The Metronome Hack A smartphone app set the tempo for a fix to bring the International Space Station (ISS) back online after a thermal system failed. For a few moments one Saturday evening in December, the Mission Control Center at Johnson Space Center went very quiet except for the sound of a metronome beat interrupting the silence every half-second. All eyes were on flight controller Mark Smith, poised to send a series of commands to the ISS orbiting Earth some 250 miles above at 17,500 miles per hour. On the sound of one beat, Smith sent a single command to station. He counted twelve more beats and on the thirteenth, he issued a second command. Mission controllers analyzed the resulting data and realized their “metronome hack” might work. It wasn’t quite perfect, but it just might work. ******* Three days earlier, at 8:23 a.m. Central Standard Time on December 11, 2013, for the second time in ISS history, Loop A of the External Thermal Control System (ETCS) stopped working. While this raised concerns since the loop contributes to thermal regulation for the ISS, the independent backup system, Loop B, was still operational. With the ISS one failure away from the possibility of evacuation, everyone involved wanted to bring Loop A back online quickly. We typically think of space as being a very cold place—and it is. At the same time, it is difficult to dissipate heat in space. On Earth, we cool our homes with air conditioners and computers with fans through the processes of conduction and convection, both of which rely on the presence of an atmosphere to occur. Space is a vacuum, which makes that impossible. In the case of ISS, its power system can generate approximately 84 to 120 kilowatts (enough to power more than 40 houses). Combined with the heat produced by six crewmembers, the ISS can get pretty warm. Loops A and B of the ETCS work to actively cool the ISS, but with one loop down, the remaining loop became a single point of failure. Flight controllers on the ground fiddled with the system to understand what had happened. They determined that a flow control valve inside Loop A’s 780-lb, refrigerator-sized pump module was broken. As a result, Expedition 38 Lead Flight Director Judd Frieling got all hands on deck to figure out how to restore nominal operations. He tasked one team with planning a spacewalk to replace the pump, a second team with the planning for a commercial cargo mission headed for station the following week, a third team with planning for the next-worse failure, and a fourth team comprised of thermal mission controllers with bringing Loop A back online without a spacewalk. Mark Smith was a member of that fourth team—and they were on to something.
Posted on: Sat, 29 Mar 2014 02:54:40 +0000
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