HOUSTON - NASA officials are confident that
a potential fuel leak in a power unit aboard the space shuttle Discovery, if
unchanged, will not hinder the spacecraft's Monday landing, a top mission
manager said Friday.
John
Shannon, NASA's deputy shuttle program manager, said that in worst case
scenario - in which fuel is leaking and not harmless nitrogen, something that
is not yet certain - one of Discovery's three auxiliary power units (APUs) may
be leaking a total of about six drops an hour, or about 100,000 times below the
fire hazard limit.
"We're okay
where we are right now," Shannon said, adding that extra checks of the APU are
planned to Sunday. "If it's hydrazine, at the current leak rate we really don't
have any concerns with using it right now."
Shuttle
APUs provide the power required for hydraulic systems that move an orbiter's
elevons, vertical stabilizer flaps, landing gear and other systems required
during landing.
To generate
power, the units use hydrazine for fuel and gaseous nitrogen to generate
pressure. Either one of those materials may be the source of the leak in APU 1.
The leak's presence is suggested by a minute yet steady drop in tank pressure,
but the aft section of Discovery that houses the unit lacks the necessary
sensors to know for sure.
"The question
is what's leaking," said Shannon, who also chairs Discovery's STS-121 Mission
Management Team, during a briefing here at NASA's Johnson Space Center. "There is some anecdotal evidence, actual even better than anecdotal evidence, that it's
nitrogen and if it is than it's no issue at all. In my view, it's a coin flip."
Based on
that uncertainty, NASA managers and engineers are assuming the worst - a hydrazine
leak - and have developed a test to determine whether drip is stable or could
degrade further. If a Sunday checkout of the system, in which engineers plan to
run the power unit briefly to see if its pressure drops more than the current
rate, is unfavorable, NASA plans to run the APU until its fuel is used up and
take it offline during reentry.
While a
shuttle can land on one APU if necessary, all three are preferred for
redundancy. APU 1, in particular, is the only unit powering Discovery's landing
gear. If it is taken offline, the shuttle will have to fire a set of
pyrotechnic charges to deploy its main and nose landing gears, Shannon said.
Shannon said if APU 1 is leaking hydrazine,
it will have lost about 13 pounds (five kilograms) - a little less than two
gallons - since Discovery's July 4
launch. According to NASA shuttle specifications, a typical hydrazine load
for each APU is about 325 pounds (147 kilograms).
If the leak
is actually gaseous nitrogen, there is no concern that APU 1's fuel tank
pressure - currently at about 230 psi - will drop below the minimum 100 psi
threshold before Discovery lands next week, Shannon added.
Either way,
there is no risk of a repeat fire such as that seen during NASA's STS-9 shuttle
flight in 1983, in which a large hydrazine leak sprayed the toxic fuel on a hot
surface and ignited two APUs, Shannon said. One other leak early in the shuttle
program - during approach and landing tests - actually seeped gallons of the
stuff into the vehicle's aft compartment, but did not ignite, he added.
But Discovery's
current leak - if it is hydrazine at all - is nowhere near as pronounced as
those previous incidents.
"Right now
I don't see any change for the landing plan," Shannon said.
Discovery's
STS-121 astronaut crew is slated to land at NASA's Kennedy Space Center on July 17 at 9:14 a.m. EDT (1314 GMT). The spaceflight is NASA's second shuttle
mission since the 2003 Columbia accident.
Engineers
also determined Friday that a thermostat glitch in the heaters for Discovery's
APU 3 is not a major concern, Shannon said. They also suspect that vehicle
telemetry suggesting a pressure increase that could damage a load-bearing bulkhead
in the spacecraft's nose during a hard landing is the result of instrumentation
error rather than actual problem, but will study the matter further, he added.
Extra
late inspections
Meanwhile,
Discovery's six STS-121
astronauts, commanded by veteran shuttle flyer Steven
Lindsey, worked through some issues of their own today as they prepared
their spacecraft to undock from the International Space Station (ISS) on
Saturday.
Problems
with the station's robotic arm delayed a late
inspection of Discovery's port wing by almost an hour Friday, prompting
flight controllers to suggest that the task be added to the STS-121 crew's
Saturday schedule. But Lindsey turned down the idea since it would make a busy
undocking day even more so.
"I think
doing an undock plus an entire Flight
Day 2 inspection tomorrow is probably not such a good idea, and I would
just as soon complete the port wing today," Lindsey told flight controllers.
Astronaut
Lee Archambault, serving as spacecraft communicator, told Lindsey that flight
controllers were willing to pull the port wing scan - which is part of two-day inspection
to determine whether tiny meteorites have struck critical heat shield areas
during Discovery's flight - from the STS-121 crew's scheduled altogether.
"It's not a
show stopper as far as the program is concerned," Archambault told the crew. "They're
happy to take it out of the program if they absolutely have to."
But Lindsey
believed his crew could perform the task, even though it would eat into time allotted
to their sleep preparations.
"We're
going to start now, and we're going to do it," Lindsey said at about 2:06 p.m.
EDT (1806 GMT). "If there's anything out there that we can delete later one,
let's delete it."
The survey
was complete by 3:39 p.m. EDT (1939 GMT), leaving only the starboard wing
leading edge and shuttle nose cap remaining for a closer look.
"It was a
tough day, but we got through it," Lindsey said. "Hopefully, we're caught up on
the timeline."
NASA
will broadcast Discovery's Saturday undocking live on NASA
TV beginning with a crew farewell ceremony at 3:38 a.m. EDT (0738 GMT). You
are invited to watch the activity via SPACE.com's
NASA TV feed, which is available by clicking here.
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