Next shuttle mission to
re-wire the space station
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: October 30, 2006
Shuttle astronauts, flight controllers and space station engineers are in the final stages of training for a complex multi-spacewalk mission in December to conduct electronic bypass surgery on the orbital lab complex and activate its main power and cooling systems.
Construction has now reached the point where an interim power system, designed to support the station during its initial assembly, needs to be phased out. With the installation of new solar arrays in September, NASA is finally ready to activate the lab's permanent power grid, a major step that sets the stage for attachment of European and Japanese research modules.
But in order to do that, the Discovery astronauts and station crew must first retract one wing of the older solar arrays providing interim electricity to the U.S. segment of the station. If that goes well - and no one knows whether the fragile array wing will, in fact, retract smoothly - commands will be sent to begin slowly rotating the new set of arrays installed in September.
Flight controllers then will power down the lab's major circuits, two at a time, while spacewalking astronauts unplug and re-plug electrical cables into different sockets.
Precisely timed computer scripts will set electro-mechanical switches to begin routing power from the new arrays and the remaining older array wing through four big circuit control boxes called main bus switching units, or MBSUs, mounted in the center of the station's main solar power truss. The MBSUs, in turn, will send array power to a host of transformers, circuit breakers and other components to provide precisely regulated 124-volt DC power to the station's myriad systems.
But the MBSUs will begin heating up as soon as the power begins flowing. In a high-stakes race against the clock, the astronauts and flight controllers must quickly activate powerful pumps to push ammonia coolant through cold plates and radiators to keep the MBSUs and other components from overheating.
"This is pretty much your classic chicken-or-egg scenario here," said Paul Hill, mission operations manager at the Johnson Space Center in Houston. "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 the cooling equipment."
It will take two virtually identical spacewalks - and hundreds of commands and cable switches - to reconfigure the station's four major circuits. There is little margin or error and a fair amount of uncertainty. While the MBSUs have at least been tested in space to confirm electrical continuity, the ammonia coolant system has never been activated or tested in space.
"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."
Should any significant problems develop - an ammonia leak, an electrical glitch, a computer commanding issue - that might prevent the activation of a cooling system, the astronauts could be forced to quickly switch back to the interim power system, cutting of the flow of electricity to the MBSUs while engineers in Houston consider alternatives.
"What makes this mission singularly unique is the choreography between what we're doing in orbit and what the ground is doing because of the amazing amount of ground commanding to go ahead and power down and re-power the ISS electrical system while we're out there doing spacewalks," said Discovery commander Mark Polansky.
"So yeah, we hold our breath like everybody else while the ground sends the commands and then wait to find out how it's going to work."
Said Bob Curbeam, the lead spacewalker on the mission: "It is very complicated. The thing I think is kind of cool about it is, we have these three spacewalks, the second two, the big player in them both is mission control. ... The pressure is on those guys.
"While they have to be very slow and deliberate about how they do it, they can't be too slow because the clock's ticking on us. There's got to be a point where they say OK, it's going to be fine, you guys press on or OK, we have to back out of it and go back to (interim) power and we have to undo everything we did and then talk about what we're going to do after that."
In that case, the pressure will quickly switch back to the astronauts.
"Our tasks are relatively straight forward as long as you don't run into any of the contingencies," he said. "And that's the big deal. We probably train for contingencies more than most crews because we know that's where the devil is."
Launch of the 117th shuttle mission, the fourth since the Columbia disaster, is targeted for Dec. 7, around 9:39 p.m. It will be NASA's first night launch since 2002, the result of a recent decision to waive a post-Columbia requirement to launch shuttles in daylight.
The daylight constraint was put in place to ensure good photo documentation of the shuttle's heat shield and external tank foam insulation during the first few post-Columbia missions. Problems seen during the first such flight in July 2005 were addressed and NASA's two most recent launches, in July and September, were relatively debris free.
Given the tank's recent performance, and experience demonstrating that on-orbit inspections can spot any damage that does occur, NASA managers cleared Discovery for launch at night.
But the Dec. 7 target date had already been booked by the Air Force for launch of a Lockheed Martin Atlas 5 rocket carrying a suite of military payloads. If the Atlas stays on track, the shuttle launch will slip a few days. NASA planners want to get Discovery off the ground by Dec. 17 at the latest to avoid any possible software issues that might arise with a shuttle in orbit during the rollover to 2007.
Another wild card for mission STS-116 is the health of the space station's Russian Elektron oxygen generator. The device broke down in September and the crew has been tapping into oxygen stored in the Quest airlock module. Spare parts were launched aboard a Russian Progress supply ship Oct. 23 for a planned overhaul.
The oxygen system is a factor for shuttle planners because of a post-Columbia requirement to provide safe haven aboard the station for shuttle crews in the event of major problems that might prevent a safe re-entry. Enough oxygen must be available to support the combined station-shuttle crews - 10 people in this case - until NASA could launch a shuttle rescue mission.
Oxygen in tanks mounted on the Quest airlock cannot be used after Nov. 1 to ensure enough is available for the three spacewalks planned during Discovery's mission. Without an operational Elektron, the crew eventually will have to burn so-called "candles" and tap into oxygen launched aboard Progress modules. But NASA planners believe enough supplies will be available for Discovery's mission to proceed.
Joining Polansky and Curbeam aboard Discovery will be pilot William Oefelein, Nicholas Patrick, European Space Agency astronaut Christer Fuglesang, Joan Higginbotham and Sunita "Suni" Williams, who will replace ESA astronaut Thomas Reiter aboard the station as part of the Expedition 14 crew.
Along with re-wiring the space station and retracting one of its existing solar array wings, the astronauts also will launch three small satellites after undocking from the space station.
