Next shuttle mission to do complex electrical work
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: September 28, 2006
NASA managers today agreed to move up the target launch date for the shuttle Discovery and mission STS-116 from Dec. 14 to Dec. 7 at roughly 9:38 p.m. EST. Agency managers have not yet formally relaxed a post-Columbia daylight launch constraint, but that issue will be discussed at the next program requirements change board meeting Oct. 5 at the Johnson Space Center in Houston.
Still unresolved is conflict with a Lockheed Martin Atlas 5 rocket carrying a military payload that currently is scheduled for launch Dec. 7 from the Cape Canaveral Air Force Station.
An overview of the STS-116 mission is posted below.
Construction has now reached the point where an interim power system, designed to support the station during its initial assembly, needs to be upgraded to support the eventual attachment of new research modules. And with the delivery of new solar arrays by shuttle astronauts earlier this month, NASA is finally ready to activate the lab's permanent power and cooling systems.
But in order to do that, the astronauts must first retract one wing of the older solar arrays providing interim electricity to the U.S. segment of the station. Flight controllers then will power down the lab's two major circuits, one at a time, while spacewalking astronauts plug electrical cables into different sockets.
With the solar arrays attached by the shuttle Atlantis' crew in September, "we'll now be generating enough power we can bring the permanent cooling system on line and start using it," said Paul Hill, mission operations manager at the Johnson Space Center. "Once we have that system active, we can also start flowing power through those switch boxes that are sitting in the middle of the truss.
"This is pretty much your classic chicken-or-egg scenario here," he said. "You have to have active cooling to the switch boxes (main bus switching units, or MBSUs) in order to route power through them. You have to have power flowing through the MBSUs in order to power their cooling equipment."
The trick, Hill said, is to power up the MBSUs, route power through them to the cooling system and get it that system activated before the MBSUs overheat.
Launch of the 117th shuttle mission, originally planned for Dec. 14, is now targeted for Dec. 7. Assuming NASA managers relax a post-Columbia daylight launch constraint, liftoff around 9:38 p.m. will be the agency's first night launch since 2002.
On board will be commander Mark Polansky, pilot William Oefelein, Nicholas Patrick, Robert Curbeam, European Space Agency astronaut Christer Fuglesang, Joan Higginbotham and Sunita Williams, who will replace ESA astronaut Thomas Reiter aboard the station.
Curbeam and Fuglesang will carry out two spacewalks to attach a short spacer segment to the station's main solar power truss and to re-wire one of the lab's two primary electrical circuits. Curbeam and Williams, who will remain aboard the outpost in Reiter's place when Discovery departs, will re-wire the other power channel during a third spacewalk.
The spacewalks are not inherently difficult as such things go. As each electrical circuit is powered down by flight controllers in Houston, the astronauts will unplug various cables and plug them back into different sockets. But no one knows how the station's myriad electrical components will respond to a shutdown and power up.
Likewise, no one really knows how the big coolant pumps will operate in the weightlessness of space to push ammonia through various lines and radiators to dissipate the heat generated by those same electrical components.
Once a power channel is shut down, components plugged into that circuit will quickly cool down. During power up, they will rapidly heat up. The thermal control system is needed to keep temperatures in a carefully controlled range and if problems develop on either side of those power downs and power ups, the spacewalkers will have to act fast to prevent potentially serious trouble.
Lead station flight director John Curry likens the power downs and power ups 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)."
The space station's electrical system was designed to operate in an interim mode during the initial stages of construction, providing power to critical systems from a set of solar arrays - P6 - mounted atop a short truss - Z1 - extending upward from the multi-hatch Unity module. An interim cooling system keeps the electrical components from overheating.
During Atlantis' flight in September, a new set of solar arrays - P4 - was mounted to the end of the station's main solar array truss, a beam that eventually will stretch the length of a football field sporting two huge sets of arrays on each end.
Two massive rotary joints, one on each side of the truss - the first was attached during Atlantis' mission - will rotate the arrays like giant paddlewheels as the station circles the globe to keep them face-on to the sun. Each side of the truss also features a set of three large radiator panels mounted on rotating platforms to dissipate the heat generated by the station's electrical systems.
The heart of the station's electrical system is made up of the MBSUs mounted inside the S0 truss, the central segment of the solar array beam. The arrays on each end of the beam will feel electrical power to the MBSUs, which in turn can distribute electricity to virtually any station system. If one set of arrays, batteries or chargers fail, the MBSUs can redistribute power from the other panels to keep critical systems safely operating.
