Author Topic: SpaceX Falcon 9 - AMOS-6 - (Pad Failure) - DISCUSSION THREAD (2)  (Read 700660 times)

Offline MikeAtkinson

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I wonder if there could be enough amplified energy to rupture something unfortunate, ie the COPV, or something very close to it, a connector perhaps.

My guess would be the connector between the COPV and the rest of the helium system, I have no inside knowledge, but it seems to me that that is where the maximum stress from any vibration would be.

Offline OnWithTheShow

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I wonder if there could be enough amplified energy to rupture something unfortunate, ie the COPV, or something very close to it, a connector perhaps.

My guess would be the connector between the COPV and the rest of the helium system, I have no inside knowledge, but it seems to me that that is where the maximum stress from any vibration would be.

And also likely the weakest point of the COPV since they had to drill through it (most likely) to attache the connector.

Offline Proponent

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What effect do low temperatures have on the strength of the CO part of the COPV?  Is it strengthened, as a traditional metal PV is?

Offline VulcanCafe

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From SpaceNews

Gwynne Shotwell- 'We currently have video on the tanks. We weren’t downloading the stream on the one that had that particular camera on this most recent event a month ago.'

Much more of interest in this article:
http://spacenews.com/spacexs-shotwell-on-falcon-9-inquiry-discounts-for-reused-rockets-and-silicon-valleys-test-and-fail-ethos/

Offline HMXHMX

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I wonder if there could be enough amplified energy to rupture something unfortunate, ie the COPV, or something very close to it, a connector perhaps.

My guess would be the connector between the COPV and the rest of the helium system, I have no inside knowledge, but it seems to me that that is where the maximum stress from any vibration would be.

And also likely the weakest point of the COPV since they had to drill through it (most likely) to attache the connector.

There is no drilling on these vessels.  The shell/liner is formed with an outlet tube welded into place; no boss.

Offline Mike_1179

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There is a temperature gradient as well as a pressure gradient in these COPV.

The bottom of the COPV is cold when immersed in LOX and area near the fill port is warm from expanded helium. Modeling this gets complicated when combined with stresses from pressure gradients and differential thermal contraction of the metal liner and the composite overwrap.

Offline Herbie

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There is a temperature gradient as well as a pressure gradient in these COPV.

The bottom of the COPV is cold when immersed in LOX and area near the fill port is warm from expanded helium. Modeling this gets complicated when combined with stresses from pressure gradients and differential thermal contraction of the metal liner and the composite overwrap.
I don't know about the internal geometry of the helium system inside the tank. Is the helium supply line immersed in LOX, so that a significant degree of cooling takes place before the He enters the COPV?

Offline virnin

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There is a temperature gradient as well as a pressure gradient in these COPV.

The bottom of the COPV is cold when immersed in LOX and area near the fill port is warm from expanded helium. Modeling this gets complicated when combined with stresses from pressure gradients and differential thermal contraction of the metal liner and the composite overwrap.

I don't think it's safe to assume the COPVs were immersed or even touching the LOX at that point in the sequence, unless there was a major change in the timeline.  The S2 LOX tank wasn't due to finish filling for several more minutes according to the FT sequence someone posted in the first thread.

Offline LastStarFighter

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There is a temperature gradient as well as a pressure gradient in these COPV.

The bottom of the COPV is cold when immersed in LOX and area near the fill port is warm from expanded helium. Modeling this gets complicated when combined with stresses from pressure gradients and differential thermal contraction of the metal liner and the composite overwrap.

I don't think it's safe to assume the COPVs were immersed or even touching the LOX at that point in the sequence, unless there was a major change in the timeline.  The S2 LOX tank wasn't due to finish filling for several more minutes according to the FT sequence someone posted in the first thread.

Weren't they experimenting with simulating longer launch windows for this static fire?

Offline envy887

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Based on the timeline the LOX tank was roughly 2/3 full. The COPVs are about 1.5 m tall and installed vertically. Seems likely they were partially immersed wet the time of failure.

