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Posted by George on April 18, 2006, 9:10 am
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http://www.space.com/news/ft_060417_shuttle_fix.html
By Todd Halvorson and John Kelly
FLORIDA TODAY
posted: 17 April 2006
10:16 am ET
CAPE CANAVERAL - NASA is taking steps to prevent a known safety hazard that
could tear apart the International Space Station and a docked shuttle,
triggering rapid depressurization of both spacecraft and killing all
aboard.
But NASA managers opted against a permanent fix because it would take at
least three years. The shuttles are to be retired in 2010.
Nonetheless, NASA safety engineers say action being taken will reduce the
already remote chance that shuttle steering jets could accidentally ignite
while an orbiter is parked at the outpost, generating enough force to rip
the joined craft apart.
What's more, an about-face by managers skeptical of the potential for
disaster indicates safety engineers are exercising renewed clout in the
wake of the 2003 Columbia accident.
"In my mind, this activity was a major success for safety. It's probably
one of the top things," said Jeffrey Williams, chairman of NASA's Shuttle
Safety Review Panel, which evaluates potential hazards and makes
recommendations to managers.
"You always have Doubting Thomases, and wherever we went with this thing,
we ran into Doubting Thomases," he said. "But the potential was identified.
We confirmed the threat."
The risk also was reported in a 2005 FLORIDA TODAY reviewof agency
documents obtained through the Freedom of Information Act.
The review showed the safety panel, and two others in the agency, had
recommended changes aimed at preventing inadvertent jet firings.
Shuttle program leaders interviewed by FLORIDA TODAY discounted the slim
chance of catastrophe, a probability between one in 10,000 and one in 1
million.
But safety experts later convinced managers that disaster could happen, and
that the potential consequences - loss of the $100 billion station, a $2
billion shuttle and their astronaut crews - called for action.
"It was a sales job," Williams said. "We didn't get the Cadillac, but I
think we came pretty darn close in getting management focused on what
needed to be happening here."
Jets steer shuttle
The problem lies within the shuttle's Reaction Control System, which
consists of 38 primary jets and six smaller thrusters in the orbiter's nose
and tail.
The system is designed to steer shuttles in space and during the dive back
through Earth's atmosphere.
Two electronics boxes called Reaction Jet Drivers route firing commands to
the thrusters from the shuttle commander's stick, the ship's computers or
Mission Control.
NASA managers have known since the early 1980s that thrusters could fire
without being commanded to do so. It has happened five times when shuttles
were not docked to other spacecraft.
NASA deemed the risk acceptable then because the crew could recover if the
shuttle were accidentally propelled through open space.
The consequences increase when two ships are linked, because the craft
could rip apart and crews would have no time to react.
Can fire when 'off'
Since 1995, when shuttles began docking at Russia's Mir space station,
astronauts have controlled the hazard by turning off shuttle jet power most
of the time the spaceships are joined in orbit. NASA continued that
practice during dockings at the international station.
New fears arose after the Columbia accident. Safety studies showed the jets
could fire even when power to the thruster system is turned off.
In a phenomenon known as "arc tracking," a short circuit in other shuttle
systems could trigger a thruster firing if defective wiring runs through
the same bundles as electrical lines leading to Reaction Jet Drivers.
That means the method NASA has used for years to control the risk - turning
off shuttle thrusters while docked to a station - could not guarantee
prevention of the problem.
Frayed wiring or the failure of transistors in the electronics boxes could
prompt an accidental firing.
So could erroneous commands from shuttle computers or devices that relay
commands from the computers to Reaction Jet Drivers.
Threat to station
Engineers also determined an unexpected firing lasting 1.5 seconds or
longer could produce enough force to snap off station solar wings or
radiators.
The hardware holding a docked shuttle to the station also could break.
NASA documents show the hazard is one of the most serious threats to the
station, on par with the collision of a visiting spacecraft, a deadly
orbital debris strike or a medical emergency.
"Here you have a potential single-point failure that could take out both
the station and the shuttle. So it no doubt is going to be their No. 1
risk," Williams said.
NASA took steps aimed at limiting the risk prior to launching Discovery
last July on its first post-Columbia mission.
Engineers developed a shuttle computer software patch designed to
automatically detect and shut down unintended thruster firings within 1.3
seconds, or before stress on the structures would increase enough to cause
catastrophic damage.
