SpaceX Investigating the SpaceX Rocket Explosion of September 1, 2016

[Jonathan Scott]

From the look of the initial explosion, I still think that it happened after LOX and fuel were ejected by a COPV failure. That's what I've been saying since I first saw the video, and that would be consistent with what SpaceX is saying.

"TechX" (whoever that is) keeps posting these videos blaming an external explosion in the LOX pipe insulation. Although their latest version of that is more plausible than the initial suggestions, I personally think that would give a different appearance to the initial explosion. I don't know whether SpaceX uses polyurethane foam insulation as suggested nor whether it has taken care to seal the outside of the insulation sufficiently to prevent any significant absorption of LOX (given that low temperatures can degrade the foam structure), so I can't judge the plausibility of various technical aspects.

Does anyone know who this "TechX" is and what they are trying to achieve? Do they by any chance sell non-organic insulated pipework for cryogenic products?

 

[mfb]

Looks like TechX makes physics simulation software.

 

[Jonathan Scott]

Looks like TechX makes physics simulation software.

Is that definitely the same TechX as the Youtube stuff?

 

[mfb]

Hmm, it matched the topic, but then it would be odd to use WolframAlpha.

 

[jim hardy]

Have i already asked this ? When does the helium bottle get filled ?
Is it really at only 4 degrees K ? source http://newatlas.com/spacex-falcon-9-explosion-helium/45594/

 

[mfb]

As far as I remember the temperature was higher. The bottles are under high pressure and you want evaporation while releasing helium - subcooling the helium does not make sense.

 

[Jonathan Scott]

There's a new update on SpaceX news: http://www.spacex.com/news/2016/09/01/anomaly-updates

Here's the most interesting bit:

The investigation team has made significant progress on the fault tree. Previously, we announced the investigation was focusing on a breach in the cryogenic helium system of the second stage liquid oxygen tank. The root cause of the breach has not yet been confirmed, but attention has continued to narrow to one of the three composite overwrapped pressure vessels (COPVs) inside the LOX tank. Through extensive testing in Texas, SpaceX has shown that it can re-create a COPV failure entirely through helium loading conditions. These conditions are mainly affected by the temperature and pressure of the helium being loaded.

 

[jim hardy]

SpaceX has shown that it can re-create a COPV failure entirely through helium loading conditions. These conditions are mainly affected by the temperature and pressure of the helium being loaded.

Oh really ?

Hmmmm Helium loading not Lox loading like i'd thought. So much for post #144 https://www.physicsforums.com/threads/spacex-needs-us.884983/page-8#post-5581600

When do they fill the helium tanks? Are they immersed in LOX when filled ? Or does LOX loading come after helium ?

 

[Jonathan Scott]

When do they fill the helium tanks? Are they immersed in LOX when filled ? Or does LOX loading come after helium ?

I think helium loading starts before LOX loading is complete. The COPVs are immersed in LOX. There was more on the Nasa forum and elsewhere about it. The timeline of a previous launch has been published, but in this case they were using a modified timeline with very late loading of super-cold LOX and helium in order to get the maximum amount in and minimize the warm-up before launch. It is thought that this modified timeline could be a major factor in the anomaly.
The extra-cold LOX could perhaps have meant that LOX in the carbon fibre layers surrounding the COPV liners was cold enough to solidify when the liner was being filled with helium. There are theories that the embedded solid oxygen could have damaged the carbon during thermal contraction or even locally somehow got under such extreme pressure that it reacted with the carbon and possibly even ignited it locally. (I don't understand the mechanism of how that could happen).

 

[jim hardy]

The extra-cold LOX could perhaps have meant that LOX in the carbon fibre layers surrounding the COPV liners was cold enough to solidify when the liner was being filled with helium.

Thanks ! I hadn't thought of LOX freezing solid.

Oxygen freezes around 54K ? Helium liquefies around 3K ? Sure, it'd freeze on that COPV just like the frost on a good Gin&Tonic.

http://www.tvu.com/PShearStrSO2web.html
Experiments have been performed to test the shear strength of solid oxygen. Shear strength measurements were made in a cryostat by pulling a rod out of solidified liquid oxygen. The approximate shear strength of solid oxygen was measured as a function of temperature, increasing from 0.31 MPa at 45 K, to 4.46 MPa at 18 K. Solid oxygen was found to undergo plastic deformation at high temperatures, becoming increasingly strong and brittle as its temperature is decreased. Data and simple experiments confirmed a similarity of engineering material properties between solid oxygen and room temperature plastics.

