Question: Is this explanation for the SpaceX 1-Sept-2016 anomaly plausible? Or most of it at least (sans LHe)?

In the recent New Atlas article SpaceX fingers helium as cause of Falcon 9 rocket explosion a detailed scenario for the September 1st 2016 anomaly (fast fire, explosion) is presented. They say "SpaceX hasn't said exactly how the helium system failed, but the catastrophic nature of the explosion points to a likely scenario."

I recommend you read it there. I will show a few snippets:

Along with the propellants is the cryogenic helium system, which are insulated gas bottles filled with liquid helium that sit submerged in the LOX to keep them at a frosty −452.2° F (−269° C, 4 K, )...

NOTE!!: The scenario here starts with liquid helium, not high pressure helium gas as is generally discussed here in SXSE. After the helium tanks were moved inside the LOX tanks where the ambient temperature is so much lower - in fact below 70K for sub-cooled LOX - , is there a chance they were then changed to liquid helium tanks instead of gaseous helium tanks?

Briefly, they propose a cascade of events where each fluid violently boils when it comes in contact with the next, warmer fluid.

...the most likely scenario is that one of the helium bottles in the LOX tank suffered a sudden, massive failure, such as bursting open or a valve failing. The helium flashed into gas in the warmer liquid oxygen and the tank suddenly overpressurized.

This caused another failure between the LOX tank and the RP-1 section. Because the failure was so fast, the bulkhead most likely ruptured or shifted aside. At this point, the LOX and the RP-1 mixed. The RP-1 that the Falcon 9 uses is super-chilled to 20° F (−7° C, 266° K), but the LOX is cooled to −340° F (−207 °C, 66° K). When they mix, the LOX immediately boiled and the pressure in the tank skyrocketed...

They continue:

It's likely that either the tank blew from the pressure and the mixture ignited, or it ignited in the tank due to an electrical spark, friction, or some other cause. The rest was a matter of combustion and gravity as the first stage, then the propellants in the satellite payload exploded, and the remaining LOX flooded into underground passages to feed more fires.

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    $\begingroup$ Isn't LHe something like 4°K? Even oxygen would freeze on contact $\endgroup$ – jkavalik Sep 25 '16 at 14:55
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    $\begingroup$ Correct, you need 4K. I've seen no indication SpaceX has switched to liquid He. Having an LHe tank inside the LOX tank would complicate the design immensely. $\endgroup$ – Hobbes Sep 25 '16 at 15:01
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    $\begingroup$ I would be extremely surprised to find that any booster utilized liquid helium in its pressurant system. I can't think of any advantage and there are plenty of disadvantages. $\endgroup$ – Organic Marble Sep 25 '16 at 17:30
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    $\begingroup$ Where this article is correct, it is not original. Where it is original, it is not correct. $\endgroup$ – Organic Marble Sep 25 '16 at 17:34
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    $\begingroup$ spaceflightnow.com/2016/09/23/… claims that the pressure in the bottles is 5500 psi (~375 bar). It wouldn't make sense to pressurize LHe to such a high pressure, as liquids are not well compressible. $\endgroup$ – oefe Sep 25 '16 at 18:23

Edit Jan 2, 2017: Well, it was a COPV bottle after all. There were buckles in some liners where super-cooled liquid oxygen pooled. From NASAspaceflight article, quoting the results of the investigation:

Each stage of Falcon 9 uses COPVs to store cold helium which is used to maintain tank pressure, and each COPV consists of an aluminum inner liner with a carbon overwrap. The recovered COPVs showed buckles in their liners. Although buckles were not shown to burst a COPV on their own, investigators concluded that super chilled LOX can pool in these buckles under the overwrap.

When pressurized, oxygen pooled in this buckle can become trapped; in turn, breaking fibers or friction can ignite the oxygen in the overwrap, causing the COPV to fail. In addition, 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.

So, what follows has useful points, but leaned too much on a statement from another NSF article that sources were extremely doubtful the COPVs were the cause.

If by 'plausible' you mean that an exploding helium bottle could have caused an explosion like the one we saw, yes. However, the article's assertion that 'it is the most likely scenario' is not plausible.

