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Subject: Discovery Launch Issues-follow up

The "intermittent" sensor problem that forced a postponement of the space shuttle Discovery's launch was found to be a little less intermittent overnight, NASA reported Thursday.

Engineers were able to reproduce the problem late Wednesday after draining the propellants out of the shuttle's external fuel tank, NASA spokesman Bruce Buckingham said. One of the sensors initially provided bad readings, just as it did during the countdown, he said.

Buckingham said thaT the recurrence of the problem vindicated the decision to delay launch. However, the test by itself could not determine whether the source of the problem was a balky sensor in the tank, or bad wiring or faulty electronics in the shuttle orbiter.

The results of Wednesday night's test, along with other data, should help mission managers decide whether the shuttle could be launched in a day or two - or not until weeks from now.

Problems with the fuel-tank sensor system have bedeviled NASA for months:
They first popped up while testing a different fuel tank back in April, but engineers could not reproduce the glitch or track down its source. Instead, they replaced components of the system with approved spares. The system checked out OK in May, and Discovery was mated to a newer, heater-equipped fuel tank in June.

Even though another full-scale tanking test was not conducted, managers felt reassured that the problem had gone away.

After Wednesday's launch scrub, NASA Administrator Mike Griffin said the off-and-on nature of the problem was frustrating. "This is an intermittent anomaly, and the hardest of all electronic problems is an intermittent one,"
he told reporters.

A matter of days or weeks?


Discovery's mission management team is still meeting to sift through the data from a variety of tests. At an afternoon news briefing, managers are expected to provide more information about the nature of the sensor glitch - and about the schedule for NASA's first shuttle mission since the catastrophic loss of Columbia more than two years ago.

If the problem can be fixed quickly, launch could be rescheduled for as early as Saturday - a prospect that looks increasingly unlikely. Even if more extensive troubleshooting is required, NASA might be able to fix things in time for launch later this month. But if the problem lies in the fuel tank rather than the shuttle's circuitry, Discovery might have to be moved back into NASA's Vehicle Assembly Building for more extensive work. That could delay the launch until September.

The glitch involved a low-level sensor that appeared to show that the tank still had plenty of liquid-hydrogen fuel - even when the system was intentionally reset to make the tank seem empty. In NASA parlance, the sensor registered "wet" when it should have shown "dry."

The low-level sensors are supposed to signal the fuel system to get ready for shutting down the shuttle's main engines if propellant levels get too low. For that reason, they're also known as engine-cutoff sensors or ECO sensors.

The liquid-hydrogen compartment in the fuel tank has four such sensors, and all four of them have to be working in order to clear the shuttle for launch, said deputy shuttle program manager Wayne Hale. During launch, two of the sensors can go out without posing a hazard, he said.

If the sensor system fails, the shuttle's propulsion system might not be able to adjust the fuel system correctly during its ascent to orbit. A false "dry" reading could lead the system to shut down the engines too soon, while a false "wet" reading could keep the engines running when they should be in shutdown mode.

Late Wednesday, when the tank was drained, three of the sensors correctly indicated that the tank was dry. But engineers found that the troublesome sensor indicated "wet," Buckingham said. They ran through a series of commands, but the sensor still gave faulty readings.

Then the testing team waited five minutes. When they ran through the commands again, the sensor started operating normally, Buckingham said. But there was no indication as to what resolved the situation - underlining the intermittent nature of the problem.

Mission focuses on safety
Hale told reporters on Wednesday that the fuel-tank concerns illustrated that NASA was taking safety seriously in the wake of the Columbia tragedy, in which seven astronauts were killed. NASA grounded the shuttle fleet after that accident and made efforts to reform its "safety culture."

"We built a very tight safety net to make sure we don't fly until it's safe," Hale said.

Discovery's 12-day mission is aimed at testing safety measures developed in the past two years - including the addition of cameras to monitor the launch, changes in the design of the fuel tank and the orbiter, and the use of a new 50-foot (15-meter) boom to inspect critical parts of the shuttle's protective skin while in orbit.

The shuttle is also due to take tons of supplies up to the international space station, and bring tons of equipment and trash back down.

While docked to the station, Discovery's spacewalkers are to test some of the tools that have been developed to repair minor damage in the shuttle's tiles or reinforce panels. During a second spacewalk, they will replace a balky guidance gyroscope on the station - and a storage platform will be installed on the station's exterior during a third outing.

Discovery's commander, Eileen Collins, has said that she and her crew are planning a tribute to Columbia and its crew, but she has held back the details to build suspense.

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Posts: 2845 | Registered: Oct 2003  |  IP: Logged | 

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So many to choose from: http://en.wikipedia.org/wiki/Starship_class

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Posts: 8473 | Registered: Apr 2003  |  IP: Logged | 

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It’s not uncommon for the SRB separation motors to blast bits (big and small) of the ET insulation (the orange stuff) off the tank. At that altitude with near zero air it’s a straight ballistic fall and can not come near the orbiter (this has been seen lots of times in the past). The problem with Columbia was it happen (the insulation came off) in dense air and got caught up and accelerated in the turbulent air flow between the ET and the orbiter. So no worries, but NASA will put on a good show (as they should). For 3 billion dollars you get the full show… :-)

The cameras work just as they should. If a strike had happened to the RCC or if a lot of tile was zapped they would have done a TAL to Spain or France right then.

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Posts: 2845 | Registered: Oct 2003  |  IP: Logged | 

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The shuttle program is being grounded until engineers determine the severity of the impact of debris that fell from the shuttle Discovery during blastoff Tuesday, NASA says. More soon.

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Posts: 2845 | Registered: Oct 2003  |  IP: Logged |