{"id":39156,"date":"2016-01-16T00:52:38","date_gmt":"2016-01-15T16:52:38","guid":{"rendered":"https:\/\/wp-productionenv-bjg9h2g2bgg5b8aa.southeastasia-01.azurewebsites.net\/news\/astronauts-repair-iss-power-channel-first-spacewalk-for-uk-ends-early\/"},"modified":"2016-01-16T00:52:38","modified_gmt":"2016-01-15T16:52:38","slug":"astronauts-repair-iss-power-channel-first-spacewalk-for-uk-ends-early","status":"publish","type":"post","link":"https:\/\/starpath.global\/news\/astronauts-repair-iss-power-channel-first-spacewalk-for-uk-ends-early\/","title":{"rendered":"Astronauts repair ISS power Channel \u2013 first spacewalk for UK ends early"},"content":{"rendered":"

A pair of astronauts aboard the International Space Station (ISS) conducted a shortened spacewalk to replace a failed electrical component, in order to restore the station to its full power generation capacity. The spacewalk was the first ever to be conducted by a British astronaut, marking a historic moment for the UK\u2019s space program. However, due to water in Tim Kopra\u2019s helmet, the EVA was terminated early.<\/p>\n

EVA background:<\/p>\n

Friday\u2019s spacewalk was necessitated by the failure of a component of the station\u2019s power generation system, called a Sequential Shunt Unit (SSU).<\/p>\n

\"2014-10-14The SSU is essentially a voltage regulator, which serves to limit the output voltage of each of the station\u2019s eight Solar Array Wings (SAWs) to a set voltage, typically 160 Volts Direct Current (VDC), as the output voltage of the arrays can vary between 130-180 VDC depending on the amount of sunlight hitting them at any given moment.<\/p>\n

Regulating the output voltage to a set level is important as it provides protection to \u201cdownstream\u201d components which could be damaged by large fluctuations in power.<\/p>\n

The SSU achieves voltage regulation by essentially disconnecting individual solar cells on the solar array. Each SAW is made up of 82 separate \u201cstrings\u201d of 400 solar cells each, all of which are connected to the SSU.<\/p>\n

\"2016-01-15-111631\"If, for instance, a SAW is generating too much power (i.e. above the set point of 160 VDC), then the SSU can disconnect any number of these strings of solar cells in order to reduce the array\u2019s overall power output to the required level.<\/p>\n

This is achieved by shorting the strings back to the array itself, which is known as \u201cshunting\u201d. As well as disconnecting individual strings, the SSU also contains 82 capacitors to \u201csmooth\u201d the output voltage of the 82 individual strings.<\/p>\n

Each one of the eight SAWs on the ISS has its own SSU, which thus makes for eight SSUs on the ISS in total, each one serving their respective \u201cpower channel\u201d, which are designated 1A, 1B, 2A, 2B, 3A, 3B, 4A, and 4B.<\/p>\n

Technology News<\/path>NASA mission updates<\/path>NASA educational resources<\/path>\n

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\"2016-01-15-111655\"In terms of physical construction, the SSU is a rectangular box installed at the base of the Mast Canister Assembly (MCA) of its corresponding SAW.<\/p>\n

On 13 November 2015, Remote Bus Isolator-1 (RBI-1) \u2013 essentially a circuit breaker \u2013 in Direct Current Switching Unit-1B (DCSU-1B) tripped open, which took down the entire 1B power channel.<\/p>\n

Although the trip occurred in the DCSU \u2013 which is a switching unit that switches ISS power sources between the solar arrays during orbital daytime, and the storage batteries during orbital night-time \u2013 the SSU (connected directly upstream of the DCSU) was the suspected cause of the trip.<\/p>\n

\"2016-01-15-111734\"Upon review, data indicated that just prior to the DCSU-1B RBI-1 trip, in excess of 2000 Amps of current was passing through SSU-1B, leading ground teams to declare the SSU failed, as that level of current is likely to have \u201cfried\u201d the SSU\u2019s internal components.<\/p>\n

As the SSU provides voltage regulation protection to downstream components, operating Channel 1B without it\u2019s SSU is not desired, and as such Channel 1B will remain down until SSU-1B can be replaced.<\/p>\n

While no power downs are necessary as all of Channel 1B\u2019s loads have been re-powered by \u201ccross-tying\u201d Channel 1B\u2019s loads with another power channel, the loss of SSU-1B leaves the ISS running on only seven of its usual eight power channels, which is not ideal since it reduces redundancy in the event of another power channel loss.<\/p>\n

