July 08, 2013
A tricky weight-saving technology for space launch vehicles suddenly seems within reach after a small composite fuel tank was able to contain 2,091 gal. of liquid hydrogen while engineers fiddled with its internal pressure and cycled its temperature from ambient down to -423F in a June 25 test at Marshall Space Flight Center. The success came a short walk from the test structure where NASA's X-33 single-stage-to-orbit testbed came to an ignominious—and expensive—end in November 1999 when its composite hydrogen tank delaminated after it was filled. The agency pulled the plug on that project, cutting its loss at almost $1 billion and leaving prime contractor Lockheed Martin to box up and warehouse the almost-complete suborbital prototype.
Now the agency's Space Technology Mission Directorate is sponsoring another composite cryotank, using the latest technology to build the tanks, and setting a goal of at least a 25% reduction in cost and 30% in weight over the aluminum-lithium tanks now in use (AW&ST Nov. 14, 2011, p. 24). So far, it looks like the effort is on target.
“All the data indicate that the tank performed just as expected, so it performed nominally, and nominally is a very good thing for us,” says John Vickers, project manager on the composite cryogenic tank technology demonstration project at Marshall.
During the day-long test, the Boeing-built subscale tank went through 20 pressure cycles from zero to 135 psi, without leaking and with strain measurements meeting expectations. Next up for testing will be a 5.5-meter-dia. tank already in fabrication at the Boeing Advanced Development Center in Tukwila, Wash. (see photo). Now that the smaller tank has validated the technology, the way is clear for Boeing to deliver the full-scale tank to Marshall this fall, with testing to begin next spring.
Both test tanks are built up with thin-ply composites that do not require a pressurized autoclave for curing. The out-of-autoclave fabrication helps hold the cost down, says Dan Rivera, Boeing's project manager on the tanks, while the thin-ply approach, already in use on satellite structures and other Boeing products, prevents microcracking that causes leaks.
“The thin-ply concept, as designed for our tank, is a real breakthrough,” says Vickers. “It's been known theoretically that thin plies could reduce permeability of the hydrogen through the laminate, but the work we've done recently has been quite comprehensive and has shown that not only can it reduce permeability through the laminate, but it can eliminate it completely.”
Boeing halved the 5.5-mil plies used previously, adopting plies weighing 70 grams per sq. meter instead of 135, Rivera says. In the 5.5-meter (18-ft.) tank, the design also will tackle the honeycomb substructure that is believed to have contributed to the X-33 tank failure.
“We have developed what we call a fluted core structure,” says Rivera. “It varies significantly from honeycomb in that the core of that structure is essentially a hollow tube. So if gases escape, they are very easily vented or purged through that hollow structure.”