PTFE Seals, PTFE Spring-Energized Seals

With the success of commercial spaceflight companies such as SpaceX, Blue Origin, and Virgin Galactic, there is an increasing demand for high performance, dependable seals. Rockets are one of the areas where harsh environment seals are needed, but also pose extremely challenging issues for success. Spring energized seals are one solution, but why?

What Makes a Modern Rocket

Successful spaceflight involves rockets, and the primary sections of a modern two-stage rocket are the first stage engine bay, first stage, second stage engine bay, second stage, and, last of all, the payload. This constitutes the most common configuration for today’s NewSpace companies. 

Such a configuration features an expendable or reusable first stage that contains 4 to 9 engines (the number of engines varies based on company design) and an expendable second stage that typically contains a single vacuum-optimized engine. The goal of the first and second stages is to produce enough thrust to achieve a targeted orbital velocity–usually around 17,500 mph– for the payload that sits on top of the rocket.

Propellants and Pressurants

Most rockets use either solid or liquid propellant. In this blog post, the focus will be on bi-propellant rockets, which are most commonly being used or developed in the United States commercial market. Bi-propellant rockets, as the name implies, use a combination of propellants. Common propellant configurations include:

• RP-1 (Highly refined kerosene)/Liquid Oxygen (LOX) (aka, Kero-Lox)
• Liquid Methane/LOX (aka, Metha-Lox or Lox-meth)
• Liquid Hydrogen/LOX (Hydro-Lox)

Pressurants and support fluids include:

• GN2 (Gaseous Nitrogen)
• Helium (He)
• GOX (Gaseous Oxygen)
• GCH4 (Gaseous Methane)

How Modern Rocket Propulsion Systems Work

For a pump-fed system, the propellants are fed from low pressure tanks into a turbopump assembly (TPA). This significantly raises the pressures to be injected into the main combustion chamber (MCC). In most cases, a small portion of the propellants are scavenged from the high-pressure side to feed a separate small combustion chamber known as a gas-generator or pre-burner and used to drive the turbine. These fuel or oxygen rich gases can then either be vented to the atmosphere or re-injected into the MCC.

Operating Conditions of a Rocket Propulsion System

Consideration of the operating conditions within a rocket propulsion system provides insight into the challenges faced by the seals.

• State 1 – Tank to Turbopump Assembly (TPA) inlet: propellants (oxygen + methane) are usually around 50 -150 psi and RP1 will be between 20 F and 80 F while the cryogenics will be between -450 F to -260 F.
• State 2 – TPA outlet: depending on the engine, pumps will raise these pressures to somewhere between 1,500 and 16,000 PSI.
• State 3 – Pre-burner: pressure will have dropped across the lines and injector – usually 8-15%, however temperatures will be between 800 -1,500 F.
• State 4: depending on the engine cycle, propellants may be in a liquid-liquid state, gas-liquid state, or gas-gas state at an array of temperatures and pressures before mixing in the MC; note that in most cases the fluids will be supercritical.
• State 5: once across the injector, the remaining propellants will combust at temperatures higher than 4000 F while pressure in the MCC may be between 50-20% of State 2 depending on system losses; note that this pressure drops quickly as the gases are pushed toward the atmosphere.

Depending upon which stage is involved, seal requirements vary greatly but high pressures and extreme temperatures will always be involved. 

Rocket Engine Seals

Rocket engine seals must perform in some of the most harsh environments imaginable and may involve wide operating temperature ranges (including cryogenic), extreme pressures, wide thermal cycling, and chemical compatibility with fuels, propellants, and pressurants. Most importantly, they must be extremely reliable. As an example, consider the just a rocket turbopump.

The image shown is a Hydro-Lox turbopump with a geared coupling used in the Aerojet Rocketdyne RL10 engine. Where it is labeled with a 1 indicates flange locations that likely use spring-energized face seals. Downstream of the outlets  will be the main valves, and they too will most likely have additional flange connections that will require seals. Areas labeled with 2 indicate other flange locations that depend on face seals of unknown makeup but likely involve hot gas connections.

Spring Energized Seals: A Rocket Sealing Solution

One of the most reliable, harsh environment sealing solutions is the spring energized seal. Unlike conventional seals, a spring energized seal includes an energizer that enables the seal lip to stay in contact with the mating surface through extreme variations in pressure and temperature,and  dimensional changes, as well as out of roundness, eccentricity, hardware misalignment, and some degree of wear. Vibration, cryogenic temperatures, and high temperatures are also an area where spring-energized seals offer outstanding performance.

They are highly durable in operating environments where other seals simply cannot survive. In fact, the performance of such seals has been well established in aviation and aerospace, including both NASA and commercial rockets. 

A wide variety of jacket materials are available, with some of the most widely used aerospace options being PTFE (trade name Teflon) and Hytrel. Materials such as Teflon and Hytrel can handle extreme temperatures, are chemically compatible with media involved, are heat resilient, provide low friction, have excellent wear characteristics, and are typically self lubricating. In addition, both materials are available in grades that provide key characteristics such as improved wear, lower friction, additional stiffness, better strength, etc.

And the same is true for spring energizers, which vary in both geometry and material used. For example, vacuum pressure and cryogenic applications often utilize V-springs (also known as V ribbon springs), high pressure environments may use coil springs, and vacuum pressure operating conditions with medium speeds may utilize helical springs. Various materials can be used for the spring, which will be enclosed within the seal jacket; because of this, the spring material will be protected from whatever media is being sealed.

Conclusion

If you are in need of spring energized seals for space applications, allow the seal specialists at Advanced EMC help you. We have a long history of providing our customers with the seals they need, including custom engineered and manufactured solutions that not only meet their specifications but also the rigorous standards that may be involved. Advanced EMC has the design, manufacturing, and testing capabilities you need to make your design a success. Contact us today to learn more.