Offline PeteW

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Early on, a reddit commenter quoted a SpaceX source as saying they had observed some "weird harmonics" in a COPV during tanking, and this could also be consistent with some operational irregularity causing unexpected phenomena.
The mention of weird harmonics has made me wonder if they have managed to unwittingly make a thermoacoustic heat engine. This could generate sound waves in the helium, powered by the temperature difference between the LOX and the ambient air. These sound waves can have very large amplitudes and could potentially damage the COPV. 

The COPV would be acting as a resonator with the valves, connectors and piping between the the stage and the GSE acting as a regenerator. The heat input would come from the warm helium being loaded, with the heat sink  being the LOX.

Obviously this is just speculation at the moment, and I don't think we don't have enough information about the details of the helium system to say anything definitive, but perhaps it will give someone at SpaceX something else to think about.


Offline Rocket Science

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Early on, a reddit commenter quoted a SpaceX source as saying they had observed some "weird harmonics" in a COPV during tanking, and this could also be consistent with some operational irregularity causing unexpected phenomena.
The mention of weird harmonics has made me wonder if they have managed to unwittingly make a thermoacoustic heat engine. This could generate sound waves in the helium, powered by the temperature difference between the LOX and the ambient air. These sound waves can have very large amplitudes and could potentially damage the COPV. 

The COPV would be acting as a resonator with the valves, connectors and piping between the the stage and the GSE acting as a regenerator. The heat input would come from the warm helium being loaded, with the heat sink  being the LOX.

Obviously this is just speculation at the moment, and I don't think we don't have enough information about the details of the helium system to say anything definitive, but perhaps it will give someone at SpaceX something else to think about.
Great first post, welcome to the forum! :)
"The laws of physics are unforgiving"
~Rob: Physics instructor, Aviator

Offline CameronD

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Early on, a reddit commenter quoted a SpaceX source as saying they had observed some "weird harmonics" in a COPV during tanking, and this could also be consistent with some operational irregularity causing unexpected phenomena.
The mention of weird harmonics has made me wonder if they have managed to unwittingly make a thermoacoustic heat engine. This could generate sound waves in the helium, powered by the temperature difference between the LOX and the ambient air. These sound waves can have very large amplitudes and could potentially damage the COPV. 

The COPV would be acting as a resonator with the valves, connectors and piping between the the stage and the GSE acting as a regenerator. The heat input would come from the warm helium being loaded, with the heat sink  being the LOX.

The "weird harmonics" could also be related to a water hammer scenario.  If the LOX is only just above the triple-point and they either suddenly, briefly, stopped loading or got a decent bubble of gas in the line, could the resulting shockwave from the collapsing bubble be too much for a nearby carbon overwrapped aluminium tin-can COPV to handle?!?
 
« Last Edit: 10/07/2016 03:44 am by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline Dante80

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Assuming an operation error (no defects or installation errors).

1. COPV being filled to operational pressure before being completely immersed.
2. Changes in the LOX fill rates (for keeping the LOX subcooled and providing a re-cycle optiob in tougher launch window scenarios).

That is the two I can think of.

Offline Stranger

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Assuming an operation error (no defects or installation errors).

1. COPV being filled to operational pressure before being completely immersed.

On the Falcon 9 has a pre-pressurization COPV?
this happens at the start or advance?

Offline woods170

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Early on, a reddit commenter quoted a SpaceX source as saying they had observed some "weird harmonics" in a COPV during tanking, and this could also be consistent with some operational irregularity causing unexpected phenomena.
The mention of weird harmonics has made me wonder if they have managed to unwittingly make a thermoacoustic heat engine. This could generate sound waves in the helium, powered by the temperature difference between the LOX and the ambient air. These sound waves can have very large amplitudes and could potentially damage the COPV. 

