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Atlantis to hangar
After its safe landing to end mission STS-115, space shuttle Atlantis is towed from the Kennedy Space Center runway to hangar 1 of the Orbiter Processing Facility for post-flight deservicing and the start of preparations leading to its next mission, STS-117.

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STS-115 landing
Space shuttle Atlantis glides to a smooth touchdown on Kennedy Space Center's Runway 33 at 6:21 a.m. to conclude the successful STS-115 mission that restarted construction of the space station.

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Soyuz TMA-9 docking
The Russian Soyuz TMA-9 space capsule carrying the Expedition 14 resident crew and space tourist Anousheh Ansari safely docks to the International Space Station's Zvezda service module.

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Expedition 14 launch
This extended duration movie follows the Soyuz rocket from the final countdown through arrival in orbit with the Expedition 14 crew. The video shows the three-stage rocket's ascent from Baikonur Cosmodrome and includes views of Mike Lopez-Alegria, Mikhail Tyurin and Anousheh Ansari from cameras inside the capsule.

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Mission of Expedition 14
The voyage of Expedition 14 aboard the International Space Station is expected to see major construction activities for the outpost. Learn more about the mission in this narrated mission preview movie.

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STS-31: Opening window to the Universe
The Hubble Space Telescope has become astronomy's crown jewel for knowledge and discovery. The great observatory was placed high above Earth following its launch aboard space shuttle Discovery on April 24, 1990. The astronauts of STS-31 recount their mission in this post-flight film presentation.

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STS-34: Galileo launch
The long voyage of exploration to Jupiter and its many moons by the Galileo spacecraft began on October 18, 1989 with launch from Kennedy Space Center aboard the space shuttle Atlantis. The crew of mission STS-34 tell the story of their flight to dispatch the probe -- fitted with an Inertial Upper Stage rocket motor -- during this post-flight presentation film.

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Atlantis on the move
Space shuttle Atlantis is transported to the cavernous Vehicle Assembly Building where the ship will be mated to the external fuel tank and twin solid rocket boosters for a late-August liftoff.

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Spacewalkers to re-wire the space station's power grid
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: October 30, 2006

With P6 successfully retracted and the port-side SARJ slowly rotating the new P4 arrays, the stage will be set for a second and third spacewalk to carry out the main power switch over that is the primary goal of Discovery's mission.

During the second and third spacewalks, the station's main electrical circuits will be powered down, channels 2/3 first and then 1/4. The output from the still-extended P6-2B wing will be switched to the main bus switching units on the solar array truss and the lab will begin drawing power from the MBSUs, downstream DDCU transformers and remote power control modules.

Complicating the work, certain command-and-control computers must remain operational throughout the power switch over, requiring the station crew to install jumpers between components in avionics racks in the lab module to provide redundancy for critical systems when a given power channel is shut down.

After the second spacewalk, by Curbeam and Fuglesang, all channel 2/3 power, provided by P6-2B and P4-2A, will be routed through MBSUs 2 and 3 on the S0 truss. After the third spacewalk, by Curbeam and Williams, all channel 1/4 power, provided by P4-4A, will be routed through MBSUs 1 and 4. The retracted P6-4B array also will be tied into channel 1/4 to provide "parachute mode" battery power if needed.

While the electrical system is being reconfigured, the main ammonia cooling system in the truss - the external active thermal control system, or EATCS - must be activated to dissipate the heat that will be generated by the electrical power system as components come on line. The large radiators on each side of the main solar array truss will begin rotating for the first time to maximize heat rejection.

The space station's solar array truss eventually will stretch the length of a football field, sporting two sets of dual-wing solar arrays on each end of the main truss. The solar array wings, or SAWs, are numbered based on their position on the station with even numbers assigned to panels on the left, or port, side of the main truss and odd numbers assigned to SAWs on the right, or starboard, side.

The recently installed P4 segment's two SAWs are numbered 2A and 4A while the P6 SAWs are numbered 2B and 4B. The S4 arrays will be designated 1A and 3A while the S6 SAWs will be known as 1B and 3B.

