Astronauts endorse NASA's fuel sensor strategy
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 22, 2005
The shuttle Discovery's crew returned to the Kennedy Space Center today to prepare for blastoff Tuesday on the first post-Columbia mission. Commander Eileen Collins, making the crew's first comments on NASA plans to possibly launch the shuttle with a known fuel sensor problem, said extensive testing and analysis had given the astronauts "a lot of confidence" and "we think they've got a great plan."
Greeting reporters under a bright sun and partly cloudy sky, Collins said "we're hoping this weather holds through all of next week, whatever day we launch."
"I hope we get this nice, dry weather and with an early morning launch, we hope we can give you a good show," she said. "We're really excited about getting this launch off, we're very prepared, and we'll be talking to you from space."
Earlier concern about tropical storm Franklin has abated and the shuttle's countdown is scheduled to begin at noon Saturday for a launch attempt at 10:39:00 a.m. Tuesday. A detailed countdown timeline is posted here.
Collins and her crewmates were in the process of strapping in aboard Discovery for a launch attempt July 13 when problems with one of the four hydrogen engine cutoff - ECO - sensors in the shuttle's external tank failed to operate properly during a routine test.
The ECO senors are part of a backup system that ensures the shuttle's main engines shut down properly and don't run too long in the event of some other problem that might prevent an on-time cutoff. A main engine likely would tear itself apart if it simply ran out of gas and NASA's current launch rules call for all four ECO sensors to be operational for a countdown to proceed.
NASA and contractor engineers have been wrestling with ECO sensor issues since a fueling test in April. See the CBS News/Spaceflight Now ECO sensor page for complete details.
Since the problem with ECO sensor No. 2 cropped up last week, engineers have been working around the clock to figure out what went wrong and how to fix it. Suspicion has focused on the sensor itself, the wiring that leads to it and on a so-called point sensor box in Discovery's engine compartment that routes sensor data to the ship's flight computers. But so far, troubleshooters have found nothing wrong other than a few subtle grounding issues. They had hoped those grounding issues might explain the sensor anomaly, but they were unable to recreate the failure signature at the launch pad.
In the meantime, NASA managers ordered engineers to swap the wiring used to connect sensors 2 and 4 to the point sensor box. If sensor 4 acts up during fueling Tuesday, troubleshooters will have strong evidence the problem is, in fact, in the wiring and not a generic problem that could affect the other sensors.
Under that scenario, NASA's mission management team likely would sign an exception to the launch commit criterion calling for four-of-four sensors and allow Discovery to launch with three. For the ECO sensor system to let the engines drain the tank, all three of the other sensors would have to "fail wet," indicating the tank still had fuel when, in fact, it was empty. The odds of that happening are considered remote.
NASA's mission management team will meet Sunday to review launch preparations and to consider the three-of-four scenario on the assumption sensor No. 4 might act up. If any of the other sensors misbehave, launch will be called off.
Speaking at the shuttle runway today, Collins gave the mission management team a vote of confidence and thanked the teams of engineers who have been troubleshooting the sensor issue since last week.
"We've ... been keeping track of the events that have been taking place and the developments of the plan to test and analyze what happened to ECO sensor 2," she said. "My crew has been listening to the technical meetings and the management meetings and that's very important for us to do that, to really understand the problem.
"It's very important for us to say that we're very proud of the work the engineers and the managers and the technicians have done over the past week and a half trying to find out what's going on with this very elusive problem. And we have a lot of confidence in what they're doing and we think they have a great plan that they're going forward with."
She said the astronauts hope to blast off Tuesday but "the launch date to us isn't that important. What's important to us is that we get through this process and that we do it right."
"We have a fantastic team of people working on this, they've been putting in very, very long hours and they've been really working hard," Collins said. "They are very dedicated people, very dedicated to the space program and getting the shuttle flying again. And we're very, very proud of the work they're doing. That really comes form the bottom of our hearts."
Posted immediately below are updated launch windows through the end of the official July 31 launch period. Because the international space station's orbit was adjusted earlier to optimize the front end of Discovery's July launch period, some of the remaining launch opportunities require relatively large rendezvous rocket firings and shorter windows. The window for July 27, for example, is less than four minutes long. The optimum window on July 29 is just 11 seconds long. Mike Wetmore, director of shuttle processing at the Kennedy Space Center, said Wednesday the current plan calls for launch attempts July 26 and 27, after which the team would stand down for 48 hours to service the shuttle's electrical generators. Two more attempts could be made on July 29 and 31 if necessary, with fuel cell servicing in between.
NASA managers are debating a launch period extension July 31, but as of this writing, no such decisions have been made.Note: To optimize ascent performance and to improve margins for East Coast aborts, NASA will target the "in-plane" launch time if possible (times in EDT):
DATE.......OPEN..........IN PLANE......CLOSE.........RENDEZVOUS 07/26/05...10:33:57 AM...10:39:00 AM...10:43:56 AM...Flight Day 3 07/27/05...10:12:26 AM...10:12:26 AM...10:16:12 AM...FD-3 07/28/05...09:45:42 AM...09:50:42 AM...09:55:42 AM...FD-3 07/29/05...09:23:11 AM...09:28:11 AM...09:28:22 AM...FD-3 07/30/05...08:57:27 AM...09:02:27 AM...09:07:27 AM...FD-3 07/31/05...08:34:56 AM...08:39:56 AM...08:44:56 AM...FD-3 08/01/05...08:09:13 AM...08:14:13 AM...08:19:12 AM...FD-3 08/02/05...07:46:41 AM...07:51:41 AM...07:56:41 AM...FD-3 08/03/05...07:20:58 AM...07:25:58 AM...07:30:57 AM...FD-3 08/04/05...06:58:26 AM...07:03:26 AM...07:08:26 AM...FD-3
See the Status Center for full play-by-play coverage.