A busy diagram from NASA's Shuttle Reference Manual shows the location and plumbing related to the orbiter's fuel cell system underneath the cargo bay liner.
This story was updated at 9:44 a.m. EDT.
CAPE CANAVERAL, Fla. - Six astronauts and NASA's shuttle Atlantis will have to wait at least one more day before launching toward the International Space Station (ISS) while engineers puzzle a fuel cell glitch aboard the spacecraft.
Atlantis' launch is now reset for no earlier than Thursday at 12:03:03 p.m. EDT (1603:03 GMT) as shuttle specialists tackle a voltage issue with one of three fuel cells which generate the electricity to power the shuttle during its 11-day ISS construction mission. The shuttle's launch window closes Friday.
"We were ready to go," NASA spokesperson Bruce Buckingham said here at the Kennedy Space Center (KSC), where Atlantis stands poised to launch at Pad 39B. "We were just waiting."
NASA mission managers held off fueling Atlantis' 15-story external tank with cryogenic liquid oxygen and liquid hydrogen propellant - originally slated for 2:30 a.m. EDT (0630 GMT) - while discussing the fuel cell glitch, which first appeared as engineers powered up the shuttle before 1:30 a.m. EDT (0530 GMT) this morning. Atlantis was slated to launch its STS-115 mission at 12:28:46 p.m. EDT (1628:46 GMT) today.
"We held off as long as we could," Buckingham said. "We're going to troubleshoot this, see if we can understand what happened and when we can rectify it."
A voltage spike in the alternating current of Fuel Cell 1 prompted the concern, NASA officials said, adding that the short occured in a coolant motor for that unit. According to NASA flight rules, all three of Atlantis' fuel cells must be functioning properly before the shuttle can launch. Engineers are drawing up options to work around, repair or replace the hardware, shuttle officials said.
Shuttle managers scrubbed Atlantis' Wednesday launch for at least 24 hours to give engineers more time to study the fuel cell issue. The shuttle had a 70 percent chance of favorable launch weather today and is expected to have the same odds over the next two days.
Shuttle managers will gather at 1:00 p.m. EDT (1700 GMT) today during a Misison Management Team meeting to discuss Atlantis' options. A press briefing is expected to follow that discussion.
By calling off today's launch attempt before loading Atlantis' shuttle fuel tank with its 526,000 gallons of super-cold fuel, NASA does avoid the typical $600,000 cost of a scrub after tanking is complete, Buckingham added.
Atlantis' STS-115 mission - commanded by veteran shuttle astronaut Brent Jett - is aimed at delivering a $371.8 million pair of massive portside trusses and two new solar arrays to the ISS. The spaceflight will mark NASA's first dedicated ISS construction effort since the 2003 Columbia accident and follows two return to flight test missions.
The last major piece of ISS hardware - the Port 1 (P1) truss - launched aboard NASA's Endeavour orbiter during the STS-113 mission in late 2002. Atlantis' Port 3/Port 4 (P3/P4) trusses are destined to be installed at the end of their P1 cousin.
Today's scrub follows several delays for Atlantis' STS-115 mission, following a lightning strike to the shuttle's launch pad and storm threats from a tropical depression last month.
Fuel cell puzzle
Each NASA space shuttle is powered by three fuel cells tucked beneath its cargo bay. The reusable, restartable fuel cells weigh about 255 pounds and run 45 inches long, 15 inches wide and about 14 inches high, according to manufacture specifications.
While the shuttle carries three fuel cells, only one is required to power the spacecraft, according to builder UTC Power. They use liquid hydrogen and liquid oxygen to produce a steady 12-kilowatt supply of power each, can put out 16 kilowatts for short periods, and generate heat and water - used for drinking by astronaut crews - as byproducts.
If extensive repairs - such as a fuel cell replacement - are needed Atlantis would likely miss its current STS-115 launch window, which closes on Sept. 8. NASA's current daylight launch restrictions for Atlantis actually allow for a launch up to Sept. 13, but shuttle officials will stand down Friday to avoid conflicts with the planned Sept. 18 launch of a Russian Soyuz spacecraft that will ferry a new crew and one space tourist to the ISS.
Wayne Hale, NASA's space shuttle program manager, said earlier this week that a study is currently underway to reevaluate the daylight launch rules for Atlantis' STS-115 mission. Those restrictions were in place to evaluate fuel tank modifications, but may not be relevant when the next lighted launch window opens on Oct. 26, Hale has said.
If a well-lit launch is not required, Atlantis could rocket toward the ISS sometime after Sept. 29, when Russia's ISS crew change mission concludes, Hale has said.
NASA officials said fuel cell glitches have afflicted several shuttle missions in the past, including: Columbia's STS-2 flight in 1981, Discovery's STS-42 mission in 1992, Endeavour's STS-69 flight in 1995, Columbia's STS-83 spaceflight in 1997 and Atlantis' STS-98 in 2001.
The STS-69 glitch also prompted NASA shuttle managers to scrub their attempted launch before loading Endeavour's external tank propellant, NASA records show. The space agency actually re-launched the STS-83 mission as STS-94 after a fuel cell issue prompted cut the planned 16-day flight down to three days. Erratic readings were seen in one of Endeavour's fuel cells before flight and developed into a full-blown problem once in orbit.
The planned five-day STS-2 mission aboard Columbia was also shortened to three days due to a fuel cell failure in flight.
It took NASA about one week to replace a fuel cell after scrubbing the planned Aug. 31, 1995 launch Endeavour's STS-69 mission, just over 11 years before Atlantis' current problems. STS-69 launched on Sept. 7 of that year.
NASA's post-MMT press briefing on Atlantis' launch scrub is expected to begin no earlier than mid-afternoon. The briefing will air live on NASA TV, and you are invited to follow the event via SPACE.com's NASA TV feed by clicking here.
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