Labs’ tools to help NASA get shuttles space-bound again
The multicolored printouts Dennis Roach (6252) rips off an extra-wide printer in his lab could be some sort of abstract art. Soft blues and greens cover most of the page, but in several areas there are spots of red surrounded by rings of orange and yellow.
To the people at NASA responsible for resurrecting the space shuttle program from last January’s Columbia tragedy, the images are a sight for sore eyes — laboratory science applied to one of the space agency’s most pressing problems: how to certify that key orbiter components are spaceworthy before allowing the remaining shuttles to fly again.
Last month, NASA funded Sandia to develop nondestructive inspection hardware, techniques, and standards that will lead to a scientifically rigorous shuttle certification process. The process would be applied prior to the shuttle program’s return-to-flight mission and before each successive mission after that.
"This could be a defect, perhaps a degradation of the wing composite resulting from an impact or abrasion of the coating," says Dennis, pointing to one of the red spots.
Phil Walkington (6252) locates the corresponding spot on a shuttle wing section sitting on a nearby table, a part salvaged from the shuttle Discovery.
"It tells us that we need to look more closely for other evidence of a defect, either on the surface or inside the composite material," Phil says.
Echoes of vibrations
The 2-D images are created using data from an ultrasonic scanner. The scanner uses beams of high-frequency sound waves to detect surface and subsurface flaws as the sound waves travel through the material and are reflected at interfaces and flaws in the orbiter’s wings. The wings are made of quarter-inch-thick reinforced carbon-carbon material with a thin silicon-carbide coating.
The wings’ leading edges are important flight safety features because they must survive the incredibly harsh environment of atmospheric reentry, during which leading-edge temperatures can reach 3,000 degrees F.
NASA scientists now believe, with Sandia’s help (Lab News, Sept. 5, 2003), that a piece of foam insulation from the shuttle’s external fuel tank fell off during launch and damaged the underside of Columbia’s right wing, which caused the wing’s internal structure to overheat, melt, and disintegrate during reentry.
Certifying for reentry
The Sandia team’s work began last May when NASA’s Shuttle Safety Advisory Board asked DOE and NASA labs to investigate and propose ways to improve nondestructive inspection (NDI) methods for certifying the flightworthiness of orbiter wing leading edges.
NASA’s goal was to obtain a turnkey inspection system that could be used in a Kennedy Space Center hangar to certify the safety of shuttle wings prior to NASA’s first return-to-flight
mission, now scheduled for spring 2005. The NDI device had to be deployed without removing the leading-edge panels from the wing. It also needed to be quickly deployable following each successive orbiter mission.
Previously, orbiter wings were inspected between missions by technicians looking at the wings carefully and perhaps running their fingers over their surfaces to feel for anomalies, says Dennis. Following the Columbia accident, NASA wanted a more rigorous inspection and certification process, one backed by hard scientific documentation.
Combination of techniques
The agency envisioned a combination of nondestructive inspection methods — including ultrasonics, eddy current, thermography, and radiography techniques — that together would lead to flight certification.
Impact damage, microcracks in the exterior coating, oxidation in the carbon-carbon layer, subsurface gaps between layers, and adhesive disbonds all need to be discovered and corrected prior to launch, Dennis says.
In addition, the advancing age of the two-decade-old orbiters gives rise to a need for more advanced monitoring techniques, he says.
"Orbiters are similar to aging commercial airliners in that way," he says. "This is an area where we think we can help NASA."
Sandia’s Airworthiness Assurance Center (AANC), funded by the FAA, develops and evaluates NDI techniques as part of its work to improve methods for certifying aging commercial airliners for continued service.
The center, managed by Airworthiness Assurance Dept. 6252, was established in 1991 as an extension of Sandia’s nondestructive testing (NDT) capabilities. Advanced Diagnostics and Testing Dept. 9122 continues to provide NDT support to Sandia projects and various external customers.
Looking for the same thing
In seven short months the Sandia team selected and assembled hardware and designed and tested a tripod mount for the ultrasonic scanner that could reach all surfaces of a shuttle wing.
NASA selected Sandia in December to lead the development of the ultrasonic inspection system and funded the project in February. NASA Langley is leading development of the eddy current and thermography systems, and NASA Marshall is leading the development of the radiography system, with Sandia as a team member.
Kyle Thompson (9122) heads Sandia’s work to support NASA Marshall’s development of radiographic (X-ray) techniques to find flaws.
"My role has been to investigate the current radiographic technologies and determine what’s the best technique to use, and also determine if it’s feasible to do an on-wing inspection and what kind of trade-offs you have to make," he says.
Dennis describes the different technical approaches as complementary. Ultrasonics, for instance, has the advantage of deeper penetration into materials, whereas other techniques might find surface cracks with more fidelity.
"For the most part, we’re all looking for the same thing," he says. "One method might find something and the others would be used to support those findings."
Return to flight
The Sandia team initially evaluated and refined their inspection methods and hardware using carbon-composite samples with known defects created by the Sandia team. Later, as part of the selection process, a NASA engineer hand-carried orbiter wing samples to all the labs involved in the project and asked that each lab try to find defects known only to NASA scientists.
"Our ultrasonic scanning method performed very well," says Phil.
The Sandia team now is developing the revised inspection and certification protocols and standards that can be readily integrated into NASA’s Shuttle Orbiter Processing Facility at Kennedy Space Center for routine inspections after each orbiter flight.
"We are hopeful that by this fall NASA will be using Sandia’s tools and approaches to safely and reliably certify an orbiter for NASA’s first return-to-flight mission and for each successive orbiter reentry exposure after that," says Dennis.
Team members include Phil, Dennis, Kyle, Kirk Rackow (6252), Dick Perry (6252 Manager), Michael Hassard (9122), Steve Younghouse (9122), Mark Garrett (9122 Manager), and Jose Hernandez (NASA).