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Seals for Space Vehicle Propulsion Systems

In addition to the growing market for commercial applications, Statista estimates that 80 countries will have their own space program by 2025. And a critical factor in the success of any space venture is the type of seals used, including those for space vehicle propulsion systems.

However, finding the optimum sealing solution that provides highly dependable performance can be formidable for several reasons, including extreme pressures and temperatures, chemical compatibility, SWaP, and outgassing, not to mention vendor requirements involving seal size manufacturing.

Extreme Pressures

Depending on the applications, the pressures faced by seals can involve not only high pressures in the range of 100 bar but ultra-high vacuum conditions as well. For example, consider a typical hypergolic rocket propulsion system where pressures can reach between 1,500 and 16,000 psi during Stage 2. To further complicate matters, pressure fluctuations make it much more challenging to retain a reliable seal. 

Extreme Temperatures

One of the challenges involved in designing seals for propulsion systems often involves wide temperature ranges from cryogenic to extreme heat. For example, the fuel tanks on many modern rockets must provide sealing at cryogenic level temperatures at which many polymeric and elastomeric materials become brittle and unreliable. And when temperatures fluctuate, traditional seal solutions can experience significant dimensional changes, leading to leaks.

Material Compatibility

Seals for vehicle propulsion systems must be compatible with the fuels involved. For example, cryogenic hyperbolic bipropellants often require seals resistant to oxidizers. Also, keep in mind that, according to a NASA Lesson Learned on Static Cryogenic Seals for Launch Vehicle Applications, seal materials must also be compatible with any anticipated purge or cleaning material that they may come into contact with.

SWaP

SWaP (Size, Weight, and Power) is a significant factor in both space vehicle engineering and aerospace in general. Designs must fall within tight constraints for weight to minimize fuel requirements and size to reduce the overall volume of the space vehicle. In addition, seals, including those for the propulsion systems, must also be optimized to meet SWaP requirements.

Outgassing

Vacuum conditions make outgassing, the release of gasses from solid materials, a serious problem. This phenomenon not only introduces new chemicals to what may already be a volatile environment but can also lead to the deposition of chemicals on critical surfaces. Therefore, NASA makes available spacecraft material outgassing data to aid in selecting safe materials.

Size

Large fuel tanks necessitate large seals. For example, some of the seals for NASA’s SLS (Space Launch System) are over 6’ in diameter, and other fuel tanks have similar requirements. Such seals are challenging to both manufacture and test. Such large seals must usually be custom manufactured with small production runs, making it difficult to balance cost and performance. In addition, not all seal vendors have the manufacturing capabilities required for creating seals of this size at the proper levels of accuracy and precision.

Spring-Energized Seals

Image from NASA

Spring-energized seals, widely used in aerospace, are a practical solution for space vehicle propulsion systems. They can retain seal integrity even when subject to extreme pressure and temperatures due to the energizer. In addition, these seals can account for misalignment, eccentricity, thermal expansion and contraction, wear issues, and other dimensional changes. And they provide a consistent force over a wide deformation range.

The most commonly used materials for spring-energized seals in space environments include PTFE, FFKM, and Hytrel. However, when outgassing is a concern, materials such as PEEK, PAI, PI, Acetal, PCTFE, and antistatic PTFE can be considered. And many of these materials are also available with fillers to enhance their properties.

There are also a variety of choices regarding the spring energizer used. For example, cryogenic and vacuum pressure environments most often implement V ribbon springs (also known as V-springs),  helical springs work exceptionally well for static cryogenic applications, and high-pressure conditions usually employ coil seals. And remember that canted coil springs can be highly customized to produce consistent forces.

Advanced EMC: Your Source for Space Vehicle Seals

When it comes to vendor concerns, Advanced EMC has the equipment and experience required to manufacture large seals accurately. Our sealing solutions team has the knowledge and understanding needed to help you at every step of selecting propulsion system seals for everything from small LEO satellites to passenger-laden space vehicles. Contact us today!

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Canted Coil Springs for Latching, Holding, and Locking

Many associate canted coil springs with spring-energized seals, but they can be used for much more. For example, canted coil springs provide a lightweight, easy-to-install alternative to traditional mechanical fasteners. 

Canted Coil Springs Fasteners

Canted coil springs can perform three different fastening tasks:

  • Latch: fastens two parts together
  • Lock: permanently fastens two parts together
  • Hold: holds and aligns two parts together

Note that both holding and latching allow for the parts to be disconnected and reconnected. However, if two pieces are locked together, they cannot be separated without either damaging the canted coil spring or the connected parts. Also, note that canted coil springs can be used for quick-release locking, which allows the lock to be bypassed for a quick, simple release. And in all fastening applications, the force required to latch, lock, or hold can be specified by the engineer and accomplished by varying the coil space, wire diameter, spring width, and height.

Advantages of Using Canted Coil Springs as Fasteners

There are several benefits to using canted coil springs instead of more traditional mechanical fasteners. For example, they are easy to install and require no special tools, shortening assembly time. They are also highly reliable and provide dependable performance over thousands of insertion/removal cycles.

Canted coil spring fasteners experience almost no compression set and provide precise, predictable forces even when subject to a wide range of temperatures. Furthermore, the individual coils compensate for mating surface irregularities, misalignment, and dimensional variations. 

Their lightweight, compact nature makes them ideal for use in applications where weight and available space are at a premium, and they can also reduce the complexity of assemblies. Finally, and perhaps most importantly, they can serve other functions besides fastening, such as grounding, service as multi-point conductors, and providing tunable EMI/RFI shielding.

Applications of Canted Coil Fasteners 

In the medical industry, canted coil springs are used for both latching and holding in various applications, including implantable and non-implantable devices and surgical devices, such as customized tools for use by specific surgeons. An example of an implantable device would be joint replacements that require highly reliable locking. 

The oil and gas industry depends on canted coil springs’ highly reliable mechanical fastening capabilities for advanced downhole tools where insertion and breakaway forces must be controllable. In the renewable energy market, canted coil springs work well as connectors within the highly confined environment of turbine nacelles. 

The automotive and aerospace industries find them an ideal approach to fastening in applications where SWaP (Size, Weight, and Power) is critical. In addition, canted coil springs are regularly used in the defense industry, where specific applications include aircraft system connectors and optoelectronics connectors. Finally, canted coil springs also work well for underwater, fluidic, and satellite/orbital connectors.

Conclusion

Canted coil springs are a simple, reliable, multi-use alternative to traditional mechanical fasteners. Benefits include the ability to compensate for dimensional changes, lightweight and compact nature, and their ability to reduce the complexity of systems. They can also serve as not only fasteners but EMI shields and multi-point conductors. And keep in mind that their use ranges from medically implantable devices where their ability to respond to particular insertion/removal forces is critical as well as the harsh and dangerous world of downhole tools in the oil and gas industry. 

FlexForce Canted Coil Springs

Advanced EMC’s FlexForce Canted Coil Springs are available in 300 Series Stainless Steel, Hastelloy, and copper alloys with gold, silver, or nickel plating. In addition to light, medium, and heavy load spring forces, the forces can be customized. Our FlexForce canted coil springs are available in wire diameters between 0.25 mm (0.010 inch) and 1.50 mm (0.059 inch) and coil widths between 1.5 mm (0.039 inch) and 15 mm (0.591 inch). However, other materials, finishes, and dimensions may be available.

 

Contact us at bvardeman@gmail.com or call us at (918)-994-7776 to learn more about the FlexForce canted coil springs and how they can benefit your designs.