O-Rings in Spaceflight | Advanced EMC Technologies
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

O-Rings in Spaceflight

Since the Challenger disaster, o-rings have come under close scrutiny in spaceflight designs and applications and they continue to play a vital role in modern spaceflight, including modern commercial spaceflight ventures such as SpaceX, Virgin Galactic, and Blue Origin.

In this week’s blog post, we will discuss o-rings in spaceflight, including problems that arise, the best materials, and more.

O-Ring Failures in Modern Spaceflight

Few would argue the importance of seals and o-rings in space shuttles and rockets. From rocket engines to the International Space Station, the ability to retain media and prevent its contamination is of vital importance. This importance was first brought to public attention through the Challenger disaster where a stiff o-ring cost multiple lives. However, o-ring issues did not end there.

In 2005, orbiter tests prior to the space shuttle Discovery’s return to flight revealed a failure that traced back to Nitrile/Buna N o-rings. Six of nine flow control valve o-rings had suffered radial cracks, with one o-ring developing problematic leak paths as a result. The cause of the o-ring issue was found to be ozone attack of Nitrile/Buna N, which is one of its susceptibilities.

Back in 2016 a Blue Origin launch was delayed by o-ring issues. Jeff Bezos reported that the rubber o-rings in the New Shephard rocket’s nitrogen gas pressurization system were leaking and had to be replaced before the launch could continue. New Shephard is the same rocket used to take Star Trek legend William Shatner on his first real space flight.

Virgin Galactic, owned by Richard Branson, discovered a very dangerous issue with the flight vehicle SpaceShipTwo when it was returned to the hangar in 2019. A critical seal running along a stabilizer on one of the wings had “come undone.” While not an o-ring, this does reinforce the importance of seals on modern spacecraft.

Operating Environment Complications for O-Rings in Spaceflight

O-rings face a very hostile environment in space, including …

  • Extreme temperatures, ranging from cryogenic to high
  • Wide temperature variation
  • Extremely high pressures and vacuum pressures
  • Vibration during launch
  • Risk of permeation depending on the media involved
  • Chemical attack from media such as fuels and lubricants
  • Potential exposure to ozone, ultraviolet, and radiation

There are other potential issues as well. For rockets in particular, one of the challenges faced when specifying o-rings involves their ability to expand fast enough to maintain a seal even when joints (a common area of use for o-rings) move away from each other. Swelling when exposed to hydrocarbon-based greases used to protect components against corrosion can be problematic as well. 

O-Ring Materials in Spaceflight

O-rings are manufactured from a diverse group of materials, including EPDM, FEPM, FFKM, FKM, Fluorosilicone, HNBR, Hytrel, NBR, Neoprene, Polyurethane, and Silicone.

Any material used in spaceflight applications, however, would need to fall within the categories of high temperature service and/or chemical service, reducing the list to materials such as …

  • FEPM (trade name Aflas)
  • FFKM (trade names Kalrez, Chemraz, Markez, and Simriz)
  • FKM (trade names Viton, Technoflon, and Fluorel)
  • Silicone. 

Keep in mind, however, that other materials may be suitable that are not included in this list and the suitability of these materials is highly dependent on the application.

FEPM O-Rings

FEPM, perhaps better known by the trade name Aflas, is a copolymer of tetrafluoroethylene and propylene and often represented as TFE/P. In addition to chemical compatibility and a degree of high temperature performance, it offers excellent ozone resistance. It is known for providing excellent performance where traditional fluoroelastomers are known to fail.

FFKM O-Rings

FFKM, often referred to by trade names such as Kalrez or Chemraz, is an excellent option for applications that involve extreme pressures, extreme temperatures, and aggressive chemicals. FFKM, which is a perfluoro elastomer material, is available in various grades that offer key properties such as low permeation, low compression set, resistance to temperature cycling, and wide ranging chemical compatibility as well as resistance to explosive decompression and plasma resistance. 

FKM O-Rings

Fluoroelastomers such as FKM, known to most people as Viton, can provide excellent resistance to fuels, lubricants, and oils. Another key characteristic of is extremely permeability when exposed to a range of substances that include oxygenated aircraft fuels. They also offer reliable performance at extremely high temperatures where non-fluorinated elastomeric materials will start to degrade.

In addition, FKM comes in various grades focusing on features such as low temperature resistance, fuel resistance without sacrificing necessary elasticity, and chemical resistance that is unaffected by extremely high temperatures. Such features combined have already made them a common choice in aerospace applications, including o-rings.

Silicone O-Rings

Silicone rubber o-rings have been used extensively by NASA and remain a popular choice for o-rings used in spaceflight applications. In fact, here’s a direct quote from NASA that dates back to 2010:

“Silicone rubber is the only class of space flight-qualified elastomeric seal material that functions across the expected temperature range.”

It is considered by many to be the best in-class elastomer choice for extremely harsh environments involving high temperatures and among its key properties is its ability to maintain critical mechanical properties in the presence of extreme heat. A potential issue related to the use of silicone for o-rings lies in its gas permeability.

Conclusion

O-rings are just as important to modern spaceflight as ever, and so is the importance of choosing the right type of o-ring. A failed o-ring, no matter how tiny it may seem, can lead to serious disaster and potential loss of life. 