I wonder if it expands or contracts on freezing ...

Sounds like they're getting close...
It's the small things of the Earth that confound the mighty.

 

[Jonathan Scott]

Oxygen freezes around 54K ? Helium liquefies around 3K ? Sure, it'd freeze on that COPV just like the frost on a good Gin&Tonic.

I don't think they use liquid helium, just helium gas cooled to get more in for the same pressure.

 

[mfb]

They don't fill in liquid helium, but it is possible that they overestimated the temperature somewhere.

 

[jim hardy]

They don't fill in liquid helium, but it is possible that they overestimated the temperature somewhere.

Ahhh thank you. As I've said often, I'm a plodder.

So if liquid oxygen at 1 atmosphere is 90 K
and they subcool it to 66K to get more density
and it surrounds the helium tank
and the helium approximates ideal gas , no phase change,

as LOX warms from 66 toward 90 K
helium tank pressure follows , from whatever pressure it had at 66K toward 90/66 = 1.36 X that pressure .

And it would be easy enough to get too much helium in there while it's cold. Especially if a temperature measurement is slow so that you think it's warming more slowly that actual and overestimate your 'headroom'.

Got it i think,
could be something as simple as sensor response time.

it'll be interesting to find what they meant by '...helium loading conditions' .

Two sayings from power plant: "Temperature taketh time" "Haste maketh waste"
thanks. old jim

 

[nikkkom]

Helium loading conditions might refer to helium ending up hotter than expected/designed for. When hot COPVs get splashed with cold LOX, composite overwrap could develop delaminations, which reduce strength and allow LOX ingress.

 

[liometopum]

Elon Musk says SpaceX finally knows what caused the latest rocket failure
http://www.theverge.com/2016/11/5/13533900/elon-musk-spacex-falcon-9-failure-cause-solved

Elon Musk Offers Icy Explanation For Spectacular SpaceX Falcon 9 Explosion
http://hothardware.com/news/musk-offers-explanation-for-falcon-9-explosion#GiSdfoWEJeV3dk7W.99

Elon Musk Says SpaceX Rocket Launches Might Resume Next Month
http://www.nytimes.com/2016/11/05/science/elon-musk-spacex-rocket-launches.htmlCould financial pressure have been a factor, as in this press release?:
Elon Musk’s SpaceX May Lose Inmarsat Launch Order
http://www.wsj.com/articles/elon-musks-spacex-may-lose-inmarsat-launch-order-1478165008

 

[jim hardy]

hmmm. they're dancing around mention of liquid helium...

from second link

According to Musk, the problem “basically involves liquid helium, advanced carbon fiber composites, and solid oxygen. Oxygen so cold that it actually enters solid phase.”

 

[Jonathan Scott]

hmmm. they're dancing around mention of liquid helium...

from second link

Yes, I think that under the pressure involved the helium is liquid when at full pressure, but I'm fairly sure it's nowhere near cold enough to be liquid at atmospheric pressure (that would be around 4K, but oxygen solidifies at around 54K). It does however seem that the helium was being loaded at below the freezing point of oxygen, because they think the failure involved liquid oxygen getting inside the composite wrap layers (presumably because of thermal contraction effects) and forming solid oxygen which can react with the carbon. They don't seem to give much away about the actual details.

 

[.Scott]

Additional details are provided in the NASA Space Flight forum (http://forum.nasaspaceflight.com/index.php?topic=41252.msg1606656#msg1606656).
LOX that manages to get between the metal helium tank and the composite overwrap flows back out as the tank is pressurized. But solid oxygen will not. This results in pure solid oxygen being forced against the carbon in the composite overwrap - apparently a condition that supports spontaneous combustion.

 

[Jonathan Scott]

Yes, I think that under the pressure involved the helium is liquid when at full pressure, but I'm fairly sure it's nowhere near cold enough to be liquid at atmospheric pressure (that would be around 4K, but oxygen solidifies at around 54K).

After catching up on the NASA Space Flight forum, I see that this has recently been discussed in there. As the critical temperature of helium is around 5K and they don't think SpaceX would be going anything like that cold, they say that the highly pressurized helium gas technically forms a supercritical fluid rather than a liquid, but Elon Musk has been referring to it as "liquid helium" anyway.