What you just described is known as "Helium Hammer" and was observed, with near catastrophic effects in early spaceflight projects dealing with cooling systems using liquid helium.
A famous incident in NASA history is helium hammer induced rupture of a burst disk on the very first infrared space observatory: IRAS.
During a test of the cryogenic telescope assembly in the Netherlands liquid helium was transferred from the filling dewar to the cryogenic helium tank of the telescope assembly. The filling line was vaccuum jacketed to prevent flash evaporation of liquid helium during transfer. However, the filling line was equipped with a tee to hook up an additional pump assembly. This short 10 centimeter tee was un-isolated. It was enough to create local flash evaporation of the liquid helium in the filling line followed by almost immediate re-condensation into a liquid. This behaviour created a large-amplitude standing-wave in the helium transfer line. The thus generated pressure waves quickly overcame the burst-disks protecting the system. Given that the cryogenic telescope assembly was a closed-off vacuum it immediately was sucked full of air upon burst-disk rupture. The cryogenic temperature of the system made for immediate and large scale water-ice build up in the system. This little incident caused a multiple-month delay in the IRAS project. The entire dewar had to be warmed up, opened up, purged, cleaned, inspected, cleaned again, re-vacuumed and re-filled again.
The solution against the pressure waves proved to be very simple: a 2.5 liter surge tank was installed in the system between the filling-line and the cryogenic helium tank. The problem never occurred again.


Oh, and by the way. I noticed this was your first post here. A great post IMO. And welcome to the forum!  :)
« Last Edit: 10/07/2016 11:30 am by woods170 »

Offline Mike_1179

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What you just described is known as "Helium Hammer" and was observed, with near catastrophic effects in early spaceflight projects dealing with cooling systems using liquid helium.
 

The helium used in F9 is not a liquid though. Would you get this same effect through simply heating then re-cooling gaseous helium?

Offline MarekCyzio


What you just described is known as "Helium Hammer" and was observed, with near catastrophic effects in early spaceflight projects dealing with cooling systems using liquid helium.
 

The helium used in F9 is not a liquid though. Would you get this same effect through simply heating then re-cooling gaseous helium?

I believe he meant bubbles in LOX. I'm not an expert, but this is similar to cavitation - bubbles forming and then collapsing. Cavitation can severly erode metals, so maybe it was too much for one of COPV's?

Offline ellindsey

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What you just described is known as "Helium Hammer" and was observed, with near catastrophic effects in early spaceflight projects dealing with cooling systems using liquid helium.
 

The helium used in F9 is not a liquid though. Would you get this same effect through simply heating then re-cooling gaseous helium?

No, but the thermoacoustic heat engine effect can happen with a purely gaseous medium, so it could have occurred here.

Offline AnalogMan

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What you just described is known as "Helium Hammer" and was observed, with near catastrophic effects in early spaceflight projects dealing with cooling systems using liquid helium.
 

The helium used in F9 is not a liquid though. Would you get this same effect through simply heating then re-cooling gaseous helium?

No, but the thermoacoustic heat engine effect can happen with a purely gaseous medium, so it could have occurred here.

Thermoacoustic oscillation (also known as Taconis oscillation) is a well known phenomenon that occurs in gas-filled tubes that are at cryogenic temperatures at one end and warm (i.e. ambient room temperature) at the other.  It is common for these to occur when dealing with liquid helium cryostats having gaseous helium filled lines attached, but can occur with other cryogens (LOX) and gasses.

Design equations for determining whether operation will be stable or lead to unwanted oscillation are well known.  The oscillations are self-sustaining and usually result in a big increase in heat load on cryogenic dewars.  They are driven by the temperature difference between the two ends of the tube (the bigger, the easier they occur), and are controlled by careful choice of tube diameter/tube length ratios.

If thermoacoustic oscillation is present, and the frequency (determined by tube lengths) matches a mechanical resonance mode and there is significant coupling possible between the two, then this can lead to problems with high-cycle fatigue of parts.

One example of acoustic resonance coupling to mechanical resonance leading to high-cycle failure was in the Flow Control Valves of the Shuttle Orbiter where the poppet in the valve fractured (chunk of metal broke off the metal sealing face) resulting in its inability to seal fully closed.  Given that this valve controlled the flow of gaseous hydgrogen from the SSME back to the LH2 tank to provide ullage pressure, this was a big deal at the time.  There are lots of NSF articles on this.

For more info see:
http://www.mtm-inc.com/ac-20110117-thermoacoustic-oscillations.html

and the attached design guideline.

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