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’s space program. However, due to water in Tim Kopra’s helmet, the EVA was terminated early.
Friday’s spacewalk was necessitated by the failure of a component of the station’s power generation system, called a Sequential Shunt Unit (SSU).
The SSU is essentially a voltage regulator, which serves to limit the output voltage of each of the station’s 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.
Regulating the output voltage to a set level is important as it provides protection to “downstream” components which could be damaged by large fluctuations in power.
The SSU achieves voltage regulation by essentially disconnecting individual solar cells on the solar array. Each SAW is made up of 82 separate “strings” of 400 solar cells each, all of which are connected to the SSU.
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’s overall power output to the required level.
This is achieved by shorting the strings back to the array itself, which is known as “shunting”. As well as disconnecting individual strings, the SSU also contains 82 capacitors to “smooth” the output voltage of the 82 individual strings.
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 “power channel”, which are designated 1A, 1B, 2A, 2B, 3A, 3B, 4A, and 4B.
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.
On 13 November 2015, Remote Bus Isolator-1 (RBI-1) – essentially a circuit breaker – in Direct Current Switching Unit-1B (DCSU-1B) tripped open, which took down the entire 1B power channel.
Although the trip occurred in the DCSU – 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 – the SSU (connected directly upstream of the DCSU) was the suspected cause of the trip.
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 “fried” the SSU’s internal components.
As the SSU provides voltage regulation protection to downstream components, operating Channel 1B without it’s SSU is not desired, and as such Channel 1B will remain down until SSU-1B can be replaced.
While no power downs are necessary as all of Channel 1B’s loads have been re-powered by “cross-tying” Channel 1B’s 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.
As 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.
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.
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.
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.
An SSU failure has happened before on the ISS – 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.
SSU-3A was subsequently returned to Earth for analysis aboard SpaceX’s 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.
EVA procedures – *FOLLOW LIVE*:
Friday’s EVA – designated US EVA-35 – 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.
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.
A few UK-born astronauts have conducted spacewalks in the past – namely Mike Foale, Piers Sellers, and Nick Patrick – 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 – a moment of huge pride and significance for UK spaceflight.
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’s EVA.
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.
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 “scoop” 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).
SSUs feature “blind mate” 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.
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.
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.
It would be acceptable to leave the SSU disconnected during this time. However, ISS teams would not want the SSU to be only partially connected, as arcing may have occurred if the SSU-to-ISS electrical connectors were close to each other.
As such, for the spare SSU installation, a minimum amount of bolt turns was required by sunrise, or else the SSU would need to be removed, with re-installation attempted again upon orbital darkness.
However, the SSU-1B R&R went to plan, allowing for a number of other tasks for the spacewalkers to complete, which, while not the primary task of the EVA, are on the ISS Program’s “EVA to-do” list.
Once both spacewalkers cleaned up the SSU-1B worksite on the S6 Truss, they both headed back to the A/L, where the failed SSU was stowed in order to be brought back inside the ISS at the end of the EVA.
Kopra headed to Node 3 to re-install a Non-Propulsive Vent (NPV), which allows the ISS to vent some of its atmospheric waste products into space in a way that does not lead to a propulsive force being applied to the station, and on the way dropped off a cable for Peake on the Port side of the Z1 Truss.
The Node 3 NPV was removed from the ISS prior to the May 2015 relocation of the Permanent Multipurpose Module (PMM) to the Node 3 Forward port, since very tight clearances existed between the PMM and the NPV, which were outside the limits for robotics operations.
However, now that the PMM is at its new home on the Node 3 Forward port, the NPV can be re-installed, although tight clearances will still remain, making access difficult for Kopra, who needed to first remove a cover plate before the NPV could be installed.
Once complete with the NPV task, Kopra translated the short distance to the Aft side of Pressurised Mating Adapter-3 (PMA-3), where he worked remove a launch restraint bracket in order to free some cables that will be disconnected on a future EVA, in prep for PMA-3’s relocation to the Node 2 Zenith port to serve as a docking port for future crewed vehicles.
However, around this time, Kopra reported a small amount of water in his helmet, which resulted in Flight Director Royce Renfrew almost immediately calling for a termination of the EVA.
The incident wasn’t close to the emergency suffered by Luca Parmitano during his terminated EVA-23. However, and per lessons learned from that incident, the EVA termination was speedy, but paced to allow for a steady translation back to Quest and a nominal repress plan.
At the time of the termination, EV-2 Peake was hard at work on a variety of cabling tasks, first heading to the Nadir side of the US Lab, where he deployed an International Docking Adapter (IDA) power cable reel, before then routing one end of the reel to Node 1 and connect it to a connector panel at that location.
Peake then routed the other end of the reel Zenith-Port across the US Lab, across the infamous S0 Truss “rats nest” of cables, to the Port side of the Z1 Truss, where he connected it to an EPIC MDM cable left for him there by Kopra.
The Enhanced Processor & Integrated Communications (EPIC) Multiplexer/Demultiplexer (MDM) cable will allow for communication between the station’s MDM control computers, which were enhanced with upgraded EPIC circuit cards in January 2012, which feature Ethernet connections to allow for communications with future Visiting Vehicles (VVs).
It is not certain how much work Peake completed before the termination, with other tasks requiring the connection of the EPIC MDM cable the Multipurpose Laboratory Module (MLM) Ethernet cable on Node 1. He was also set to return to the cable reel on the Lab and route the final leg of cable to the Zenith-Forward side of Node 2, where he was to leave the cable coiled for future connection to IDA-3 once it arrives at the station.
With both spacewalkers sent back to Quest, get-ahead tasks – in the form of partially routing the EPIC MDM cable, removing a Station-to-Shuttle Power Transfer System (SSPTS) cap, and breaking torque on the S6 Truss batteries in prep for their future replacement – will be conducted on a future EVA.
(Images: via NASA and L2)
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