This article states one of the extravehicular activity (EVA) incidence that happened in NASA space station. It also highlights how NASA carefully considers checklists for the emergency repairs and EVAs. EVAs – spacewalks – are planned months, even years in advance. The astronauts who are scheduled to perform them spend hour upon hour practicing them in the controlled environment of the Johnson Space Center in Houston. Expedition 46 Commander Scott Kelly and Flight Engineer Tim Kopra successfully repaired the International Space Station's Mobile Transporter rail car during a three-hour and 16-minute spacewalk—the third for Kelly, and the second for Kopra. The smoothness of the EVA is a testimony to NASA’s ability to handle emergencies. The major lesson learned from the EVA is that a thorough worksite inspection checklist should be made to ensure crew do not leave ISS in a poor configuration after an EVA.
NASA doesn’t do drama.
That statement may come as a surprise to anyone who has been to the movies in the past couple years. In The Martian, released last October, a team of NASA scientists and astronauts improvise to bring a stranded spacefarer home. The 2013 movie Gravity told the story of a pair of NASA astronauts who escaped the in-orbit destruction of a space shuttle and struggled to make their way to another vehicle.
NASA may love the publicity movies like those bring, but the actual flesh-and-blood space agency hates the sort of improvisation they depict. Extravehicular activities—spacewalks—are planned months, even years in advance. The astronauts who are scheduled to perform them spend hour upon hour practicing them in the controlled environment of the Johnson Space Center in Houston. Every action is as tightly choreographed as a Soviet May Day parade.
So when astronauts Scott Kelly and Tim Kopra opened the airlock of the International Space Station on December 21, it may have looked like an ordinary day in orbit, but it was really a step into somewhat uncharted territory. But they had no choice. Without the emergency repair that Kelly and Kopra were to perform, there was a possibility that the station could tear itself apart.
The International Space Station is comprised of modules, some of which are pressurized and can be inhabited by the crew without space suits. Most of those pressurized modules are strung together along one axis; the ISS's architects call that the forward-aft or x axis. (In spite of the terminology, the forward-most Node 2 or Harmony module does not always lead the station in orbit.)
Extending through the port-starboard or y axis is the other major structural component, an unpressurized truss that stretches more than 350 feet. Made of a lightweight aluminum alloy, the dozen pieces of the Integrated Truss Structure form the backbone of the station and house the modules as well as equipment such as the solar panels and electrical subsystems. The truss is what makes the station a single functioning unit instead of just a number of separate modules, nodes, orbital replaceable units, and other elements spinning about in the same orbit.
Running along the length of the truss is a set of rails, and on those rails runs the Mobile Transporter. The MT is a trolley that carries the Space Station Remote Manipulator System, which is the large robotic limb known as the Canadarm2, and its smaller two-armed cousin, the Special Purpose Dexterous Manipulator, or Dextre, which is designed for finer motions. The MT carries both robots and any cargo they are holding to one of the eight designated work sites built into the truss. The MT-Canadarm2-Dextre system weighs about 11,000 pounds.
“We rely on the Mobile Transporter to position Canadarm2 and Dextre at outboard worksites on the extreme edges of the ISS that would not otherwise be reachable,” said Laura Lucier, a NASA robotics flight controller. “We use them as a platform for extravehicular activities (spacewalks), to capture re-supply ships, such as the Cygnus, Falcon, and Japanese HII transfer vehicle, and dock them to the ISS, to install science experiments, to take visual surveys of and
“WE TRIED TO MOVE THE MT FOR 10 OR 11 HOURS UNTIL WE FELT WE HAD EXHAUSTED ALL OPTIONS.” —LAURA LUCIER, NASA ROBOTICS FLIGHT CONTROLLER
repair and replace items on the ISS. We position the robots on the MT and drive them up and down the truss members or ‘rails’ of the station to reach equipment on the extreme ends of the station.”
Each rail that the MT runs along has a different shape and function. The rails are offset along the zenith-nadir axis of the space station, perpendicular to both forward-aft and port-starboard. The “top” rail has a simple, flat flange that the MT's drive wheels roll over. The drive wheels are held against the rail by other wheels that reach around to the other side of the flange. Only one drive wheel is engaged at a time.
The “bottom” rail has a more convoluted shape. The MT has two wheels at each bottom corner to constrain it in the nadir-zenith axis and three others at each corner that hold its place in the forward-aft axis. The only directions left for the MT to move are port and starboard.
When the MT is latched down at one of its worksites, the drive wheels are disengaged from the rail; at each nadir MT corner, four strong latches are clamped hard to the station truss. The truss is also beefed up at those sites, so as to resist the forces and torques applied as the robotic arms pull and twist various payloads. Those locations also provide the only spots on the truss to park the MT to ride out changes in attitude or altitude, when a crew vehicle or resupply vessel docks, or when the station needs to jet out of the way of a piece of space debris—and space debris is an ever-present possibility.
“When we are at a worksite and clamped down,” said Laura Merritt, NASA's Deputy Systems Manager for ISS Structures and Mechanisms, “we understand the load path very well. The MT has a set of structural load limits. The forces in the +x and –x directions are in the hundreds of pounds, and moments of inertia are in the low thousands of inch-pounds. The force paths are pretty linear.
“When we’re not at a worksite,” Merritt continued, “there are several degrees of freedom. Depending on the direction of the load and how high the peak load is, you can get high loads in several different directions. You go from linear to non-linear analysis of the load paths. The forces when the MT is not latched down are 3,800 pounds in each direction and the moments range from 108,000 inch-pounds to 146,000 inch-pounds. That could do some damage. The concern is that this can create little cracks in the truss beam. Then it becomes a long-term loading issue and reduces the lifetime of the beam.”
