[sarex] Shuttle Mission STS-116: A Hard Wire Job
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Fri Oct 13 09:57:08 PDT 2006
SUBMITTED BY ARTHUR N1ORC - AMSAT AC #31468
Shuttle Mission STS-116: A Hard Wire Job
NASA has said it over and over again: The coming missions to finish the
International Space Station are among the hardest and most complex ever.
But if you ask the astronauts and engineers which of the final 14
assembly flights may be the most complex, many would point to
Discovery's next mission, set to launch in December.
JSC2006-E-23034 : Mark Polansky and William Oefelein"What makes this one
singularly unique is the fact that we're going to rewire the space
station," Mark Polansky, Discovery's commander, said.
Image to right: STS-116 Commander Mark Polansky (left) and Pilot William
Oefelein participate in a training session in the fixed-base shuttle
mission simulator at Johnson Space Center, Houston. Image credit: NASA
Since it went into orbit in 1998, the space station has been running on
a temporary electrical system. Lead Space Station Flight Director John
Curry compared it to the way you might build a house on the ground –
until your electricity is hooked up, you probably plug your saws into a
generator. That's basically what the astronauts building and living on
the station have been doing for the past eight years.
But with the installation of two new electricity-generating solar array
panels in September, all the pieces are now in place to switch to the
permanent system. At your house, it would just be a matter of unplugging
the saw from the generator and plugging it back into the wall. But in
space, it's not that easy.
"Everything will be fine – if nothing breaks," Curry said.
The plan is to send astronauts out on two spacewalks, each devoted to
rewiring half of the station. Though it sounds complicated, that part
shouldn't be too difficult. Spacewalks are inherently dangerous and
should only be done if there is no alternative, Polansky said, but as
spacewalks go, these are pretty straightforward. The astronauts will
head outside, wait for the team on the ground to send commands to switch
off the power, and then unplug the power cables and plug them in new
places. There might be the occasional stiff cable to deal with – that
can happen in the minus 200 degrees Fahrenheit of space – and the
process will likely be slow, but not especially complex.
The real pressure, Polansky said, will be on those back in Houston.
"I hold my breath every time we do spacewalks because you never know
what can happen," he said. "So I'll definitely be watching. But I don't
think I'll be as worried as the guys in Mission Control and the folks
who have been working on the hardware will be. I think I'll have a lot
of company in the worry department."
Curry confirmed that suspicion. He's been training for this mission for
six years, and he said his team couldn't be any more prepared. But when
asked what about the mission keeps him awake at night, he had no trouble
coming up with a list.
"My team is the one that has to turn everything back on and get it
running," he said.
JSC2003-00016 : Robert Curbeam and Christer Fuglesang participate in
spacewalk training Image above: Mission Specialists Robert Curbeam and
Christer Fuglesang, wearing training versions of the Extravehicular
Mobility Unit spacesuit, participate in an underwater simulation of
extravehicular activity scheduled for the STS-116 shuttle mission to the
International Space Station. Image credit: NASA
Theoretically, everything should go fine. But there are a few things
that could cause some big hitches. Several of them have to do with the
unpredictability of equipment that's been in space for years. For
instance, before any of the rewiring can be done, half of the solar
array that's been providing the temporary electricity must be folded up
to make room for the new solar arrays to rotate. That's never been tried
before, and it may not be as easy as it sounds.
"It's been sitting out there taking thermal cycles (moving from minus
200 degrees Fahrenheit to plus 200 degrees Fahrenheit every 45 minutes)
since November 2000," Curry said. "It's like a map – if you keep a map
out in your car for six years and then you decide to fold it up again,
you may get some waves in it or it may not fold back the same way at all."
Many of the main components of the electrical system have been flying
that long and could cause similar large headaches. It's impossible to
know for sure if the equipment will work until the power has been turned
off, rewired and turned back on. And if it doesn't work, the astronauts
can't leave it like that – the essential systems on the station would be
running on whichever half of the station has power, but without both
halves they won't have any backup.
That's not a position anyone wants to leave the station in for any
longer than necessary. So, if Mission Control flips the switch and the
lights don't all come on, the astronauts will have to try and fix
whatever the problem is before they run out of time. If they can't, then
it's back to square one.
"Then I have to tell the crew, stop what you're doing and undo
everything," Curry said.
To avoid that, plans addressing possible problems are made well in advance.
"You put all your energy into being successful and doing it safely,
while making sure you do it efficiently," Lead Shuttle Flight Director
Tony Ceccacci said. "And then you step back after you get that completed
and say, ‘What if?'"
JSC2006-E-33306 : Robert Curbeam with virtual reality hardware Polansky
said the crew spends a lot of time training for those what ifs.
"They run through scenario after scenario," Polansky said. "Today we're
going to do a main bus switching unit checkout, tomorrow we're going to
do a pump module remove and replace. The next day we're going to a
replace a direct current-to-direct current converter unit box. None of
which we ever plan to do on orbit."
Image to left: STS-116 Mission Specialist Robert Curbeam uses virtual
reality hardware to rehearse some of his duties on the upcoming mission
to the International Space Station. Image credit: GO
But, Ceccacci said, as much as you would like to, you can't plan for
every contingency. You look at the most probable failure, impact to
mission, complexity of recovering, then determine if you should spend
the resources to develop the fix.
If the old solar array won't fold up, the astronauts won't be able to do
any of their rewiring spacewalks until they either fold it up manually
or jettison it. If the pump that keeps the electrical system cool
doesn't work, there would only be enough time to rewire half of the
station after replacing it. But he believes NASA is up to the challenge.
"Everyone has stepped up, is prepared, and is confident that this
mission will be very successful," Ceccacci said. "As with all complex
assembly flights, it's going to be interesting."
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