It is always risky trying to get an accurate analysis of a very complex subject from a news source that isn't dedicated to that area. Gigantic explosions always get a lot of coverage, and a lot of news outlets have covered this that hardly ever mention space industry events. New Atlas reports on these kinds of things regularly, but there are several inaccuracies in the article. In response to the same update from SpaceX on the investigation, two of the top magazines dedicated to the space industry published long articles: NASA Space Flight and Space Flight 101. These articles use a bit of lingo so are somewhat harder for the general public to follow. Some googling might be needed, but not so much as to be onerous. Most of what follows is taken from them, with a few additional sources which are listed.

Here is what SpaceX's statement says:

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. [Updated 09/24: At this time, the cause of the potential breach remains unknown.].

There are lines and connectors leading to the helium bottles, so a breach doesn't mean it was in the bottle. Also, because the bottles were being filled at the time, a breach doesn't mean the a piece failed, per se, in the sense that, if the problem was (for instance) that a pump in the ground equipment failed, it could have caused pressure waves to travel down the lines and into the bottles, pushing them beyond their design limits. In that case it isn't the 'fault' of the lines or the bottles.

Certainly because the helium bottles hold so much pressure, if one failed suddenly it could well look like the explosion that happened. The problem is, why would it do that. This is an important point.

The pressure vessels that hold the helium, the COPVs, have had problems in the past. They are spheres made of Kevlar embedded in a polymer, with a plastic or metal liner, that are filled with very cold helium gas up to a pressure of about 5500 psi. Failure of a strut holding one caused the loss of the CRS-7 vehicle when the COPV was thus breached.

That failure was during flight, when the structure is subjected to a lot of vibration and high acceleration. The helium vessel did not explode as a result, though it was at full pressure. In the AMOS-6 case, the COPVs had only been partially filled, and were not under any of the stresses that occur during launch. The NASA Space Flight article, says 'sources note they are extremely skeptical a COPV could be at fault, due to the amount of focus placed on them after the CRS-7 failure' - which is to say, because they'd lost a vehicle due to a breach in a COPV, they worked to make sure that wouldn't happen again. So, such a helium bottle exploding when it isn't even full, is under no extra g-force, and there is no vibration, would be very hard to explain. Something coming from elsewhere might have subjected such a bottle to excessive forces that led to a rupture in it (or in a line leading to it) but then the issue is that something, not the bottle itself.

In the CRS-7 flight, there was 0.9 seconds between the first indication of a problem and the loss of telemetry. In the AMOS-6 case, there was only 0.09 seconds between signs of a problem and the loss of telemetry. COPVs hold so much pressure that a massive rupture is like a small bomb going off, so it is awfully tempting to focus on them (and I have speculated a lot about it myself). However, there is a lot at stake in this investigation, and the FAA, NASA, and the Air Force are taking a much more active hand in it than they did last time. If the sources used by NASA Space Flight - and they get the best sources as the magazine is highly respected - say they are extremely skeptical a COPV caused this, then I am too.

  • $\begingroup$ Thank you for taking the time to bring together such a thorough discussion! The two articles you've linked are definitely worth taking the time the read. $\endgroup$ – uhoh Sep 26 '16 at 2:34
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    $\begingroup$ Interesting, sounds like a design flaw, and although I have not followed it closely, I thought SpaceX was claiming this failure was purely caused by operational issues. $\endgroup$ – Organic Marble Jan 2 '17 at 16:08
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    $\begingroup$ Possibly changes in procedure (lower temperature LOX or helium) uncovering a design flaw that wasn't an issue with the old procedure. $\endgroup$ – Hobbes Jan 2 '17 at 16:32
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    $\begingroup$ The NSF article did in fact comment right after the quoted bit that SpaceX intends to stop using super-cooled LOX and helium until newly designed COPVs prove themselves capable of handling the stresses. $\endgroup$ – kim holder Jan 2 '17 at 21:08
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    $\begingroup$ @OrganicMarble the part about not using supercooled LOX should be fixed now to not using extremely cooled helium. There is nothing suggesting reverting to "normal" LOX in the SpaceX statement and it is maybe not even possible to do because the engines and pumps are optimized for the more dense fluids now. $\endgroup$ – jkavalik Jan 3 '17 at 16:59

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