Pipes made of graphite-fiber/epoxymatrix composites have been shown to be suitable as conduits for liquid hydrogen. Conceived for as replacements for heavier metallic liquid-hydrogen feedlines in spacecraft, pipes like these could also be advantageous in terrestrial applications in which there are requirements to minimize weights.
Heretofore, feedlines for cryogenic applications have been fabricated from conventional metals. The present pipes are fabricated from composite materials. These composite pipes are also fabricated without metal flanges or other metal end fittings. Elimination of the metal components reduces weight.
The figure depicts an all-composite material spacecraft liquid-hydrogen feedline that was constructed and tested. The design of this feedline incorporated five features that would have to be proven to demonstrate practicality. These features and their rationale were the following:
- No Metal Liner — While the absence of a metal liner made the feedline weigh less than it otherwise would, it raised a concern over potential leakage. However, no leakage was observed in ground or flight tests.
- Composite-Material 45° and 90° Elbows — It was necessary to demonstrate the manufacture and use of composite-material elbows because feedlines are often routed along twisting three-dimensional paths, making it essential to include elbows.
- Composite-Material Flanges — A metallic feedline typically consists of a pipe with a flange at each end. The flanges are often the heaviest components of metallic feedlines. Hence, the use of composite material flanges in addition to a composite tubular section saves considerable weight.
- Joint Between Two Composite-Material Tube Sections — Some feedlines can be so long or have such a complex geometry that they cannot be fabricated as single pieces. It is necessary to fabricate them by joining shorter feedlines end to end. The inclusion of a joint between two composite-material tube sections demonstrates the practicality of this approach to the fabrication of long or complex shaped composite-material feedlines.
- Joint Between a Composite-Material and a Metallic Tube — In a typical cryogenic system, there is at least one location where a feedline must be joined to a metal component (e.g., a bellows). Such joining was demonstrated in the prototype by including an adhesive bond between the composite-material pipe and a titanium end fitting.
This work was done by Philip Tygielski of Marshall Space Flight Center.