NASA hopes to make another attempt at launching space shuttle Discovery as early as Sunday after problems with fueling equipment at the launch pad forced a scrub of the countdown seven hours before tonight's scheduled launch. However, before NASA can make another launch attempt, technicians will have to repair or replace a balky vent valve that allowed flammable hydrogen gas to escape into the atmosphere around Discovery today, creating a potentially explosive situation at the pad.
READ: GASEOUS HYDROGEN VENT LINE ARM FACT SHEET VIDEO: POST-SCRUB LAUNCH STATUS BRIEFING VIDEO: CAMERAS CAPTURE CLOSEUP VIEWS OF GH2 VENT ARM VIDEO: TIMELAPSE VIDEO OF RSS RETRACTION EARLY THIS MORNING VIDEO: TIMELAPSE VIDEO OF THE SUN SETTING BEHIND DISCOVERY ALL-ACCESS SUBSCRIBERS: REAL-TIME VIDEO OF RSS RETRACTION THIS MORNING - 1200KBPS ALL-ACCESS SUBSCRIBERS: THE SUN SETS AT PAD 39-A - 1200KBPS "we're really trying hard to make the 15th if we can. The 16th might just be the day that's out there," Space shuttle Integration Manager and Chairman of the Mission Management Team Mike Moses told reporters at a news conference this evening. The leak is located at the end of the gaseous hydrogen vent arm in the ground umbilical connector plate (GUCP). This is where the hydrogen vent line attaches to the external tank and is comprised of a valve and two metal seals, one on the outside of the tank and one on the vent arm. The hydrogen vent arm extends from the fixed service structure to the external tank, attaching to a port on the tank located behind the left solid rocket booster. Explosive bolts fire at liftoff to separate the arm from the tank, which then falls down and back toward the FSS where it is arrested by a catch net. As the external tank is filled with supercold liquid hydrogen, the heat from the tank (at ambient temperature) causes the hydrogen to slowly warm up and some of it evaporates. The gaseous hydrogen builds up inside the tank to pressurize it. However, if too much liquid hydrogen evaporates, the pressure would increase to the point the tank would rupture. To prevent this from occurring, excess hydrogen is released through a venting system. Because of its highly flammable nature, gaseous hydrogen can't be simply dumped into the atmosphere. Instead, it is routed through the vent arm, down the FSS and away from the launch pad where it is burned off at the hydrogen flare stack. Everything had appeared nearly perfect for launch today before the hydrogen leak occurred. Even Florida's notoriously unpredictable weather was idea, with the forecast giving a 95% probability of acceptable conditions at launch time. Fueling began on schedule at 11:56 a.m. EST. As the hydrogen tank was being filled, operators in launch control opened the vent valve to release excess gaseous hydrogen. That was when consoles in the firing room showed an off-nominal pressure profile in the tank indicative of a small leak in the vent valve. 
(Images above: A closeup of the gaseousygen vent valve and vent valve arm location. Phot Credit: NASA) During de-tanking, the valve was cycled again to no avail, confirming that the problem is indeed a hardware failure in the system. The current plan is for technicians to go into the area once the tank has been safed, inspect the valve for any obvious signs of damage and then remove and replace the valve and associated seals. The entire process takes several days, so the earliest NASA could launch Discovery will be Sunday evening. First, the hydrogen tank must be "inerted". Even though offloading the liquid hydrogen from the tank can be completed in a few hours, some liquid remains pooled at the bottom of the tank below the 5% fill level. The residual fuel is allowed to boil off and be removed through the gaseous hydrogen vent valve. This process takes 18-20 hours to complete. Once that operation is finished, workers will be able to access the area again, sometime Wednesday afternoon. Replacing the valve assembly is relatively straightforward and should take about two days to complete. NASA has spare hardware in storage at Kennedy Space Center which technicians will swap with the failed system. On Saturday morning, the vent arm and vale will be re-mated to the external tank, followed by a two-stage torque process to tighten the seals. After the valve is mated to the tank, the seals are pressed together with a torque device. Then they are allowed to settle before torque is reapplied to tighten the connection even more. One final torque is applied after another pause. The torque procedure normally takes 30 hours. However, NASA has tested an alternative method that will complete the task in 15 hours instead and achieve a sealed connection of the same quality. Assuming the valve replacement goes well, NASA hopes to be in a good posture to pick up the countdown early Sunday morning at the T-11 hour mark, leading to a liftoff at 7:43 p.m. EST. The flight of STS-119 will have to be shortened because of the launch delay. The Russian Space Agency is in final preparations for the launch of Expedition 19 on board a Soyuz rocket on March 26. ISS rules prohibit the docking of a shuttle to the station while the Soyuz is there during a swap of resident crews because of crew workload issues during the handover process and the ability of the station to support 13 people on-board for an extended period of time. Since the Soyuz is the primary means by which crews are rotated to and from the space station, program officials are against delaying the launch. That means Discovery will have to leave the station before the Soyuz arrives. Launching on the 15th will result in a mission shortened to 11 days and only 3 spacewalks. If the launch takes place on the 16th, then the mission will have to be shortened to 10 days and only 1 or 2 EVA's. The 17th is the last day the shuttle can launch before having to stand down for the Soyuz. A launch on March 17 would result in a short 9-day mission with a single EVA to support the installation of the S6 truss. Under each of those scenarios, the work that was to take place on the cancelled spacewalks will be done by the Expedition 19 crew after the Expedition 18/19 handover. If NASA is unable to get Discovery off the ground before the 17th, the earliest the mission can launch is in the April 7-13 timeframe, after the departure of the Soyuz spacecraft. (The Spacearium / Space Media Corporation)
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