 

[jim hardy]

Thanks !

I guess if it's in there as a compressed liquid and gets warmed up, its pressure will follow saturation curve ?
i looked for a curve for helium but the ones i found didn't go to temperature so high as that of LOX at 1 atm .
Helium atoms being not sticky seems they'd act like an ideal gas very difficult to liquefy, that's why it condenses at only 5 degrees absolute? Somewhat higher temperature at pressure but still pretty doggone cold ?

Elon Musk has been referring to it as "liquid helium" anyway.

Maybe "Liquefied" would be a better term?

I just can't get past the thought of overpressurizing that helium tank by warming it with LOX.

A straightforward explanation will come out sooner or later .

old jim

 

[.Scott]

I guess if it's in there as a compressed liquid and gets warmed up, its pressure will follow saturation curve ?
i looked for a curve for helium but the ones i found didn't go to temperature so high as that of LOX at 1 atm .
Helium atoms being not sticky seems they'd act like an ideal gas very difficult to liquefy, that's why it condenses at only 5 degrees absolute? Somewhat higher temperature at pressure but still pretty doggone cold ?


Maybe "Liquefied" would be a better term?

I just can't get past the thought of overpressurizing that helium tank by warming it with LOX.

A straightforward explanation will come out sooner or later .

old jim

Here's a chart posted in that other forum:

LOX at 1 atmosphere will freeze at 54.36K; higher at higher pressures.

 

[Greg Bernhardt]

Elon Musk’s SpaceX says it has found cause of rocket explosion and plans to fly again Sunday
https://www.washingtonpost.com/news...ocket-explosion-and-plans-to-fly-again-sunday

 

[mfb]

http://www.spacex.com/news/2016/09/01/anomaly-updates.

Liquid or solid oxygen that went under parts of the helium containers (COPV) and lead to an ignition of the carbon fibers.

They'll go back to an older fueling procedure to avoid that, as long-term plan they want to redesign the COPV.

It will be a busy year for SpaceX, and we'll see how many things of their ToDo-list they can get:
In addition to returning to flights, SpaceX wants to launch one of the landed boosters soon (~February-April?), and the maiden flight of Falcon Heavy is planned for 2017 as well. It has 3 cores, one of them could also be a landed booster.
They want to make one final upgrade to the booster (the last update was the "full thrust" version) to get a bit more performance out of it, and to make re-use easier. The fairing that protects the payload is currently thrown away - a few million dollars per launch because it needs huge expensive oven to cure. They plan to recover it with parachutes.
Dragon V2, the manned version of the Dragon capsule, could have its maiden flight in 2017, and a manned flight in 2018.

 

[mheslep]

A novel aspect of the SpaceX launch procedures included fueling close to launch, made necessary (in part?) because of the super cooled liquid design SpaceX chose. Manned flights in the past were fueled prior to crew load. The accident, occurring during fueling, drew further attention to SpaceX procedures. I'm curious how SpaceX intends to address the fueling sequence as they proceed into manned flight.

October:

...Earlier this month, Gwynne Shotwell, the company’s president and chief operating officer, said investigators believed the cause likely was an operational issue, versus a design or manufacturing problem. One of the biggest questions, according to industry officials, is how the helium tank interacts with the surrounding supercooled liquid oxygen. The process is unfamiliar to most of the industry because such a supercooled oxidizer isn’t typically used on big rockets...

http://www.wsj.com/articles/spacex-probe-into-blast-focuses-on-fueling-issues-1477042203

From the WaPo article today:

The company said that in the short term, it plans to change the way it loads fuel. Eventually, it plans to change the design of the pressure vessels to prevent buckling.

Change fuel loading how? Fuel early? Time would be a factor with supercooled O2.

From the SpaceX link today:

...The corrective actions address all credible causes and focus on changes which avoid the conditions that led to these credible causes. 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. In the long term, SpaceX will implement design changes to the COPVs to prevent buckles altogether, which will allow for faster loading operations.

The new COPV 'configuration' is unspecified, as are the 'prior' 'helium loading operations'.

 

[Q_Goest]

I don't know where this guy got his info but it appears to jive with what I've seen published from SpaceX, plus the explanation and conclusion reached seems believable.

 

[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.