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

[meekGee]

Here's a way to interpret what they said about buckling.

The statement says "The recovered COPVs showed buckles in their liners. " - which means these are not elastic bucklings - that's for sure..

They say LOX accumulated in the buckles, and being trapped there, created the ignition source.  SOX, they say, would have exacerbated the issue.  This means to me that trapped LOX or SOX could have caused the problem, but it's easier for the SOX to get trapped in those pockets, and that of course makes sense. LOX would sometimes be able to squeeze out, but SOX couldn't.

Carbon Fiber is strong only in tension (or rather, axially).  If there's localized stress in another direction (as would result from such pockets) some fibers may break, but apparently not enough of them to cause structural failure.

But if enough of them break, their tension gets converted to fast snapping motion, and maybe that's the friction issue they're talking about.  Like a bunch of little match strikes. The mechanical energy stored in the fibers gets released, and it goes either into friction heat in the resin, or compression heat in the fiber itself.

One fiber is not a problem, since there's plenty of bulk around it to take the heat, but if enough of them go, it can cause ignition, failing more fibers, etc.

Maybe.

[strangequark]

3) Minuteman uses small titanium alloy helium tanks for nozzle vectoring. They are positioned near the nozzle. Larger versions would likely work well within a LOX environment, but qualification burns would be essential

For the record, Titanium is not generally considered oxygen compatible. You could perhaps argue for its acceptability around stagnant, low pressure LOX, but it reacts quite vigorously.

[Fred Bonyea]

There is another failure mode in filament wound bottles that I have not seen discussed. During filament winding, buckling and fretting can create voids, micro-channels and resin-rich pockets. Epoxy and other resins have higher linear expansion coefficients than carbon fibers. Fibers are wound under tension, and this tension is locked into the matrix as the resin cures adding strength, the same way steel cables are held in tension inside of suspended concrete floors.

If there is an area that is resin rich, the resin rich area will be under more stress as temperatures change than areas where the resin and fibers are evenly distributed. I haven't worked with low temperature applications, but in very high temperature environments, the additional strain in resin rich regions can result in coning or spawling.

The amount of resin used while winding is difficult to control; as the viscosity of most resins is constantly increasing; an extruder setting used on one day may need to be changed the next, or even on the next shift.

Again, I have been out of the industry for a long time, but carbon fiber winding is still a blend of careful science and skilled art. 

[vulture4]

Thermal stress is certainly possible, but AFAIK it would not require oxygen to permeate through the composite material. SpaceX has indicated that oxygen collected between the buckled liner and the outer composite layer. This could only have ocurred if LOX permeated all the way through the composite material of the COPV. If this is happening in the second stage, it is probably also happening in the booster. If LOX is present between the layers of the lamination, it will expand greatly as the tank warms after flight, pushing apart the layers of composite and making reuse impossible. This was one of the problems that led to termination of the X-33 program.

TMK only the New Shepherd reuses linerless composite cryogenic fuel tanks. Presumably they have licked the problem of cryogenic intrusion. It is hard to say whether Mr. Bezos would be willing to share the details of their construction, however.

[ChrisWilson68]

Thermal stress is certainly possible, but AFAIK it would not require oxygen to permeate through the composite material. SpaceX has indicated that oxygen collected between the buckled liner and the outer composite layer. This could only have ocurred if LOX permeated all the way through the composite material of the COPV. If this is happening in the second stage, it is probably also happening in the booster. If LOX is present between the layers of the lamination, it will expand greatly as the tank warms after flight, pushing apart the layers of composite and making reuse impossible. This was one of the problems that led to termination of the X-33 program.

TMK only the New Shepherd reuses linerless composite cryogenic fuel tanks. Presumably they have licked the problem of cryogenic intrusion. It is hard to say whether Mr. Bezos would be willing to share the details of their construction, however.

My understanding is that this is not an issue because as long as the oxygen doesn't solidify, it will simply move back through the composite material as it expands.  If the composite is permiable to oxygen, it works both ways -- easy in, easy out.