"We have these big boxes in the middle of the space station, big switch boxes," Hill said of the MBSUs. "You've got four pairs of solar arrays (when the station is complete) and you've got all these finger-thick copper wires that run from the solar arrays to the middle of the truss. Those are the boxes that, for an assembly-complete station, you want all your power flowing from and then going down to our converters that then flow power to individual pieces of equipment.
"In order to reconfigure the electrical system and the cooling system so we have the permanent cooling system up and we're flowing all power through these main switching boxes, we've got to power off a hell of a lot of equipment so we can safe those individual copper lines, disconnect them and reconnect them to where we want them. That will be a case where we'll have to power off almost all the U.S. segment one way or the other throughout that whole process."
That process begins when the left wing of the P6 array - known as the 4B wing - is retracted during Discovery's mission to permit the newly installed P4 panels to rotate as needed to track the sun. The right wing of P6 - the 2B wing - will be retracted during the next shuttle flight early in 2007. If all goes well, P6 will be moved, bolted to the P5 spacer truss and its 2B/4B solar blankets re-extended during a shuttle flight next August.
"Until now, P6 has been parked in an interim location in the middle of the truss on top of Unity," Hill said in an email. "Not only has it provided electricity, but it used a smaller, temporary cooling system for the U.S. segment until the MBSUs and the more robust permanent cooling system were installed and activated.
"This set up has worked very well, but it also means we have many electrical and cooling lines connected to the early or temporary systems, rather than the permanent or assembly-complete architecture. In order to finally connect all of the powered equipment to the their permanent power and cooling sources, the vast majority of the U.S. equipment must be powered down, the connections physically switched by spacewalking astronauts, then powered back up.
"This is a long, choreographed activity in order to ensure at least one of every critical component remains powered throughout the reconfigurations," he said. "The important element in all of this is that the additional power provided by the new solar arrays and the additional cooling the permanent cooling system provide are necessary before we can install the Japanese and European research laboratories which are also coming in the next year."
The P6 port wing retraction is scheduled for the day between the first and second spacewalks. The night before, flight controllers will begin moving critical station operations from hardware powered by the 4B solar array (through channel 1/4) over to hardware powered by the 2B solar array (through channel 2/3). The goal is to get the total load on the 4B array to around 6.5 kilowatts.
On retraction day, flight controllers will execute a procedure known as "seamless power channel handover," which will move the 6.5-kilowatt load remaining on the P6-4B wing to the newly installed P4-4A wing on the left end of the main solar array truss. Once the handover is complete, the P6-2B wing will be providing power to station channel 2/3 while P4-4A will be powering channel 1/4.
At that point, commands will be sent to retract the left side P6-4B solar array wing.
"The guys who built it, they designed it to retract and they expect it to retract," Hill said. "But for a solar array that's been hanging out in the breeze, where if we took a micrometeoroid strike and dinged one of those structural elements, there's a real good chance that we're not going to be able to fully retract those arrays. We'll see what we get."
Assuming P6-4B successfully retracts, flight controllers will configure its fully charged batteries to operate in "parachute mode." While the three nickel-hydrogen batteries cannot be recharged with the port array retracted, they can provide up to eight hours of emergency power to channel 1/4 if any problems crop up with the P4 arrays.
The interim P6 power system routed direct current electricity to six DC-to-DC converter units (DDCUs) in the Destiny laboratory module and two on the Z1 truss. To switch the station to its permanent power system, the Discovery astronauts will carry out two spacewalks in concert with complex ground commanding. A third spacewalk - the first EVA of the mission - will be devoted to hooking up the P5 spacer truss that will serve as the attachment point for P6 when those arrays are moved next year.
During the second and third spacewalks, the station's main electrical circuits will be taken off line, channel 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 DDCUs will be connected to the MBSU output.
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 keep critical systems active when other elements of a given power channel are shut down.
After the second 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. The large radiators on each side of the main solar array truss will begin rotating for the first time to maximize heat rejection.
Getting the thermal control system activated as quickly as possible is critical to prevent components from getting too cold or, in the case of the MBSUs, too hot.
"The ISS control team has to balance that race against overheating the MBSUs with the power downs necessary to disconnect and reconnect the power cables and cooling lines and the subsequent equipment reactivation necessary before moving on to the next set of power cables and cooling lines," Hill said.
