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

[Roy_H]

Excuse me for not reading all of this thread. But since the focus seems to be on helium, how does a helium failure (by any means) cause an explosion on the outside of the rocket?

[DaveS]

Excuse me for not reading all of this thread. But since the focus seems to be on helium, how does a helium failure (by any means) cause an explosion on the outside of the rocket?

Nobody has said the failure was external to the launch vehicle. It's the still very much possible that the failure originated internally. The tanks that are used to pressurize the propellant tanks are located inside the actual propellant tanks and are called Composite Overwrap Pressure Vessels (COPVs).

[Roy_H]

Excuse me for not reading all of this thread. But since the focus seems to be on helium, how does a helium failure (by any means) cause an explosion on the outside of the rocket?

Nobody has said the failure was external to the launch vehicle. It's the still very much possible that the failure originated internally. The tanks that are used to pressurize the propellant tanks are located inside the actual propellant tanks and are called Composite Overwrap Pressure Vessels (COPVs).

Ok, then how does helium released into oxygen create and explosion? Over pressure could rupture the tank, but that is not an explosion. AFAIK helium and oxygen are not combustible gases. Wouldn't it just rapidly vent without burning? And if it did, wouldn't we see a jet of burning gas coming out the side? What we do see is a ball of exploding gas on the side of the TE.

[DaveS]

Excuse me for not reading all of this thread. But since the focus seems to be on helium, how does a helium failure (by any means) cause an explosion on the outside of the rocket?

Nobody has said the failure was external to the launch vehicle. It's the still very much possible that the failure originated internally. The tanks that are used to pressurize the propellant tanks are located inside the actual propellant tanks and are called Composite Overwrap Pressure Vessels (COPVs).

Ok, then how does helium released into oxygen create and explosion? Over pressure could rupture the tank, but that is not an explosion.

They're  currently looking at a large breach in the cryogenic helium system of the second stage liquid oxygen tank. The wording seems to suggest a burst of one or more of the COPVs that are located in the second stage LOX tank. The "explosion" might just be a mixing of the LOX and RP-1. For example, Challenger is always described as having exploded when there was no explosion of any kind, only a large cloud of LOX and LH2 from the disintegrated External Tank with some localized mixnings of N2O2 and MMH from the orbiter RCS and OMS propellant tanks. 

[Herb Schaltegger]

Excuse me for not reading all of this thread. But since the focus seems to be on helium, how does a helium failure (by any means) cause an explosion on the outside of the rocket?

Nobody has said the failure was external to the launch vehicle. It's the still very much possible that the failure originated internally. The tanks that are used to pressurize the propellant tanks are located inside the actual propellant tanks and are called Composite Overwrap Pressure Vessels (COPVs).

Ok, then how does helium released into oxygen create and explosion? Over pressure could rupture the tank, but that is not an explosion. AFAIK helium and oxygen are not combustible gases. Wouldn't it just rapidly vent without burning? And if it did, wouldn't we see a jet of burning gas coming out the side? What we do see is a ball of exploding gas on the side of the TE.

Even discarding the possibility of a ruptured COPV with flying fragments of carbon fiber/epoxy chunks and bits of metal liner flying around inside the LOX tank, an overpressure alone could well rupture the common bulkhead, very thoroughly mixing RP1 and LOX and destroying the stage; the specific ignition event is almost a formality at that point.

[CameronD]

Ok, then how does helium released into oxygen create and explosion? Over pressure could rupture the tank, but that is not an explosion. AFAIK helium and oxygen are not combustible gases. Wouldn't it just rapidly vent without burning? And if it did, wouldn't we see a jet of burning gas coming out the side? What we do see is a ball of exploding gas on the side of the TE.

Even discarding the possibility of a ruptured COPV with flying fragments of carbon fiber/epoxy chunks and bits of metal liner flying around inside the LOX tank, an overpressure alone could well rupture the common bulkhead, very thoroughly mixing RP1 and LOX and destroying the stage; the specific ignition event is almost a formality at that point.

The only problem I see with this scenario is that, from the video, the size and behaviour of the LOX cloud escaping from the LOX vent during the filling process is not appreciably changed until the entire structure is well "ruptured".   Since filling was in progress (LOX in, GOX out) I would have expected we'd see the effect of any overpressure event in the LOX tank at the vent first, long (relatively speaking) before the explosion cloud.

(The above doesn't preclude the same COPV failure scenario in the RP-1 tank though)

[FinalFrontier]

more thing

... . They have done this in the past after failures, CRS 7 included (IE upgrading to FT ect, using the time to just redesign things all the way around before flying again).

