What NASA will learn from dissecting Atlantis, Discovery, and Endeavour
- By Greg Freiherr
- Air & Space magazine, May 2012
(Page 3 of 4)
NASA tried to remedy the problem by giving wires a protective covering, an arduous process that may not have had the desired effect. “So we want to go back and look at the wiring to see if there’s anything to be learned,” says Francisco J. Hernandez, deputy chief engineer for orbiter propulsion and power subsystems. “The wiring methods and design are very similar to what is done in the aircraft world, so this is something that is not only potentially applicable to future space vehicles but also maybe to the aviation community.”
In addition, engineers will examine shuttle avionics for signs of “tin whiskers,” wispy extrusions of tin molecules in circuit-board soldering that expand in zero gravity. Knowing the extent of their growth over time could help operators of satellites that use the same tin soldering predict when the satellites could fail. “Long whiskers are a great way to short out a circuit,” says Shannon. “We had a satellite short out, and we think tin whiskers was the most likely cause.”
Orbiter autopsies may also benefit the space station: Cooling systems there use a highly corrosive type of ammonia that was also used on the shuttle. So engineers are examining orbiter cooling systems for signs of damage. “These kinds of failures sneak up on you, and they are not necessarily easy to find,” says Thomas M. Simon, commercial crew assistant chief engineer. “So even if we find [only] a couple issues, the relative cost of these studies compared with losing a mission or setting up an entire test program is a big savings to the agency.”
Analyzing orbiter elevons could yield dividends as well. Each orbiter has four of these control surfaces, two on the trailing edge of each wing. Elevons roll the spacecraft left and right, and pitch the nose up and down. During a pre-launch test for STS-101, which launched on May 19, 2000, the actuators that controlled Atlantis’ elevons failed. NASA followed up by testing every actuator on every orbiter and discovered that in some cases hydraulic fluid had accumulated in tight spaces instead of flowing freely. The problem was traced to microscopic silt, contaminants in the hydraulic system that impede flow like plaque slows blood in the body.
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.