The use of hypergolic bipropellants such as RP1/LOX have proven to be an efficient approach to seals for rocket engine propellant. However, they require highly reliable, leak-proof seals to keep them separate before actual launch, in part because the bi-propellants will ignite when they come into contact with each other. 

Critical Factors for Rocket Engine Seals

There are four critical factors for rocket engine seals that apply regardless of the type of propellant used: safety, weight, cost, and cost per kilogram of payload. 

Regardless of whether space flight is privately or federally funded, safety remains the main priority. The seals used in rockets and rocket engines must be highly reliable with predictable behavior for every possible environment in which they will be operational, including the temperatures, pressures, and media involved. Seals in general have been problematic for rockets in the past, and accidents with hypergolic bipropellants are certainly not unheard of, with spills having occurred at Johnson Space Center, White Sands Test Facility, and Edwards Air Force Base. 

Weight has been a major concern in aeronautics and space flight for decades. The weight of the rocket is directly related to the fuel required for launch, and the combined weight and fuel reduces the payload that can be carried.Payload is a determining factor for the commercial viability of the rocket. Lightweight seal materials with high specific stiffness and/or specific strength are in high demand.

Cost is another major factor, and some approaches to sealing within rocket propulsion systems are more expensive with determining factors including the type of seal (e.g., spring energized vs traditional seals) and the material involved (PTFE, Torlon, Silicone) . For custom solutions, the size, manufacturing method, and production run are also key. 

Also, keep in mind that the commercial viability of a rocket is often determined by cost per kilogram of payload. Striking a balance between reliability, weight, and cost to achieve a lost cost per kilogram of payload is a challenging aspect of specifying rocket seals.

Hypergolic Bipropellants

Bipropellants use a mixture of two propellants, a fuel and an oxidizer, and fall into one of two categories: hypergolic and non-hypergolic. Hypergolic bipropellants will  ignite when the oxidizer and fuel come into contact with each other, while non-hypergolic propellants require a separate ignition source.

For example, the last several iterations of Blue Origin (which recently took actor William Shatner into space) rocket engines have been using the follow the following bipropellants:

• BE – 2: Kerosene + Peroxide
• BE – 3PM: Hydro-LOX (Liquid Hydrogen + Liquid Oxygen)
• BE – 3U: Hydro-LOX
• BE – 4: LNG-LOX (Liquified Natural Gas + Liquid Oxygen)
• BE – 7: Hydro-LOX

Bipropellants that include liquified oxygen, hydrogen, or natural gas are considered cryogenic because of the extremely low temperatures required to keep these materials in liquid form.

Cryogenic Hypergolic Bipropellant Seal Challenges 

When cryogenic hypergolic bipropellants are used, the cryogenic portion must be stored at extremely low temperatures (e.g., storing LOX before it is fed into the MCC). More conventional sealing materials such as traditional elastomers and uncoated metals do not provide the necessary performance at the cryogenic temperatures involved, in part due to the brittle behavior exhibited at extremely cold temperatures.

To further complicate matters, these cryogenic bipropellants will come into contact with seals as they travel through the various stages of a rocket, including compressors, pumps, ducts, joints, manifolds, and valves. And after ignition, extremely hot temperatures are involved. 

Specifying and Designing Rocket Engine Seals

In the context of cryogenic hypergolic fuels, there are some specifications that must be carefully considered, in addition to typical seal characteristics. These include …

• Wide Operating Temperature Range
• Lower Temperature Limit
• Thermal Cycling
• Cleanliness
• Chemical compatibility
• Wear
• Low Friction
• Surface Finish of Glands and Grooves

Seals that come into contact with oxidizers need to be resistant to the aggressive effects of oxidizing liquids, including long-term exposure. In addition, for seals exposed to either extreme heat or cold, their change in dimensions must also be accounted for during the design phase.

Rocket Engine Seal Solutions

One of the most popular options for rocket engine seals are spring-energized seals, which include a spring-energizer that keeps the seal jacket in contact with the sealing surface in a wide range of environments where traditional seals would fail. 

Another potential seal option would be encapsulated o-rings, which have proven themselves in a wide variety of cryogenic applications. These o-rings have a stainless steel, FKM, or silicone energizer encased within a durable, chemically compatible material. 

As to the most commonly used materials with spring-energized seals and encapsulated o-rings, both FFKM and PTFE provide excellent mechanical and chemical characteristics that suit the harsh operating environment of rocket engine seals in general, and cryogenic hypergolic bipropellant fueled rock engines in particular. Traditional elastomers often do not have the needed performance at cryogenic temperatures, which is why polymer or perfluoroelastomer materials are often preferred. 

FFKM, trade name Kalrez or Viton, is a perfluoroelastomer that offers excellent performance in the extremely harsh environments of rocket engine seals. It is highly resistant to the effects of oxidizers and has very good chemical compatibility. Another commonly used material for hyperbolic bipropellant sealing solutions is PTFE, better known by the trade name Teflon. It provides outstanding chemical compatibility, extremely low friction, and the necessary mechanical and thermal characteristics to provide reliable sealing in extreme environments.

Conclusion

When hypergolic cryogenic bipropellants are used in rocket applications, factors such as safety, weight, cost, and cost per kilogram of payload must be balanced with the various challenges involved with designing and specifying reliable seals. Two potential solutions to the issues related to rocket seal design are spring-energized seals and encapsulated o-rings, both with jackets of FFKM or PTFE.

If you are looking for a rocket sealing solution, let the engineers and experts in the Advanced EMC seal group lend their knowledge and expertise. They will work with you from the early design phase onward to find the seal type, geometry, and materials you need for a successful design. Contact them today!