For the record CRS7 was the last of the previous design and the next flight was always going to be FT, that was a done deal long before the failure.

FT would have used the same struts and same material for the struts more likely than not had that failure not happened. Additionally, I (and I am not alone) remain unconvinced a strut failing was the actual failure mode as opposed to the liner or part of the COPV where the strut attached failing because it de-laminated.

We will never know because it would produce almost exactly the same readings on flight instrumentation, but we do know some struts in the same batch were found to have material defects in post accident investigation.

Here is why it doesn't matter though: the bottom line was the helium pressurization system failed due to quality control lapse. The same thing happened again here though its probably for a very different technical reason.

The issue being the event chain and result were the same and have a very common problem even though actual failure/material failure is different.

But respectfully, as much as you and others may be unconvinced that a failed strut caused CRS7 , I doubt that you have access to the data that is required for coming to a rational conclusion. As I understand it, there was a fluctuating pressure signal that was incompatible with a catastrophic COPV failure and which could only be explained in terms of a broken strut allowing the strongly buoyant COPV to bend a pressure line. If you have evidence or knowledge to the contrary, please share it - please don't just declare it unsatisfactory in your judgement if you have no data to base your judgement on.

I'm also unconvinced by your declaration that there was a quality control lapse in this latest anomaly. Where is your evidence that there was no design flaw, no mishandling, no environmental factor...?

I was going to write pretty much the same post. FF's posts are full of conjecture based on no evidence whatsoever. He doesn't have access to the reports from the SpaceX engineers on CRS7 (The accident report states it was a strut failure - I tend to believe the actual official report on the subject), or any view SpaceX quality control process, so his claims are absolute conjecture, and bad conjecture at that.

If of course there is evidence that the report is wrong, or that SpaceX quality procedures are ineffective, then please cite them. Whilst I won't be happy of bad news like that, if the citation is valid then there is no argument.

Both of you are outright lying to yourselves if it is not painfully obvious what the issue is. Conjecture? Two lost vehicles and payloads in less than a year and the same system caused both failures though the exact failure mode is different the same part of the vehicle (lox tank) failed twice explosively and catastrophically that is fact not conjecture. The conjecture on my part is exact mode but that hardly matters when you are blowing up hundreds of millions of dollars every year.


I understand the passion people have for SpaceX but that is a ridiculous statement at best.

My "conjecture"  regarding whether or not they were entirely truthful or even themselves knew for sure that the strut itself failed as opposed to a component of the tank where the strut attaches is entirely plausible and worth discussing considering we have a company losing vehicles and then simultaneously stating they want to go fly again in just a few months before they even know why they lost the vehicle


Do you see anyone else be them nasa ULA or any other company or agency getting away with that? Should we just ignore these things because it's SpaceX and God forbid someone call into question what they are telling us? Let me remind everyone here again SpaceX is a private company they are not required to tell me you or anyone else everything about a failure and they have released only limited amounts at best of detailed data regarding the previous incident (as you were quick to point out)



One more thing,  how do you you know they were going to stop using an existing off the shelf supplier for their COPV struts and change their testing procedures for said struts prior to crs 7? Or are you seriously implying they would have changed those components drastically had there been no incident just for no reason on a whim?



None of these are declarations they are my opinions and take on the subject nothing more.  I've said this many times and I highly doubt anybody on this forum is trying to offer more than educated views, nobody here is declaring anything. We are shooting in the dark at raccoons with one hand tied up as to exact failure modes and contributing factors as none of us have access to the telemetry. That being said however,  what is a fact is two lox tanks failed in a way that destroyed two vehicles payload included in under 24 months across two different variants of this vehicle the second variant being a version that was supposed to have designed out possible trouble spots including the upper stage issues. There is an issue with either quality control,  flow procedures,  or both otherwise regardless of contributing factors the issue should have been caught or prevented entirely. Failures in the mechanical frontier are never just one thing it's an event chain that propagated because of a series of issues. Engineering 101. This is the point I have been getting at this entire time and this is what I meant from the first when I said I was disappointed with SpaceX. They are trying to be better than the rest and we should expect better.



All of this said I do conceed I'm out on limb regarding crs 7 being possibly more than or other than just the strut for exact failure but that is not the point I was getting at.

[FinalFrontier]

Hopefully this will spur development of a reusable methane upper stage since it uses a different technique entirely. But SpaceX isn't going to abandon the entire design like people are suggesting.

Aren't those two sentences somewhat contradictory?

He meant entire vehicle design/pressurized system design will not be abandoned just iterated out or upgraded.

