NASA prescribed regular exercise: moving the actuator at varying speeds to force the fluid to circulate better and dislodge the silt. Hooking external filters to the hydraulics removed the silt from the lines. The desilting restored full motion to the actuators, but NASA engineers worried that the devices might have experienced other types of degradation.
Over the last 10 years, every shuttle elevon actuator except two had been refurbished, so NASA could not analyze them for the effects of long-term use. Atlantis has its original actuators; Hernandez expects to get his hands on at least one. “There are types of degradation that don’t get to the point where they affect top-level functionality,” says Hernandez. “The idea is to understand this degradation and to build techniques to catch and prevent degraded hardware before it manifests at a top level of functionality.” Knowing how space shuttle actuators degrade could, again, mean a lot to the aviation industry, which employs similar hardware in commercial and military aircraft. By publishing articles in scholarly journals and agency technical publications, NASA engineers will share knowledge gained from the orbiter autopsies.
In addition to their degraded actuators, the orbiters have aging plumbing lines leading into the main engines that could prove troublesome. Years ago, routine inspections revealed cracks in the flow liners of the feed lines, which carry liquid hydrogen from the external tank to the shuttle main engines. “If they had broken off, they could have gone into the engine and caused a catastrophic failure,” says Seriale-Grush.
NASA replaced the flow liners, but had no way of knowing if there were cracks higher up. While the orbiters were still flying, removing the 12-inch-diameter feed lines would have cost millions of dollars, an unjustifiable expense considering there was no evidence of damage. It would be like removing a person’s colon to see if there was cancer, just because the patient was getting old.
Borrowing a page from modern medicine, NASA decided to go another way. Technicians threaded borescopes into the shuttles’ plumbing to look for cracks. Technicians performed these fuel-line colonoscopies regularly, searching for the tiniest signs of damage. The examinations kept the shuttles flying, held costs down, and minimized risk. Studying the feed lines now will show just how much risk remained.
Whenever possible, NASA used nondestructive testing. For example, engineers used thermography—the imaging of infrared radiation—to scan orbiters during landing to evaluate heat distribution across the reinforced carbon-carbon wing leading edges. Nondestructive testing has a role even now that the shuttles have been retired. During its stay in the Vehicle Assembly Building, Discovery was scanned with a laser technique known as photogrammetry, which plotted each surface point on the underside of the spacecraft, digitally capturing the slight asymmetries that had crept into its manufacture some 30 years ago. With this information, engineers can develop a computer simulation of Discovery in hypersonic flight. Data generated by the simulation will then be compared with data from the orbiter’s actual hypersonic flight. “We want to see whether our prediction tools can reproduce what the flight data tell us,” says Charles H. Campbell, a NASA deputy principal investigator. “That’s never been done before with the orbiter.”
Campbell and his team are especially interested in modeling the thermal changes that occur when the air surrounding a vehicle flying hypersonically changes from smooth to turbulent in a process known as boundary layer transition. “We can’t test this on the ground,” says Campbell. “You have to be in flight.”
The boundary layer transition might, however, be computer modeled, using the hypersonic data acquired during Discovery’s flights and the optical mapping data obtained from the photogrammetry. Computer simulations based on this information would not take the place of the real thing, but they could help predict the behavior of future hypersonic vehicles and aid in the development of next-generation spacecraft.
The digital mapping will also afford NASA engineers the opportunity to aid in expanding the Discovery museum exhibit with computer models. And the engineers are doing everything possible to make the orbiter look just like it did when it landed. All signs of Discovery’s surgeries will be hidden and its empty spaces filled by replacement parts, which might not fit exactly—at first.
“We’ll push a little here and push a little there,” says Mike Parrish, vehicle operations chief. “We’ll work through it, and make sure it gets done right.” If all goes well, Discovery will be as successful an artifact as it was a research vehicle during nearly 30 years of flight.