Author Topic: SLS Core Stage team recovering from consequences of weld pin change  (Read 8160 times)

Offline Chris Bergin

https://www.nasaspaceflight.com/2017/05/sls-core-stage-recovering-weld-pin-change/

- By Philip Sloss -

This is a pretty big deal. Philip - as like the site's editorial tone - didn't go dramatic with it and let the quotes do the talking, but we're waiting to follow this up per the decision they'll make.

The potential of taking the "bad" tank out to the parking lot and pressurizing it to see if it doesn't go pop something you don't expect to hear.

Offline eric z

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 Thanks NSF for the fantastic article, for once on a technical rather than a policy issue. I may have to go back to school to understand it, but I would like to point out "Now's the time" to root out these issues and the fact the team there is doing just that gives me a great feeling. Good luck to them; and, once again, great reporting!

Offline whitelancer64

Interesting article - good to know they are already working on parts for Core Stage 2, at least they have those parts to fall back on with the original welding technique, if that is determined to be necessary.
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Offline RonM

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Great article.

Why all the new tooling and techniques to build the core stage? Couldn't they have used the older Shuttle tank tooling? SLS is supposed to be a Shuttle derived vehicle. Would have save a lot of time and money.

Offline Jimmy_C

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This is one of my favorite articles on NSF. It explains how small engineering decisions may have large unforeseen consequences. It is disappointing to have yet another delay to SLS. But the article itself is very well written and quite informative. Well done.
« Last Edit: 05/08/2017 10:04 PM by Jimmy_C »

Offline Coastal Ron

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I echo others when I say this is a terrific and informative article. Two specific issues stood out for me:

"In the case of the Core Stage structures, the technology needed to weld the thickness of the material for the two propellant tanks is pushing the state of the art.  “There’s going to be a lot of PhD’s coming out of this, in the weld arena,” Doering noted.  Both the Core Stage LH2 and LOX propellant tanks are thicker welds than anyone is currently doing."

And:

"“We were finding random areas in which the strength of the weld is below the design requirement,” Doering said.  “And it is not predictable and it’s not something you can find through any kind of NDE, non-destructive evaluation technique.  When you dissect the weld itself and do the metallography, the materials analysis on it, you can see what the issue is – it creates a little brittle layer at the very top which you rely a lot for your strength on elongation of the material."

Depending on doing something "state of the art" is not inherently bad, since that's how "state of the art" is moved from the possible to the practical.  But it's not without risks, and that's what we're seeing here.

And if this risk does continue to cause schedule and safety concerns then it can be directly pinned on the Senators that originally spec'd out what the SLS had to do.  Because Congress did not ask NASA or industry for an evaluation of their spec's, Congress was assuming that the Ares V concept could be de-scoped down the Ares IV/SLS - even though the Ares V was never fully thought out when Michael Griffin mandated the design in 2005.

We've been seeing similar challenges playing out with the Orion for the past decade because it too is pushing the "state of the art" for how big capsules can be built and provide a safe ride throughout the entire operating envelope.  And the Orion is also the brainchild of Michael Griffin, so the problems both of these programs are having can directly pinned to assumptions he made.

It will be interesting to see if Boeing can find the root cause for the random weld failures, but if they are finding the limits to friction stir welding then maybe SpaceX is right in moving over to carbon fiber construction for their upcoming Interplanetary Transport System (ITS) - especially since SpaceX won't have to deal with LH2 issues.
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Offline Coastal Ron

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Great article.

Why all the new tooling and techniques to build the core stage? Couldn't they have used the older Shuttle tank tooling? SLS is supposed to be a Shuttle derived vehicle. Would have save a lot of time and money.

No, the SLS is not "Shuttle derived".  They share the same diameter, and they both use the same type of solid-rocket boosters, but otherwise the load paths and other considerations require completely different designs for the tank/stage assemblies, and they require different manufacturing methods.

Kind of like comparing a Boeing 737 to a Boeing 777 - they look similar, but they are built completely different in how they are built.
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Offline Lars-J

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Great article.

Why all the new tooling and techniques to build the core stage? Couldn't they have used the older Shuttle tank tooling? SLS is supposed to be a Shuttle derived vehicle. Would have save a lot of time and money.

No, the SLS is not "Shuttle derived".  They share the same diameter, and they both use the same type of solid-rocket boosters, but otherwise the load paths and other considerations require completely different designs for the tank/stage assemblies, and they require different manufacturing methods.

