HOUSTON - NASA officials are confident thata potential fuel leak in a power unit aboard the space shuttle Discovery, ifunchanged, will not hinder the spacecraft's Monday landing, a top missionmanager said Friday.
JohnShannon, NASA's deputy shuttle program manager, said that in worst casescenario - in which fuel is leaking and not harmless nitrogen, something thatis not yet certain - one of Discovery's three auxiliary power units (APUs) maybe leaking a total of about six drops an hour, or about 100,000 times below thefire hazard limit.
"We're okaywhere we are right now," Shannon said, adding that extra checks of the APU areplanned to Sunday. "If it's hydrazine, at the current leak rate we really don'thave any concerns with using it right now."
ShuttleAPUs provide the power required for hydraulic systems that move an orbiter'selevons, vertical stabilizer flaps, landing gear and other systems requiredduring landing.
To generatepower, the units use hydrazine for fuel and gaseous nitrogen to generatepressure. 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 necessarysensors to know for sure.
"The questionis what's leaking," said Shannon, who also chairs Discovery's STS-121 MissionManagement Team, during a briefing here at NASA's Johnson Space Center. "There is some anecdotal evidence, actual even better than anecdotal evidence, that it'snitrogen and if it is than it's no issue at all. In my view, it's a coin flip."
Based onthat uncertainty, NASA managers and engineers are assuming the worst - a hydrazineleak - and have developed a test to determine whether drip is stable or coulddegrade further. If a Sunday checkout of the system, in which engineers plan torun the power unit briefly to see if its pressure drops more than the currentrate, is unfavorable, NASA plans to run the APU until its fuel is used up andtake it offline during reentry.
While ashuttle can land on one APU if necessary, all three are preferred forredundancy. APU 1, in particular, is the only unit powering Discovery's landinggear. If it is taken offline, the shuttle will have to fire a set ofpyrotechnic 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 twogallons - since Discovery's July 4launch. According to NASA shuttle specifications, a typical hydrazine loadfor each APU is about 325 pounds (147 kilograms).
If the leakis actually gaseous nitrogen, there is no concern that APU 1's fuel tankpressure - currently at about 230 psi - will drop below the minimum 100 psithreshold 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 shuttleflight in 1983, in which a large hydrazine leak sprayed the toxic fuel on a hotsurface and ignited two APUs, Shannon said. One other leak early in the shuttleprogram - during approach and landing tests - actually seeped gallons of thestuff into the vehicle's aft compartment, but did not ignite, he added.
But Discovery'scurrent leak - if it is hydrazine at all - is nowhere near as pronounced asthose previous incidents.
"Right nowI don't see any change for the landing plan," Shannon said.
Discovery'sSTS-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 shuttlemission since the 2003 Columbia accident.
Engineersalso determined Friday that a thermostat glitch in the heaters for Discovery'sAPU 3 is not a major concern, Shannon said. They also suspect that vehicletelemetry suggesting a pressure increase that could damage a load-bearing bulkheadin the spacecraft's nose during a hard landing is the result of instrumentationerror rather than actual problem, but will study the matter further, he added.
Meanwhile,Discovery's six STS-121astronauts, commanded by veteran shuttle flyer StevenLindsey, worked through some issues of their own today as they preparedtheir spacecraft to undock from the International Space Station (ISS) onSaturday.
Problemswith the station's robotic arm delayed a lateinspection of Discovery's port wing by almost an hour Friday, promptingflight controllers to suggest that the task be added to the STS-121 crew'sSaturday schedule. But Lindsey turned down the idea since it would make a busyundocking day even more so.
"I thinkdoing an undock plus an entire FlightDay 2 inspection tomorrow is probably not such a good idea, and I wouldjust as soon complete the port wing today," Lindsey told flight controllers.
AstronautLee Archambault, serving as spacecraft communicator, told Lindsey that flightcontrollers were willing to pull the port wing scan - which is part of two-day inspectionto determine whether tiny meteorites have struck critical heat shield areasduring Discovery's flight - from the STS-121 crew's scheduled altogether.
"It's not ashow stopper as far as the program is concerned," Archambault told the crew. "They'rehappy to take it out of the program if they absolutely have to."
But Lindseybelieved his crew could perform the task, even though it would eat into time allottedto their sleep preparations.
"We'regoing 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 surveywas complete by 3:39 p.m. EDT (1939 GMT), leaving only the starboard wingleading edge and shuttle nose cap remaining for a closer look.
"It was atough day, but we got through it," Lindsey said. "Hopefully, we're caught up onthe timeline."
NASAwill broadcast Discovery's Saturday undocking live on NASATV beginning with a crew farewell ceremony at 3:38 a.m. EDT (0738 GMT). Youare invited to watch the activity via SPACE.com'sNASA TV feed, which is available by clicking here.
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