NASA debates adding EVA to retract balky solar wing
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 14, 2006
The Discovery astronauts are gearing up for a critical spacewalk today to begin re-wiring the international space station while engineers debate whether to add an unplanned spacewalk early next week to help coax an unruly solar array into full retraction.
Astronauts Robert Curbeam and Christer Fuglesang, Sweden's first man in space, are scheduled to begin a planned six-hour spacewalk at 3:12 p.m., their second in three days. The goal of this excursion is to switch two of the station's four major electrical circuits over to the lab's permanent power system.
The other two circuits will be re-wired during a spacewalk Saturday by Curbeam and newly arrived station astronaut Sunita "Suni" Williams. Both spacewalks are required to route electricity from the station's main arrays through big switching units and transformers on the solar array truss that make up the lab's permanent power system.
Before activating the permanent power system, the astronauts Wednesday attempted to remotely fold up one wing of the solar arrays that provided interim power. The retraction was required to provide the clearance needed for newly installed arrays on the end of the power truss to begin rotating to track the sun.
But the P6-4B array wing refused to cooperate, with its pleated blind-like slats kinking and failing to smoothly fold up. After repeated extension-retraction cycles over more than six-and-a-half hours, the astronauts finally managed to retract the blankets about halfway. That was enough to permit the newly installed P4 arrays to rotate as required. At that point, the astronauts were told to shift gears and prepare for today's spacewalk.
Flight Director John Curry said the partially retracted array was in a safe configuration and that shuttles and Russian spacecraft could dock and undock without causing any structural problems should engineers decide to postpone any corrective activity.
But flight controllers are studying a variety of options for an unplanned spacewalk by Curbeam and Fuglesang to complete the retraction Monday or Tuesday. They opted not to add any repair activities to today's spacewalk or Saturday's excursion because of the critical nature of the re-wiring work and the possibility of additional problems that might require action.
Kirk Shireman, deputy manager of the station program at the Johnson Space Center in Houston, said the solar array problem was particularly frustrating because it looked so easy to solve. On Earth, that is.
"The hard part, you probably saw it at the very beginning when we had that (blanket) fold problem, you look at it and say, boy, if I could just touch it right there it would fold correctly and retract."
Shireman said engineers are looking "at what are the rules, what things should we be worried about when we go out there and touch it ... whether it's a tool, perhaps modifying some of our tools to prevent damage to the arrays and certainly to protect the astronauts from any hazard. These panels, there is glass, these cells have glass in them, they build up charge, so we want to make sure we don't have a potential shock hazard. ... We're going to go through and do an analysis on all those things before we'll send somebody out to do anything with these arrays."
Curbeam and Fuslegang were trained for several solar array contingencies and Curry said access to the blankets was not an issue.
"The good thing is, all the failures that we had today, if you want to call it a good thing, were all in the area near the base of the blanket box," he said. "So you could position a crew member who could actually position himself at the font of one of these blankets. He could cover that wing, for example. You would probably be talking about putting the second crewman on (a robot) arm, more likely, and doing it from the other side. So it's physically possible. You could physically put two guys there if you had to and then you talk about what you do when you get them there and what they're allowed to do, whether they touch it themselves with a glove or whether they touch it with a tool. But it is possible."
During today's spacewalk, Curbeam and Fuglesang will work on and around the central truss segment in the station's main solar array truss atop the Destiny laboratory module. They will unplug 19 thick electrical connectors and replug 17 to re-route solar power to channels 2/3. They also will reposition two equipment carts that are towed along the truss by the station's robot arm-carrying mobile transporter. The spacewalk is relatively straight forward; the real challenge today is in mission control, where flight controllers will be sending hundreds of computer commands in a precise sequence to activate critical systems without causing problems for the operational circuits.
The main solar array truss is mounted atop the Destiny laboratory module at right angles to the long axis of the station's pressurized modules. The central S0 truss segment sits in the middle atop the lab, flanked by the S1 and P1 truss elements. S0, S1 and P1 house the major electrical components of the permanent electrical system: Four main bus switching units, or MBSUs, and transformers called DC-to-DC converter units - DDCUs - that serve to step down and regulate solar array power to levels needed by station equipment.
S1 and P1 also house the station's two independent cooling systems, each of which include large ammonia tanks, a nitrogen gas pressurization system and a massive pump module to pushes ammonia coolant through cold plates and heat exchangers and out into deployable radiators, three on S1 and three on P1. To maximize heat rejection, the radiators are mounted on a rotating beam that can point them toward the cold of deep space.
In September, the crew of mission STS-115 attached two new truss segments to the left side of the solar array beam. The first, P3 (there is no P2) features a powerful solar alpha rotary joint, or SARJ, while the second, P4, includes a new set of solar arrays that stretch 240 feet from tip to tip.
The solar array truss eventually will feature two SARJ joints, one on each side, to rotate the station's solar arrays like giant paddle wheels as the lab complex circles the Earth. That rotation, 360 degrees every 90-minute orbit, will keep the arrays generally face on to the sun. The orientation of the blankets can be fine tuned by so-called beta gimbal assemblies, or BGAs, that automatically adjust the pitch of each solar array wing like the orientation of an airplane propeller can be adjusted in flight.
The P3 SARJ was successfully activated Wednesday and the P4 arrays are now rotating to track the sun. Flight controllers also pressurized one of the two ammonia coolant loops in preparation for activation later today to provide cooling to the MBSUs and other components.
The space station's electrical system was designed to operate in an interim mode during the initial stages of construction, using the P6 arrays mounted on the end of a short segment known as Z1 that extends upward from the Unity module. Up until now, P6 provided power to six DDCUs and two others in the Z1 truss. An interim cooling system kept the electrical components from overheating.
During today's spacewalk and again on Saturday, the station's main electrical circuits will be powered down, channels 2/3 first and then 1/4. The output from the new P4 arrays and the still-extended P6-2B wing will be routed 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 today's spacewalk, 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, 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. Large radiators on the port 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.
Electrical power system components
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 today's spacewalk, 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 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 (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 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."
In an interview, shuttle commander Mark 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.