SpaceX Falcon 9 - AMOS-6 - (Pad Failure) - DISCUSSION THREAD (2)

[Fred Bonyea]

When did SpaceX say they saw buckling prior to this failure? I must have missed an update because I don't see that anywhere?

"Microbuckles" always occur during filament winding at stress points where there is a change in the geometry - especially where the diameter is small, and a fixture or attachment fitting is being woven into the structure. The filament must be held at high tension, and the resin slightly lubricates the fibers, allowing outer wraps to apply warping pressure to the inner wraps through the simple mechanics of a lever-arm. They do not show up in any testing medium I am aware of, but can be found by dissecting a case and looking under a powerful microscope. In areas where this is likely to occur, there are always additional overwraps to insure structural integrity.

[matthewkantar]

When did SpaceX say they saw buckling prior to this failure? I must have missed an update because I don't see that anywhere?

"Microbuckles" always occur during filament winding at stress points where there is a change in the geometry - especially where the diameter is small, and a fixture or attachment fitting is being woven into the structure. The filament must be held at high tension, and the resin slightly lubricates the fibers, allowing outer wraps to apply warping pressure to the inner wraps through the simple mechanics of a lever-arm. They do not show up in any testing medium I am aware of, but can be found by dissecting a case and looking under a powerful microscope. In areas where this is likely to occur, there are always additional overwraps to insure structural integrity.

The SpaceX update indicated that the buckling was in the liners, not in the composite material, if I read it correctly.

Matthew

[mn]

When did SpaceX say they saw buckling prior to this failure? I must have missed an update because I don't see that anywhere?

"Microbuckles" always occur during filament winding at stress points where there is a change in the geometry - especially where the diameter is small, and a fixture or attachment fitting is being woven into the structure. The filament must be held at high tension, and the resin slightly lubricates the fibers, allowing outer wraps to apply warping pressure to the inner wraps through the simple mechanics of a lever-arm. They do not show up in any testing medium I am aware of, but can be found by dissecting a case and looking under a powerful microscope. In areas where this is likely to occur, there are always additional overwraps to insure structural integrity.

If 'Microbuckles always occur' then it was not an anomaly and there's was nothing for SpaceX to investigate further?

+ what matthewkantar said.

[strangequark]

No it's not.

Bucking is a very serious failure.  It's very hard to predict so it's reasonable to be surprised that it's happening, but if you find that it's happening - especially in a flight critical component - it's not reasonable to continue to fly with the knowledge that it's happening despite your expectations that it wouldn't be.

If you find the spar in your airplane has some buckling during testing or test flying, you wouldn't fly that plane again until you had changed the design to eliminate the buckling and replaced the one you were flying.  The results of buckling are too hard to predict to assume that even detailed analyses could predict the results.  It's a non-linear phenomenon and your original detailed analysis didn't predict that it would even happen.

I've seen buckling during high cycle fatigue testing and in single cycle proof loading.  It never ends well.  And it always ends with a design modification.

To start with a disclaimer, I have no insight into the design of the SpaceX COPV, or the specific buckling here.

However, saying that buckling is always disallowable is flat out wrong. It's ironic that you used aircraft as an example, because buckling is used as a design feature in some aircraft spars. Specifically, Wagner beams are designed to buckle as part of normal use to decrease structural weight.

[matthewkantar]

Some of are typing past each other. "Buckling" in a composite layer is different from "buckling" in the aluminum inner shell.

-It may be that the carbon fiber always has some buckling in it as a result of how it is laid down.
-It may be true that this is impossible to identify without sectioning the composite.

But those facts, or non facts, (I don't know) are not urbane to this thread, as the buckling seems to have been in aluminum, not in composite overwrap. The voids in question seem to have been between the buckled aluminum and the over wrap. Sorry if I am repeating.

Matthew

[Lee Jay]

-It may be that the carbon fiber always has some buckling in it as a result of how it is laid down.

That's called waviness or wrinkling.  Buckling is from compression.

[Lee Jay]

However, saying that buckling is always disallowable is flat out wrong. It's ironic that you used aircraft as an example, because buckling is used as a design feature in some aircraft spars. Specifically, Wagner beams are designed to buckle as part of normal use to decrease structural weight.

We're talking about unexpected, undesigned, local buckling such as panel buckling here.