The four sets of solar arrays are essentially identical. In each set, solar power flows from two SAWs into a sequential shunt unit. Power coming into the SSU can vary from 130 to 180 volts DC depending on a variety of factors, including blanket degradation, shadowing, etc.

SSU output can be adjusted as required, but it typically will be set at 160 volts and passed on to an integrated equipment assembly, or IEA. The SSU routes excess power back to the SAWs to be dissipated as heat and it also can be used to isolate a set of SAWs from the power grid if necessary.

Because each solar array wing powers a separate station circuit, the IEAs in each array include two sets of electronics. A direct current switching unit (DCSU), containing six high power switches, routes SAW electricity from the SSU into battery charge/discharge units that regulate the flow of power to and from six batteries, three for each SAW.

When the array's SAWs are in sunlight, the DCSU sends solar power to the MBSUs, through the SARJ, and also into the batteries to charge them up. As the station moves into Earth's shadow, the DCSU begins adding battery power to the flow going to its MBSU to maintain the proper voltage. When the arrays are completely eclipsed, the DCSU sends battery power alone to the MBSU in a continuous, automatic procedure.

The DCSU, the battery chargers and other components in each array's integrated electronics unit are cooled by ammonia circulated through cold plates and then routed to a single deployable radiator. Each of the four sets of arrays that eventually will be attached to the station include its own ammonia cooling system, which is independent of the main cooling systems in the S1 and P1 truss segments.

Electricity from the solar arrays is known as "primary power." The MBSUs take that primary power and route it to transformers known as DDCUs, which lower the voltage to a precisely controlled 124 volts DC. This so-called "secondary power" is then directed to the station's myriad electrical systems using numerous electro-mechanical switches known as remote power controllers.

The eight solar array wings on the completed space station will feed power through separate lines to the MBSUs. For redundancy, power from four SAWs will flow to a pair of major circuits - 1 and 4 - while power from the other four SAWs will be directed to a second pair of circuits - 2 and 3.

During the second spacewalk planned for Discovery's mission, flight controllers will power down components on the 2/3 channel and Curbeam and Fuglesang will re-plug cables on the truss to route solar array power to the channel 2/3 MBSUs and associated equipment in S0. DDCUs in the lab module will be disconnected from the interim P6 power system. The output from P6 will be connected to the MBSU inputs and the DDCUs on the truss and in the lab module will be connected to MBSU output. It will take about two hours to reach this point.

Next, flight controllers will power up the MBSUs and DDCUs in the permanent electrical distribution system and verify they are working properly, a procedure that will take about a half hour. Ammonia coolant loop B then will be activated to cool the electronic gear. The same procedures will be carried out for power channels 1/4 during the third and final planned spacewalk when coolant loop A will be activated.

Once the MBSUs are powered up, cooling must be activated within a few hours to prevent potentially serious damage.

"On EVA 2 we're going to turn off the 2/3 channel," Curry said. "Some boxes, it's just a loss of redundancy and on other boxes, the stuff is actually physically off. So when you've got things that are physically off, you've got what we call passive thermal limits, meaning it's stuff getting too cold and how long can it go without breaking the hardware?

"Of course, most of the things like that, you have to go based on analysis because when you turn the thing off, you no longer have telemetry on it to tell you whether it's doing well or doing poorly. So we spent a lot of time trying to figure out what are the things that are the biggest risks. And if you look at the way the EVAs are planned out, we broke up the power downs into two sections, one where we do the first set of wiring, which is for things - the majority of the stuff that can make it without power for the rest of the EVA, meaning five or six hours.

"There were three other things that were not in that category, mostly related to the comm system and to the cameras on the outside of the vehicle. So those ones we (do) late. Comm is obviously very important and you don't want to go a long period of time without the comm stuff. So the S-band antennas and the KU-band system, we do those very late so the amount of time they went without power was less than an hour. That was all passive thermal. So I think we're fine on passive thermal.

