Despite rumors that the Bush Administration will order NASA to stop its reusable launch vehicle (RLV) work in favor of developing new expendable vehicles to visiting the moon and Mars, Northrop Grumman engineers working on RLV-related technologies are confident their work will find its niche.

NASA's Next Generation Launch Technology (NGLT) program is focused on developing RLV technologies to support future space vehicles. Tod Palm, cryogenic fuel tank project leader for Northrop Grumman, is working under NGLT to develop new methods of creating large, reusable composite fuel tanks. The company's NGLT contracts are set to expire at the end of May.

"With the recent rumors out there ... about what's going on with a moon mission or a Mars mission, any of those missions is going to require significant payload-to-orbit capability," Palm told The DAILY. "Of course there are no particulars [yet], but we're pretty enthused around here because I think it's going to involve very big tanks to get that amount of payload to orbit. Just picture a winged Saturn vehicle or a piggybacked version of that."

In October, Palm's team delivered to NASA the first composite fuel tank structure to be made without an autoclave, the pressurized oven normally used to heat, shape and cure composite structures. Using a technique called Ultrasonic Tape Lamination (UTL), an ultrasonic vibration is imparted to the composite layers that compacts the laminate and eliminates air bubbles, a job usually done by the autoclave.

"When finished, rather than being put in an autoclave for final compaction, you can use a vacuum bag oven cure," Palm said. Using UTL, large reusable composite tanks can be made 10 to 25 percent lighter than comparably sized single-use aluminum tanks, according to Northrop Grumman.

NASA's vision for a reusable launch vehicle called for a tank up to 27.5 feet in diameter and 80 feet long, which would be too large to cure in any standard autoclave, Northrop Grumman says. Although NGLT has been aimed at reusable tanks, expendable tanks could use the same technology, according to Palm.

"Our architecture guys have also discussed drop tanks or expendable, disposable tanks similar to what shuttle does with the external tank," he said. "And the same technologies would apply. As far as composite tanks go, I think it will be an enabling technology for the next generation of launch vehicle, whatever that may be.

"I think the composite tanks can really save a significant amount of weight and that may enable incorporating a more robust [thermal protection system] so we avoid the type of issue that the shuttle's had, by leveraging the weight savings you get off the tanks and the airframe," Palm said.

UTL advances

The next step for Northrop Grumman will be to combine the device that lays down the composite layers and the UTL device into a single attachment. For now, the composite layering device and the UTL head must be swapped out periodically.

Without the need to fit the structure in an autoclave, size is no longer a limitation on continuous composite structures, according to Palm. "We don't have to spend tens of millions on an autoclave," he said. "We've shown that if they want a 30-foot diameter tank or a 35-foot diameter tank to meet what their mission profile is, then we could provide that."

Non-autoclave composite structures also have applications to aircraft and other aerospace systems, according to Palm. Large composite wings or fuselage structures could be created without joints, thus enhancing strength and saving money, he said.