Spacewalkers to complete station re-wiring today
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 16, 2006
Astronauts Robert Curbeam and Sunita "Suni" Williams are gearing up for a critical spacewalk today to finish re-wiring the international space station. If time is available and no major problems develop, they also plan to inspect a balky solar array and perhaps shake it a bit to free up a stuck guide wire that is preventing its full retraction.
NASA's Mission Management Team met early today to debate the possibility of adding a fourth, unplanned spacewalk to Discovery's mission Monday if the spacewalkers are unsuccessful but as of this writing, no final decisions have been made.
Today's spacewalk, scheduled to begin around 2:42 p.m., is the 76th devoted to station assembly and the third for Discovery's crew. Going into today's excursion, 64 U.S., Russian, Canadian, French, German and Swedish astronauts had logged 455 hours and 50 minutes building and maintaining the outpost since assembly began in 1998. Curbeam and Christer Fuglesang logged 11 hours and 36 minutes during the first two spacewalks of Discovery's mission. Today's outing will be Curbeam's sixth over two missions and Williams' first.
During a spacewalk Thursday, Curbeam and Fuglesang re-wired two of the station's four primary circuits - channels 2 and 3 - allowing electricity from the still-extended P6-2B array and the newly installed P4-2A blankets to flow through main bus switching units 2 and 3 at the heart of the station's permanent power system.
With power flowing through MBSUs 2 and 3, flight controllers successfully activated one of two coolant systems - coolant loop B - that use ammonia to carry heat away to a set of big radiators.
Today, Curbeam and Williams will connect the output from the P4-4A solar wing to power channels 1 and 4. Stored electricity in the batteries charged by the partially retracted P6-4B array also will be tied into channels 1/4 to provide eight hours of emergency "parachute mode" battery power if needed.
As was the case Thursday, once power starts flowing through MBSUs 1 and 4, flight controllers must activate the second ammonia cooling system - loop A - to prevent the switching units and other equipment from over heating. While flight controllers work through the complex electrical system and cooling loop activation, Curbeam and Williams will move a set of Russian space debris shields to the station, along with a grapple bar needed during future assembly work.
Thursday spacewalk to re-wire channels 2/3 was completed a full hour ahead of schedule. If Curbeam and Williams are just as efficient today, they will have time to move up to the top of the station for an up-close inspection of the partially retracted P6-4B solar blankets.
The problem cropped up Wednesday, when the astronauts attempted to retract the blankets as part of work to switch the space station over to its permanent power system.
The P6 array, which features two wings - 2B extending on the right side of the station and 4B on the left - stretches 240 feet from tip to tip. It was mounted on the station six years ago to provide interim power during the initial stages of assembly.
To activate the station's permanent power system, the Discovery astronauts needed to retract the left wing of P6 to clear the way for the newly installed P4 arrays to begin rotating to track the sun. The right wing of P6, the 2B panel, will be retracted next March and in September, P6 will be moved to its permanent position on the left end of the station's main solar array truss next to P4.
But the P6-4B wing refused to cooperate and despite more than six-and-a-half hours of trying, the astronauts were only able to retract the panel about halfway. That was enough to permit the P4 array to rotate as required, but not enough to provide the desired long-term structural stability.
On Friday, the astronauts attempted to shake the stuck slats loose by rotating the array's central mast to set up oscillations in the flexible blanket. German astronaut Thomas Reiter even tried exercising with bungie cords in a bid to set up vibrations in the station structure that might jostle the unruly array. But the efforts had no discernible effects.
NASA managers are reluctant to add a dedicated spacewalk to Discovery's mission because that would force the astronauts to forego a planned heat shield inspection after undocking from the station. Others argue the array should be fixed now and not deferred to the next shuttle mission or to the station crew's already busy schedule.
(Editor's Note: The following is background on the space station's electrical and cooling systems for readers who might want a quick refresher. This is a shortened version of the electrical system overview that first ran here as part of a more detailed mission preview.)
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. 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. See illustration.
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. See system graphic.
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.
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 can 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. ... If I can't get (a) 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 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 (transformers) will overheat. It's just a matter of time."
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," said station flight director John Curry. "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 (run them without fluid)," 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 a 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 (another spacewalk) to R-&-R the pump.
"That's the part that concerns me, infant mortality," Curry said. "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."
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.