A lightweight composite-material tank suitable for the storage and transport of liquid oxygen and other cryogenic liquids has been developed. The tank includes a metallic liner compatible with liquid oxygen, a graphite/ epoxy overwrap, an insulating layer of lightweight polyurethane foam, an aromatic polyamide/epoxy overwrap for resistance to abrasion and impact, and one or two lightweight fittings or bosses. Tanks of this type have been made with diameters ranging from 6 to 36 in. (15 to 91 cm) and lengths ranging from 1 to 4 ft (30 to 122 cm).
Fabrication of such a tank begins with the formation of a eutectic-salt mandrel in the desired size and shape of the interior tank volume. The eutectic salt must be chosen so that the mandrel will withstand the temperature of curing at a subsequent stage of fabrication described below.
The salt mandrel is fitted with two lightweight bosses as follows: First, a thin-walled aluminum boss is machined and plated with a thin coat of nickel. Next, a graphite/epoxy composite boss is cast and fit over the aluminum boss. The salt mandrel is sealed and later coated with nickel by electroforming. The nickel layer bonds with the boss end fittings and, together with the end fittings, encloses and defines the interior tank volume; that is, the nickel layer becomes the metallic tank liner. The thickness of the nickel layer is typically 5 mils (≈ 0.13 mm), but the layer can easily be deposited to a different thickness as needed for a specific application.
Optionally, the bosses could be machined from pure aluminum and aluminum or poly(tetrafluoroethylene) could be used as the liner material in place of nickel. An aluminum liner could be deposited by wire arc spraying or vacuum deposition; a poly(tetrafluoroethylene) liner could be deposited by flame spraying or powder coating. An aluminum or poly(tetrafluoroethylene) liner is preferable to a nickel one if the fluid to be contained is highly concentrated hydrogen peroxide.
The metal-coated mandrel is wrapped with multiple layers of graphite fibers impregnated with an epoxy resin that, preferably, is compatible with liquid oxygen. In an application in which there is a risk of corrosion at the interface between the metallic liner and the graphite/epoxy, a fine layer of glass/epoxy could be wrapped around the metallic liner first. The overwrap structure can be filament-wound, manually laid with fabric, or a combination of both. The wrapped workpiece is cured in an autoclave or an oven, typically at a temperature of ≈100 °F ( ≈38 °C) and pressure of ≈100 psi ( ≈689 kPa). After the cure, the salt mandrel can be washed out from within by use of water; alternatively, the mandrel can be left in place temporarily.
The polyurethane foam is sprayed over the workpiece, then machined to the desired wall thickness and profile. The aromatic polyamide/epoxy composite is wrapped over the foam, then cured. A very thin rubberized coat can also be sprayed or brushed on the cured aromatic polyamide/epoxy overwrap for additional protection against abrasion.
This work was done by Thomas K. DeLay of Marshall Space Flight Center.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
MSFC Commercialization Assistance Lead
Refer to MFS-31379.