Launch News [Sep.1, 2016] Falcon 9 explodes on the Launch Pad

[Urwumpe]

Why don't they pressurize the LOX tank with its own gas?

Requires a lot of power of the rocket engine, so enough heat is available to turn enough LOX into gas. And you can't use it for Kerosene.

 

[Thunder Chicken]

Spaceflight101.com: SpaceX resorts to ‘Creative’ Testing in Falcon 9 Explosion Investigation

It sounds like a very complex pressure and thermal stress environment for the COPVs. If the COPV is pressurized to 6000 psi and only a part of it is immersed in LOX, I could see Mohr's circle getting pretty big pretty quickly.
Composites acting as thermal insulators would tend to maintain steep temperature gradients in the tank wall, maximizing thermal stresses. Going back to an all metal pressurization tank with greater thermal conductivity would diffuse out those thermal gradients during tank fill. Maybe just putting a thin metal casing on the outside of the COPV would serve this purpose without adding too much weight or complexity.

 

[orb]

SpaceX: Anomaly Updates:

October 28, 4:00pm EDT

The Accident Investigation Team continues to make progress in examining the anomaly on September 1 that led to the loss of a Falcon 9 and its payload at Launch Complex 40 (LC-40), Cape Canaveral Air Force Station, Florida.

Since the incident, investigators from SpaceX, the FAA, NASA, the US Air Force and industry experts have been working methodically through an extensive fault tree to investigate all plausible causes. As part of this, we have conducted tests at our facility in McGregor, Texas, attempting to replicate as closely as possible the conditions that may have led to the mishap.

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.

SpaceX’s efforts are now focused on two areas – finding the exact root cause, and developing improved helium loading conditions that allow SpaceX to reliably load Falcon 9. With the advanced state of the investigation, we also plan to resume stage testing in Texas in the coming days, while continuing to focus on completion of the investigation. This is an important milestone on the path to returning to flight.

Pending the results of the investigation, we continue to work towards returning to flight before the end of the year. Our launch sites at Kennedy Space Center, Florida, and Vandenberg Air Force Base, California, remain on track to be operational in this timeframe.

{...}

 

[ADSWNJ]

Would love to see some video of those fault repro test rigs exploding at will!

 

[RGClark]

Would love to see some video of those fault repro test rigs exploding at will!

I hope they go back to including video cameras in the tanks.

Bob Clark

 

[Thunder Chicken]

A lot of articles indicating that this has been "solved", something about the LOX actually freezing into a solid due to low helium temperatures, somehow making things go kaboom, but nothing on a reputable space news site yet. All the descriptions have been mangled by writers that don't understand science.

This site has some text attributed to Musk:

http://up-ship.com/blog/?p=33338

We are close to figuring it out. It might have been formation of solid oxygen in the carbon over-wrap of one of the bottles in the upper stage tanks. If it was liquid it would have been squeezed out but under pressure it could have ignited with the carbon. This is the leading theory right now, but it is subject to confirmation.

The other thing we discovered is that we can exactly replicate what happened on the launch pad if someone shoots the rocket. We don’t think that is likely this time around, but we are definitely going to have to take precautions against that in the future. We looked at who would want to blow up a SpaceX rocket. That turned out to be a long list. I think it is unlikely this time, but it is something we need to recognize as a real possibility in the future.

O2 ice formation inside the laminate?

 

[MaverickSawyer]

Wouldn't surprise me. The idea of mixing LOX and carbon fiber without some sort of minimally-combustible (there's no such thing as a non-combustible material when LOX is involved :lol: ) liner makes me skittish.

 

[PhantomCruiser]

Anyone else ever had to watch "The Man from LOX" movie? That's why aierdales don't polish their boots.
They seem to be hinting at a RTF before years end, but I would be surprised.

 

[RGClark]

It sounds like a very complex pressure and thermal stress environment for the COPVs. If the COPV is pressurized to 6000 psi and only a part of it is immersed in LOX, I could see Mohr's circle getting pretty big pretty quickly.
Composites acting as thermal insulators would tend to maintain steep temperature gradients in the tank wall, maximizing thermal stresses. Going back to an all metal pressurization tank with greater thermal conductivity would diffuse out those thermal gradients during tank fill. Maybe just putting a thin metal casing on the outside of the COPV would serve this purpose without adding too much weight or complexity.

