Pad technicians trying to fix hydrogen vent line leak
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: March 13, 2009;
Updated after news conference
As expected, engineers troubleshooting the hydrogen vent line leak that grounded the shuttle Discovery Wednesday were unable to detect any problems using helium at ambient temperatures, officials said today. While no obvious "smoking guns" have been found, engineers are hopeful that replacing the seals and internal components in the shuttle-vent line interface will resolve the problem and clear the way for launch.
If an obvious problem is not found during inspection of the vent line umbilical plate hardware, engineers may not know whether their repair work succeeded until Discovery's external tank is re-filled with rocket fuel.
Fueling is scheduled to begin at 10:18 a.m. Sunday for a launch attempt at 7:43:44 p.m. Shuttle weather officer Kathy Winters predicts an 80 percent chance of good weather Sunday, with a chance of low ceilings that could cause problems. The outlook drops to 70 percent "go" Monday and only 40 percent go Tuesday in the wake of an expected frontal system.
Flight planners at the Johnson Space Center in Houston, meanwhile, are continuing to evaluate a variety of options to maximize mission content if Discovery encounters any additional delays and launch slips to Monday or Tuesday. If Discovery isn't off the ground by Tuesday, the flight will be delayed to around April 7, after an upcoming Russian mission to rotate space station crew members using Soyuz ferry craft.
Already running more than a month behind schedule because of suspect hydrogen flow control valves, Discovery must undock from the station by March 25 to make way for the Soyuz mission. For a launch Sunday, the crew will have to eliminate one of four planned spacewalks to undock on time.
For a launch Monday, flight plans that include a day set aside for heat shield inspections at the station would feature additional reductions: two spacewalks if a "focused" inspection is required and the crew docks on the third flight day; one EVA if they dock on flight day four. For a Tuesday launch, only one spacewalk would be possible if a focused inspection is required.
But if the heat shield inspection is not needed - and the astronauts won't know that until the fifth or sixth day of the mission - flight planners likely would be able to add one of the lost spacewalks back into the crew's flight plan while still preserving an undocking on March 25.
For a Sunday launch, only three spacewalks are possible under any scenario and that assumes the gaseous hydrogen leak that grounded the shuttle Wednesday can be fixed.
The problem cropped up when sensors detected hydrogen leaking from a vent line attached to the side of the shuttle's external tank. A valve in the tank cycles periodically to reduce pressure when hydrogen gas builds up during the fueling process. The gas is vented overboard and routed to a "flare stack" where the excess hydrogen is burned away.
During fueling Wednesday, when the liquid hydrogen section of the tank was nearly full and the vent line was chilled to low temperatures, a leak developed at the vent line/shuttle interface. That interface, known as a ground umbilical carrier plate, or GUCP, passed leak checks at ambient temperatures and appeared to be leak free during the early stages of fueling. The problem only showed up when the tank was 98 percent full and the line was extremely cold.
"We had to wait until yesterday, about mid afternoon, before we could even get into the launch pad because of the external tank venting off and being inerted," Leinbach said. "Once we did get in, we did a very detailed measurement of the ground umbilical carrier plate where we had the leak. All those measurements turned out fine. We then went into a leak check, a standard leak check we always perform when we mate that guy up and that leak check, at ambient conditions, we saw no leaks, which is what we expected."
The ground umbilical plate is on the end of a swing arm that drops away when the shuttle takes off. After disconnecting ordnance, engineers began a detailed tear down and inspection of the suspect hardware.
"We don't have any smoking guns yet," Leinbach said. "We did see one seal that may have a slight nick on it. Not sure if that's the real cause of the issue yet. That's a first report, you've got to give us another six or eight hours before we can really declare whether that's the culprit or not.
"We are going to change out all the components in that system as we had planned to do. Whether we found any issues or not, the plan was to change out the soft goods, the seals, etc., and then get back into the re-mating of that line to the external tank and get into the launch configuration. So we are prepared, if we find nothing, to go ahead and reassemble, reconnect and get back into a launch configuration."
But engineers cannot test the vent line components under cryogenic conditions. Leak tests are carried out using helium at ambient temperatures.
"The proof of the pudding will be when we get into external tank loading," Leinbach said. "It's going to be that thermal effect on the seal, if the seal's the problem, the thermal effect at that location is what we'll be looking for.
"We're going to go through a normal loading, we're not going to change our loading sequence at all," he said. "There will be a lot of eyes on some (data) plots when we get into that topping scenario. We'll know Sunday afternoon."
Here is a countdown timeline for Sunday, starting with a two-hour "hold" prior to the start of fueling (in EDT)
TIME..........EVENT 08:18 AM......Begin 2-hour built-in hold (T-minus 6 hours) 08:28 AM......Safe-and-arm PIC test 09:18 AM......External tank ready for loading 09:41 AM......Mission management team tanking meeting 10:18 AM......Resume countdown (T-minus 6 hours) 10:18 AM......Liquid oxygen/hydrogen transfer line chilldown 10:28 AM......Main propulsion system chill down 10:28 AM......LH2 slow fill 10:58 AM......LO2 slow fill 11:03 AM......Hydrogen ECO sensors go wet 11:08 AM......LO2 fast fill 11:11 AM......Crew medical checks 11:18 AM......LH2 fast fill 01:13 PM......LH2 topping 01:18 PM......LH2 replenish 01:18 PM......LO2 replenish 01:18 PM......Begin 2-hour 30-minute built-in hold (T-minus 3 hours) 01:18 PM......Closeout crew to white room 01:18 PM......External tank in stable replenish mode 01:33 PM......Astronaut support personnel comm checks 02:03 PM......Pre-ingress switch reconfig 02:30 PM......NASA TV coverage begins 03:18 PM......Final crew weather briefing 03:23 PM......Crew suit up begins 03:48 PM......Resume countdown (T-minus 3 hours) 03:53 PM......Crew departs O&C building 04:23 PM......Crew ingress 05:13 PM......Astronaut comm checks 05:38 PM......Hatch closure 06:08 PM......White room closeout 06:28 PM......Begin 10-minute built-in hold (T-minus 20m) 06:38 PM......NASA test director countdown briefing 06:38 PM......Resume countdown (T-minus 20m) 06:39 PM......Backup flight computer to OPS 1 06:43 PM......KSC area clear to launch 06:49 PM......Begin final built-in hold (T-minus 9m) 07:19 PM......NTD launch status verification 07:34:44 PM...Resume countdown (T-minus 9m) 07:36:14 PM...Orbiter access arm retraction 07:38:44 PM...Launch window opens 07:38:44 PM...Hydraulic power system (APU) start 07:38:49 PM...Terminate LO2 replenish 07:39:44 PM...Purge sequence 4 hydraulic test 07:39:44 PM...IMUs to inertial 07:39:49 PM...Aerosurface profile 07:40:14 PM...Main engine steering test 07:40:49 PM...Oxygen tank pressurization 07:41:09 PM...Fuel cells to internal reactants 07:41:14 PM...Clear caution-and-warning memory 07:41:44 PM...Crew closes visors 07:41:47 PM...Hydrogen tank pressurization 07:42:54 PM...Booster joint heater deactivation 07:43:13 PM...Shuttle GPCs take control of countdown 07:43:23 PM...Booster steering test 07:43:37 PM...Main engine start (T-6.6 seconds) 07:43:44 PM...Booster ignition (LAUNCH)