They've done multiple firings of these stages and not had to replace the helium bottles.

[Nomadd]

The X-33 problem was the hydrogen tank.

[Toast]

The X-33 problem was the hydrogen tank.

And, for what it's worth, is a problem that has since been solved. My understanding is that the SpaceX problem was different in that the temperature of the helium and LOX, in conjunction with the pressurization of the LOX tank, allowed the temporary formation of SOX crystals. Slight intrusion of oxygen into the composite was not problematic by itself, the real issue was (as ChrisWilson68 stated) that the oxygen solidified and became trapped.

[mn]

The SpaceX update says: "investigators determined that the loading temperature of the helium was cold enough to create solid oxygen (SOX), which exacerbates the possibility of oxygen becoming trapped as well as the likelihood of friction ignition"

SpaceX is saying that the formation of SOX exacerbates the problem, which implies that even with just plain LOX there is still a problem.

Which also means that it's not entirely clear how they are solving the problem.

[CameronD]

SpaceX is saying that the formation of SOX exacerbates the problem, which implies that even with just plain LOX there is still a problem.

Which also means that it's not entirely clear how they are solving the problem.

I thought it was clear enough:

"In the short term, this entails changing the COPV configuration to allow warmer temperature helium to be loaded, as well as returning helium loading operations to a prior flight proven configuration based on operations used in over 700 successful COPV loads" {emphasis mine}

[tleski]

I guess, we will not solve this puzzle without reading the actual report. Don't you think that it may be just an unfortunate choice of words in a public statement ( vs. actual detailed technical description)?

[MarekCyzio]

My understanding is that this is not an issue because as long as the oxygen doesn't solidify, it will simply move back through the composite material as it expands.  If the composite is permiable to oxygen, it works both ways -- easy in, easy out.

They've done multiple firings of these stages and not had to replace the helium bottles.

I;m going to repeat my theory - previous hellium load procedures were not causing buckling. Only when SpaceX used more aggresive load procedures, buckling occured and did not get detected. Buckling alone does not cause rocket to explode, but significantly increases chances of such explosion. LOX can become trapped in the cavity and compressed to a point where it either becomes solid or just causes ripping of carbon fibers.

[dglow]

I;m going to repeat my theory - previous hellium load procedures were not causing buckling. Only when SpaceX used more aggresive load procedures, buckling occured and did not get detected.

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

[mn]

SpaceX is saying that the formation of SOX exacerbates the problem, which implies that even with just plain LOX there is still a problem.

Which also means that it's not entirely clear how they are solving the problem.

I thought it was clear enough:

"In the short term, this entails changing the COPV configuration to allow warmer temperature helium to be loaded, as well as returning helium loading operations to a prior flight proven configuration based on operations used in over 700 successful COPV loads" {emphasis mine}

If SOX was the only problem then this is clear. Otherwise it's not clear to me.

[vulture4]

>>friction ignition<<

Friction generating heat?  Ignition of what? Graphite requires extraordinary heat to ignite, even in pure oxygen, as NASA demonstrated back in the Sixties. Certainly not at cryogenic temperatures. The COPVs obviously did not burn up as they were found after the explosion. BTW Falcon COPVs have been found in Brazil after re-entry of the second stage and disintegration of the LOX tank. Despite a high-speed descent through the atmosphere followed by ground impact, they incurred only minimal impact damage.

[strangequark]

>>friction ignition<<

Friction generating heat?  Ignition of what? Graphite requires extraordinary heat to ignite, even in pure oxygen, as NASA demonstrated back in the Sixties. Certainly not at cryogenic temperatures. The COPVs obviously did not burn up as they were found after the explosion. BTW Falcon COPVs have been found in Brazil after re-entry of the second stage and disintegration of the LOX tank. Despite a high-speed descent through the atmosphere followed by ground impact, they incurred only minimal impact damage.

Graphite might be hard to light, but epoxy burns just fine.

Blunt, light objects reentering an 80% nitrogen atmosphere is not a comparable case.