The Atmospheric Neutral Density Experiment, or ANDE, will measure the composition and density of the thin air in low-Earth orbit to help satellite operators improve their understanding of atmospheric drag. The Microelectromechanical System-Based PICOSAT Instpector, or MEPSI, will test techniques for assessing spacecraft damage due to natural or man-made environmental threats. Finally, a U.S. Naval Academy student satellite known as the Radar Fence Transponder, or RAFT, will test an experimental communications technology. MEPSI and RAFT will be launched on the same day Discovery undocks from the station while ANDE will be deployed the following day.
Oefelein, Curbeam and Fuglesang were part of a different crew that was in training for the same mission in the summer of 2003. The Columbia disaster put station assembly on hold and that crew eventually was broken up. But the two spacewalkers and their in-cabin director - Oefelein - stayed together. Polansky, Higginbotham and Patrick were added in 2005 along with Williams, who is hitching a ride to join the Expedition 14 crew aboard the station.
Polansky is a veteran of one previous spaceflight while Curbeam has two shuttle missions to his credit, including three spacewalks. The rest of the crew members are rookies.
"It's kind of an unfair name, it has some connotations," Polansky said. "The fact of the matter is Joannie, who has never flown before, has worked here for 10 years. She showed up at the same time I did. Nick and Billy O, those two guys, they've been here for over eight years and they've got a lot of experience. Christer was in the same class that Joannie and I were in and he was an astronaut for ESA before we ever got selected.
"So these guys, while not flown, have been working as astronauts for a minimum of eight years. So they've got a lot of knowledge. The only thing they don't have is knowing exactly what it's going to be like to do this job on orbit."
Discovery will be launched into the plane of the space station's orbit and if all goes well, Polansky will guide the shuttle to a docking with pressurized mating adapter No. 2 on the forward end of the Destiny laboratory module two days later.
The international space station currently consists of six pressurized modules. At the back end of the outpost is the Russian Zvezda command module featuring two solar arrays and an aft docking port that can accommodate Progress supply ships or Soyuz crew capsules. An airlock module called Pirs is attached to a downward-facing port on Zvezda's front end. Zvezda's forward port is attached to the Russian Zarya module, a supply and propulsion segment equipped with its own pair of solar arrays.
Zarya's front end is bolted to a pressurized mating adapter that, in turn, is attached to NASA's Unity module, a multi-hatch node with six ports. Its starboard port is occupied by the U.S. Quest airlock module while its upper zenith port accommodates the Z1 truss and the P6 solar arrays that provide interim power. Unity's downward facing port is used by cargo modules brought up by the shuttle and its port hatch is home to another pressurized mating adapter that will be relocated later in the assembly sequence.
Unity's forward port is attached to the Destiny laboratory module. On the forward end of Destiny is another pressurized mating adapter used as a docking port by visiting space shuttles. On top of the lab module is the station's multi-segment solar array truss, which is mounted at right angles to the long axis formed by the pressurized modules.
The 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: The 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 deep space and away from the sun.
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 (solar array orientation diagram).
The same day Discovery docks with the space station, the astronauts will use the shuttle's robot arm to pull a short spacer segment - P5 - from the orbiter's cargo bay. The 4,110-pound tress segment will be handed off to the station's robot arm and positioned near the left end of the main solar array truss.
The next day, Curbeam and Fuglesang will carry out a spacewalk to connect P5 to the outboard side of the P4 solar array segment. The spacer measures 11 feet long, 14 feet 11 inches wide and 13 feet 11 inches tall. Next year, the P6 arrays now providing interim power atop the Z1 truss will be moved to the left end of the power truss and bolted to P5.
NASA planners initially planned for Curbeam and Fuglesang to manually move P5 into position and then bolt it into place. But due to an oversight, planners did not realize the current position of P4 would leave the spacewalkers with just two inches of clearance while moving the spacer into place.
As a result, Williams and Higginbotham, operating the station's robot arm from inside the Destiny module, will position P5 while Curbeam and Fuselgang, positioned near P4, provide visual guidance.
"They move it in for the mating and then Christer and I actually bolt it manually," Curbeam said. "It doesn't have the automatic (attachment) system. They'll bring it in really, really close, we'll take the launch locks off, which actually just protect the mating bolts, and once we take the launch locks off ... we drive the bolts to permanently connect them."
Precision work with the robot arm will be required to get P5 into place. During the final stages of the move, it will pass within two inches of the sequential shunt unit on the P4 solar array segment, "which all the juice from the solar array goes through," Curbeam said. "Not a good thing to hit."
"As a matter of fact, that's why we ended up doing it robotically. The first idea was to grab it and put it in by hand. But the problem was, when they did the evaluation, they had the SARJ angle wrong. The sequential shunt unit was in the wrong orientation for mating. So they thought they had plenty of clearance, they said you can do this by hand. Then I went like, wait a minute, no, I'm looking at it and I see two inches of clearance, maybe and inch and a half. And they went like, no. Then we did our homework and like yes!
"I said there's no way - and it's a blind mate if you do it EVA - there's no way I can sit here with a straight face and tell (lead station flight director) John Curry that I wasn't going to hit that thing. So I told him, I said John, I'm sorry but we've got to look at a different way of doing this because I can't tell you we're not going to hit it. It was a blind stick.
"I think everyone is comfortable with the arm install now," Curbeam said. "But in the beginning, they said hey, this is not what we had planned. I said I know it's not, but I don't feel comfortable doing this EVA because if we do it ourselves we are going to hit something. ... if we hit that SSU, that could turn into a really, really bad scene quickly."
Once the first spacewalk is out of the way, the astronauts and ground controllers will turn their attention to the heart of mission STS-116: electrical bypass surgery to wire in the new solar arrays.