New fuel tank design linked to shuttle disaster

New Scientist ^ | 4-16-03 | Jeff Hecht

[Prince Charles]

New fuel tank design linked to shuttle disaster

 

13:20 16 April 03

 

NewScientist.com news service

 

The Columbia shuttle disaster may been triggered by a combination of a new external fuel tank design and ageing of the spacecraft, a Columbia Accident Investigation Board press conference was told on Tuesday.

Although some remaining alternatives remain to be ruled out, the CAIB's focus is now on two crucial factors. First, it appears that a new fuel tank design led to foam falling off during liftoff. A one-kilogram block was seen striking Columbia's left wing during launch.

Secondly, the damage this impact caused may have been made worse by an unexpectedly high degree of wear on wing components. The loss of a key wing component is thought to have allowed hot plasma to enter and destroy the shuttle's wing during re-entry on 1 February.

CAIB member Scott Hubbard did add a note of caution: "Currently the data are not all pointing in the same direction." But CAIB hopes that a series of new tests on shuttle spare parts and models will resolve the questions that remain.

Shedding events

NASA's problems with foam only began recently, when it shifted to the new "ultra lightweight" external tank used in the Columbia launch, said CAIB member Steve Turcotte.

Before the change, the foam had "very few stringer cracks", up to 15 centimetres long, he said. "We don't know if this caused the shedding events," Turcotte said, but the tanks had not shed foam in the 10 years before the new design was introduced.

New analysis of the launch film, also presented on Tuesday, showed the foam hit the leading edge of the wing about half a metre further out than had been thought, on panels 7 and 8.

Analysis of recovered debris shows severe heat damage and erosion to the reinforced carbon composite of panels 8 and 9. That could indicate that the T-shaped seal between the two panels failed after being hit by the foam, Hubbard said. Only two small bolts hold it in place.

Radar images from the second day of Columbia's mission show an object falling off near this part of the wing. If the T-seal broke off, it would have left a gap about three centimetres wide for hot plasma to enter the wing on re-entry, CAIB chairman Hal Gehman said.

Cracks and pinholes

Perhaps most ominously for NASA, CAIB reported finding more wear than expected to the reinforced carbon composite on the remaining shuttles. Zinc contaminants from the shuttle launch platform have oxidised pinholes in the wing edges. Expansion and contraction are also causing tiny cracks.

Gehman says CAIB will urge NASA to replace current visual tests with thorough nondestructive testing of hidden as well as exposed structures before any future shuttle launches.

However, NASA Administrator Sean O'Keefe recently told Associated Press: "There is no technology right now to do effective nondestructive testing."

Finally, CAIB announced that it expects to end its search for shuttle debris in east Texas by the end of April. About 70,000 pieces of debris have already been found - 36 percent of the orbiter's dry mass. However, investigators will continue hunting further to the west for pieces that fell from the shuttle earlier in its re-entry.

[Regulator]

Always have to be careful when you're changing things in Engineering...

"There is no technology right now to do effective nondestructive testing."

Say what, Sean? It's a whole big industry. Maybe y'all could, like, qualify that statement.

[Prince Charles]

Bingo. When the Clinton NASA folks decided to go "green" with a new foam, they introduced a new failure mode, apparently without even bothering to test. Absolute incompetence or negligence is the only characterization that's appropriate.

[TheBattman]

Ping....Thought you would appreciate.

[Windcatcher]

I know the ULET has different foam, but I didn't think it was the primary factor that made it lightweight. The ultralightweight tank is made by Lockheed, and when it first came out, I was working at a (recent) Lockheed spinoff at the time. The company newsletter said that the weight reduction came from composite construction (the normal tank is made of aluminum alloy). The primary purpose of the tank was to enable shuttles to ferry cargo to the ISS. Normally, this meant the newer shuttles like Atlantis and Endeavour, which were built with newer designs and/or techniques, and weigh less than Columbia. Nevertheless, different (lighter) foam could have been an issue. Another possibility is that, since the materials used in the ULET are so different, perhaps it demanded a different adhesive between foam and tank. Or, even if the adhesive was the same, perhaps the thermal expansion properties of the foam and/or tank (or adhesive, if it was different) could have been different enough to cause separation. Just some speculation on my part.

