Lightweight composite structures, manufactured “out of autoclave” without pressurized curing, are a major goal in NASA’s latest technology road map, but the shape of the tanks is bringing a degree of difficulty to this process.
Engineers working on two different NASA-backed composite cryogenic tank demonstrations at Marshall Space Flight Center, Huntsville, Alabama, say strong composite structures can be produced with heat-curing alone, as long as there are open edges that can vent water vapor and other gases that would otherwise create voids when the composite material hardens. In a cylindrical launch-vehicle propellant tank with a dome on the end, those edges do not exist.
“During cure, your volatiles, your entrapped air, all these things that make porosity or voids, travel down those fiber channels to the edges,” says Justin Jackson, who was project engineer on the 5.5-meter (18-ft.) composite liquid-hydrogen tank tested at the NASA field center near Huntsville. “So, by trapping off those edges, all of that now has to go through the thickness of the laminate. It is just a more torturous path. You don’t get it out as efficiently as you would traversing down the fiber.”