"The bigger concern is the active thermal, and it's not just the MBSUs but it's all the truss equipment. There's a whole bunch of boxes that are in line for power, your MBSUs, your DDCUs and then your RPCMs. The DDCUs and the MBSUs are cooled by cold plates that have this ammonia running through them. Right now, on the permanent system the ammonia is still sitting in the tanks, we haven't started pushing it through the system yet because we're waiting until we need it."

While the MBSUs can be cross tied to route power to different circuits in case of failures, the ammonia systems are independent and not connected to protect against a micrometeoroid impact that might rupture a line and take out the entire system.

But that lack of connectivity means a problem with loop A or B will take out two of the station's four primary electrical circuits.

"This is the one that from a station design perspective I wish they had plumbed it, cross tied it, because the pump and all the ammonia that's on the port side of the vehicle, that cools the 2/3 side, and then the pump and the ammonia tank and all that that's on the starboard side on the S1 truss, cools the 1/4 channel. So if a pump goes down or doesn't ever come up, the way that the guys when they designed the vehicle felt they got away with it, they said hey, I've got four power channels and so it's OK to lose two power channels and still be OK from a redundancy perspective.

"The problem is, that's not exactly the way the station's built, there are certain things that are wired to the 2/3 side and certain things that are wired to the 1/4 side. So they didn't cross tie the plumbing. So what that means is, on EVA-2 when I go to activate loop B's pump, if loop B's pump doesn't come up, if I have any kind of glitch - and I want you to know this in case it happens - there's a clock that I will be running for the MBSUs and the DDCUs and just in general and then I have to compare that clock against the crew's (spacesuit) clock, how long they can actually stay out.

"If I can't get that pump running within a certain amount of time, I have to save time on the back end of the spacewalk to allow the crew to unwire what they did before and to back back out again. If I left the wiring the way it was and the pump never got up to speed and I sent the crew back in, the MBSUs and the DDCUs will overheat. It's just a matter of time."

Engineers initially concluded the MBSUs would overheat within an hour or so, but a later assessment using a qualification unit showed the devices could operate without cooling for five to six hours.

"So we think we're OK," Curry said. "I have telemetry on the MBSUs and I have telemetry on the DDCUs. So I have numbers in the flight rules that tell me thou shalt not let the temperature of the MBSUs or the DDCUs get above this certain number."

The DDCU limit is 140 degrees Fahrenheit while the limit on the MBSUs is 115 degrees.

"In terms of EVA requirements, it takes about two hours for the crew to get to the point where they're ready for us to power stuff back up again," Curry said. "We power all that stuff down so they don't shock themselves, they make the connections and then they tell us they're clear and we're ready for activation."

Lead station electrical officer Dave Crook "then activates a script that powers on a whole bunch of stuff really fast because obviously, we're racing against the clock we talked about earlier. So in the first 20 minutes, I'll know if the copper path worked. If any of those things don't work, I've also got a number that we can check against the limits on the suits, if one of the MBSUs fail or one of the DDCUs fail, we can do an R&R. And that would be during that specific EVA because hopefully, I have enough time for that. We've choreographed how that would work. There's an MBSU spare as well and that could be done in real time."

It will take about 20 minutes for the computer commands to execute, rerouting power to the MBSUs and downstream DDCUs. It will take another 45 minutes to an hour to activate each ammonia cooling system.

"The problem is, we don't want to cavitate the pumps," Curry said. "You have to get the ammonia pushed through the system at the proper pressure and the operating pressure of the pump is like 376 psi so we've got to get that pump up to minimum number before we can start trying to activate it so we don't cavitate. So that takes a little bit of time."

Adding up the numbers, Curry's team will know if power and cooling are active within about and hour to an hour and a half. While a spare MBSU or DDCU could be installed during the same spacewalk, trouble with an ammonia pump unit would cause a significant impact on the mission.

"Let's say the pump doesn't come up, or say I got bit by some software feature like what happened (when a SARJ commanding problem cropped up during the September shuttle mission)," Curry said. "If I can't figure that out within a short period of time, then I have to back out because I couldn't get the cooling done and there's not enough time to do the R-&-R of the pump.

