When commercial aircraft cruise at 12,000 meters, rockets penetrate the atmosphere, and satellites operate in vacuum orbits, extreme conditions involving drastic temperature changes, dramatic pressure drops, intense radiation, and severe vibrations become the norm. Ensuring the safety of these vehicles is partly dependent on thousands of sealing points distributed throughout the fuselage, engines, and fuel systems. These seals must prevent fuel leaks, hydraulic failures, or cabin pressure instability over decades, within a temperature range from -65°C to +200°C or even higher. In this ultimate test of reliability, high-performance silicone rubber stands out as the "silent guardian" of aerospace sealing systems due to its wide-temperature elasticity, low-temperature flexibility, and weathering stability.

I. Performance Requirements under Extreme Conditions

Aerospace sealing materials need to meet:

Wide-temperature elasticity: Maintain sealing force between -55°C (stratosphere) and +200°C (near engines);

Low compression set: Have a rebound rate >80% after long-term compression to avoid leakage caused by "cold flow";

Resistance to media: Withstand erosion by Jet A aviation kerosene, MIL-H-5606 hydraulic oil, deicing fluids, etc.;

Low outgassing: Do not release volatiles in vacuum or low-pressure environments to prevent contamination of optical lenses or sensors;

Flame retardancy: Pass FAR 25.853 or OSU heat release tests without producing molten drips when burning.

Common rubbers like NBR harden at low temperatures, while FKM, although oil-resistant, becomes brittle. However, phenyl silicone rubber significantly improves low-temperature resistance and radiation resistance through the introduction of phenyl groups on the side chains, making it the preferred choice for high-end applications.

II. Typical Application Scenarios

Fuselage and Cabin Door Seals

Silicone rubber O-rings or custom-shaped seals are used for cabin windows, emergency doors, cargo doors;

At altitudes above 8,000 meters, the pressure difference between inside and outside the cabin reaches 0.5 bar, requiring seals to withstand continuous compression without creeping;

The low glass transition temperature (Tg ≈ -115°C) of silicone rubber ensures softness and conformity in extremely cold environments.

Engines and Auxiliary Power Units (APUs)

Although main seals often use FKM, some sensor interfaces and cable penetrations use silicone;

Its excellent electrical insulation properties can prevent interference with high-voltage ignition systems;

High-temperature areas use ceramic-filled silicone rubber, capable of short-term endurance up to 300°C.

Fuel and Hydraulic Systems

Despite average oil resistance, silicone rubber is widely used in secondary seals that do not directly contact fuel;

For example, gaskets for fuel pump housings, seals for filter end caps, working alongside FKM primary seals in redundant designs;

New fluorosilicone rubber (FVMQ) is used in direct contact with aviation kerosene, offering both flexibility and oil resistance.

Spacecraft and Satellites

In vacuum environments, NASA strictly limits total mass loss (TML <1.0%) and collected volatile condensable materials (CVCM <0.1%);

High-purity addition-cured silicone rubber certified by ASTM E595 is used for solar panel hinges, antenna feed-through seals;

It offers superior atomic oxygen and UV radiation resistance compared to most polymers.

III. Material Innovation and Certification Systems

Phenyl content adjustment: When the molar ratio of phenyl is between 5–15%, tensile strength at -70°C increases threefold;

Nano-reinforcement: Adding fumed silica or carbon nanotubes enhances extrusion resistance and wear resistance;

Certification standards include AMS 7254 (silicone rubber specifications), MIL-DTL-25988 (fluorosilicone rubber) in the US; EN 2282, AIRBUS AIMS standards in Europe;

All aerospace-grade silicones must provide batch traceability and comprehensive performance testing reports.

IV. Challenges and Future Directions

Dynamic seal lifespan: Actuator cylinders of landing gears experience high-frequency reciprocating motion leading to wear; self-lubricating silicone composite materials are being developed;

Smart seals: Embedding fiber optic sensors for real-time monitoring of seal stress and temperature;

Green alternatives: Reducing platinum catalyst usage to promote sustainable aerospace material development.

Conclusion

At ten thousand meters altitude, without the roar of engines or blinking alarms, there are only those silent silicone rubber seals remaining soft in minus sixty-degree nights and reliable after hundreds of takeoffs and landings. They are unseen but represent the fundamental promise of flight safety. It is these countless small yet resilient flexible defenses that allow humans to safely traverse clouds and head towards the stars—because true safety often lies hidden in the least noticeable gaps.


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