerosene) and Liquid Oxygen (LOX), which ignite spontaneously upon contact. Seals act as the guardians, preventing premature mixing and ensuring that each stage of the propulsion process occurs without incident.

However, the challenges faced by these seals are formidable. The cryogenic nature of LOX requires materials that can withstand extremely low temperatures. Traditional elastomers, commonly used in sealing applications, fall short of the necessary performance at such cryogenic conditions. This necessitates the use of advanced materials and innovative designs.

Hypergolic propellants, while efficient, introduce another layer of complexity. These substances are highly reactive and can ignite on contact, meaning any leak or failure in the seals could result in catastrophic consequences. Therefore, seals must be engineered to maintain their integrity under these extreme conditions, providing an unyielding barrier that ensures the safe handling of these volatile compounds.

Navigating the Extremes: Temperature and Pressure

Seals in rocket engines are exposed to a wide range of temperatures and pressures, from the cryogenic storage of LOX at temperatures as low as -450°F to the blistering heat of 4,000°F in the main combustion chamber. They must maintain their integrity and functionality across these extremes, ensuring no leaks or failures occur.

To put this into perspective, consider the temperature fluctuations a seal must endure during a typical rocket launch. Initially, the seals must function at cryogenic temperatures, ensuring the propellants remain securely stored. As the rocket ignites and propellants are fed into the combustion chamber, the seals are suddenly subjected to extreme heat and pressure. This rapid change can cause thermal expansion and contraction, which must be accounted for in the seal’s design to prevent material fatigue and failure.

The journey of propellants through the various stages of a rocket – from tanks to turbopump assemblies, compressors, pumps, ducts, joints, manifolds, and valves – exposes seals to different states of matter, pressures, and temperatures. Each stage requires seals to adapt and perform consistently, making their reliability paramount. This is where advanced materials like PTFE (Polytetrafluoroethylene), PEEK (Polyether ether ketone), and specialized elastomers come into play, providing the necessary resilience and flexibility to handle these demanding conditions.

Balancing Weight and Cost

In the aerospace industry, every gram counts. The weight of a rocket is directly proportional to the fuel required for launch, impacting the payload capacity and, ultimately, the commercial viability of the mission. Seals must therefore be lightweight yet robust, made from materials that offer excellent strength-to-weight ratios.

Spring Energized PTFE Seal

Advanced sealing materials like PTFE and PEEK are favored not only for their durability but also for their lightweight properties. PTFE, known for its low friction and high resistance to chemicals and temperature extremes, is often used in applications where traditional materials would fail. PEEK, on the other hand, offers exceptional mechanical strength and can withstand high pressures and temperatures, making it ideal for critical sealing applications in rocket engines.

Cost is another critical factor. Advanced sealing solutions, while more expensive, provide higher reliability and performance. The choice of materials, whether PTFE, PEEK, or specialized elastomers, and the type of seal, such as spring-energized encapsulated seals, play a significant role in the overall cost. Custom solutions tailored to specific requirements further drive up costs, but the investment is justified by the enhanced safety and reliability they offer.

The Art of Specifying and Designing Seals

Designing seals for rocket engines involves a meticulous consideration of various factors. The seals must withstand thermal cycling, maintain cleanliness, ensure chemical compatibility, and exhibit low friction and wear. The surface finish of glands, grooves, and the seal jacket must be flawless to prevent any points of failure.

Thermal cycling, the repeated heating and cooling cycles that seals undergo during a mission, can cause material degradation over time. Engineers must select materials that not only withstand these cycles but also maintain their sealing properties. Cleanliness is another critical factor, as any contamination can compromise the seal’s effectiveness and potentially cause leaks.

Chemical compatibility is essential, especially when dealing with aggressive propellants like LOX and RP-1. The seals must resist chemical attack and degradation, ensuring they maintain their integrity throughout the mission. Low friction and wear properties are equally important, as they reduce the risk of seal damage during the dynamic operations of the rocket.

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A Glimpse into the Future

As space exploration evolves, the demands on sealing technology will continue to grow. The development of reusable rockets, new propellant combinations, and extended missions into deeper space will require even more advanced and resilient seals. The pursuit of these innovations is not just about enhancing performance but also about ensuring the safety and success of every mission.

Reusable rockets, such as those being developed by SpaceX and Blue Origin, present unique challenges for sealing technology. The seals must not only perform flawlessly during multiple launches and re-entries but also endure the rigors of refurbishment and reuse. This requires seals that can maintain their properties over extended periods and through numerous thermal cycles.

New propellant combinations, including methane-based propellants, introduce additional complexities. Methane, while offering advantages in terms of performance and availability, can pose different challenges for seals compared to traditional RP-1 and LOX. Engineers must develop seals that are compatible with these new propellants, ensuring they provide the same level of reliability and safety.

Extended missions into deeper space, such as those planned for Mars and beyond, will push the limits of sealing technology. The seals must withstand the harsh conditions of space, including extreme temperatures, radiation, and vacuum, while maintaining their sealing properties over long durations. This will drive the development of new materials and designs that can meet these unprecedented challenges.

Conclusion

In the grand tapestry of space exploration, seals may seem like minor players, but their role is nothing short of heroic. They stand as the unsung guardians, ensuring that each component of a rocket operates flawlessly, propelling humanity toward new frontiers. As we continue to push the boundaries of what is possible, the technology behind these seals will remain a cornerstone of our journey to the stars.