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Space shuttle main engine nozzle leaks being studied
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: September 4, 2009


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Engineers are looking into what might have caused more than 300 microscopic leaks in the nozzle of a space shuttle main engine used to help launch Endeavour in July. But officials Friday characterized the issue as relatively minor, saying the leaks were well below any threshold that could cause an in-flight problem.


One of Endeavour's main engines is seen here after removal. Credit: NASA-KSC
 
The issue was discovered during post-landing testing Aug. 27 in which helium gas was pumped through the 1,080 coolant tubes that make up the engine nozzle. Hydrogen is routed through the tubes during engine operation to keep the metal from overheating.

Jerry Cook, manager of the main engine project at the Marshall Space Flight Center in Huntsville, Ala., said engineers found some 340 tiny leaks near the top of engine No. 2045's nozzle. Engine No. 2045 was used as Endeavour's center engine, mounted directly under the ship's vertical stabilizer.

"They are microscopic," Cook said in a telephone interview. "As a matter of fact, the total leak rate on the nozzle from all these leaks is so small that it's not even the equivalent of one ruptured tube. They are tiny. The total leak rate in the 340 hot wall leaks that we have is 0.6592 pounds per second. So it's not even equivalent of one tube rupture, nothing you could see in the ascent performance going up hill at all."

Even so, the sheer number of leaks "is definitely out of the ordinary," he said. "We've got a lot of theories, but honestly, we don't know exactly what caused it. We've got a team down there now doing an investigation."

One possible contributor is corrosion caused by lengthy exposure to the salty environment at the shuttle's sea-side launch pad.

"We're not sure if that's what caused it," Cook said. "We went in and cleaned them off prior to launch, went in and did a leak check, no problems there. So these are small, microscopic holes we see. We've mapped the tubes and we'll see what kind of repair we can do on them."

In the meantime, while the investigation continues, nozzle No. 2031 will be taken out of service. Cook said repairs near the top of the nozzle, near the main engine combustion chamber, can be difficult.

Depending on how the investigation plays out, "we could potentially lose a nozzle from the inventory," Cook said. With just six shuttle flights remaining before the fleet is retired late next year, a lost nozzle could force NASA to shuffle nozzles from engine to engine, but it would "not put us in a situation where we cannot support the manifest."

At the Johnson Space Center in Houston, meanwhile, space station Flight Director Ron Spencer provided a bit more insight into how NASA flight controllers assess the threat posed by orbital debris, and why the International Space Station's orbit was not repositioned even though a large piece of rocket debris passed within eight tenths of a mile earlier Friday.

The debris, from an Ariane 5 rocket launched in 2006, is in a highly elliptical orbit. Radar tracking earlier this week showed it would pass close by the space station around 11:07 a.m. EDT. Flight controllers initially looked into raising the station's orbit but eventually ruled out any such move.

Spencer is the flight director who wrote the rules governing debris avoidance maneuvers, or DAMs.

"The way we do all of this is we get tracking data from Space Command on the objects that are a threat to the space station," he said. "And some of those objects we can track very well, some of them we can't track as well. This one we could track very well, so even though the miss distance was only 1.3 kilometers, we were very confident in our tracking of it so we knew it was going to be a near miss without a threat of collision.

"We actually calculate a probability of collision for all of these items, which factors in the uncertainty we have as to where this object is going to be. Since this object was so well tracked, we calculated a probability of collision of zero. So we knew we were OK."

NASA monitors an imaginary volume around the space station roughly the shape of a pizza box measuring 0.466 miles thick and 15.5 miles square.

"Initially, we have a screening box, which is .75 kilometers radial miss, which would be up or down, by 25 kilometers in cross track, which would be left or right, by 25 kilometers down track, which is either in front or behind us," Spencer said.

"Space Command will alert us of any debris objects out there that are going to get that close to us. Then they increase tasking on those objects to try to get a better solution and decrease the uncertainty. Then we calculate a probability of collision based on the data Space Command gives us. If the probability of collision is greater than 10 to the minus five (1-in-100,000), then we will begin to start looking at taking action."

For the debris encountered Friday, no action was required.

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