Expedition 39 astronauts Rick Mastracchio and Steve Swanson have successfully removed and replaced a failed Multiplexer-Demultiplexer (MDM) on the central S-0 truss of the International Space Station (ISS). In one of the shortest EVAs ever undertaken by U.S. astronauts, the two men spent 96 minutes outside the station and marched crisply through their task, even pausing at one stage to acquire the first-ever photographs of a berthed SpaceX Dragon cargo craft from a spacewalker’s perspective. Although EVA-26 was short, it pushed up Mastracchio’s cumulative EVA time to 53 hours and four minutes in nine career spacewalks, which now makes him the fifth most experienced spacewalker in the world.
As described in yesterday’s EVA-26 preview article, this excursion became necessary following the 11 April failure of the EXT-2 MDM box, which provides backup capability to its primary sibling on the S-0 truss. According to Flight Director Brian Smith, who led the “Team Four” efforts at the Johnson Space Center (JSC) in Houston, Texas, to respond to this critical event and re-establish the ISS in “a good risk posture,” efforts were made to power-up EXT-2 for a routine health check, but proved unsuccessful. Mr. Smith explained in a recent NASA press briefing that if EXT-2’s primary sibling also failed, it would cause the loss of Solar Alpha Rotary Joint (SARJ) controllability, the loss of commanding capability for the thermal control loops, and also threaten to impair NASA’s ability to power-up redundant equipment. In the case of visiting vehicles, including SpaceX’sDragon, such failures would have a direct impact upon the ability of these craft to dock or berth with the ISS.
According to Mr. Smith, the team experienced problems powering-up the EXT-2 MDM and was forced to declare it as having failed. The box has been aboard the S-0 truss since it was launched and installed aboard the ISS by the crew of shuttle mission STS-110, back in April 2002. It was responsible for routing commands to various systems on the station’s inboard truss, including the cooling system, radiators, and the Mobile Base System (MBS). Late on 12 April, NASA managers decided to press ahead with forward planning for EVA-26, and it was hoped to execute the contingency spacewalk as early as the 22nd, just before the planned undocking of Russia’s Progress M-21M cargo craft on the 23rd for two days of Kurs-NA (“Course”) navigation system tests. As circumstances transpired, EVA-26 was moved until 23 April and occurred shortly after the Progress M-21M undocking.
EVA-26 involved Extravehicular Mobility Units (EMUs) No. 3011, which was used by Italian astronaut Luca Parmitano on the dramatic EVA-23 in July 2013, and No. 3005. Both have been equipped with new fan pump separators and were considered to be the best prepared of the three EMUs—the other is suit No. 3010—aboard the ISS at the present time. A new fan pump separator for 3010 arrived as part of the payload aboard SpaceX’s CRS-3 Dragon on Easter Sunday.
After an early breakfast, Rick Mastracchio (designated “EV1”) and Steve Swanson (“EV2”) entered the Quest airlock, and at 4:45 a.m. EDT embarked on a well-trodden path of pre-breathing on masks, during which time the inner “equipment lock” was depressed from its ambient 14.7 psi down to 10.2 psi. Assisted by Expedition 39 Commander Koichi Wakata, their next step was to don and begin purging their EMUs at about 6:30 a.m. EDT, after which the Quest atmosphere was repressurized to 14.7 psi. They undertook a nominal pre-breathing protocol, lasting about 50 minutes, followed by a further 50 minutes of In-Suit Light Exercise (ISLE), which began at about 7:30 a.m. EDT. This required Mastracchio and Swanson to flex their knees for four minutes, rest for one minute, then repeat over and over until the 50 minutes were over, as part of efforts to remove nitrogen from their bloodstreams in a shorter time period and avoid an attack of the “bends.”
With EVA-26 scheduled to begin at about 9:20 a.m. EDT, efforts ran smoothly, although slightly behind the timeline. The fully-suited spacewalkers and their equipment were transferred into Quest’s outer “crew lock” by about 9:15 a.m. EDT, and the hatches between the two locks were closed and sealed shortly afterward. Depressurization was briefly halted at about 9:40 a.m., whilst at 5 psi, for leak checks, after which the process resumed until the crew lock reached a condition of near-vacuum. A few minutes before the spacewalk was due to begin, the pressure hit 0.2 psi and Mastracchio and Swanson were given the go-ahead to open the outer, Earth-facing (or “nadir”) hatch. EVA-26 officially commenced at 9:56 a.m. EDT, when the astronauts transferred their space suits’ life-support utilities over to internal battery power. Thus began the 179th EVA dedicated to the construction and maintenance of the ISS since December 1998.
