SpaceX’s Crew Dragon spacecraft static fire anomaly - THREAD 3

[edzieba]

>
The valve that separates the pressurant system from the propellant systems is what leaked during ground processing. NTO leaked passed this leaky valve into the pressurant system.
It is this leaky valve, separating pressurant system and propellant systems, that has been replaced by a burst disc.

This is not supported by any official public (or L2) information from SpaceX or NASA. Do you have an alternate source that the "leak during ground processing" was from the same valve that was destroyed? Additionally, removing a check valve would be extremely unlikely, for the reasons previously stated.

And here I thought SpaceX laid it all out in gory detail in this update.

Exactly:

Evidence shows that a leaking component allowed liquid oxidizer – nitrogen tetroxide (NTO) – to enter high-pressure helium tubes during ground processing. A slug of this NTO was driven through a helium check valve at high speed during rapid initialization of the launch escape system, resulting in structural failure within the check valve.

Emphasis mine. SpaceX very carefully DO NOT name what component leaked to allow the passage of NTO into the pressurisation system during ground processing. They very specifically DO name the the component that failed as a result of NTO within the pressurisation system.
If these were the same component, they would have named it as such. They did not, and it is therefore pure assumption that it was the same component. As ground processing involves a large amount of external hardware being hooked up to all fluid and gas systems, there are a very large number of potential components that could have allowed NTO to get into a Helium line.
Neither SpaceX nor NASA have named what component caused the original leak.

[Cherokee43v6]

>
The valve that separates the pressurant system from the propellant systems is what leaked during ground processing. NTO leaked passed this leaky valve into the pressurant system.
It is this leaky valve, separating pressurant system and propellant systems, that has been replaced by a burst disc.

This is not supported by any official public (or L2) information from SpaceX or NASA. Do you have an alternate source that the "leak during ground processing" was from the same valve that was destroyed? Additionally, removing a check valve would be extremely unlikely, for the reasons previously stated.

And here I thought SpaceX laid it all out in gory detail in this update.

Exactly:

Evidence shows that a leaking component allowed liquid oxidizer – nitrogen tetroxide (NTO) – to enter high-pressure helium tubes during ground processing. A slug of this NTO was driven through a helium check valve at high speed during rapid initialization of the launch escape system, resulting in structural failure within the check valve.

Emphasis mine. SpaceX very carefully DO NOT name what component leaked to allow the passage of NTO into the pressurisation system during ground processing. They very specifically DO name the the component that failed as a result of NTO within the pressurisation system.
If these were the same component, they would have named it as such. They did not, and it is therefore pure assumption that it was the same component. As ground processing involves a large amount of external hardware being hooked up to all fluid and gas systems, there are a very large number of potential components that could have allowed NTO to get into a Helium line.
Neither SpaceX nor NASA have named what component caused the original leak.

How else would it end up on the pressurization side of the valve due to a leak?  Would not common sense dictate that those sides be completely isolated from each other except at the location of the valve?

[edzieba]

>
The valve that separates the pressurant system from the propellant systems is what leaked during ground processing. NTO leaked passed this leaky valve into the pressurant system.
It is this leaky valve, separating pressurant system and propellant systems, that has been replaced by a burst disc.

This is not supported by any official public (or L2) information from SpaceX or NASA. Do you have an alternate source that the "leak during ground processing" was from the same valve that was destroyed? Additionally, removing a check valve would be extremely unlikely, for the reasons previously stated.

And here I thought SpaceX laid it all out in gory detail in this update.

Exactly:

Evidence shows that a leaking component allowed liquid oxidizer – nitrogen tetroxide (NTO) – to enter high-pressure helium tubes during ground processing. A slug of this NTO was driven through a helium check valve at high speed during rapid initialization of the launch escape system, resulting in structural failure within the check valve.

Emphasis mine. SpaceX very carefully DO NOT name what component leaked to allow the passage of NTO into the pressurisation system during ground processing. They very specifically DO name the the component that failed as a result of NTO within the pressurisation system.
If these were the same component, they would have named it as such. They did not, and it is therefore pure assumption that it was the same component. As ground processing involves a large amount of external hardware being hooked up to all fluid and gas systems, there are a very large number of potential components that could have allowed NTO to get into a Helium line.
Neither SpaceX nor NASA have named what component caused the original leak.

How else would it end up on the pressurization side of the valve due to a leak?  Would not common sense dictate that those sides be completely isolated from each other except at the location of the valve?

There is multiple pieces of ground side support equipment that must
- Perform initial fills of fuel, oxidiser, and Helium (along with other sundry internal gasses & fluids)
- Drain after pre-flight hot-fire tests (if any)
- Purge after pre-flight hot-fire tests (if any)
- Perform refill of fluids & gasses and repressurise after hot-fire tests (if any)
- Maintain and top-up and fluids/gasses while capsule is on the pad (via umbilical plate)
- Safe systems on recovery from the ocean after splashdown
- Drain fluids and gasses after recovery
- Purge and clean all lines
- Refill and repressurise for subsequent testing/operations

That involves numerous quick-disconnects, valves, tanks, pumps, tanks, etc, on multiple pieces of equipment. There are ample opportunities for a component failure or process failure to move fluids where they shouldn't be, without any failures of any hardware on the capsule itself.

