SpaceX Investigating the SpaceX Rocket Explosion of September 1, 2016

[Dotini]

Video of successful launch shows details of strongback, fueling process,

 

[jim hardy]

The squeak and pop are further support, to me, that the whole thing started on the surface of the rocket. I don't think that the squeak and pop could have come from inside the rocket.

I played it back with good headphones and volume turned all the way up.
If the sound is 12 seconds behind the video

there are several squeaks 1:06, 1:09, 1:12 at moment explosion on video , the big squeak at 1:17 followed by pop at 1:18, followed by what sounds like a bird singng at 1:20, .
1:12 through 1:23 there is a rapid clicking or tinkling sound that is easy to miss because the video is so pyrotechnic.

If I'm right about the 12 second delay, the explosion sound is at 1:24 just before payload hits the ground. Thereafter booms dominate.

I'd say from the sounds something started 18 seconds before visible explision. Could be birds between mike and rocket, or it could be thin aluminum crinkling as it gets cooled rapidly by a LOX leak. Ever listen to an air cooled motorcycle engine cool down? It clicks and ticks quite a lot...

I've forgot now which site had sound synchronized with video to remove the delay.
If this Rowvid one is synchronized then I'm dead wrong (again) , it's just the explosion woke up the neighborhood birds.
http://rowvid.com/?v=_BgJEXQkjNQ

old jim

 

[Dotini]

Video presents case for ignition promoted on the strongback preceding the explosion of the Falcon rocket carrying AMOS-6.

 

[Jonathan Scott]

As I've said before, it appears that ignition occurred outside the rocket, either on its surface or further out.
If the indicated pipe is indeed for LOX (others have suggested it is an air duct) then it does look vulnerable, but I don't understand why a LOX pipe would be going higher than the LOX umbilical.
I haven't viewed the entire video, but what is the suggested fuel source? Yes, many things will burn strongly in pure oxygen, but the initial flame spreads so rapidly it suggests a significant amount of pre-mixed fuel and oxygen.

 

[Dotini]

I haven't viewed the entire video, but what is the suggested fuel source?

Haven't viewed the video? It's only 9 minutes. The authors of the video suggest the fuel source as leaked kerosene or pipe insulation, seals, gaskets, or even the metal itself, potentially saturated or caked in recondensed LOX. In a previous video it is discussed how strongback kerosene pipes may at times flow in the reverse direction.

 

[Jonathan Scott]

Haven't viewed the video? It's only 9 minutes. The authors of the video suggest the fuel source as leaked kerosene or pipe insulation, seals, gaskets, or even the metal itself, potentially saturated or caked in recondensed LOX. In a previous video it is discussed how strongback kerosene pipes may at times flow in the reverse direction.

I'd like to hear something definitive about the locations of the LOX and RP-1 pipes in the strongback. Does anyone have any official information?

The umbilicals going across to the rocket are certainly lower down than the ignition point so there doesn't appear to be an obvious point in having pipes higher than that. I would not have expected leaking kerosene (which is of course still liquid at ambient temperature) to mix enough with any form of oxygen to explode initially with that much speed and energy. One can see how it burns later; the part which has been hit with LOX burns fiercely, but the part which is mixing with air looks more like a rain of fire. If it were leaking as a spray from a small aperture under pressure, it's possible that it could mix well with the oxygen, but as RP-1 loading was supposed to be complete and the wind was away from the strongback it would seem strange to me that fuel under pressure was still in the vicinity.

 

[Dotini]

The specific configuration of the Falcon and its strongback may have changed between JCSAT-16 and AMOS-6. But Mr Musk seems loathe to share such details.

 

[jim hardy]

How do they get the hydrazine into the payload ? Is it in the countdown sequence ?

I read hydrazine self ignites with LOX .
In that climate a squirt or drip of it down into a cloud of O2 boiloff seems way more plausible ignition source than static electricity.

 

[mfb]

Hydrazine should have been in the satellite before they put it into the rocket. It is liquid at room temperature, no need to have some fancy fueling steps later.

 

[mfb]

http://www.spacex.com/news/2016/09/01/anomaly-updates, bold from me:

At this stage of the investigation, preliminary review of the data and debris suggests that a large breach in the cryogenic helium system of the second stage liquid oxygen tank took place. All plausible causes are being tracked in an extensive fault tree and carefully investigated. Through the fault tree and data review process, we have exonerated any connection with last year’s CRS-7 mishap.

They want to have the launch pad repaired and ready by November.