\"2014-10-14As such, a replacement of SSU-1B, which is located externally on the S6 Truss, was desired. However, it was not possible to replace the SSU immediately, as SSU Removal & Replacements (R&Rs) cannot be conducted in direct sunlight.<\/p>\n

This is because ISS teams do not wish to remove a potentially damaged SSU while solar arrays to which it is connected are generating power, as this will involve disconnecting the SSU from live connectors, which could potentially cause electrical arcing.<\/p>\n

Removing the SSU while the solar arrays are in darkness will solve this problem, however. The best time to remove an SSU will be during a low beta-angle period, which is where sunlight hits the solar arrays side-on, which leads to longer periods of orbital darkness where the arrays are not in sunlight.<\/p>\n

\"\"This would give spacewalkers the maximum amount of orbital darkness per orbit in order to perform the R&R. A beta angle of -10 degrees is desired for an SSU R&R, and the next available period after the 13 November SSU-1B failure that satisfied this requirement was the 12-18 January. As such, the 15 January was chosen, where a beta angle of -8.5 degrees would occur.<\/p>\n

An SSU failure has happened before on the ISS \u2013 once in September 2012, where one of the 82 capacitors inside SSU-3A burned out, leading to the SSU being recovered with no impacts to its operation, and again in May 2014, where SSU-3A went down, leading to it being replaced via EVA in October 2014.<\/p>\n

SSU-3A was subsequently returned to Earth for analysis aboard SpaceX\u2019s Dragon vehicle, and ISS teams would also like to return SSU-1B to Earth once it is removed from the ISS. There is currently one spare SSU aboard the ISS, and one spare on the ground.<\/p>\n

EVA procedures \u2013 *FOLLOW LIVE*:<\/p>\n

Friday\u2019s EVA \u2013 designated US EVA-35 \u2013 began at 12:48 PM GMT, with NASA astronaut Tim Kopra as EV-1, wearing the suit with the red stripes, and British astronaut Tim Peake as EV-2, wearing the all-white suit. Kopra is making the third EVA of his career, while Peake is making his first.<\/p>\n

\"2015-12-14-191601\"However, Peake is making more than a personal first on this EVA, as he is also be making a first on behalf of his nation, as the spacewalk marks the first time in history that a UK national has ventured into the vacuum of space.<\/p>\n

A few UK-born astronauts have conducted spacewalks in the past \u2013 namely Mike Foale, Piers Sellers, and Nick Patrick \u2013 however they all did so as NASA astronauts, while Peake is the first to do a spacewalk representing the UK, with a Union Jack on the sleeve of his EMU \u2013 a moment of huge pride and significance for UK spaceflight.<\/p>\n

\"2016-01-15-113634\"ISS Commander Scott Kelly is serving as the Intravehicular (IV) crewmember for the EVA, and NASA astronaut Reid Wiseman serving as CAPCOM in Houston, as he conducted the October 2014 EVA to R&R SSU-3A, and so has experience of performing the primary task of Friday\u2019s EVA.<\/p>\n

Upon egressing the Quest Airlock (A\/L), the first task for the two spacewalkers was to translate out to the S6 Truss, which was the worksite for the SSU-1B R&R, with EV-1 Kopra retrieving an Articulating Portable Foot Restraint (APFR) from the Starboard Crew Equipment Translation Aid (CETA) cart along the way.<\/p>\n

Once at the worksite, Kopra set-up and ingressed the APFR, and Peake positioned himself to observe SSU-1B. Kopra then attached a handling aid called a \u201cscoop\u201d to SSU-1B, and once in orbital darkness, removed the SSU from the S6 Truss via driving a single bolt with the aid of his Pistol Grip Tool (PGT).<\/p>\n

\"2016-01-15-111912\"SSUs feature \u201cblind mate\u201d electrical connectors that are made and broken automatically upon driving their single bolt, hence no manual cable connections are necessary. Once the failed SSU was removed, a spare SSU was installed in its place, again via the driving of a single bolt.<\/p>\n

A 31-minute period of orbital darkness was available to complete the SSU R&R. During the October 2014 EVA to R&R SSU-3A, a stiff bolt caused the task to run long. However, this time, all proceeded smoothly.<\/p>\n

If the spacewalkers had encountered any difficulty, and the sun rose before they could complete the SSU R&R, they would have had to wait approximately one hour until the next period of orbital darkness until they could resume the R&R.<\/p>\n

See Also<\/h4>\n