If you are looking for a reliable o-ring solution for an aerospace or spaceflight application, contact the sealing group here at Advanced EMC. Our team will work with you to explore all possible solutions, including materials beyond those discussed here. Give us a call today and let our team put their expertise to work for you.

Spring Energized Seals for Aerospace and Defense
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Spring-Energized Seals for Aerospace and Defense

In aerospace and defense applications, seals can be found in gearboxes, flap actuators, aircraft braking systems, actuators, turbopumps, cryogenic refueling systems, and hydraulic systems. They are necessary for land and ground defense systems, seaborne systems, ships, UAVs, airborne systems, and aircraft. But finding a sealing solution that provides the right mix of reliability and performance in the harsh environments associated with such applications can be challenging. However, there is a solution: spring-energized seals.

Spring-Energized Seals

Spring-energized seals are designed with a stainless steel spring designed to keep the seal lip in contact with the sealing surface despite wear, eccentricity, pressure changes, dimensional changes, and out-of-roundness. Spring-energized seals are often implemented when other sealing solutions have failed.

When the right jacket material is chosen, spring-energized seals can also handle some of the harshest operating conditions, including exposure to corrosive or reactive chemicals, extreme temperatures, wide temperature and pressure ranges, and environments where lubricants cannot be used.

The Challenges of Aerospace and Defense Sealing Solutions

Because both aerospace and defense applications are mission-critical, seal failure is simply not an option. Seal performance is key, even in extremely harsh environments that can destroy traditional polymer and elastomer seal solutions.

Many times, aerospace and defense engineers must achieve a balance between the friction of a seal and its sealing effectiveness. This can be difficult because friction increases power consumption which in turn can increase the fuel or power needed. A low coefficient of friction is a must for the seal jacket material used, especially if it is a dynamic seal of one that involves oscillating movements.

Applications that involve oscillation, such as pan/tilt, pod, and gimbal seals, must have both low friction but prevent stiction. Repetitive, precise, accurate movement has to be taken into account for some of these applications.

Aerospace and defense sealing environments often involve corrosive or reactive media that is not only dangerous if it leaks but can destroy the materials used for seals. Chemical compatibility and resistance to media such as fuels, oils, acids, bases, and other reactive chemicals is a must.

There is, however, another factor to be accounted for: abrasive media. Many seals are exposed to environmental contamination such as dirt, dust, sand, and other abrasive particles. An effective seal must not only be able to keep such contaminants outside, but not be destroyed in the process.

Extreme temperatures (and temperature ranges) can also pose a serious challenge. Applications can involve cryogenically cold temperatures down to -460°F to extreme heat at 600°F. And some operating environments may involve temperatures that vary widely, making dimensional stability a key element in seal design for aerospace and defense equipment. Temperatures are not the only environmental factor that can be extreme, however. Operating pressures can be extreme, ranging from 20 KSI to vacuum pressures.

Spring-Energized Seals for Aerospace and Defense

Spring-energized seals are highly reliable, even when things go wrong. They provide dependable performance in extremely harsh conditions, especially when the right seal jacket material is chosen.

Materials such as PTFE or PEEK have extremely low coefficients of friction and are self-lubricating. When combined with spring-energized seal design, the result is a low-friction seal that does not compromise the effectiveness of the seal and prevents potential issues with stiction and stick-slip behavior. These materials are also self-lubricating, making them ideal for situations where temperature or media rules out the use of traditional lubricants.

In addition, PTFE and PEEK are both highly resistant to chemical attack, with PTFE being the most chemically compatible polymer on the market. Combining the reliability of a spring-energized seal with the compatibility of materials such as PTFE or PEEK means a high performance seal that can survive in the presence of harsh media.

Spring-energized seals can account for seal wear and changes in the surface condition of the sealing surface without compromising the integrity of the seal. When combined with an abrasion resistant jacket material, they become an ideal sealing solution for applicants that involve abrasive media and contamination.

Both PEEK and PTFE can handle the extreme temperatures involved in aerospace and defense sealing applications. Not only do they possess a wide range for operating temperatures, they are also dimensionally stable. Dimensional stability combined with the ability of a spring-energized seal to account for dimensional changes make for an excellent sealing solution when there are extreme temperatures and wide temperature differentials involved.

Extreme pressures are another area where spring-energized seals outperform traditional seals. Whether its negative vacuum pressures or high 20,000 psi pressures, spring-energized seals can provide reliable performance — even when the environmental pressure varies significantly. Keep in mind that the spring-energizer keeps the seal lip in contact with the sealing surface.

Where Spring-Energized Seals Are Used

Spring-energized seals are already being used in landing gears, where they have proven invaluable for cylinders and hydraulic pumps. They offer the precision and performance needed for metering valves and fuel pumps, not to mention their use with actuators as they prevent environmental contamination. Spring-energized seals can also be found applications involving gimbals and pods, where their low coefficient of friction and self-lubrication prevents issues with sticking.

Conclusion

Spring-energized seals with PTFE or PEEK as the jacket material provide a high-performing solution to many sealing applications in the demanding environments of the aerospace and defense industries. Keep in mind that custom jacket profiles and spring-energizers can also be engineered to meet specific project needs and extensive testing can be performed to ensure that they are in compliance with related military standards.