SpaceX Return to Fight and what was the anomaly

[HVM]

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.

Anomaly Updates

 

[Grendal]

So really, they know what happened but they still have to jump through all of the hoops to get back up and running. Now they will need to prove they have a procedure that allows them to continue using the helium and COPVs as they have done before.

A new stage has left the Hawthorne factory for testing at McGregor.

 

[RDoc]

It will be very interesting to see what the problem was. They may have a nasty situation where the combination of near freezing LOX surrounding the He tank which needs to be loaded with cyro temp gas at high pressure is at the edge of a dangerous temp/pressure regime. That is, it will take very careful control to make it safe, the default condition may be dangerous.

 

[miimura]

If the problem is LOx freezing inside the layers of carbon over-wrap, why not load and pressurize the Helium before loading the LOx? Wouldn't that prevent the LOx getting in the carbon over-wrap because it's already at pressure? The timing for the Helium loading should be less critical since it's in a pressure vessel, not just in the huge LOx tank that is basically the structure of the rocket.

 

[Mike1080i]

If the problem is LOx freezing inside the layers of carbon over-wrap, why not load and pressurize the Helium before loading the LOx? Wouldn't that prevent the LOx getting in the carbon over-wrap because it's already at pressure? The timing for the Helium loading should be less critical since it's in a pressure vessel, not just in the huge LOx tank that is basically the structure of the rocket.

As I recall, the LOX is used to keep the Liquid Helium cool. The LHe would warm and expand faster if the COPVs were not already submerged in LOX.

 

[miimura]

As I recall, the LOX is used to keep the Liquid Helium cool. The LHe would warm and expand faster if the COPVs were not already submerged in LOX.

I don't think the Helium is liquified, they want it as a pressure source, hence the COPV tanks. It also has a boiling point that is nearly 100C lower than Oxygen at standard pressure.

The explanation above, was that pressurized Helium, when transferred to an empty vessel, would supercool due to expansion. If the LOx was already surrounding the COPV, some of the oxygen that seeped into the carbon layers could freeze into solid oxygen.

 

[Mike1080i]

I don't think the Helium is liquified, they want it as a pressure source, hence the COPV tanks. It also has a boiling point that is nearly 100C lower than Oxygen at standard pressure.

You're right! I guess I shouldn't be posting when I should be asleep. I think the Cryo Helium line in the timeline must have had my sleepy brain a tad confused tonight!

The explanation above, was that pressurized Helium, when transferred to an empty vessel, would supercool due to expansion. If the LOx was already surrounding the COPV, some of the oxygen that seeped into the carbon layers could freeze into solid oxygen.

That seems to be the case. The solution might be a change in the tanking procedure that avoids the Helium loading dragging the tank temperature below 55K. Since the LOX is used to keep the Helium cool, I wouldn't think they will move Helium loading too far in advance of the LOX.

 

[Ben W]

Why not just add salt to the liquid oxygen to lower its freezing point?

Obviously the chemistry is different, but possibly "doping" the LOX with something could lower its freezing point without compromising its effectiveness as an oxidizer?

 

[oneday]

The explanation above, was that pressurized Helium, when transferred to an empty vessel, would supercool due to expansion.

Doesn't the opposite happen with helium? It heats during expansion?

This is a copied from wiki:
Helium has a negative Joule-Thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand. Only below its Joule-Thomson inversion temperature (of about 32 to 50 K at 1 atmosphere) does it cool upon free expansion.[13] Once precooled below this temperature, helium can be liquefied through expansion cooling.

 

[ggr]

The Oxygen that the COPV tank is immersed in, is just above 50K. But the Helium is not at anything like 1 atmosphere; it's at extremely high pressure. I found the following at SpaceX fingers helium as cause of Falcon 9 rocket explosion:

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, ). When the Falcon 9 second stage fires, turbopumps suck the LOX and RP-1 into the engine's combustion chamber, but the pumps can't do the job alone because the second stage fires in zero gravity, so the propellants need to be forced in at ignition. Furthermore, the pumps would soon cause a vacuum to form inside the tanks and the propellants wouldn't flow properly. This is where the helium comes in.

If this is accurate (I have no reason to believe it isn't, it cites SpaceX themselves as the source), the Helium is loaded as a liquid, not a gas, so expansion cooling wouldn't be an issue. Now I'm just confused...