"The pump weighs a lot, it's 1,500 pounds, so that's a complex remove-and-replace scenario. That would take an entire dedicated EVA to do that. There's plan I've got in place where if the pump didn't come up to speed on EVA-2, then we would give the MMT (Mission Management Team) folks a day to think about it and then the next day after that, we would then use EVA-3 to R-&-R the pump."

In that case, EVA-2 "would end up being a waste of time," Curry said. "That's the part that concerns me, infant mortality. Every time you start up a new system you always learn something. Something could come up to bite us. The problem is, I've only got about an hour to figure that out. The pressure's on the ground. That's the difference between this flight and most others. This is a simple task for the crew. All they have to do is hook up a cable."

Polansky agreed, saying flight controllers will be under pressure to make quick decisions if things go wrong.

"You can't just sit there and say, 'I did this and that happened therefore it must have been because of what I did.' It could be because of a lot of things, you've got to be careful not to jump to the wrong conclusion," Polansky said. "And oh by the way, you don't have a day to think about it because people are sucking on their oxygen and using up their (carbon dioxide-absorbing lithium hydroxide). There's a very short fuse here."

Because of safety requirements and the toxic nature of ammonia, electrical components inside the station's pressurized modules are cooled by water circulating through cold plates. That water is then routed to heat exchangers tied into external ammonia loops and radiators.

In the near term, the primary external ammonia system will only be used to cool electrical components mounted on the solar array truss.

NASA planners initially considered having Discovery's crew complete the electrical switch over as well as the plumbing changes necessary to switch the module heat exchangers from interim to permanent cooling. But given the complexity of the electrical work, the cooling system re-plumbing was deferred to early next year when Williams and station commander Michael Lopez-Alegria will tie the module heat exchangers into the primary cooling system during two spacewalks.

"The first spacewalk will be one of the external power loops," Williams said in a NASA interview. "We call it loop A. We will switch it from using a radiator and cooling loop system on the P6 to its more permanent cooling system out on the truss. And that involves changing some electrical connectors in the 'rat's nest,' which is the area between Z1, S0 and the Lab. Itıs a small area that has a lot of electrical connections and a lot of fluid connections; it's this very tight space.

"As it's planned, both me and Michael Lopez-Alegria will be inboard in this small little area. On the first loop reconfiguration, I'll do the electrical connectors then he will follow that with the big fluid jumpers to switch the fluid lines from P6 to the external. Following that, we will be up on the P6 truss; they will retract the starboard radiator, which we've been using for the early external thermal control system. The ground will be doing that but our role in that is to cinch it down, because it needs to stay down and compacted so the radiators aren't moving around as the space station is rotating.

"He'll be up on the zenith side; I'll be on the nadir side of the radiator. We will wait and watch the radiator retract - potentially it could need a little bit of a push from us at the very end - and then we cinch bolts around the edge of it that we need to, simultaneously, connect to make sure that the radiator is contained nicely. That's the main portion of EVA number one."

The second is virtually identical to the first, but focusing on making the required loop B fluid connections and retracting the aft radiator on P6.

Curry likens the power downs and power ups planned for Discovery's mission to switching a house under construction from temporary generator power to utility power. But in this case, the computer commands needed to make that happen must be carried out with extraordinary precision.

"On 115 (the most recent shuttle flight), the EVA tasks, the robotics tasks, were without question more difficult than the similar tasks we do on 116," Hill said. "But we're doing a hell of a lot more commanding to the hardware outside the vehicle on 116 than we (did on STS-115)."

No specific task in this process is overly complex or challenging. But the sheer number of commands that must be sent, the complexity of the power system hardware and the close coordination required between the astronauts and flight controllers will make shuttle mission STS-116 the most complex station assembly flight yet attempted.

"All this reconfiguration we have to do on 116, those are big steps," Hill said. "It doesn't sound like much. It sounds pretty mundane and nerdy. We're sending a bunch of commands, changing over electrical and thermal controls. That flight right there and that choreography is something that when we first came up with this sequence in 1994, we all sat back and said, 'how are we going to figure this one out?'

"Today, the folks who have been leading that effort feel pretty good they've got their arms around it but they're ... keeping their fingers crossed that everything goes well."

THE END


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