From what I recall, Columbia has only visited the ISS once, after an overhaul in which engineers managed to shave some weight off the orbiter. I didn't think they had used the ULET for its final mission, however, since it was just a normal science mission, not an ISS mission, and the ULET costs more than the ET. The ULET also looks a bit different; the ET has the normal orange foam, but the foam on the ULET is more tan in color. I don't suppose anyone can verify which tank was used, or is the article verification in itself?

[BenLurkin]

Sad.

Also typical of the Clintoon era.

[Ernest_at_the_Beach]

New motto!

Going Green can kill!

[Prince Charles]

I'm not positive but I think they did use the lightweight tank since this was still the heaviest re-entry to date (more than 250,000 pounds I think).

[Windcatcher]

Here's a decidedly off-the-wall thought: in the very beginning (STS-1 to STS-4 or so) the tank was painted white. The paint was eventually scrapped because it added an additional 600 lbs. With our luck, we'll discover that some NASA engineer back in '75 or so figured out that a cold-resistant rubbery latex (or something similar) would provide just enough tension to ensure that the foam stayed on.

[polemikos]

Absolute incompetence or negligence is the only characterization that's appropriate.

Actually, the technical term is "to Clintonize."

[VRWC For Truth]

In other words, we don't want to spend money on safety. It's not cost effective. Non-destructive testing is effective but it will increase cost. You can chrome plate a turd, but it's still a turd.

[NormsRevenge]

Ping

[MediaMole]

STS-107 used one of the older "lightweight" tanks, not the newer "ultra lightweight" tanks.

[XBob]

thanks for the ping. Xlinton strikes again.

[RIGHT IN SEATTLE]

leave the exploration to the stupid BMW dummies!

[bonesmccoy]

Thanks for the ping.

While it is gratifying to see that the CAIB is adopting a testing protocol similar to that which I proposed here just a few days after the disaster; the story leaves me feeling heavy-hearted.

Obviously, the bird would not have flown if the engineers really understood what was occuring. I suppose this is the risk we take for spaceflight.

We lost more people on the orbiter Columbia than we have on any particular day in the battlefields of Iraq.

One fights for the right scientific knowledge.

Another fights for the right to gain that knowledge.

It is hard to accept the price we pay for progress.

Yet, our nation presses forward with optimism and hope.

If O'Keefe does not believe that non-destructible testing is technically feesible, the birds need to be grounded and we need to build a new manned space vehicle with modern technology.

[brityank]

Thanks for the ping, Norm.

... a new fuel tank design ...

IIRC, there were three types of External Tank. The original ET, a Light Weight ET (LWET), and the Ultralight ET (ULET). The original ET has not been used since the early flights through about 1993, and there were two LWETs left -- one used on STS-107 Columbia, and the last one is being used by the CAIB for the investigation. The rest are the ULETs. The changes were made to increase the payload to orbit.

[brityank]

See my note at #17; most of the info and links to Michaud are in bonesmccoy's thread.

[brityank]

... O'Keefe does not believe that non-destructible testing ...

There are myriads of companies large and small that have grown up in the ND field, yet it still isn't widely known. It may be that his comment reflected his impression that there may not be a current outfit with the background and facilities to do such on as large a unit as the ET or shuttle wing, but I'm sure somewhere someone would handle it. What's that old saw; "Money talks, bullshit walks!" -- seems NASA is adept at doing both.

[snopercod]

Thanks for the ping. The New Scientist is displaying it's usual ignorance. As several have stated, STS-107 used the Lightweight external tank, not the SLWT.

Also, what makes the SLWT tank lighter is that it is fabricated out of Aluminum-Lithium alloy, not straight Aluminum alloy.

Originally, the Al-Li alloy was imported from Russia, since no American firms could make it, supposedly. Actually, that was just another hidden welfare payment to Russia...