Kind of like comparing a Boeing 737 to a Boeing 777 - they look similar, but they are built completely different in how they are built.

The proper and more accurate way to phrase it is "Shuttle contractor derived".

Offline Kansan52

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Wonderful article. It really explained that it wasn't corner cutting but cutting edge. Maybe 'weding edge'?

Offline RonM

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Great article.

Why all the new tooling and techniques to build the core stage? Couldn't they have used the older Shuttle tank tooling? SLS is supposed to be a Shuttle derived vehicle. Would have save a lot of time and money.

No, the SLS is not "Shuttle derived".  They share the same diameter, and they both use the same type of solid-rocket boosters, but otherwise the load paths and other considerations require completely different designs for the tank/stage assemblies, and they require different manufacturing methods.

Kind of like comparing a Boeing 737 to a Boeing 777 - they look similar, but they are built completely different in how they are built.

The proper and more accurate way to phrase it is "Shuttle contractor derived".

And there's the whole problem. Instead of something practical like the Direct Jupiter rocket that would have used modified Shuttle tanks, Congress served up a huge serving of pork to the contractors. We end up with the contractors using bleeding edge technology (most expensive) to build a rocket based on 40 year old engineering.

The history of STS replacement going back to the Clinton administration through today; X-33, Ares V, and SLS. Unless Boeing and friends can get their act together we're looking at another failed program. It would be funny except for the waste of billions of dollars.

Offline Coastal Ron

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The proper and more accurate way to phrase it is "Shuttle contractor derived".

And there's the whole problem. Instead of something practical like the Direct Jupiter rocket that would have used modified Shuttle tanks, Congress served up a huge serving of pork to the contractors. We end up with the contractors using bleeding edge technology (most expensive) to build a rocket based on 40 year old engineering.

I originally got active on space blogs because I thought DIRECT sounded like a good idea, but as I ultimately learned replacing one expensive government transportation with another would not have been in our best interests since it would have suppressed private sector solutions like what we're seeing from SpaceX and Blue Origin.

Plus there was nothing for a government space transportation system to do after the completion of the ISS - and still isn't much to do more than a decade later with the SLS.

Quote
The history of STS replacement going back to the Clinton administration through today; X-33, Ares V, and SLS. Unless Boeing and friends can get their act together we're looking at another failed program. It would be funny except for the waste of billions of dollars.

There are situations where it makes sense for the U.S. Government to do something on it's own, but for transporting cargo and crew to space U.S. industry has proven that it is ready to take over that task - now it's just a matter of our politicians allowing that to happen...
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Offline Rocket Science

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Just a note on this joyride: recall that we were going from the ET 8.4m, then CxP proposed 10m, the SLS went with 8.4m for cost cutting and other efficiencies I'm sure...
"The laws of physics are unforgiving"
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Offline eric z

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 Could this issue have a bearing on whether to go-for-broke on the first mission? Wonder how this type of news gets processed in the astronaut office?

Offline TomH

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Very informative article. I do think Doering could have made a better analogy by comparing the stirring pin to a bit in a router rather than the bit in a drill.

Offline ChrisWilson68

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It's disturbing that they think it's OK to go ahead and use one of the bad tanks for structural testing, because it means that what they're testing is not the same as what they'll be flying.

We already know that switching from pin design 1 to pin design 2 had some unintended consequences that aren't understood.  We know that using pin design 2 gives certain kinds of weaknesses, and nobody knows why.  So how can we be sure pin design 1 doesn't have different kinds of weaknesses that pin design 2 does not?  By only doing structural testing on the tank built with pin design 2, they'll be missing any unexpected weaknesses in tanks made with pin design 1 that structural testing would have caught.  But they plan to fly the tanks built with pin design 1, which will never have gone through structural testing.

The only safe thing to do is throw away both of the first two H2 tanks and use the next two for structural testing and flight.

Offline catdlr

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It's disturbing that they think it's OK to go ahead and use one of the bad tanks for structural testing, because it means that what they're testing is not the same as what they'll be flying.

We already know that switching from pin design 1 to pin design 2 had some unintended consequences that aren't understood.  We know that using pin design 2 gives certain kinds of weaknesses, and nobody knows why.  So how can we be sure pin design 1 doesn't have different kinds of weaknesses that pin design 2 does not?  By only doing structural testing on the tank built with pin design 2, they'll be missing any unexpected weaknesses in tanks made with pin design 1 that structural testing would have caught.  But they plan to fly the tanks built with pin design 1, which will never have gone through structural testing.

