Investigating the SpaceX Rocket Explosion of September 1, 2016

[Jonathan Scott]

From some of the popular descriptions, people might think that the explosion was directly caused by the COPV overwrap catching fire when solid oxygen was crushed against it. However, that wouldn't have enough energy to do much immediate damage on its own, as there was very little material to burn. All it did was cause the COPV failure (and perhaps provided a source of ignition for the main fire later, although many other things could also have done that). As Scott Manley also mentioned, it isn't even certain that the solid oxygen caused a fire. Perhaps it could have merely created ridges underneath the overwrap layer which overstressed it, causing a split.

The second step of the problem was presumably that the pressure shockwave from the helium released suddenly by the failed COPV split the second stage (probably along some sort of seam), opening up the LOX tank and the top part of the fuel tank, ejecting a LOX-rich mixture. (Note of course that LOX does not burn on its own, and fuel will only burn slowly in the absence of LOX).

Finally, that mixture caught alight outside the second stage, creating a flame front which initially ran through it at very high speed. However, once the flame front had reached the limit of the expanding shower of fuel and LOX, it slowed down significantly. I think this means that at the time the mixture was ejected, it was not on fire and was moving significantly slower than the subsequent flame front. I must admit I can't see any sign of anything being sprayed out in the previous two or three frames, which one might expect from that theory, but there's a lot of cloudy stuff around anyway.

The above is my personal interpretation of the details, which is mostly the same as my initial guess earlier in this thread of a COPV failure. I suggested that buoyancy forces or stresses due to thermal contraction could be responsible for the COPV failing at that point. Obviously the buoyancy forces during launch would be higher, so that didn't seem likely, but I think the thermal contraction idea was close.

Does anyone know of any additional information on this apart from the SpaceX anomaly update on their own website? Was anything interesting posted to those Nasa forums, which I don't have the time to read through?

 

[jim hardy]

Effects from sequence of filling is not yet clear to me.
Carbon fiber i believe to have virtually zero coefficient of thermal expansion
but aluminum's is 22 micro per K . So the liner could shrink by ~1/2% when cooled to LOX temperature?

What I've been missing is helium's critical point is so doggone cold we need only consider gas phase ?

A COPV built at room temperature
when cooled
will see its aluminum liner shrink and pull away from the overwrap
unless it's already pressurized enough to stretch the aluminum out to overwrap

So,
if it's cooled before pressurized , there's movement between liner and overwrap and LOX can get into the annulus
if it's pressurized before cooled , there's no annulus formed, but there's a balancing act because the fill gas also shrinks by gas law reducing pressure so you have to add more to keep pressure up.

A question i have , that's probably answered already,
Just how far subcooled was their LOX ? 25K to 50K should double the pressure in COPV. Surely they monitor that ?

plodding along,

old jim.

 

[mfb]

Oxygen has a melting point of 55 K and atmospheric pressure, slowly increasing with pressure. You don't want the oxygen to get colder than that, and you cannot fill in oxygen colder than 55 K.

 

[Jonathan Scott]

What I've been missing is helium's critical point is so doggone cold we need only consider gas phase ?

That diagram appears to relate to the rare helium-3. You need a diagram for the usual helium-4. But the general conclusions hold anyway.

 

[mheslep]

...
However, that wouldn't have enough energy to do much immediate damage on its own, as there was very little material to burn

The overwrap is carbon and resin. Ignition of the first 30 grams of carbon in pure O2 produces a megajoule. In a rocket built at the margin to save weight, I think a very little carbon combustion is required to burst the tank. With O2 loose along with an ignition source, all the structure burns.

 

[jim hardy]

That diagram appears to relate to the rare helium-3.

Oops !

it's difficult to find a chart that goes above ~10K. Thanks...

Oxygen has a melting point of 55 K and atmospheric pressure, slowly increasing with pressure. You don't want the oxygen to get colder than that, and you cannot fill in oxygen colder than 55 K.

okay, Thanks. It's becoming clearer to me.

What Elon said was the temperature was just above freezing and was -340°F or -207°C.
(reply #30 at https://forum.nasaspaceflight.com/index.php?PHPSESSID=pht92iam9tpu1iocsng4l1ak41&topic=39072.20 )

-207 is 66K ?
from this

Boiling point is 90K

warming an ideal gas from 66 to 90 should raise its pressure by 90/66 = 1.36
i don't know yet how close is helium in those tanks to ideal

plodding along,

old jim

 

[jim hardy]

this appears to be "Hitchiker's Guide to Helium"

but it's still downloading...

http://nvlpubs.nist.gov/nistpubs/Legacy/TN/nbstechnicalnote631.pdf

 

[Jonathan Scott]

The overwrap is carbon and resin. Ignition of the first 30 grams of carbon in pure O2 produces a megajoule. In a rocket built at the margin to save weight, I think a very little carbon combustion is required to burst the tank. With O2 loose along with an ignition source, all the structure burns.

My point is that the energy of any reaction involving a patch of the overwrap and solid oxygen (at least up to the point where it lost integrity) would have been small compared with the energy released by the resulting COPV failure when the overwrap split, and that of course would be small compared with the energy released by the subsequent burning of the LOX / fuel mixture. Just based on orders of magnitude, I don't think that the pressure shock wave from a small amount of burning overwrap material would itself have caused immediate splitting of the 2nd stage, but that from a COPV failure would have easily been able to do so.

Although I understand that immersing almost anything in LOX will enable it to burn rapidly and fiercely, I still feel it's more likely that any exothermic reaction between the overwrap and solid oxygen would have been very localized, along a stress ridge or similar produced after buckled liner was pushed back into shape by the helium.