SLS Core Stage team recovering from consequences of weld pin change

[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.

[eric z]

 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!

[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.

[RonM]

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.

[Jimmy_C]

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.

[Coastal Ron]

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.

[Coastal Ron]

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.

[Lars-J]

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".

[Kansan52]

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

[RonM]

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.

[Coastal Ron]

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.

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...

[Rocket Science]

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...

[eric z]

 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?

[TomH]

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.

[ChrisWilson68]

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.

[catdlr]

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?

[Lars-J]

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.

[WBY1984]

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. 

[woods170]

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.

[AncientU]

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.