The only safe thing to do is throw away both of the first two H2 tanks and use the next two for structural testing and flight.


Or saved them for eventual museum pieces?
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Offline Lars-J

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It's disturbing that they think it's OK to go ahead and use one of the bad tanks for structural testing, because it means that what they're testing is not the same as what they'll be flying.

We already know that switching from pin design 1 to pin design 2 had some unintended consequences that aren't understood.  We know that using pin design 2 gives certain kinds of weaknesses, and nobody knows why.  So how can we be sure pin design 1 doesn't have different kinds of weaknesses that pin design 2 does not?  By only doing structural testing on the tank built with pin design 2, they'll be missing any unexpected weaknesses in tanks made with pin design 1 that structural testing would have caught.  But they plan to fly the tanks built with pin design 1, which will never have gone through structural testing.

The only safe thing to do is throw away both of the first two H2 tanks and use the next two for structural testing and flight.

I think this may be a symptom of the *extremely low* production rate. There is no mass production, which means that every item will be basically hand built (by machines) with ever changing configurations as they find things to improve. And since the production rate is so low, every item costs a fortune, so you can't waste them and start over, even though it would have been cheaper to build several of them to begin with.

How can you ever hope to build something reliable if you only build it once every two years? And some of the hardware they are working on now won't fly for another six years (optimistically). Madness.

Offline WBY1984

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It's getting more and more difficult over time to feel anything positive about Orion/SLS. With this issue, and the possibility of making the first flight manned, I doubt it'll fly before 2020. What an utter, horrible mess.  :'(

Offline woods170

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Just a note on this joyride: recall that we were going from the ET 8.4m, then CxP proposed 10m, the SLS went with 8.4m for cost cutting and other efficiencies I'm sure...
It went with 8.4m to support the claim that SLS is not Ares V redux. That and the silly black-and-white paint scheme.
Additionally: an 8.4m core with two SRB's was the best option to prevent a major re-do of the LC-39B flame trench, because this configuration is exactly as wide as the shuttle configuration was.
« Last Edit: 05/09/2017 08:05 AM by woods170 »

Offline AncientU

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It's disturbing that they think it's OK to go ahead and use one of the bad tanks for structural testing, because it means that what they're testing is not the same as what they'll be flying.

We already know that switching from pin design 1 to pin design 2 had some unintended consequences that aren't understood.  We know that using pin design 2 gives certain kinds of weaknesses, and nobody knows why.  So how can we be sure pin design 1 doesn't have different kinds of weaknesses that pin design 2 does not?  By only doing structural testing on the tank built with pin design 2, they'll be missing any unexpected weaknesses in tanks made with pin design 1 that structural testing would have caught.  But they plan to fly the tanks built with pin design 1, which will never have gone through structural testing.

The only safe thing to do is throw away both of the first two H2 tanks and use the next two for structural testing and flight.

The error seems to have been not following the proof sequence -- confidence article --> STA --> flight article -- and finishing the testing at each stage before going to next.  This is basic engineering, not cutting edge.  This is exactly why we spend so much to have a fool-proof process.  Changing the configuration (pin design) during a process proof sequence is also Engineering 101.  (Were they trying to hurry/catch up because the vertical assembly tooling was screwed up and delayed start of welding?)

But now it seems that they are short-cutting the process again... maybe they need to hurry so we can get the manned EM-1 flying before good will runs out. :( 

Kinda scary that Shuttle-era management practices that directly resulted in lost crews are still running the show.
« Last Edit: 05/09/2017 11:08 AM by AncientU »
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Offline Rocket Science

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Just a note on this joyride: recall that we were going from the ET 8.4m, then CxP proposed 10m, the SLS went with 8.4m for cost cutting and other efficiencies I'm sure...
It went with 8.4m to support the claim that SLS is not Ares V redux. That and the silly black-and-white paint scheme.
Additionally: an 8.4m core with two SRB's was the best option to prevent a major re-do of the LC-39B flame trench, because this configuration is exactly as wide as the shuttle configuration was.
That was a "wee bit" of a sarcasm quote on my part which doesn't translate well over the internet I'm afraid... ;)
« Last Edit: 05/09/2017 01:47 PM by Rocket Science »
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Offline Proponent

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I think this may be a symptom of the *extremely low* production rate.

Yes -- and many of Orion/SLS's problems stem from the low flight rate.  It fundamentally does not make sense to build a system that is too expensive to fly with some frequency.

