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Lightweight, Low-CTE Tubes Made From Biaxially Oriented LCPs

CTEs can be tailored by tailoring biaxial orientations.

Tubes made from biaxially oriented liquid-crystal polymers (LCPs) have been developed for use as penetrations on cryogenic tanks. ("Penetrations" in this context denotes feed lines, vent lines, and sensor tubes, all of which contribute to the undesired conduction of heat into the tanks.) In comparison with corresponding prior cryogenic-tank penetrations made from stainless steels and nickel alloys, the LCP penetrations offer advantages of less weight and less thermal conduction. An additional major advantage of LCP components is that one can tailor their coefficients of thermal expansion (CTEs). The estimated cost of continuous production of LCP tubes of typical sizes is about 1.27/ft (4.17/m) [based on 1998 prices].

LCP tubes that are compatible with liquid oxygen and that feature tailored biaxial molecular orientation and quasiisotropic properties (including quasiisotropic CTE) have been fabricated by a combination of proprietary and patented techniques that involve the use of counter rotating dies (CRDs). Tailoring of the angle of molecular orientation is what makes it possible to tailor the CTE over a wide range to match the CTEs of adjacent penetrations of other tank components; this, in turn, makes it possible to minimize differential-thermal expansion stresses that arise during thermal cycling.

The fabrication of biaxially oriented LCP tubes by use of CRDs is not new in itself. The novelty of the present development lies in tailoring the orientations and thus the CTEs and other mechanical properties of the LCPs for the intended cryogenic applications and in modifications of the CRDs for this purpose.

The LCP tubes and the 304-stainless-steel tubes that the LCP tubes were intended to supplant were tested with respect to burst strength, permeability, thermal conductivity, and CTE. The following conclusions were drawn from the tests:

  • The thermal conductivities of the LCP tubes ranged from 0.21 to 0.40 W/(m·K) — 98 percent smaller than those of the corresponding 304-stainless-steel tubes.
  • The CTEs of the LCPs were fully tailorable down to an exceptionally low value of 5.8 × 10–6 (°C)–1.
    It was necessary to fabricate an LCP tube with a wall thickness of 0.080 in. (˜2.0 mm) to obtain a flexural strength equal to that of a 304-stainless-steel tube with a wall thickness of 0.028 in. (˜0.7 mm). Even with its greater thickness, the LCP tube weighed 53 percent less than did the stainless-steel tube.
  • The LCP tubes exhibited exceptionally low permeability: The average rate of transport of oxygen though an LCP tube having dimensions typical of those of a cryogenic tank at a temperature of –183 °C was found to be of the order of 4 × 10–21 cm3 (standard temperature and pressure) per day.
  • All of the LCP tubes exceeded the hydrostatic-burst-strength requirement by at least 400 percent.

This work was done by Leslie Rubin, Frank Federico, Richard Formato, John Larouco, and William Slager of Foster-Miller, Inc., for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4-8, 21000 Brookpark Road, Cleveland Ohio 44135. Refer to LEW-16780.