Yes. While aluminum at such pressures would be higher mass for the helium tanks, I can't believe it would be that much higher as a proportion of the overall stage dry mass.

Bob Clark

 

[boogabooga]

[quote = Elon Musk]
The other thing we discovered is that we can exactly replicate what happened on the launch pad if someone shoots the rocket. We don’t think that is likely this time around, but we are definitely going to have to take precautions against that in the future. We looked at who would want to blow up a SpaceX rocket. That turned out to be a long list. I think it is unlikely this time, but it is something we need to recognize as a real possibility in the future.

If that's really from Elon, he may want to follow basic manners and keep that to himself, unless they have actual evidence.

 

[Hlynkacg]

Why? It's not as if any accusation was made. Also, TIL that there is a job where you get paid to shoot at rockets in the name of science. Where do I send my resume?

 

[Thunder Chicken]

After some more reading and thinking this makes more sense. LOX could permeate the carbon fiber (by itself not a problem). The problem comes in when the liquid helium is introduced to the initially-empty COPV. Pressurized liquid helium is introduced at a temperature above the freezing point of oxygen, but when it is first supplied to the tank, it suddenly drops in pressure, creating a saturated gas-vapor mixture that is at a much colder temperature than the freezing point of oxygen (this is similar to the process of refrigeration where liquid refrigerant at room temperature undergoes a pressure drop, creating a cold saturated mixture that absorbs heat from the refrigerated space).

The solution might be as simple as charging the COPV tanks up to a certain minimum pressure before beginning the LOX fill. Small variations in tank fill timing between the helium and LOX tanks, and perhaps variations in the permeability of the COPV fibers, could make all the difference. They may have dodged this bullet several times and not realized it.

---------- Post added at 10:28 PM ---------- Previous post was at 10:22 PM ----------

If that's really from Elon, he may want to follow basic manners and keep that to himself, unless they have actual evidence.

He specifically says that this isn't likely. But they were tasked with exploring a possible fault tree, and they found that this was a potential means to make the rocket explode. Just because it didn't cause this explosion does not mean it could not cause a future one. In many accident investigations you find the root cause of the accident and discover an additional list of failure modes that maybe hadn't been considered previously. It's all valuable information and leads to smarter processes. I don't think he's pointing fingers at anyone here.

 

[Urwumpe]

If that's really from Elon, he may want to follow basic manners and keep that to himself, unless they have actual evidence.

He does not say anything new at all... he just makes it sound like they have never even considered it. :lol:

 

[boogabooga]

Why? It's not as if any accusation was made.

We looked at who would want to blow up a SpaceX rocket. That turned out to be a long list.

I think the accusation is implied.

I'm not saying that you're stealing from me, but I'm going to start hiding my valuables when you come over.

But they were tasked with exploring a possible fault tree, and they found that this was a potential means to make the rocket explode.

No kidding- you intentionally shoot a fueled rocket, and it explodes. Quite a "fault tree."

Just because it didn't cause this explosion does not mean it could not cause a future one.

Just because the tooth fairy didn't cause this explosion, does not mean it could not cause a future one.

In many accident investigations you find the root cause of the accident and discover an additional list of failure modes that maybe hadn't been considered previously.

If we are in the realm of deliberate human intervention to cause a failure, the list of "failure modes" that hadn't been considered previously becomes huge. How would you construct a fault tree to cover every scenario? What happens if the sabotage is deliberately disguised to appear to have a different cause?

 

[Zatnikitelman]

I think the accusation is implied.

I'm not saying that you're stealing from me, but I'm going to start hiding my valuables when you come over.

An accusation requires direction. He didn't say "x shot the rocket" that's just a possible failure mode that they hadn't previously considered, probably because of see below.

No kidding- you intentionally shoot a fueled rocket, and it explodes. Quite a "fault tree."

A fire/explosion requires oxygen, fuel, and a heat source to ignite. Just like the myth of shooting a gas tank (petrol or LPG), I find it doubtful that simply shooting a more conventional rocket stage would cause an explosion. Most likely, since it seems that applying LOX at high pressure to a carbon material is being hinted at as the cause, they are considering a bullet on its way into a tank containing LOX because that would apply pressure and heat to LOX in contact with carbon.

Just because the tooth fairy didn't cause this explosion, does not mean it could not cause a future one.

We're talking about the realm of realistic here. I'm sure aliens could come down and zap the rocket and make it explode too, but it's not realistic to consider that.