[Jimmy_C]

This is a very interesting article called, "Explosion Hazard from a Propellant-Tank Breach in Liquid Hydrogen-Oxygen Rockets:"

https://web.njit.edu/~muratov/hazards.pdf

Fig 13 lists possible scenarios and risks. I wonder how applicable it is to kerolox, but I figure the engineering risk assessment can inform people's guesses about the AMOS-6 incident. In particular, I wonder if cavitation and a shockwave from an exploding COPV could work as the sources of some of the types of combustion seen in the video.

[FinalFrontier]

There's a thought that's been going around in my head for the past three weeks about the two LOV events when it comes to the structural differences between S1 and S2... I call it simply, "what's the same, what's different"... S1 solid bird from launch to landing, S2 cantankerous... Why is this "if" there is so much commonality in materials, production technique, tooling, employees assembling them, handling and transportation, testing...etc?

Then the question Glen would be what happens during the integrated test that would cause two S2 failures, one in flight, one on the pad and are those unrelated?

Well, my career involvement in rocketry advanced as far as Estes Rockets, so take my thoughts with a grain of salt,

but, based on having built complex systems that sometimes get simplified because the customer didn't need EVERYTHING...

It would seem to me like S1 is like the absolute attention getter from an engineering perspective.  9 times as many engines, bigger tanks, more He systems, grid fins, landing legs, precision landing guidance, has to do re-entry and fly a gazillion times.  It's a complex beast and the best and the brightest engineering goes into that.

The S2 is a scaled down copy, 1 engine extended nozzle, small tanks, carries a tiny payload...  It's really boring by comparison.

I can easily picture a case where the senior engineer says, here, take this, it's validated on S1, it should work on S2, and the junior engineer does exactly that. 

I try to imagine simply the He fill system for S1 and compare it to the same system for S2.  Should be exactly the same, except, not as much He gets loaded, not as much plumbing, not as many COPVs.  Would it really be that the He process is the same in both cases, except S2 gets less He than S1?  Would the couplings be the same, the flow rates, the target load pressures, the expected COPV & plumbing temperatures, etc. etc.?

S2 is a totally different rocket than S1 albeit shares as many parts as possible with S1.  Sharing parts is good economics.  Do they share the same processes and do they have equal validation and oversight and QA?  Certainly your brightest engineers did the work on S1.  Did the same engineers do the work on S2?

In the fault tree analysis, is there a little box next to each item that says, "same as S1" and did that mean it didn't need the same analysis, design review that it got when it was defined for S1?

I don't claim any knowledge of the root cause, or even of the possible process failures that lead to it, but I know from personal experience, it's very easy to assume a working subsystem will work in a different context and end up being very surprised when it doesn't.

Or is it the case that S1 is overbuilt with increased structural margins for re-usability and S2 is pushing the minimal margins to extract maximum performance to make up for it...

This is the exact issue I've been getting at as well. Quality does not appear to be uniform .

But this is from the outside looking in.

[Kabloona]

Ok, then how does helium released into oxygen create and explosion? Over pressure could rupture the tank, but that is not an explosion. AFAIK helium and oxygen are not combustible gases. Wouldn't it just rapidly vent without burning? And if it did, wouldn't we see a jet of burning gas coming out the side? What we do see is a ball of exploding gas on the side of the TE.

Even discarding the possibility of a ruptured COPV with flying fragments of carbon fiber/epoxy chunks and bits of metal liner flying around inside the LOX tank, an overpressure alone could well rupture the common bulkhead, very thoroughly mixing RP1 and LOX and destroying the stage; the specific ignition event is almost a formality at that point.

The only problem I see with this scenario is that, from the video, the size and behaviour of the LOX cloud escaping from the LOX vent during the filling process is not appreciably changed until the entire structure is well "ruptured".   Since filling was in progress (LOX in, GOX out) I would have expected we'd see the effect of any overpressure event in the LOX tank at the vent first, long (relatively speaking) before the explosion cloud.

(The above doesn't preclude the same COPV failure scenario in the RP-1 tank though)

One possible sequence of events that could be consistent with no observable change in LOX venting:

1. Helium system component ruptures, but leak rate of Helium is limited by the small diameter of the outlet line from the COPV, so the rate of change in LOX tank pressure is negligible compared to the speed of the next two steps, ie:
2. Shrapnel from the ruptured component pierces the common bulkhead...
3. LOX/RP-1 explosion ensues.

In this scenario, the initial "overpressure" event might have occurred inside the helium system itself, leading to component failure and bulkhead piercing/explosion before LOX ullage pressure had time to rise significantly.