Here is a video of those helium bottles bursting (on purpose in the video), 6 psi and 18 psi should be 6000 psi (~40 MPa, about the pressure SpaceX uses) and 18000 psi (~120 MPa):



COPV = Composite Overwrapped Pressure Vessel, titanium with carbon fiber. Fragments of carbon fiber in liquid oxygen is a very explosive mixture.

 

[.Scott]

SpaceX reports that the explosion was caused by Helium overpressure, but the cause of that is still being investigated.
http://www.spacex.com/news/2016/09/01/anomaly-updates

The timeline of the event is extremely short – from first signs of an anomaly to loss of data is about 93 milliseconds or less than 1/10th of a second. The majority of debris from the incident has been recovered, photographed, labeled and catalogued, and is now in a hangar for inspection and use during the investigation.

At this stage of the investigation, preliminary review of the data and debris suggests that a large breach in the cryogenic helium system of the second stage liquid oxygen tank took place. All plausible causes are being tracked in an extensive fault tree and carefully investigated. Through the fault tree and data review process, we have exonerated any connection with last year’s CRS-7 mishap.

 

[.Scott]

For those following the NASA Space Flight (NSF) forum, they have used this SpaceX announcement to start a new thread on the subject (and closed the previous one). It is now: http://forum.nasaspaceflight.com/index.php?topic=41252.0

By the way, the NSF forum had focused on external causes to the incident - so most of those posters were way off. A notable exception was "Jim" - not surprisingly, someone with quite some background in the field.

 

[Dotini]

Apparently they think the root cause could yet be traced back to the pad.

https://www.yahoo.com/news/spacex-releases-details-recent-rocket-181133330.html
The root cause of the accident may also change the amount of time SpaceX has to stand down from launching. If the issue is related to the rocket itself, it's possible the redesign and testing process would force a more extended grounding, while if the problem is traced back to the pad, it might be a quicker fix, industry experts have said.

 

[mfb]

Sure, ground equipment has no strong weight and size limits, that is way easier to change.

 

[jim hardy]

I did some reading about aluminum and titanium at cryogenic temperatures.
The exotic alloys of titanium you have to use are very sensitive to how hot you make the weld joint during fabrication. The heat affected zone adjacent weld must remain below 1800F or it's brittle and cannot be annealed by slow cooling. Something about alpha and beta phases in the metals crystal structure.

The article was dated 1974, long enough ago that some new little fabricator shop might not be aware ? All their gray-hairs retired, or it got outsourced overseas ?

I'm no metallurgist just had one course about 1966 and remember a few terms so won't embarrass myself by trying to explain further.

If i can find the article again will link it. Who made that COPV tank ?

 

[Jonathan Scott]

Who made that COPV tank ?

I believe SpaceX now make their own COPVs.

 

[jim hardy]

found the article

https://app.aws.org/wj/supplement/WJ_1974_03_s117.pdf

Conclusions
Welding procedures for Ti-6211 were established using the pulsed GMAW process and sound quality welds were produced. Hot ductility tests made with the Gleeble did not show any anomalous behavior and did not indicate a susceptibility to hot cracking. Detailed examination of the heat-affected zone using synthetic specimen techniques revealed that some problems may be anticipated. Poor impact strengths, lower than those of the weld fusion zone are developed in the areas of the heat-affected zone immediately adjacent to the fusion zone; specifically those regions which reach temperatures above 1800 F. The structure developed in these regions in equilibrium a-platelets with a' martensite within coarse former beta grains produced as a result of rapid cooling from a temperature at which it is fully beta.

i don't know if that's even the same alloy they used for their tank

what it does tell me is welding those specialty alloys is an acquired skill.

Maybe they'll find some pieces of the tank and tell us how it failed..

 

[Jonathan Scott]

i don't know if that's even the same alloy they used for their tank

The SpaceX COPVs are made of aluminium (or aluminum, depending on your local preference), which is commonly used for COPVs but not usually for ones which are immersed in LOX.
There have been some comments which suggest that aluminium could cause problems in this case, which I understand is due to aluminium's thermal expansion/contraction profile being less compatible with that of the overwrap layer.

 

[jim hardy]

i'm just curious about it. Part "old fire horse" reaction of a guy who spent too many years as a troubleshooter to not be curious, and i had that one metallurgy course 50 years ago...

LOX is -193C at 1 atmosphere, i don't know what temperature Spacex keeps theirs.

surely Spacex engineers know their cryogenic alloys
A quick search turned up
http://www.totalmateria.com/Article23.htm

Low-Temperature Properties. Aluminum alloys represent a very important class of structural metals for subzero-temperature applications and are used for structural parts for operation at temperatures as low as -270oC.

