"We will come back in late November and give it another shot," said Launch Director Michael Leinbach. "I'm disappointed for the team today, for sure. But as we always say, and it's absolutely the truth, we're going to fly when we're ready and clearly we were not ready to fly today. So we'll come back another day and try it again."

If the problem, which originated in the Gaseous Umbilical Connector Plate (GUCP), and if a crack that was discovered in the external tank foam insulation can be fixed in time, Discovery could potentially be targeted for launch at 4:05:46 a.m. EST - adding an unexpected final night launch to the shuttle manifest.

The leak, described as "significant" appeared while the launch team was loading the external tank with supercold liquid hydrogen in preparation for today's scheduled launch. Initially detected at a rate of 35,000 parts per million (3.5% concentration in ambient atmosphere), just below the requirement of a maximum 4 percent, it rapidly increased to over 60,000 parts per million.

The potentially dangerous leak was actually higher than that, possibly much higher, since the sensors top out at 60,000 ppm and the leak rate was still increasing when the sensors went off-scale.

Similar leaks in the GUCP delayed two missions three times in 2009, STS-119 and STS-127.

However, shuttle Launch Director Mike Leinbach said this leak appeared to be different than the previous ones. The leak today developed sooner than the previous ones, while the liquid hydrogen tank was still being loaded, and was much greater. Leinback expressed hope that the different characteristics will mean there's something obviously wrong with the connection at the GUCP that can be readily repaired.

The GUCP is a plate at the end of the liquid hydrogen vent line arm that butts up against the side of the external tank at the hydrogen vent port. While the tank is loaded with fuel, liquid hydrogen constantly boils off and has to be vented. Due to the extremely flammable nature of hydrogen, it isn't simply vented into the open atmosphere as the liquid oxygen is. Instead, it is captured by the vent line and routed down the Fixed Service Structure gantry, down the concrete launch pad and to a flare stack at the perimeter of the launch complex where it is safely burned off.


(Photo: Hydrogen vent arm and gaseous umbilical connector plate. Credit: NASA)

Engineers tried to correct the leak by cycling the vent valve open and closed several times in the hopes of forcing the vent line to make a good seal. When those efforts were unsuccessful, engineers continued monitoring the fueling process while gathering data.

Several minutes later, Leinbach officially called a scrub and the time-consuming process of detanking the shuttle began.

The hydrogen leak is particularly puzzling after extensive analysis and changes that were made in the wake of the three previous incidents. Engineers determined that those leaks were caused by a slight misalignment of the GUCP and external tank fitting.

To correct that, they changed the seal from Teflon to a more flexible material that would seat better. They also installed shims on the plate to provide additional force to keep a tight fit with the tank and counteract the misalignment.

Meanwhile, workers inspecting the external tank discovered a large, 7-inch long crack in the orange foam insulation near the ribbed intertank region.

The damage is located near the forward bipod attach fitting between the orbiter and external tank. Even had the hydrogen leak not developed, the foam damage could have potentially stopped the launch because it's location is on the side of the tank facing Discovery's delicate thermal protection tiles.


(Photo: Crack in foam insulation of the external tank. Credit: NASA)

A chunk of foam breaking off the external tank during the launch of space shuttle Columbia on the STS-107 mission in January 2003 punctured a hole in its left wing and led to the destruction of the orbiter on re-entry and the loss of its crew of seven astronauts.

It's still unknown what caused the crack or how, or even if, it will be repaired. The location is very difficult to reach on the launch pad and may require the shuttle to be rolled back to the Vehicle Assembly Building for repairs.

All that is up in the air, however, until technicians can gain access to the launch pad. After Discovery's external tank was drained of propellants, it has to outgas and residuals left inside. The fill and drain valve is located near the 5% full point in the hydrogen and oxygen tanks and so the remaining few percent is left to boil off naturally, a process that takes almost 24 hours. Until that's complete, workers are not permitted to access the launch pad.

They should finally be allowed back to the pad sometime Saturday afternoon and will begin work immediately to determine the causes of both problems and what corrective action is required.

Initially, the Mission Management Team considered options for a quick repair of the leak and a final launch attempt on Monday, the last day of the current launch window.

That option was quickly taken off the table since workers won't get their hands on the hardware until Saturday and it's still to be determined exactly what has to be done in terms of repair work. In past instances, it took at least four days to turn the shuttle around and set up for the next launch attempt.

With the timeline extremely tight, Mike Moses, chairman of the launch Mission Management Team, decided that would be too risky and put too much time pressure on workers and decided to forego anymore launch attempts until the next window opens on November 30.

"We have a lot to do before we settle in on a new launch date, and we've said this before, we're going to make sure we fix the problems and worry about the launch later. Nov. 30th is the first time the window opens, and we'll get the results from the problems and some of the other problems we've been having all along," said Moses.

Leinback stressed that one of his primary concerns is putting the launch team under too much pressure or rushing any fixes for the sake of meeting an arbitrary launch window.

"We want to do the right thing for this vehicle," said Mike Leinbach, the shuttle launch director. "We were presenting an option to the program that might have gotten to Monday, but we all know in our hearts the right thing to do is to take this slowly, understand this issue and go fix it."

Discovery's six astronauts, commander Steven Lindsey, pilot Eric Boe, and mission specialists Michael Barratt, Nicole Stott, Timothy Kopra Alvin Drew, flew back to Houston this afternoon to await a new launch date for what will be space shuttle Discovery's final mission before it, and the other two orbiters, are retired from service.

The next launch window extends from November 30 until December 5. Even if, and it's still a big "if", the repair work can be completed in time, managers might opt to wait until the next window opens in late February - currently the window slated for Endeavour to make its final flight.

At issue is the workload the combined shuttle and station crews would face with a launch this year. While the station is currently occupied by six people, three of those are scheduled to return to Earth in several weeks. If Discovery lifts off during the upcoming window, it would be docked to a space station staffed by only three people. As a result, some of the experiments and tasks slated for STS-133 would have to be deferred or cancelled altogether.

Complicating matters, NASA is gearing up to officially manifest one more flight for Atlantis next June, the final mission in the space shuttle program. While congress has authorized the mission, no funds have been appropriated for it. Delays to STS-133 and STS-134 make it more likely that congress will change its mind and decline to fund the additional mission, especially if the delays result in the mission slipping into Fiscal Year 2012 which begins on October 1, 2011.

(The Spacearium / Zero-G News)