As detailed in the EVA-26 timeline, Mastracchio was first out of the airlock, followed by Swanson, and their first few minutes were spent checking the integrity of each other’s suits and tethers. Carrying their tools and the 50-pound (22-kg) replacement MDM in an Orbital Replacement Unit (ORU) bag, the two men translated swiftly from Quest “up” the Crew and Equipment Translation Aid (CETA) Spur to the S-0 worksite. They were in position by 10:10 a.m. EDT, a mere 15 minutes into the EVA. The Mobile Transporter, which normally occupies a position directly over the failed MDM, had been moved and Mastracchio and Swanson set to work stowing their ORU bags and equipment.
Mastracchio entered a “heads-down” orientation, directly in front of the failed MDM, to inspect it and found no visible evidence of physical damage. He removed a scoop-like handling tool and by 10:20 a.m. EDT was hard at work removing two outer, “tie-down” bolts on the failed MDM with his Pistol Grip Tool (PGT). He then released the center “jacking” bolt, which enabled him to smoothly slide out the failed unit. Mastracchio handed the MDM to Swanson, who stowed it in the ORU bag for subsequent return to Earth.
Working well ahead of the timeline for their planned 2.5-hour EVA, the spacewalkers inspected the MDM “cold plate” and the “blind” electrical connectors on the back face of the replacement MDM, then set to work installing it into position. By 10:50 a.m. EDT, less than one hour since leaving Quest, the new unit had been installed and successfully powered-up and appeared to be operating normally. In the meantime, Swanson was assigned a short “get-ahead” task to cut a cable which threatened to interfere with robotic access to the Secondary Power Distribution Assembly (SPDA) on the upper face of the S-0 truss. The spacewalkers photographed their work—even acquiring an impressive view of SpaceX’s Dragon, berthed at the nadir interface of the Harmony node—and returned to the airlock. EVA-26 officially ended at 11:32 a.m. EDT after 96 minutes.
Less than a half-hour into today’s spacewalk, Mastracchio eclipsed Russian cosmonaut Fyodor Yurchikhin by moving up from No. 6 to No. 5 on the list of the world’s most experienced spacewalkers. He had supported nine EVAs—three aboard STS-118 in August 2007, three aboard STS-131 in April 2010, and three aboard the current Expedition 38/39, including two contingency spacewalks in December 2013—and has now accrued 53 hours and four minutes in the vacuum of space. He is surpassed by John Grunsfeld, who has 58 hours and 30 minutes cumulative time; Jerry Ross, with 58 hours and 32 minutes cumulative time; Mike Lopez-Alegria, with 67 hours and 40 minutes cumulative time; and Russian cosmonaut Anatoli Solovyov, the world record-holder, with over 82 hours.
Steve Swanson, though further down on the list, is also a seasoned EVA veteran and completed his fifth career spacewalk today. He has now spent 27 hours and 58 minutes working in a pressurized suit in the vacuum of space.
EVA-26 was the third contingency spacewalk to be performed by U.S. astronauts since the much-publicized EVA-23 in July 2013, during which water intruded into Italian astronaut Luca Parmitano’s helmet. In the following weeks and months, investigators dug in to uncover the root cause of this life-threatening incident. NASA’s Mishap Investigation Board (MIB) reported its findings in February 2014, although several “open” issues still remain to be closed on the engineering fault tree, ahead of the expected resumption of “planned” U.S. EVAs with a pair of spacewalks involving Steve Swanson, fellow NASA astronaut Reid Wiseman, and Germany’s Alexander Gerst in July.
Until those issues are fully closed-out, ISS Program Manager Mike Suffredini is adamant that he does not want U.S. Orbital Segment (USOS) astronauts performing any non-critical tasks until the problems with the EMUs are fully understood and the hazard work is completed. For that reason, in response to questions about why Mastracchio and Swanson will not be assigned any major “get-ahead” tasks, Mr. Suffredini asserted that they would only be outside for as long as it took to complete the MDM removal and replacement operation. Faced with other queries about the apparent frequency of contingencies aboard the space station—most recently EVAs-24 and 25 in December 2013 to remove and replace a failed pump module—Mr. Suffredini added that ISS Program planning anticipated about six to eight EVAs per year at this stage to handle faults and equipment failures, but that the systems were actually proving considerably more reliable and robust than expected.