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

[jgoldader]

Last time they identified the root cause as "this strut broke due to bad materials/manufacturing". If they now have that strut laying bent and sooty but unbroken in a hangar I think they are allowed to state that...

Hopefully they also have pieces of the failed part(s) of the helium system that are useful for forensic analysis.  For all we know, they've recovered all but one COPV in good shape and have the last one showing a big hole somewhere (either a big blowout hole, or a failed opening for plumbing, etc.).  There are many top-notch people working this problem, and they need to be given time to figure it out.

[Kaputnik]

Summarising what some knowledgeable NSF members posted in the last thread:
- Falcon is unusual (possibly exceptional) in having LOX-immersed COPVs
- some voices at NASA were not wholly convinced that SpaceX had truly got to the root of CRS-7, therefore it could be premature to completely rule out any connection
- SpaceX's COPVs are unusual in having Al liners, rather than Ti, which has the potential for differential thermal expansion issues.

The thing that strikes me as very odd is that there has not been a He system failure in a first stage yet. These stages have been fired many more times than the second stages have, so you would predict that failures would crop up there instead- unless there are fundamental differences in the He systems?

[Herb Schaltegger]

Why would this not have been seen (detected by strain gauges in the system) during filling in McGregor prior to full-duration test firing there? Even if LC-40 GSE is different and fills at a different rate, the resonant frequency of the helium system doesn't change based on the force used to excite it, right?

Do you know there are strain gauges and accelerometers in the helium system? I don't.

[SWGlassPit]

Summarising what some knowledgeable NSF members posted in the last thread:
- SpaceX's COPVs are unusual in having Al liners, rather than Ti, which has the potential for differential thermal expansion issues.

Does anybody know if autofrettage is used in the construction of those COPVs?  My first thought is that would help mitigate thermal expansion issues, but the more I think about it, the less sure I am.

The thing that strikes me as very odd is that there has not been a He system failure in a first stage yet. These stages have been fired many more times than the second stages have, so you would predict that failures would crop up there instead- unless there are fundamental differences in the He systems?

I would argue that two data points isn't enough to rule out a coincidence.

[Herb Schaltegger]

The thing that strikes me as very odd is that there has not been a He system failure in a first stage yet. These stages have been fired many more times than the second stages have, so you would predict that failures would crop up there instead- unless there are fundamental differences in the He systems?

Different acceleration profiles, more mass-margin in a first stage, probably substantially so to allow for reuse ... plus stuff I can't think of off the top of my head.

[guckyfan]

The two second stage incidents are unrelated. Probably both could have happened as well on the first stage.

The first problem with the He system way back was at the first stage. After that they took COPV production in house.

[John Santos]

People keep saying COPV.  The SpaceX statement doesn't say "COPV".  It says "cryogenic Helium system".  There is way more to this system than just the tanks (and the struts that support them.)  There is lots of piping, valves, regulators, and a supply system from the TEL.

Edit: correct quote from SpaceX

[Jim]

The thing that strikes me as very odd is that there has not been a He system failure in a first stage yet. These stages have been fired many more times than the second stages have, so you would predict that failures would crop up there instead- unless there are fundamental differences in the He systems?

There was a static fire incident in the first stage related to pressure system

[woods170]

A large breach in the cryogenic helium system, sufficient to rupture the second stage in less than 1/10th of a second means one thing, and one thing only IMO: a COPV blew spectacularly.

Given that we know that SpaceX has had prior problems with LOX immersed COPV's (other than the CRS-7 incident), I think it is a safe bet that either one of the below design changes will be implemented:

- COPV's will be moved out of the LOX tank, possibly into an interstage-like extension of the upper stage
- COPV's will be redesigned to be LOX hardened (option least adviced given that composites and LOX don't like each other very much)
- COPV's will be replaced by different tankage. For instance all-metal tanks.

Anyone of those changes is IMO going to eat into F9's performance.

[woods170]

People keep saying COPV.  The SpaceX statement doesn't say "COPV".  It says "high pressure Helium system".  There is way more to this system than just the tanks (and the struts that support them.)  There is lots of piping, valves, regulators, and a supply system from the TEL.

IMO the only part of the high pressure helium system that has the energetic potential to pop the upper stage in less than 1/10th of a second is a pressurized COPV letting go.

[SWGlassPit]

The two second stage incidents are unrelated.

I wonder if they really are.  Investigation teams have been wrong about root causes before.

To clarify: I'm on board with the idea that a failed strut was not the cause of this event.  They didn't have the luxury of being able to recover hardware from that.  Is it possible, though, that the strut wasn't actually part of the failure chain on the CRS-7 event and that they just also happened to find bad struts in their supply chain?  More to the point: once the strut fault idea became the leading idea, were all other branches of the fault tree conclusively ruled out?