Offline bob the martian

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It's disturbing that they think it's OK to go ahead and use one of the bad tanks for structural testing, because it means that what they're testing is not the same as what they'll be flying.

We already know that switching from pin design 1 to pin design 2 had some unintended consequences that aren't understood.  We know that using pin design 2 gives certain kinds of weaknesses, and nobody knows why.  So how can we be sure pin design 1 doesn't have different kinds of weaknesses that pin design 2 does not?  By only doing structural testing on the tank built with pin design 2, they'll be missing any unexpected weaknesses in tanks made with pin design 1 that structural testing would have caught.  But they plan to fly the tanks built with pin design 1, which will never have gone through structural testing.

The only safe thing to do is throw away both of the first two H2 tanks and use the next two for structural testing and flight.


Or saved them for eventual museum pieces?

Tear them down to examine the welds and add some data points.  Like the man said, they're in uncharted territory here, and if they can't use the tanks as intended, they should at least get some useful data out of them. 

I do find it a bit worrying that they are in a situation where such a seemingly minor change can have such major consequences.  Makes you wonder what other surprises are lurking. 

Offline butters

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Is the SLS tank significantly thicker than the ET, or is it just an unprecedented thickness for friction stir welding?

Offline woods170

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Is the SLS tank significantly thicker than the ET, or is it just an unprecedented thickness for friction stir welding?
Both significantly thicker (given that it holds substantially more propellant than the shuttle ET) as well as that the thickness of the weld is pushing the current state-of-the-art for friction stir welding.

Offline Tez

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Plus the tanks are now the thrust structure between the main engines and the upper stage instead of just fuel tanks as in the ET.
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Offline SimonFD

Plus the tanks are now the thrust structure between the main engines and the upper stage instead of just fuel tanks as in the ET.

The tanks were also the thrust structure with shuttle. The load from the SSME's was transfered to the tanks via the orbiter's lower attachment point. The SRB thrust was applied at the intertank (as with SLS).
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Offline Tez

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Agreed, but there will be more thrust and a lot more loading with an upper stage on top of the new structure, hence the thicker walls.
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Offline Unobscured Vision

If they're seeing voids in their weld, then there's a high probability that there will be areas elsewhere that they'll find that the welds will fail when put under load. The forces will transfer to those areas, which now have to do extra work to hold together, and will ultimately exceed biaxial stress limits (since we've got two factors of stress). Snappola. That weld will fail, and the tank will unzip along that point of failure. Then the voids become the weakest part of the welded section and contribute to the ripping action from the blowout.

So yeah, the voids are a big problem, but not because they're voids (as such). It's the force transfer that those voids introduce elsewhere along the weld. Those forces need to be as uniform as possible, and that requires a consistent weld.

Yeah -- gotta love second year of Mat. Engineering. I actually see how all of this works, now.  8)
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Offline spacenut

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In order not to waste a tank, would they overweld (manually) the voided areas?  I know it would add some weight, but why waste a tank? 

When I inspected pipeline welding, any voided areas were ground out and re-welded manually.  The x-rays would show voids, cracks, etc, that had to be redone. 

Offline Ben the Space Brit

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I might be wrong, spacenut, but I think the problem is that the voids can only be detected through destructive testing. Consequently, the tanks are unsalvageable.
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Offline ChrisGebhardt

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I might be wrong, spacenut, but I think the problem is that the voids can only be detected through destructive testing. Consequently, the tanks are unsalvageable.

The tank is not "unsalvageable" per se.  The article on site states that evaluations are ongoing into how to save the tank.  Whether or not they can "save" it in a manner that makes them confident enough to use it on a subsequent flight to EM-1 is a different matter.
« Last Edit: 05/10/2017 03:27 PM by ChrisGebhardt »

Offline AncientU

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I might be wrong, spacenut, but I think the problem is that the voids can only be detected through destructive testing. Consequently, the tanks are unsalvageable.

The tank is not "unsalvageable" per se.  The article on site states that evaluations are ongoing into how to save the tank.  Whether or not they can "save" it in a manner that makes them confident enough to use it on a subsequent flight to EM-1 is a different matter.

Rewelding aluminum is notoriously difficult -- it tends to crack at ends of welds, so frequently you wind up 'chasing' a flaw a long way down a seam.  Since the process is so sensitive to the skill of the individual welder (individual people doing hand work), you could not ever 'fix' the STA, test it, and then proceed to the flight article with any confidence that another perfect set of repairs had been done.  FSW takes this hand work variable out of the equation (for the most part).

