Author Topic: SLS Core Stage team recovering from consequences of weld pin change  (Read 8195 times)

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. 

Offline FutureSpaceTourist

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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).
"Oops! I left the silly thing in reverse!" - Duck Dodgers

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