The seal does not crack or leak at cryogenic temperatures.
Typical lighter-than-air vehicles utilizing a superpressure design such as balloons, aerostats, or blimps, have one or more fittings attached to the gas containment skin that can serve as load attachment points or inflation/vent ports. These fittings are often sealed to the skin with a silicone gasket and a room temperature vulcanizing (RTV) adhesive. This type of seal works very well over the temperature range encountered in the Earth’s atmosphere (–60 to +40 °C). However, balloons designed to operate at Titan or Mars would encounter temperatures much colder than those found on Earth, making this type of seal inadequate.
A new balloon fitting seal design, rated to operate down to cryogenic temperatures, was developed by designing a fitting capable of precisely compressing an expanded polytetrafluoro ethylene (ePTFE) gasket to prevent helium leakage. ePTFE is routinely used as a gasket material in cryogenic pipeline applications because it does not crack under stress down to temperatures as low as –268 °C. This gasket material does not exhibit the typical glass transition properties of amorphous materials, which can become brittle and crack when loaded at temperatures below their glass transition temperatures.
The end fitting is used as a fill-port for a superpressure cryogenic balloon prototype. The fill-port end fitting is a stainless steel component that has a circular flat region that holds the gasket in compression against the balloon film material; a raised annular section on the circular flat region that compresses the seal by 67% in the assembled configuration; a 37° cone connection on the fill port that seals with a common AN fitting attached to the gas supply; and a threaded stem that uses a nut and flat washer to compress the seal during installation into the balloon. The fitting uses a Gore GR expanded PTFE gasket that is 0.063-in. (≈1.6-mm) thick to seal the balloon film material against the balloon fitting.
The new seal applies technology used in cryogenic piping systems to create a helium tight balloon fitting capable of operating down to at least –180 °C (limit of testing). The seal incorporates a feature (the raised annulus) that was specifically designed to compress a continuous portion of the gasket to the optimum thickness so that it does not leak due to thermal contraction difference between the seal materials at cryogenic temperatures. There is a plateau in the deflection-stress curve when the gasket is compressed to 67% of its original thickness. The plateau in the compression curve assures that a gap will not be created in the seal due to differences in thermal contraction between the stainless steel end fitting and the ePTFE gasket as the temperature drops to cryogenic values. A constant pressure is maintained between the gasket and the end fitting over a wide temperature range, ensuring the seal does not leak.
Since the raised annulus feature compresses only a portion of the gasket, it is self-evident when the seal is properly seated during assembly. The torque required to turn the compression nut sharply increases when the gasket is seated in the fitting. This makes it easy to determine when the seal is sufficiently tightened in assembly.
This work was done by Jeffery L. Hall and Michael T. Pauken of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49165
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