[FinalFrontier]

This is a very interesting article called, "Explosion Hazard from a Propellant-Tank Breach in Liquid Hydrogen-Oxygen Rockets:"

https://web.njit.edu/~muratov/hazards.pdf

Fig 13 lists possible scenarios and risks. I wonder how applicable it is to kerolox, but I figure the engineering risk assessment can inform people's guesses about the AMOS-6 incident. In particular, I wonder if cavitation and a shockwave from an exploding COPV could work as the sources of some of the types of combustion seen in the video.

Another user offered a theory on this in thread one I will try and repost it here later but basically the short answer is yes it can but it depends on exact conditions in the tank and propellant and how much potential energy you had at the failure pressure on the copv at the time.

[CameronD]

Ok, then how does helium released into oxygen create and explosion? Over pressure could rupture the tank, but that is not an explosion. AFAIK helium and oxygen are not combustible gases. Wouldn't it just rapidly vent without burning? And if it did, wouldn't we see a jet of burning gas coming out the side? What we do see is a ball of exploding gas on the side of the TE.

Even discarding the possibility of a ruptured COPV with flying fragments of carbon fiber/epoxy chunks and bits of metal liner flying around inside the LOX tank, an overpressure alone could well rupture the common bulkhead, very thoroughly mixing RP1 and LOX and destroying the stage; the specific ignition event is almost a formality at that point.

The only problem I see with this scenario is that, from the video, the size and behaviour of the LOX cloud escaping from the LOX vent during the filling process is not appreciably changed until the entire structure is well "ruptured".   Since filling was in progress (LOX in, GOX out) I would have expected we'd see the effect of any overpressure event in the LOX tank at the vent first, long (relatively speaking) before the explosion cloud.

(The above doesn't preclude the same COPV failure scenario in the RP-1 tank though)

One possible sequence of events that could be consistent with no observable change in LOX venting:

1. Helium system component ruptures, but leak rate of Helium is limited by the small diameter of the outlet line from the COPV, so the rate of change in LOX tank pressure is negligible compared to the speed of the next two steps, ie:
2. Shrapnel from the ruptured component pierces the common bulkhead...
3. LOX/RP-1 explosion ensues.

In this scenario, the initial "overpressure" event might have occurred inside the helium system itself, leading to component failure and bulkhead piercing/explosion before LOX ullage pressure had time to rise significantly.

Hmm...  but assuming the He in the COPVs is at a ridiculously high pressure, surely any release into the LOX tank, however small, would result in a large rate-of-change in LOX venting?

Assuming it hasn't been raised before (very likely I missed it) and along the line of your "initial overpressure event inside the helium system itself" idea, is there any mechanism by which one or more COPV's could implode causing the same effect?  When is the helium system pressurised anyways?

EDIT: If an empty/blocked/faulty unpressurised COPV imploded while the LOX tank was filling, they might not pick that up on telemetry - but the end result would be the same.

[Katana]

Excuse me for not reading all of this thread. But since the focus seems to be on helium, how does a helium failure (by any means) cause an explosion on the outside of the rocket?

Nobody has said the failure was external to the launch vehicle. It's the still very much possible that the failure originated internally. The tanks that are used to pressurize the propellant tanks are located inside the actual propellant tanks and are called Composite Overwrap Pressure Vessels (COPVs).

Ok, then how does helium released into oxygen create and explosion? Over pressure could rupture the tank, but that is not an explosion. AFAIK helium and oxygen are not combustible gases. Wouldn't it just rapidly vent without burning? And if it did, wouldn't we see a jet of burning gas coming out the side? What we do see is a ball of exploding gas on the side of the TE.

Compatibility problems between composite and LOX, a famous problem of research.
Any impact, friction or electrostatic sparks could cause ignition.

http://arc.aiaa.org/doi/abs/10.2514/6.2008-1910

[Steven Pietrobon]

OK, here's some data to chew on. I found this link on the coefficient of thermal expansion for carbon fibre in relation to other materials. I've converted the imperial units to metric, with the coefficients in µm/(mK). I've also added Titanium from the Engineering Tool Box, which seems like a perfect fit for carbon fibre. The values for Steel and Aluminium are from the Engineering Tool Box.

http://www.christinedemerchant.com/carboncharacteristics.html
http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

Steel   12.0
Aluminium    22.2
Kevlar   5.4 or lower
Carbon Fiber woven    3.6 or less
Carbon fiber unidirectional   -1.8 to 14.4+
Fiberglass   12.6-14.4
Titanium  8.6

"Carbon fiber can have a broad range of CTE's, -1.8 to 14.4+, depending on the direction measured, the fabric weave, the precursor material, Pan based (high strength, higher CTE) or Pitch based (high modulus/stiffness, lower CTE)."