[Space Ghost 1962]

Excuse me, but cryochemistry is what this is all about. Please don't neglect it by omission. You'll miss the whole show.

And LOX has many interesting non-intuitive properties including CF/resins/metals ...

[cscott]

>>friction ignition<<

Friction generating heat?  Ignition of what? Graphite requires extraordinary heat to ignite, even in pure oxygen, as NASA demonstrated back in the Sixties. Certainly not at cryogenic temperatures. The COPVs obviously did not burn up as they were found after the explosion. BTW Falcon COPVs have been found in Brazil after re-entry of the second stage and disintegration of the LOX tank. Despite a high-speed descent through the atmosphere followed by ground impact, they incurred only minimal impact damage.

Some of the copvs were found intact.  I don't think i ever heard anyone say that *all* of them were intact.  It only takes one to have a bad day.

[woods170]

>>friction ignition<<

Friction generating heat?  Ignition of what? Graphite requires extraordinary heat to ignite, even in pure oxygen, as NASA demonstrated back in the Sixties. Certainly not at cryogenic temperatures. The COPVs obviously did not burn up as they were found after the explosion. BTW Falcon COPVs have been found in Brazil after re-entry of the second stage and disintegration of the LOX tank. Despite a high-speed descent through the atmosphere followed by ground impact, they incurred only minimal impact damage.

Some of the copvs were found intact.  I don't think i ever heard anyone say that *all* of them were intact.  It only takes one to have a bad day.

In fact it was confirmed by SpaceX that at least one COPV was not intact. COPV #2 in the US LOX tank blew itself to pieces.

[vulture4]

>>friction ignition<<

Friction generating heat?  Ignition of what? Graphite requires extraordinary heat to ignite, even in pure oxygen, as NASA demonstrated back in the Sixties. Certainly not at cryogenic temperatures. The COPVs obviously did not burn up as they were found after the explosion. BTW Falcon COPVs have been found in Brazil after re-entry of the second stage and disintegration of the LOX tank. Despite a high-speed descent through the atmosphere followed by ground impact, they incurred only minimal impact damage.

Some of the copvs were found intact.  I don't think i ever heard anyone say that *all* of them were intact.  It only takes one to have a bad day.

In fact it was confirmed by SpaceX that at least one COPV was not intact. COPV #2 in the US LOX tank blew itself to pieces.

What condition was it in? How many pieces? Rupture of the tank due to mechanical disruption of some of the composite material could do something similar. But was it burned? I don't guess SpaceX has released any images.

[woods170]

>>friction ignition<<

Friction generating heat?  Ignition of what? Graphite requires extraordinary heat to ignite, even in pure oxygen, as NASA demonstrated back in the Sixties. Certainly not at cryogenic temperatures. The COPVs obviously did not burn up as they were found after the explosion. BTW Falcon COPVs have been found in Brazil after re-entry of the second stage and disintegration of the LOX tank. Despite a high-speed descent through the atmosphere followed by ground impact, they incurred only minimal impact damage.

Some of the copvs were found intact.  I don't think i ever heard anyone say that *all* of them were intact.  It only takes one to have a bad day.

In fact it was confirmed by SpaceX that at least one COPV was not intact. COPV #2 in the US LOX tank blew itself to pieces.

What condition was it in? How many pieces? Rupture of the tank due to mechanical disruption of some of the composite material could do something similar. But was it burned? I don't guess SpaceX has released any images.

What condition that COPV is in does not really matter. What matters is that it failed due to friction ignition of pooled LOX/SOX in between the overwrap and the liner. We can determine for a fact that the COPV was at least locally burned because LOX/SOX on itself does not burn. It needs a fuel. And that fuel was present: aluminium liner, carbon wrap, resin matrix. Don't forget that just about anything will burn in a pure oxygen environment.
So, was the COPV burned? Yes. Was the COPV blown to pieces? Very likely given the highly energetic nature of the event and the fact that it failed structurally while pressurized.