Hybrid versions of the high-performance overwraps used on some low-pressure tanks have been proposed. The hybrid overwraps would weigh less than do the corresponding nonhybrid overwraps, as explained below.

In current practice, the overwrap material used on all areas of a tank is a composite that contains graphite fibers. The minimum tape thickness of this overwrap material is about 0.005 to 0.007 in. (≈0.13 to 0.18 mm); consequently, the thickness of the low-angle-helical (L.A.H.) wrap on each end surface of the tank cannot be made less than about 0.010 to 0.014 in. (≈0.25 to 0.36 mm). However, thinner L.A.H.-wrap thicknesses are needed on the ends of most tanks of this type; in such cases, the excess thicknesses of carbon-fiber-based overwraps give rise to unnecessary weight penalties.

According to the proposal, the material for the L.A.H. end wraps would be a composite based on polybenzoxazole (PBO) fibers. The minimum tape thickness of the PBO-fiber-based overwrap material would be about 0.001

in. (≈0.025 mm), and the strength and elastic modulus of PBO fibers is almost equal to that of graphite fibers; as a result, no weight penalty would exist for an end wrap based on PBO fibers.

Also according to the proposal, the hoop wrap on the cylindrical portion of a tank would still be made of the conventional graphite-fiber-based composite material. This would not entail an unnecessary weight penalty because the needed hoop-wrap thickness is greater than the minimum graphite-fiber tape thickness.

This work was done by Joseph C. Lewis of Caltech for NASA's Jet Propulsion Laboratory. No further documentation is available. NPO-20443