The difference between carbon fibre and Aluminium ranges from 7.8 to 24 µm/(mK). If say there was say a 100 K temperature difference over 1 m, this would result in a length difference from 0.78 to 2.4 mm.

A possible solution to using COPVs is to use liquid Helium, like in the Ariane 5 core stage. At 300 K, gaseous Helium has a density of 0.164 kg/m³. At 4 K, liquid Helium has a density of 147 kg/m³. According to Spaceflight101

http://spaceflight101.com/spacerockets/falcon-9-ft/

Falcon 9 v1.2 has an RP-1 mass of 32,300kg and LOX mass of 75,200k. At subcooled temperatures, corresponding densities are 826.5 kg/m³ and 1253.9 kg/m³. This gives a total volume of 99.1 m³. Musk says they use 2% ullage. This increases total volume to 101.1 m³. For Gaseous Helium, this requires a Helium mass of 16.6 kg. This would require a volume of only 0.113 m³ as a liquid, equivalent to a sphere 0.6 m in diameter. This could easily be stored at the base of the second stage.

For the first stage, we have 123,100 kg of RP-1 and 286,400 kg of LOX. Total volume is 385.1 m³, Liquid Helium volume is 0.43 m³ and liquid Helium sphere diameter is 0.936 m. Maybe this could be placed in the interstage, fitting within the second stage nozzle.

[Kaputnik]

Noob question, in relation to the above:
In order to store the He as a liquid, would this imply massively greater pressure, or lower temperatures? Assuming the latter, does that suggest better insulation and/or active cooling?

[woods170]

Noob question, in relation to the above:
In order to store the He as a liquid, would this imply massively greater pressure, or lower temperatures? Assuming the latter, does that suggest better insulation and/or active cooling?

Lower temperatures. Interatomic forces in helium are very low. That is why Helium remains a liquid from the point where it turns into a liquid down to absolute zero. This is the behavior at atmospheric pressure. Adding high pressure will eventually result in Helium turning solid.

[Herb Schaltegger]

For the first stage, we have 123,100 kg of RP-1 and 286,400 kg of LOX. Total volume is 385.1 m³, Liquid Helium volume is 0.43 m³ and liquid Helium sphere diameter is 0.936 m. Maybe this could be placed in the interstage, fitting within the second stage nozzle.

That kind of depends on the details of that center pusher, I'd guess. Have we ever seen a good photo of this mechanism, and how it works? I don't think any photos of landed stages have shown much at all about what's inside the interstage. All the photos are taken from at or near ground level looking up, or from a distance. Then the beanie cap is installed before lowering the stage to horizontal ... I guess the interstage internals are pretty darn proprietary (and may be ITAR as well, since the internals there are basically how to make an IRBM into a precision-guided device).

tl;dr: maybe, if the pusher mech - and everything else in there (avionics, grid fin hydraulics, stage latches, etc. - aren't in the way.

[hrissan]

Falcon 9 v1.2 has an RP-1 mass of 32,300kg and LOX mass of 75,200k. At subcooled temperatures, corresponding densities are 826.5 kg/m³ and 1253.9 kg/m³. This gives a total volume of 99.1 m³. Musk says they use 2% ullage. This increases total volume to 101.1 m³. For Gaseous Helium, this requires a Helium mass of 16.6 kg. This would require a volume of only 0.113 m³ as a liquid, equivalent to a sphere 0.6 m in diameter. This could easily be stored at the base of the second stage.

Tanks are pressurized at more than 1 atm, let's say 3 atm, so you need 3x helium more than you counted. 350 liters instead of 113.

Liquid helium tank is neither small nor lightweight. See specs here http://www.cryofab.com/products/spec_sheets/Cryofab_CMSH_Series_Specs_350.pdf

[Rocket Science]

This is a very interesting article called, "Explosion Hazard from a Propellant-Tank Breach in Liquid Hydrogen-Oxygen Rockets:"

https://web.njit.edu/~muratov/hazards.pdf

Fig 13 lists possible scenarios and risks. I wonder how applicable it is to kerolox, but I figure the engineering risk assessment can inform people's guesses about the AMOS-6 incident. In particular, I wonder if cavitation and a shockwave from an exploding COPV could work as the sources of some of the types of combustion seen in the video.

Another user offered a theory on this in thread one I will try and repost it here later but basically the short answer is yes it can but it depends on exact conditions in the tank and propellant and how much potential energy you had at the failure pressure on the copv at the time.

Was it what I posted about focused shock detonation?
post#1156
https://forum.nasaspaceflight.com/index.php?topic=30981.1140