The problem as the spacewalk started on December 21 was that the MT wasn’t at a worksite. It was sitting on ordinary truss, and it was stuck. It was just four inches from a worksite, but it might as well have been miles. With the Canadarm2 and Dextre attached, all of those very large moments could have been in play if the station had to jet out of the way of debris—or undergo the docking of another spacecraft.
And a Russian Progress resupply vehicle had already launched and was due to dock in just two days.
Mission Control first discovered the problem with the Mobile Transporter on December 17. “We were trying to translate the MT to get to a worksite to do some tasks,” Lucier said. “But we couldn’t translate.” The MT has drive motors to move it along the truss. “Those motors were turning, but we weren’t moving—the electric switches that show us movement weren’t switching. So first, we tried the motors again. We tried separate drivewheels—lowered the first and raised the second.
“We didn’t give up easily in Mission Control,” Lucier said. “We tried all sorts of things. We tried to move the MT for 10 or 11 hours until we felt we had exhausted all options.
“We had also been doing camera surveys, which is how we discovered that the Crew and Equipment Translation Aid cart brake was on.”
Two CETA carts are coupled to the MT, one on each side. They also attach to the truss and move along it whenever the MT moves. Crewmembers can detach them from the MT and use them to manually move along the truss without the MT during extravehicular activities. The crew also uses them to perform various EVA tasks when the MT and the other two robots aren’t needed.
In this case, the camera survey showed that a hand brake on the starboard-side CETA cart had been engaged, thus preventing the MT from moving. There was no way to disengage the hand brake remotely.
Having the scheduled Progress re-supply vehicle come up and loiter around the station a while before
THE ANALYSIS WASN’T FAVORABLE, SO NASA WENT INTO HYPERDRIVE.
it could dock was a possibility, but one NASA was not keen on. Loitering takes fuel, and that fuel was intended to support the ISS. NASA engineers also engaged in a parallel effort to see if the Progress resupply vehicle could dock with the MT where it was, or if the Canadarm2 could “walk off” the MT and then pick up the Dextre arms to get them off the MT.
“We were looking for ways to lessen the mass of the system by getting the robots off of the MT,” Lucier said. Less mass on the MT would mean less potential for it to apply destructive force in the event the ISS suffered an unexpected jolt during docking.
“Ongoing analysis was being run to determine if ISS would be in an OK position to dock,” said Alex Kanelakos, the lead Spacewalk Officer in Mission Control. “If the analysis was very favorable, we could delay the EVA task until the scheduled EVA this January.”
But the analysis wasn’t favorable, so NASA went into hyperdrive.
Usually EVAs are planned months or even years in advance. Not this one. Late Thursday night, December 17, “the team kicked off their efforts to make an EVA feasible for the following Monday,” Kanelakos said.
“We basically had 72 hours to check out our spacesuits and hardware, perform fit-checks, configure all the tools, write the procedures, train the crew on the procedures, and execute an EVA,” he said. “EVA does perform planned spacewalks in which we know well in advance that we are going to perform certain tasks. We can train the crew and optimize the procedures so that when we go out EVA, we are extremely efficient with the crew time. We risk crew life every time we send an astronaut out EVA. In this instance, we did not have the benefit of being able to create these efficiencies.”
To do the EVA on such short notice, “the EVA tasks were chosen to minimize tool and airlock configuration, minimize required crew study time, minimize ISS reconfiguration, and have acceptable break-out points during any phase,” Kanelakos said.
The ground team also performed a scuba dive so they could tell the astronauts what the best body orientation was to provide the approximately 20 pounds of force needed to release the CETA cart brake. Though the brake was set by hand, it had to be released with a pedal. And when the brake was released, there would be a mechanical reaction force— a pushback—to prepare for as well. To help address these issues, a CETA cart down on Earth was also used to make videos for the astronauts and explain to management the goals of the EVA.
The goal was for the emergency repair to be as routine as possible.
By the time astronauts Scott Kelly and Tim Kopra pressed the pedal to release the brake, NASA's prep work had drained most of the drama out of the emergency. The astronauts handled the minor pushback force with no trouble. And to make the best use of EVA time, after they took care of the MT problem, Kelly and Kopra did some other tasks as well, including opening some doors so the Canadarm could perform future replacement activities, routing several cables in preparation for the International Docking Adapter and the Russian Multipurpose Laboratory Module, and retrieving some hardware needed for another EVA to be done later in 2016.
The Progress vehicle docked as scheduled with no problems, meaning it didn’t burn fuel the station needed for re-boosts, attitude adjustments, and other orbital maneuvers.
The smoothness of the EVA is a testimony to NASA's ability to handle emergencies—and make no mistake, this was one—with a calmness the agency strives to project. Unless you knew what it meant to plan an EVA in two days, you’d have never known anything out of the ordinary was going on.
It was fitting that Kelly was one of the astronauts who fixed the brake problem. On an EVA a month earlier, Kelly had tied down the brake handles to fix them in position and make sure they would stay clear of any MT worksites and stay disengaged. In tying down the brake handles, Kelly inadvertently engaged the one on the starboard side. Thus, it was appropriate that he also released it.
Will NASA change anything as a result of this emergency?
“The major lesson learned from the EVA is that a thorough worksite inspection checklist will be made to ensure crew do not leave ISS in a poor configuration after an EVA,” Kanelakos said.
Averting crisis through the use of an inspection checklist—that is so NASA.