 

[RDoc]

I'm pretty sure the He is under high pressure, not liquid. That article is the only one I've ever seen that talks about liquid He and a heat exchanger. Keeping helium liquid requires a very good insulating container, but COPVs aren't insulated at all, they are pressure vessels.

 

[Mike1080i]

The Oxygen that the COPV tank is immersed in, is just above 50K. But the Helium is not at anything like 1 atmosphere; it's at extremely high pressure. I found the following at SpaceX fingers helium as cause of Falcon 9 rocket explosion:

If this is accurate (I have no reason to believe it isn't, it cites SpaceX themselves as the source), the Helium is loaded as a liquid, not a gas, so expansion cooling wouldn't be an issue. Now I'm just confused...

Me too!
KSC supports Helium storage and distribution for KSC and CCAFS. It appears that KSC stores it as liquid then distributes it as a high pressure gas to the pads.

This countdown timeline for the Falcon 9 FT mentions both Cryo Helium and Helium Cryo.

• T-0:25:00 All three Cryo Helium Pumps active
• T-0:13:00 Stage 2 LOX Flow Adjustment for Helium Cryo Load

 

[Grendal]

Here's an article that brings it all together:

SpaceX prepares Falcon 9 test; AMOS-6 investigation narrows | NASASpaceFlight.com

RTF:
"Currently, a Falcon 9 first stage is understood to be patiently waiting at Vandenberg Air Force Base in California. It is believed this Falcon 9 will launch the first mission since the AMOS-6 incident on a December launch that may involve the Iridium NEXT flight."

LC 39A:
"A placeholder date of December 17 was cited on planning Cape documentation over a week ago. The mission was cited as for Echostar-23, although – as expected – SpaceX is yet to provide specifics to its upcoming manifest until after the AMOS-6 investigation has concluded."

 

[Grendal]

Even more confirmation of a December RTF:

Inmarsat, juggling two launches, says SpaceX to return to flight in December - SpaceNews.com

At this point the biggest question is which will be the first launch. You have Iridium Next out of Vanderberg or Echostar 23 out of LC-39A.

Flip a coin?

 

[Grendal]

Musk: SpaceX explosion toughest puzzle we've ever had to solve

Direct from Elon. And here is an article outlining what he said:

Musk predicts mid-December return to flight for Falcon 9 - SpaceNews.com

 

[ecarfan]

Elon said in that interview that "frozen oxygen" had "never happened before in the history of rocketry", isn't that because no other rocket company had ever deliberately cooled down the onboard LOX propellant to as low a temperature (with the intent of it remaining a liquid and not a solid) as SpaceX has for recent launches?

 

[Grendal]

Elon said in that interview that "frozen oxygen" had "never happened before in the history of rocketry", isn't that because no other rocket company had ever deliberately cooled down the onboard LOX propellant to as low a temperature (with the intent of it remaining a liquid and not a solid) as SpaceX has for recent launches?

I would take it that he meant unintentionally. I'm sure if someone really wanted frozen (SOX?) oxygen that they could make and use it. But no one else has intentionally worked with supercooled LOX as part of their normal procedure. SpaceX is definitely breaking ground there to maximize efficiency of their system. Supposedly the supercooled LOX helps the turbopumps work much better as well as densifying the fuel.

 

[Grendal]

EchoStar expects Jan. 8 or 9 SpaceX launch, confronts Brazil and EU deadlines - SpaceNews.com

So we have two possible launches coming soon. Iridium comes first out of Vandenberg and then EchoStar 23 out of 39A in Florida. The launch schedule incorrectly says that EchoStar will launch from SLC-40. That won't be repaired by then.

 

[Grendal]

SpaceX will be handing in the initial report explaining what they believe went wrong to cause the September 1st explosion to the various agencies needed to okay the RTF.

There have been a number of reports of explosions at McGregor. It is not verified but it is speculated that SpaceX has done a number of tests that highlight and recreate the fueling procedure issue. This would fit with the information that SpaceX is moving forward and the initial report will soon be handed over.

Initial Report About SpaceX September Rocket Explosion Imminent

 

[ecarfan]

Thanks @Grendal. Not being a WSJ subscriber I can't access that article. "handing in" to whom? Will the report be publicly available?