Note: Steel or stainless steel rework is vastly more simple than aluminum.  I suspect that above pipeline experience was working with these materials.
« Last Edit: 05/10/2017 04:46 PM by AncientU »
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Offline AncientU

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« Last Edit: 05/10/2017 06:47 PM by AncientU »
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Offline spacenut

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Yes, aluminum is far more difficult for even a human welder than steel.  Pipelines were steel. 

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Quote
NASA Watch‏ @NASAWatch 26s26 seconds ago

Sources report #NASA looking at using the EM-2 SLS launch vehicle for EM-1 mission due to hydrogen tank issues on EM-1 vehicle @NASA_SLS

https://twitter.com/NASAWatch/status/864146709399646208

Offline A_M_Swallow

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If the EM-3 tanks are used for EM-2 then a extra set of tank will be needed for EM-3. Extra cost.

Offline Ben the Space Brit

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Depends on schedule, IMO. They're effectively bringing forward the due date for the next set of tanks so they may end up just doing two units instead of one.
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Offline AncientU

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If the EM-3 tanks are used for EM-2 then a extra set of tank will be needed for EM-3. Extra cost.

There is no contract for an EM-3 vehicle, is there?
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Offline Ben the Space Brit

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As I understand it, EM-3 is the last mission that The Powers That Be have officially confirmed would happen to date (even if details are sparse).
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Offline MP99

Is the SLS tank significantly thicker than the ET, or is it just an unprecedented thickness for friction stir welding?

https://www.nasaspaceflight.com/2013/02/sls-new-buckling-standards-drops-super-light-alloy/

This is my only article written for NSF, but seems to be relevant here.

Shuttle's ET went through revisions to lighten it, once to the Light Weight Tank, then to Super Light Weight Tank.

SLWT introduced Al-Li alloys, orthogrid structures, and (IIRC) friction stir welding.

SLS has reverted back to Al alloys because Al-Li suffers from brittleness​ when formed into cylindrical sections (limiting its thickness) and is a pain to weld. They gave up on Al-Li for some parts of SLWT over time.

Thicker Al panels could be used on SLS because they took the stresses of being formed better, and thicker panels result in deeper stiffening ribs. Since these are stronger, a thick Al panel ends up being lighter than a thin Al-Li panel once it's beefed up to cope with SLS's loads.

But, that thickness seems to be contributing to these FSW issues.

Although Al-Li tanks would be thinner, the hassles of welding it might just have landed them with similar problems, anyway.

Basically, engineering trades aren't necessarily as simple as they first seem.

Cheers, Martin

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Offline Steven Pietrobon

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SLWT introduced Al-Li alloys, orthogrid structures, and (IIRC) friction stir welding.

They didn't use friction stir welding on SLWT. The SLWT was first flown in 1998, way before friction stir welding became common. They used variable polarity plasma arc welding.

https://app.aws.org/wj/may/chienar.htm

"The early shuttle tanks were welded using gas tungsten arc welding. Hibbard said, "In the early 1980s, we developed with NASA Marshall the variable polarity plasma arc (VPPA) welding process." That process creates a keyhole in the metal and the molten metal then solidifies at the back of the hole as the torch progresses. "We used those processes for the 2195, plus a hybrid process where the plasma arc alternates current and does some cathodic cleaning at the torch," he said."
Akin's Laws of Spacecraft Design #1:  Engineering is done with numbers.  Analysis without numbers is only an opinion.

Offline rsnellenberger

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Friction stir welding was used for SLWT beginning with ET-134, flown on STS-130.

https://www.nasa.gov/centers/marshall/news/news/releases/2010/10-010.html

Online AnalogMan

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Friction stir welding was used for SLWT beginning with ET-134, flown on STS-130.

https://www.nasa.gov/centers/marshall/news/news/releases/2010/10-010.html

Small correction (as noted in the linked press release) - the first SLWT to fly with FSW was ET-132 flown on STS-128.  It only had two barrel sections of the LH2 tank welded this way (out of four total).  ET-134 was the first with all LO2 and LH2 tank longitudinal joins of the barrel sections welded by FSW.

Incidentally the first FSW pathfinder LH2 barrel section was produced in October 1998 using SLWT panels.
« Last Edit: 05/24/2017 03:39 PM by AnalogMan »

Offline psloss

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