"That means the clock is ticking on the MBSUs as they begin heating up without cooling, and much of the equipment that is powered off require power to keep them from getting too cold and breaking circuit boards and other sensitive components."
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."
Here is a timeline overview of Discovery's mission as it now stands, based on a Dec. 7 liftoff. Readers are advised the flight plan remains a work in progress and changes are expected. A more detailed flight plan is available here.
All times in EST and elapsed time from launch:
DATE/EST DD HH MM EVENT _____________________________________________________________ 12/07/06 Thu 09:38 PM 00 00 00 STS-116 Launch (flight day 1) 12/08/06 Fri 01:53 AM 00 04 15 Shuttle robot arm (RMS) checkout Fri 01:58 AM 00 04 20 External tank video downlink Fri 03:38 AM 00 06 00 Crew sleep begins Fri 11:38 AM 00 14 00 Crew wakeup Fri 04:03 PM 00 18 25 Wing leading edge, nose cap survey 12/09/06 Sat 03:08 AM 01 05 30 Crew sleep begins Sat 11:08 AM 01 13 30 Crew wakeup Sat 06:41 PM 01 21 03 Discovery docks with space station Sat 09:43 PM 02 00 05 P5 truss segment grapple by shuttle arm Sat 10:28 PM 02 00 50 P5 handoff from shuttle arm to station arm 12/10/06 Sun 03:08 AM 02 05 30 Crew sleep begins Sun 11:08 AM 02 13 30 STS crew wakeup Sun 04:03 PM 02 18 25 EVA-1: Airlock egress Sun 04:28 PM 02 18 50 EVA-1: P5 launch lock removal Sun 05:28 PM 02 19 50 EVA-1: P5 attached to P4 Sun 10:08 PM 03 00 30 EVA-1: Airlock repressurization 12/11/06 Mon 02:38 AM 03 05 00 STS/ISS crew sleep begins Mon 10:38 AM 03 13 00 STS crew wakeup Mon 01:53 PM 03 16 15 P6 port panel retraction (1 bay) Mon 06:23 PM 03 20 45 P6 port wing full retraction begins Mon 06:38 PM 03 21 00 P6 port wing full retraction complete 12/12/06 Tue 02:38 AM 04 05 00 STS/ISS crew sleep begins Tue 10:38 AM 04 13 00 STS crew wakeup Tue 03:33 PM 04 17 55 EVA-2: Airlock egress Tue 03:58 PM 04 18 20 EVA-2: Channel 2/3 power reconfig Tue 05:33 PM 04 19 55 MCC: EPS final activation Tue 09:23 PM 04 23 45 EVA-2: Airlock repressurization 12/13/06 Wed 02:08 AM 05 04 30 STS/ISS crew sleep begins Wed 10:08 AM 05 12 30 STS crew wakeup Wed 05:33 PM 05 19 55 Joint crew news conference Wed 06:03 PM 05 20 25 Crew off duty time begins 12/14/06 Thu 02:08 AM 06 04 30 STS/ISS crew sleep begins Thu 10:08 AM 06 12 30 STS crew wakeup Thu 03:03 PM 06 17 25 EVA-3: Airlock egress Thu 03:28 PM 06 17 50 EVA-3: Channel 1/4 power reconfig Thu 06:08 PM 06 20 30 MCC: EPS final activation Thu 09:03 PM 06 23 25 EVA-3: Airlock repressurization 12/15/06 Fri 01:38 AM 07 04 00 STS/ISS crew sleep begins Fri 09:38 AM 07 12 00 STS crew wakeup Fri 12:38 PM 07 15 00 Transfers resume 12/16/06 Sat 01:38 AM 08 04 00 STS/ISS crew sleep begins Sat 09:38 AM 08 12 00 STS crew wakeup Sat 04:18 PM 08 18 40 Discovery undocks from space station 12/17/06 Sun 12:38 AM 09 03 00 STS/ISS crew sleep begins Sun 08:38 AM 09 11 00 STS crew wakeup Sun 07:08 PM 09 21 30 Crew off duty time begins 12/18/06 Mon 12:08 AM 10 02 30 Crew sleep begins Mon 08:08 AM 10 10 30 Crew wakeup Mon 03:57 PM 10 18 19 Deorbit ignition Mon 05:03 PM 10 19 25 Landing