Below zero, most aluminum alloys show little change in properties; yield and tensile strengths may increase; elongation may decrease slightly; impact strength remains approximately constant. Consequently, aluminum is useful material for many low-temperature applications.

The chief deterrent is its relatively low elongation compared with certain austenitic ferrous alloys. This inhibiting factor affects principally industries that must work with public safety codes. A notable exception to this has been the approval, in the ASME unfired pressure vessel code, to use alloys 5083 and 5456 for pressure vessels within the range from -195 to 65oC. With these alloys tensile strength increases 30 to 40%, yield strength 5 to 10% and elongation 60 to 100% between room temperature and -195oC.

The wrought alloys most often considered for low-temperature service are alloys 1100, 2014, 2024, 2219, 3003, 5083, 5456, 6061, 7005, 7039 and 7075. Alloy 5083-O which is the most widely used aluminum alloy for cryogenic applications, exhibits the following cooled from room temperature to the boiling point of nitrogen (-195oC):

• About 40% in ultimate tensile strength
• About 10% in yield strength.

Retention of toughness also is of major importance for equipment operating at low temperature. Aluminum alloys have no ductile-to-brittle transition; consequently; neither ASTM nor ASME specifications require low-temperature Charpy or Izod tests of aluminum alloys. Other tests, including notch-tensile and tear tests, assess the notch-tensile and tear toughness of aluminum alloys at low temperature characteristics of welds in the weldable aluminum alloys.

Compared with other alloys, alloy 5083-O has substantially greater fracture toughness than the others. The fracture toughness of this alloy increases as exposure temperature decreases. Of the other alloys, evaluated in various heat-treated conditions, 2219-T87 has the best combination of strength and fracture toughness, both at room temperature and at -196oC, of all the alloys that can be readily welded.

Alloy 6061-T651 has good fracture toughness at room temperature and at -196oC, but its yield strength is lower than that of alloy 2219-T87. Alloy 7039 also is weldable and has a good combination of strength and fracture toughness at room temperature and at -196oC. Alloy 2124 is similar to 2024 but with a higher-purity base and special processing for improved fracture toughness. Tensile properties of 2124-T851 at subzero temperatures can be expected to be similar to those for 2024-T851.

Several other aluminum alloys, including 2214, 2419, 7050 and 7475, have been developed in order to obtain room-temperature fracture toughness superior to that of the other 2000 and 7000 series alloys. Information on subzero properties of these alloys is limited, but it is expected that these alloys also would have improved fracture toughness at subzero temperatures as well as at room temperature.

Fatigue Strength. Results of axial and flexural fatigue tests at 106 cycles on aluminum alloy specimens at room temperature and at subzero temperatures indicate that, for a fatigue life of 106 cycles, fatigue strength is higher at subzero temperatures than at room temperature for each alloy. This trend is not necessarily valid for the tests at higher stress levels and shorter fatigue lives, but at 106 cycles results are consistent with the effect of subzero temperatures on tensile strength.

It'll be interesting to see if they used one of those mentioned..Looks like cooling the tank ought to shrink it more than i'd imagined
http://www.noao.edu/ets/gnirs/SDN0013-02.htm

i'm no expert, just surfing net for background.old jim

 

[Dotini]

IMO the problem is solved by TechX. They have identified the explosive foam insulation on the strongback as combining with venting LOX, with ignition promoted from corona discharge. Then a couple of the COPV tanks were dislodged from their bosses from the concussive explosion and flying strongback shrapnel. A much safer insulation is discussed. And there you have it. SpaceX will be back in business soon!

 

[Jonathan Scott]

I've viewed their earlier stuff and I'm very sceptical. It is certainly true that it seems a bit risky having a lot of pure oxygen around flammable materials, but the time factors don't seem consistent with that.

The amount of material which catches fire in the first frame and the implied speed of the flame suggests a significant amount of LOX plus some fuel was already mixed. I still think it's more likely that some major internal failure within the helium high pressure system caused an external rupture around the common bulkhead area and that the ejected LOX and fuel then caught light.

 

[jim hardy]

I've viewed their earlier stuff and I'm very sceptical.

seconded.

I'm betting on a weld failure. Metallurgy around welds in aluminum is troublesome. Still haven't heard what alloy COPV is made from.
https://app.aws.org/wj/supplement/WJ_1987_03_s73.pdf
Is it true they used a colder LOX on this rocket, 66K ? About 20 degrees colder ?

 

[Dotini]

Seedy conspiracy theories are tossed into the case by the Washington Post:
https://www.washingtonpost.com/amph...b60514-874c-11e6-a3ef-f35afb41797f_story.html

At a conference in Mexico earlier this week, Musk said that finding out what went wrong is the company’s “absolute top priority,” but he said what caused the explosion is still unknown.