If we are in the realm of deliberate human intervention to cause a failure, the list of "failure modes" that hadn't been considered previously becomes huge. How would you construct a fault tree to cover every scenario? What happens if the sabotage is deliberately disguised to appear to have a different cause?

It's called degree of risk. You protect against what you can reasonably protect against rather than trying to protect every single possibility. The most secure computer in the world is the one unplugged from network and power, welded into a meter-thick steel box, encased in concrete, buried deep in the ground, and the site protected by an army. But it's not a useful computer in the slightest.

 

[Thunder Chicken]

Both the Challenger and Columbia accident investigations considered the possibility of sabotage. That possibility may have been quickly dismissed, but it was a possibility. NASA has has suffered some instances of sabotage, so they consider it and take the possibility seriously.

In these investigations, you need to consider all of the possible causes of failure without prejudice. Some whack-job wanting to make something go all explodey might consider popping a few rounds at a fully fueled rocket for kicks. The possibility was investigated and found not to be a cause.

 

[ADSWNJ]

After some more reading and thinking this makes more sense. LOX could permeate the carbon fiber (by itself not a problem). The problem comes in when the liquid helium is introduced to the initially-empty COPV. Pressurized liquid helium is introduced at a temperature above the freezing point of oxygen, but when it is first supplied to the tank, it suddenly drops in pressure, creating a saturated gas-vapor mixture that is at a much colder temperature than the freezing point of oxygen (this is similar to the process of refrigeration where liquid refrigerant at room temperature undergoes a pressure drop, creating a cold saturated mixture that absorbs heat from the refrigerated space).

The solution might be as simple as charging the COPV tanks up to a certain minimum pressure before beginning the LOX fill. Small variations in tank fill timing between the helium and LOX tanks, and perhaps variations in the permeability of the COPV fibers, could make all the difference. They may have dodged this bullet several times and not realized it.

---------- Post added at 10:28 PM ---------- Previous post was at 10:22 PM ----------



He specifically says that this isn't likely. But they were tasked with exploring a possible fault tree, and they found that this was a potential means to make the rocket explode. Just because it didn't cause this explosion does not mean it could not cause a future one. In many accident investigations you find the root cause of the accident and discover an additional list of failure modes that maybe hadn't been considered previously. It's all valuable information and leads to smarter processes. I don't think he's pointing fingers at anyone here.

Still trying to figure out this myself. There appear to be 2 theories for how this solid oxygen causes a problem:

1. The solid oxygen permeating the COPV wrap gets trapped as the COPV presses up (rather than getting squeezed out). At a critical pressure, the oxygen combusts with the carbon and kapow.

2. The solid oxygen still gets trapped, but this time it ruptures the integrity of the COPV as is presses up, causing a surge in O2 pressure in the tank and a big kinetic event with enough carbon shards in pure oxygen to kapow it a different way.

Do either of both of these make sense? Any chemists out here figuring out this at a more technical level?

 

[Urwumpe]

I would dismiss 1. While there is a lot of oxygen and PAN, there is almost zero energy. LOX and PAN are cold. Some chemist might be better suited now to calculate this, but I doubt there is enough energy for sustaining a combustion, even if you would start one.

I can't find any data on flammability of PAN in a high oxygen environment.

 

[Kyle]

FAA has signed off on the AMOS-6 failure investigation ahead of the Iridium NEXT-1 Launch.

http://spaceflightnow.com/2017/01/06/faa-signs-off-on-spacex-mishap-investigation/

While the inquiry did not identify a single root cause for the mishap, the company’s engineers determined a high-pressure helium vessel inside the Falcon 9’s second stage liquid oxygen tank burst. The helium tank — called a composite overwrapped pressure vessel, or COPV — most likely failed when liquid oxygen filled a void created when the vessel’s liner buckled, and breaking fibers or friction ignited and led to the rupture of the second stage.

SpaceX's solution as I understand it is to load the helium at a warmer temperature.

 

[Messierhunter]

FAA has signed off on the AMOS-6 failure investigation ahead of the Iridium NEXT-1 Launch.

http://spaceflightnow.com/2017/01/06/faa-signs-off-on-spacex-mishap-investigation/



SpaceX's solution as I understand it is to load the helium at a warmer temperature.

Scary thread to bump right after they did the hotfire test for the Iridium launch; I thought at first it happened again.