NASA Confident Potential Leak Won’t Hinder Shuttle Landing

This diagram depicts Discovery's auxiliary power unit (APU) system, with two areas currently under study highlighted. CREDIT: NASA.

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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