“We’ve eliminated all of the obvious possibilities for what occurred there,” he said. “So what remains are the less probable answers.”

 

[jim hardy]

Two questions are nagging at my alleged vrain oops my alleged brain

First one stems from high school chemistry

Inside that Helium tank...
pv = nrt

Pcold X Vcold = nr Tcold
Phot X Vhot = nr X Thot

Pcold = Phot X Vhot/Vcold X Tcold/Thot

if that aluminum data is right, it shrinks 20% from room temp to ~ -200c
then vcold/vhot = around 0.8³ = 0.512, call it 1/2

Pcold = Phot X 2 X Tcold/Tot , since Tcold is maybe 70K and Thot maybe 300K, Pcold = Phot X 2 X 70/300 = 0.45 Phot
the tank should lose pressure as it cools because the gas contracts more than the aluminum does.

Okay that was one question i had. How does tank respond to cooling, does its stress increase or decrease and it looks like it decreases overall.Next question is one of heat transfer ,
Does the helium inside the tank cool quickly ?
or is there a time when the aluminum skin is stretched tighter around the helium while cooling progresses inward through the gas as if through layers of an onion ?
That takes time and pressure won't fall so quickly as if cooling of the gas were immediate..
Seems the tank's skin would be sensitive to rate of cooling because the tension in it depends on its own shrinkage versus that of the gas it surrounds.
Skin gets cold first.

Surely they've calculated that out.
I assume LOX tank fill process include measurement of helium tank pressure ?

just rambling, sometimes such musings help one along in troubleshooting.

 

[nsaspook]

Seedy conspiracy theories are tossed into the case by the Washington Post:
https://www.washingtonpost.com/amph...b60514-874c-11e6-a3ef-f35afb41797f_story.html

At a conference in Mexico earlier this week, Musk said that finding out what went wrong is the company’s “absolute top priority,” but he said what caused the explosion is still unknown.

“We’ve eliminated all of the obvious possibilities for what occurred there,” he said. “So what remains are the less probable answers.”

I liked the UFO one better than the 'Grassy Knoll' gunman.

 

[Dotini]

I liked the UFO one better than the 'Grassy Knoll' gunman.

I wasn't aware of any alien conspiracy - what has Mr Musk done to offend them? Remember, those are birds and bugs flying over the rocket. But Mr Musk has real rivals, critics and potentially enemies in a wide variety of industries, government agencies and companies right here on Earth. If in fact there was a conspiracy against the AMOS-6 mission, then not only Musk, but Zuckerberg and the state of Israel were also victims. And now they're your mortal enemies if you are the guy with the laser or rifle on top of the ULA building. For now, better to blame the shoddy, LOX-infused strongback pipe insulation and a stray spark, or some other one-in-million metal failure, despite the amusement value inherent in conspiracies.

 

[Dotini]

More video and analysis of the strongback piping arrangement from TechX.

 

[Jonathan Scott]

More video and analysis from TechX.

I'll admit they do have some plausibility, if their technical facts are correct. It appears (correct me if I'm wrong) that they are saying that during the LOX loading, the outsides of the insulated pipes could get cold enough for some of the venting cold oxygen to recondense to liquid and get absorbed into the insulation. Presumably this absorbed liquid would then decrease the effectiveness of the insulation and cause the outside to get even colder, amplifying the effect and eventually creating a significant amount of a known explosive mixture which could be set off very easily.

I previously found their explanation implausible because of the size of the initial explosion, but if it is possible that a large volume of the insulating material had absorbed liquid oxygen before the explosion then that could fit the effects.

I'd agree that the first flash and the fact that it did not expand significantly after that could be explained as an external explosion primarily involving a solid form of fuel (insulation with absorbed liquid oxygen), in which case the next step would be caused by the shockwave from the initial explosion damaging the second stage and breaking the helium pressure system, leading to total destruction.

 

[Jonathan Scott]

This was the earlier TechX calculation of the potential explosive yield from polyurethane foam insulation soaked in condensed LOX:

 

[mfb]

We believe that the composite over wrapped pressure vessel [the helium bottle], known as a COPv, let go in the tank. What caused it, the exact reason it let go, we’re still investigating. I don’t believe it was a ground system cause, but we’re still looking at the data.
[...]
The more than likely — the overwhelmingly likely — explanation is that we did something to that rocket. And we’re going to find it and we’re going to fix it.

Source

If it is some problem of the procedure, it can be very easy to fix - load something slower/faster, adjust some lines of code to keep something at a different temperature or something like that.