[matthewkantar]

-It may be that the carbon fiber always has some buckling in it as a result of how it is laid down.

That's called waviness or wrinkling.  Buckling is from compression.

That is sort of my point, there are many definitions of buckling in this thread, and no agreement as to definition. Makes the discussion unproductive.

Matthew

[CameronD]

-It may be that the carbon fiber always has some buckling in it as a result of how it is laid down.

That's called waviness or wrinkling.  Buckling is from compression.

That is sort of my point, there are many definitions of buckling in this thread, and no agreement as to definition. Makes the discussion unproductive.

Precisely.. Anyways, there's a launch coming up in a few days, so we'll know soon enough whether or not they still have a problem.
 

[JohnWT]

The load bearing component in COPVs is the carbon fibre wrap.  The buckling, as I understand it, is in the aluminium liner.  So comparisons with buckling in load bearing structures are irrelevant.
As has been discussed upthread, LOX soaking into the wrap is inevitable, whether it is also into voids caused by buckling of the aluminium or not.  The problem is what happens if that LOX freezes.

[savuporo]

Precisely.. Anyways, there's a launch coming up in a few days, so we'll know soon enough whether or not they still have a problem.

Not really though. If they previously had a 15% probability of blowing things up and reduced it to 12%, a single launch will not show this.

[sdsds]

I am sorry if it has been discussed before and I missed it. But how many cycles of loading extra-cold helium into a COPV had SpaceX completed successfully before the AMOS-6 loading failed catastrophically?

[dglow]

Eight missions so at least 16 prop loads. Add a few more than that, since Orbcomm and SES each had scrubs for static fire and/or launch, IIRC.

[MarekCyzio]

Eight missions so at least 16 prop loads. Add a few more than that, since Orbcomm and SES each had scrubs for static fire and/or launch, IIRC.

We do not know when did they change their hellium load procedures. So the actual number may be smaller.

[envy887]

Eight missions so at least 16 prop loads. Add a few more than that, since Orbcomm and SES each had scrubs for static fire and/or launch, IIRC.

We do not know when did they change their hellium load procedures. So the actual number may be smaller.

The loading procedures were changed for JCSAT-16, but we don't know what they changed from or changed to.

We also don't know how many cycles they completed at McGregor using the same or equivalent procedures.

[dglow]

Eight missions so at least 16 prop loads. Add a few more than that, since Orbcomm and SES each had scrubs for static fire and/or launch, IIRC.

We do not know when did they change their hellium load procedures. So the actual number may be smaller.

sdsds may need to clarify the exact question implied by 'extra-cold helium'.    The numbers above simply count from the start of F9 FT flights, beginning with the Orbcomm RTF.

It would be useful to know when SpaceX changed the number of COPVs in S2. I've asked previously, but not yet seen a response.

[matthewkantar]

It would be useful to know when SpaceX changed the number of COPVs in S2. I've asked previously, but not yet seen a response.

From previous page:

We know SpaceX have returned to using four COPVs. Does anyone recall when they first moved to three?

I found from webcast video that three COPVs were first at F9-016 with ABS-3A/Eutelsat 115 West B. Previous flights had four vessels.

[dglow]

Ah, I missed it – thank you, Raul!

Neither Eutelsat nor the missions which followed had any scrubs, according to our launch log. So: six loadings with extra-extra-cold helium.

[mn]

As has been discussed upthread, ... The problem is what happens if that LOX freezes.

As has been mentioned upthread, the SpaceX update says the LOX freezing (aka SOX) exacerbates the problem. in simple english that means it is not the root problem.

I know that SpaceX/Musk mentioned several times earlier in the investigation that the cause was SOX, but based on the final update it seems the conclusion was not so clear, yet it seems many on this thread continue to repeat it as if it were fact.

I mentioned this earlier, nobody came back to say otherwise, if anyone is still convinced SOX was the root cause please speak up and convince me, I'll be happy to hear. (SOX seems like a much easier problem to solve so everyone prefers to focus on that).

Sorry if i'm being cranky today, please forgive me.

[matthewkantar]

Ah, I missed it – thank you, Raul!

Neither Eutelsat nor the missions which followed had any scrubs, according to our launch log. So: six loadings with extra-extra-cold helium.

I don't think we can assume all six FT propellant loadings were identical. AMOS-6 may have been a first. SpaceX has said that tweaks to the loading process were ongoing.

Matthew