Power-producing solar arrays were repositioned to reduce structural loads
and prevent possible damage.
Changes made
Those measures will be taken on future shuttle flights, too. But safety
experts convinced once-reluctant managers to carry out other recommended
changes that were resisted prior to Discovery's launch last summer.
Chief among them: intensified inspections of miles of wiring in bundles
containing electrical lines linked to Reaction Jet Drivers.
Special measures are being taken to protect the wiring from chafing so
insulation doesn't wear away and expose conductors that could short
circuit.
NASA took apart and examined a Reaction Jet Driver and determined that the
electronics boxes are not susceptible to age-related failures, Williams
said.
Safety experts also recommended, and managers approved, new procedures
during flight.
a.. Ground controllers will scour telemetry data for signs of potential
electrical shorts before shuttles dock with or depart from the station.
b.. Crews will power some orbiter systems prior to docking, trying to
uncover any lurking electrical shorts before a shuttle arrives at station.
c.. The amount of time Reaction Jet Drivers are electrically powered
before a docking or undocking will be cut from one hour to 20 minutes.
d.. "Keep-out zones" will be established for astronauts conducting
spacewalks near shuttle nose and tail thrusters.
a.. A shuttle will not use its 38 larger primary jet thrusters when
docked to the station unless the outpost must be hauled to a higher orbit
and the orbiter's six, smaller thrusters are out of commission.
Future danger
As a group, the measures are expected to keep shuttle and station crews
safe during the agency's second post-Columbia test flight, scheduled to
launch July 1.
Once outpost assembly resumes and the station grows larger, renewed dangers
will arise.
The larger the station, the less force it will take to break apart the
outpost and a docked orbiter if shuttle jets fire inadvertently.
NASA aims to continue extensive inspections and modifications to Reaction
Jet Driver wiring as well as cables sharing the same bundles.
A permanent fix -- redesigning the Reaction Jet Driver -- would cost an
estimated $36 million and take at least three years to complete.
NASA's shuttle fleet remains scheduled for retirement no later than Sept.
30, 2010.
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Posted by Bob Haller on April 18, 2006, 10:02 am
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how about all the other circuit boards growing whiskers, and one could
cause a bad day.
then we have the redesigned tank shedding more foam than the original
design.
the shuttle is past its end of life, lets shut it down now before it
kills again...........
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Posted by Brian Gaff on April 19, 2006, 3:41 am
Please log in for more thread options Hmm, well, aging systems is something which will need to be researched very
well if we intend to have long term colonies in inhospitable places in the
Solar System. At the moment, LEO is all that has given experience, and I've
been amazed at how many bits of crucial gear have failed. Gyros, Air
processing gear, Window systems and they seem to have a lot of problems with
lights!
As for the problems of short circuits, well, that is something that will
always occur, I guess the best you can hope for is to make the wiring
accessible, and replicablreplaceablee. Electronic boxes like the ones on the
Truss surely need to be situated inside a pressurised environment in future.
Brian
--
briang1@blueyonder.co.uk
From my IMAP account
Brian Gaff
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Posted by John Doe on April 19, 2006, 5:32 pm
Please log in for more thread options Brian Gaff wrote:
show/hide quoted text
> As for the problems of short circuits, well, that is something that will
> always occur, I guess the best you can hope for is to make the wiring
> accessible, and replicablreplaceablee. Electronic boxes like the ones on the
> Truss surely need to be situated inside a pressurised environment in future.
NASA made a conscious decision to put electric/electronic gizmos
outdoors. However, they were designed to be repleacable without EVA
with the robotic arm and robotic "hand" that is still waiting to be launched.
Consider pumps for the ammonia cooling system. You probably don't want
those to be indoors.
Russians put their batteries indoors, and this has proven to be a good
decision since they had to be replaced on a few occasions already. The
US batteries are outdoors, but seem to be reliable.
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Posted by Bob Haller on April 19, 2006, 10:12 pm
Please log in for more thread options hazardous stuff would probably be better off indoors, in compartments
sealed from the rest of the station for safety.
this would save dangerous spacewalks, add accesability, saving service
time and in a double sealed compartment is probably safer that outdoors
where space debris might be more of a issue
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> always occur, I guess the best you can hope for is to make the wiring
> accessible, and replicablreplaceablee. Electronic boxes like the ones on the
> Truss surely need to be situated inside a pressurised environment in future.