Engineers assess space station's structural integrity
BY STEPHEN CLARK
Posted: September 13, 2010
While politicians in Washington debate the future of America's space program, engineers across the globe are dutifully verifying the International Space Station can safely survive another two decades in orbit.
The $100 billion outpost has been under construction for nearly 12 years, and officials previously penciled in 2015 as the end of its mission. But a retirement that soon would cut much of the station's potential scientific and engineering value just a half-decade after it was finally completed.
Announced by the White House in February, the proposed continuation of ISS operations was met with widespread praise. All of the project's international partners are in favor of the extension.
The decision sparked a one-of-a-kind engineering effort to certify the space station to operate until 2028, the 30th anniversary of the launch of the first element of the complex.
"That's 30 years after we launched the first piece," said Kirk Shireman, NASA's deputy space station program manager. "Some of our experts said that's about as much as we think we'll get. So far, we haven't seen any show-stoppers."
It's a tough job made even more challenging by the team's inability to inspect the station. Unlike the 50-year-old B-52 bomber, which the U.S. Air Force intends to fly until 2040, the International Space Station can't be returned to the ground to check for fractures or stress.
"We do appreciate the lack of hands-on (access)," said Mark Mulqueen, the space station vehicle manager at Boeing Co. "We can't rely on ground inspections to go into our formulation of extension."
The partners picked a recertification date beyond 2020 to avoid redoing the analysis if politicians extend the station again.
"The president's budget proposal said we would get extended through 2020, but because it takes so long to do this analysis that we're talking about, you really hate to do it to 2020, and then five or six years down the road, they say, 'Hey, how about going beyond,'" Shireman told Spaceflight Now.
The space station's major modules and truss elements were built to survive 15 years in flight, so engineers must convince themselves the components can withstand double their design lives.
"It's a goal right now to try to double the life from 15 years to 30 (years)," Mulqueen said in an interview. "We feel confident in that because of the conservative nature of the initial loads and the way the station's been operating."
Engineers are looking at anything that could weaken the station's structure. The analysis is expected to continue through at least 2011, according to Shireman.
"It's thermal cycling, every time the sun comes up and goes down, it's docking events, it's berthing events," Shireman said. "In some cases, it's exercising and that puts cyclic loads on the station."
Teams are separately analyzing the station's computer network, spare parts, and operations strategies to optimize them for more years of work.
Boeing is the prime contractor for the U.S. segment of the station, which also includes the Russian-built Zarya module, the first piece of the outpost launched in November 1998.
That process started before the proposed station life extension because Zarya's warranty runs out in 2013, its 15th birthday. The space station was previously due to be retired in 2015.
"This was started about three years ago," Mulqueen said. "We said let's test it with a whole other full life environment. That would be another 15 years."
The testing was successful, according to Mulqueen.
"We haven't bought off through all the Russian agencies that we need signatures from, but we did complete the testing with adequate performance," Mulqueen said. "We're just inspecting all the welds and everything."
Engineers will next turn their attention the Unity module, a Boeing-built connecting node that was added on a space shuttle mission in December 1998. Unity's 15-year design life also runs out in 2013.
Other modules launched in the first few years of the station program, such as the Destiny laboratory and the Quest airlock, must also be analyzed before components that arrived in recent years.
The Russians are also reviewing the Zvezda service module, which was added to the complex in 2000.
"You have to look at the calendar on when the elements were launched and on-orbit," Mulqueen said. "The cycle count really starts counting as it's launched and on-orbit."
Newer modules, including European and Japanese contributions, won't hit the end of their design lives until the 2020s.
The reviews include thousands of structural components that throw up red flags in an initial screening. The biggest focus is on pressure shells, truss segments and solar arrays, according to Mulqueen.
"This is going down to the individual piece parts and fasteners, that level of detail on some of this analysis," Shireman said.
The first step is to compare the predicted effects of thermal cycles, docking loads and other stresses to measurements of the the actual forces collected by sensors aboard the space station.
"We can compare those and repeat our fatigue and fracture analyses of our hardware, and thus be able to predict additional years of service for the station," Mulqueen said.
In most cases, the observed loads on the station were much less severe than forecasted, according to Mulqueen.
"We can look at how we really flew versus how we the thought we were going to fly," Shireman said. "The loads from docking events, for instance, aren't nearly as bad as we thought it was going to be. That translates into extra life on the structure."
By taking into account the actual stress generated by changing temperatures, dockings and other events, engineers can refine their models of how the station's structure responds.
"No. 1, we look at our analysis methods and assumptions," Shireman said. "They tend to be very conservative initially, so we could make them less conservative."
"It's called fracture mechanics, and we actually inspect the structure as it's being built," Shireman said. "The inspection method says I can find a crack down to some minimal length, and if the crack is smaller than this minimum length, then I won't be able to detect it."
Station officials can't rely on crew members to inspect for fractures in space.
"The way fracture mechanics works is if we assume we have a crack at that minimum length, then we run it through all these cycles, and then you look (at the model) to see if that crack has grown," Shireman said. "If the crack has grown to the point where the structure would fail, then you say it wouldn't have enough life."
Airlines and airplane contractors commonly inspect aircraft for such fractures, but with the space station out of reach more than 200 miles up, engineers rely on complex models to predict their growth in orbit.
"The requirement says that you have to actually have four times the life, so when we certified the structure for 15 years originally, what we really did is say it has four times 15 years of life," Shireman said. "That four times factor is because it's still a pretty crude science."
So what happens if engineers find a weak spot?
"If it proves we don't have full capacity in an area, then we would either limit the station based on that critical link, or we'll go look at areas of how we operate the station," Mulqueen said.
Those operations changes could be for the crew's exercise protocols or the docking locations of visiting vehicles.
Engineers could also design struts or structural doublers to place over suspect areas.
"I don't think we're going to have wholesale issues," Shireman said. "What we'll have is a tiny rib on a piece of structure could have crack growth and might fail, so we'll have to go deal with that particular piece."