[Cherokee43v6]

>
The valve that separates the pressurant system from the propellant systems is what leaked during ground processing. NTO leaked passed this leaky valve into the pressurant system.
It is this leaky valve, separating pressurant system and propellant systems, that has been replaced by a burst disc.

This is not supported by any official public (or L2) information from SpaceX or NASA. Do you have an alternate source that the "leak during ground processing" was from the same valve that was destroyed? Additionally, removing a check valve would be extremely unlikely, for the reasons previously stated.

And here I thought SpaceX laid it all out in gory detail in this update.

Exactly:

Evidence shows that a leaking component allowed liquid oxidizer – nitrogen tetroxide (NTO) – to enter high-pressure helium tubes during ground processing. A slug of this NTO was driven through a helium check valve at high speed during rapid initialization of the launch escape system, resulting in structural failure within the check valve.

Emphasis mine. SpaceX very carefully DO NOT name what component leaked to allow the passage of NTO into the pressurisation system during ground processing. They very specifically DO name the the component that failed as a result of NTO within the pressurisation system.
If these were the same component, they would have named it as such. They did not, and it is therefore pure assumption that it was the same component. As ground processing involves a large amount of external hardware being hooked up to all fluid and gas systems, there are a very large number of potential components that could have allowed NTO to get into a Helium line.
Neither SpaceX nor NASA have named what component caused the original leak.

How else would it end up on the pressurization side of the valve due to a leak?  Would not common sense dictate that those sides be completely isolated from each other except at the location of the valve?

There is multiple pieces of ground side support equipment that must
- Perform initial fills of fuel, oxidiser, and Helium (along with other sundry internal gasses & fluids)
- Drain after pre-flight hot-fire tests (if any)
- Purge after pre-flight hot-fire tests (if any)
- Perform refill of fluids & gasses and repressurise after hot-fire tests (if any)
- Maintain and top-up and fluids/gasses while capsule is on the pad (via umbilical plate)
- Safe systems on recovery from the ocean after splashdown
- Drain fluids and gasses after recovery
- Purge and clean all lines
- Refill and repressurise for subsequent testing/operations

That involves numerous quick-disconnects, valves, tanks, pumps, tanks, etc, on multiple pieces of equipment. There are ample opportunities for a component failure or process failure to move fluids where they shouldn't be, without any failures of any hardware on the capsule itself.

Every one of those sound like either 'ground processing equipment' failures or 'ground processing procedure' errors.  Show me anything that clearly mentions changes to either of those to mitigate?  The only known change is the valve... ergo, in the absence of other corroborating evidence, the valve was the source.

You're trying to over-complicate at a time when occam's razor applies.

[edzieba]

Show me anything that clearly mentions changes to either of those to mitigate?  The only known change is the valve...

In the aforelinked SpaceX blog post, SpaceX stated they were implementing several mitigations, of which the burst discs are one.

Again, SpaceX and NASA have been very public about the "unknown unknown" discovery of NTO/Titanium interaction at high pressure, but silent of any specifics of root cause analysis (for the NTO got there) or mitigations beyond the burst discs. I would not expect any public disclosure until at least one successful Dragon 2 processing flow & flight to 'prove out' those mitigations.

[gongora]

These are some of Chris G.'s tweets from the press conference that went along with that press release:

https://twitter.com/ChrisG_NSF/status/1150852224651792387

Have already replaced valve with burst disc.  #SpaceX #CrewDragon

https://twitter.com/ChrisG_NSF/status/1150853548982317067

Great questions from @lorengrush about design.  Hans confirms it is a design issue but also a feature of the check valves.  Burst Disc they have now is safer going forward. Didn't expect it to be an issue, so why they went with valve v. burst disc. - Hans. #SpaceX #CrewDragon

That one press release is not the only source of information.

[Remes]

We always look at the main path (all simplified): Helium tank -> pressure regulator -> oxydizer tank -> check valve -> oxydizer valve -> super draco. But there should be a bladder in both propellant tanks. With that bladder there should never cross anything the path to the He lines.

What about the purge path? That is directly connecting the He tank -> valve -> check valve -> oxyidizer lines
While fueling, some oxydizer could go past the check valve. Either the He is pressurized last (which I would assume), or  the line is anyway low pressure due to the upstream valve. The slug builds up before the check valve and when the chamber is prepurged, the slug hits the check valve.

A probable solution to the mystery leaking component: "...that a leaking component allowed liquid oydizer...   ...a slug...was driven through a helium check valve...":
I have seen poppet valves for space application which main force comes from the pressurant itself. 2 piston areas, one bigger, one smaller, both supplied by the pressurant. If a small solenoid vents the bigger area, the smaller will push the valve open. If no pressurant is applied (e.g. refueling and He-tanks are empty), there is only a weak spring force. Maybe to small, to seal it. (But even a non pressurized o-ring might fall in that category).