[SWGlassPit]

People keep saying COPV.  The SpaceX statement doesn't say "COPV".  It says "high pressure Helium system".  There is way more to this system than just the tanks (and the struts that support them.)  There is lots of piping, valves, regulators, and a supply system from the TEL.

IMO the only part of the high pressure helium system that has the energetic potential to pop the upper stage in less than 1/10th of a second is a pressurized COPV letting go.

I'm not sure.  Couldn't a burst in the high pressure plumbing responsible for charging up those COPVs result in a breach in the common bulkhead of sufficient energy to allow enough mixing between LOX and RP-1 and make a nice boom?

[John Alan]

People keep saying COPV.  The SpaceX statement doesn't say "COPV".  It says "high pressure Helium system".  There is way more to this system than just the tanks (and the struts that support them.)  There is lots of piping, valves, regulators, and a supply system from the TEL.

Agreed...
I opinioned way back in thread #1 it was a high pressure fitting that burst and took down the rocket...
The flash was chunks of the fitting going thru the side of the tank and the AL/LOX flash fire that resulted...
There were three objects tracked going away early at high speed... My opinion was fitting in three pieces...
SO... what they stated today does not prove or disprove my opinionated guess of old...

Time will tell... 

On edit...
Thread #1 post as discussed above...
http://forum.nasaspaceflight.com/index.php?topic=30981.msg1578767#msg1578767

[woods170]

People keep saying COPV.  The SpaceX statement doesn't say "COPV".  It says "high pressure Helium system".  There is way more to this system than just the tanks (and the struts that support them.)  There is lots of piping, valves, regulators, and a supply system from the TEL.

Agreed...
I opinioned way back in thread #1 it was a high pressure fitting that burst and took down the rocket...
The flash was chunks of the fitting going thru the side of the tank and the AL/LOX flash fire that resulted...
There were three objects tracked going away early at high speed... My opinion was fitting in three pieces...
SO... what they stated today does not prove or disprove my opinionated guess of old...

Time will tell... 

Your analysis does not rule out a COPV letting go. The parts observed flying away at high speed may be parts of a blown COPV.

[spacekid]

So the COPV but they rule out any connection with the former incident which is important.

That's assuming they were correct in their root cause for the incident last year. Once they determine a root cause in this incident, I would want to re-evaluate the original incident and see if it or a variation of it could cause a similar failure as seen last year.

They still want to fly again in November which sounds optimistic.

Yeah. I would estimate 6 to 9 months if it's a quality control issue, 9 to 12 month if they have to redesign something or fabricate a long lead item.

[John Alan]

People keep saying COPV.  The SpaceX statement doesn't say "COPV".  It says "high pressure Helium system".  There is way more to this system than just the tanks (and the struts that support them.)  There is lots of piping, valves, regulators, and a supply system from the TEL.

Agreed...
I opinioned way back in thread #1 it was a high pressure fitting that burst and took down the rocket...
The flash was chunks of the fitting going thru the side of the tank and the AL/LOX flash fire that resulted...
There were three objects tracked going away early at high speed... My opinion was fitting in three pieces...
SO... what they stated today does not prove or disprove my opinionated guess of old...

Time will tell... 

Your analysis does not rule out a COPV letting go. The parts observed flying away at high speed may be parts of a blown COPV.

I added link to the 9/5/16 post where I stated fitting... and excluded COPV... see above... 

[Jim]

.
The flash was chunks of the fitting going thru the side of the tank and the AL/LOX flash fire that resulted...

It was a RP-1/LOX fire, not AL

[John Alan]

.
The flash was chunks of the fitting going thru the side of the tank and the AL/LOX flash fire that resulted...

It was a RP-1/LOX fire, not AL

Call it RP-1/LOX/AL fire then... my opinion... 

[matthewkantar]

IMO the only part of the high pressure helium system that has the energetic potential to pop the upper stage in less than 1/10th of a second is a pressurized COPV letting go.

The strut incident destroyed the second stage in flight. If we believe the strut was at fault, a COPV failure was not needed to breech the the second stage.

Matthew

[Rocket Science]

So the way that was worded, it did not say if it originated inside or outside of the LOX tank.

So could the first visible flash be an effect, not the cause? Common bulkhead failure before the LOX tank burst due to a sudden pressure increase in the LOX tank.

Said the same thing Kevin many moons ago on thread one, that what we saw as an effect and not the cause... Score one for the "near-sighted squirrel" perhaps...

Edit:typo