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

[mmeijeri]

It should, Titanium is used everywhere for NTO. The fact it can burn it I don't think is well known. Titanium is generally pretty non-reactive.

Interesting tweets on this topic by Ben Brockert:

https://twitter.com/wikkit/status/1150855184924336128

SpaceX update on the disappearing Dragon. (link: https://www.spacex.com/news/2019/07/15/update-flight-abort-static-fire-anomaly-investigation) spacex.com/news/2019/07/1… “It is worth noting that the reaction between titanium and NTO at high pressure was not expected” is a frightening sentence; titanium is well known to have poor oxidizer compatibility and be easy to set on fire. “resistant to N2O4 except under impact... found that Ti impacts sporadically under reasonably well controlled test conditions; the ignition freq. is increased markedly by Ti filings or glass particles on the impact surface” Compatibility of Materials w/ Rocket Prop and Oxidizers

[ZChris13]

Interesting that yet again (as with the COPV) a type of combustion that was not expected was discovered.

Agree with others that this is certainly advancing the state of the art in this area.

I am not sure how burst disks can take the place of check valves if there is a need for material flow in one direction.

I read it like this: they only pressurize the system (popping the discs) when it's go-time and S is actively HTF. Once the Super Dracos are on fire and prop is flowing you don't need to worry about oxidizer backing up into your helium line.
It's another item on the list that needs replacing after an active abort, instead of the passive check valves that'll seal themselves. I guess it can go on the list after new pairs of underwear for all involved.
Upthread a few comments calling out how this could make testing more difficult have good points.

[mmeijeri]

Might also impact the cost of reuse if they ever switch to propulsive landings.

[kevinof]

They wont (do propulsive landings) so not an issue.

Might also impact the cost of reuse if they ever switch to propulsive landings.

[brettreds2k]

Great article released today. I didn’t know SpaceX updated the cause today.

https://techcrunch.com/2019/07/15/spacex-and-nasa-detail-cause-of-dragon-test-failure-crewed-flight-this-year-looks-increasingly-difficult/amp/

[SWGlassPit]

It should, Titanium is used everywhere for NTO. The fact it can burn it I don't think is well known. Titanium is generally pretty non-reactive.

Interesting tweets on this topic by Ben Brockert:

https://twitter.com/wikkit/status/1150855184924336128?s=20

SpaceX update on the disappearing Dragon. (link: https://www.spacex.com/news/2019/07/15/update-flight-abort-static-fire-anomaly-investigation) spacex.com/news/2019/07/1… “It is worth noting that the reaction between titanium and NTO at high pressure was not expected” is a frightening sentence; titanium is well known to have poor oxidizer compatibility and be easy to set on fire. “resistant to N2O4 except under impact... found that Ti impacts sporadically under reasonably well controlled test conditions; the ignition freq. is increased markedly by Ti filings or glass particles on the impact surface” Compatibility of Materials w/ Rocket Prop and Oxidizers

This: it's in a DTIC memo from the 60s (page 9).  I don't know why it's "unexpected" for titanium to ignite when NTO is rammed into it at high pressure.

[dglow]

This: it's in a DTIC memo from the 60s (page 9).  I don't know why it's "unexpected" for titanium to ignite when NTO is rammed into it at high pressure.

It's unexpected because SpaceX apparently didn't review the literature.

[mmeijeri]

It's unexpected because SpaceX apparently didn't review the literature.

NASA review evidently also didn't catch the issue. I wonder if Orion and CST-100 use titanium check valves and if they use burst disks. I do know that check valves are known to sometimes leak, because I vaguely recall reading a description of the propulsion system of some historical spacecraft (perhaps Apollo) that used burst disks precisely for this reason.

[ulm_atms]

This: it's in a DTIC memo from the 60s (page 9).  I don't know why it's "unexpected" for titanium to ignite when NTO is rammed into it at high pressure.

It's unexpected because SpaceX apparently didn't review the literature.

If you want to go down that route...NASA didn't either as NASA let DM-1 use this config.  Lets not go down that path in this thread please. It will just muddy it all up.

I am glad however that we learned in ~4 months what happened, that it seems easily fixable, and that lessons were learned and no lives lost to learn them.  Good day!

[arachnitect]

Additionally, the SuperDraco thrusters recovered from the test site remained intact, underscoring their reliability.

...so nobody's gonna point out how ridiculous this line is?

[Craftyatom]

Bridenstine is still not too happy about the communication issue: https://spacenews.com/bridenstine-says-leadership-changes-linked-to-urgency-in-nasas-exploration-programs/

SpaceX suffered the loss of that Crew Dragon spacecraft in April during preparations for an in-flight abort test. Both the company and the agency faced criticism for the lack of openness in the ongoing investigation into that incident, and Bridenstine vowed that the process will work differently in the future.

“That criticism largely comes from me as well,” he said, noting there was “no communication” from SpaceX immediately after the accident. “That can’t happen again.”

Bridenstine said that a new process is now in place for communications in the event of another mishap. “Within a couple of hours, we’re going to do a press conference and get as much information out to the public as soon as possible.”

It seems Bridenstine means communication with the public. It's been previously reported there was lots of communication immediately between SpaceX and NASA.

I wonder whether this might be a case of "ask your mother".  SpaceX expected NASA to be the public-facing element of the investigation, since they're the regulatory and customer side of the agreement.  NASA, meanwhile, expected SpaceX to do its own public communications, since it was their hardware.

Think about it this way: NASA clearly doesn't want to be the one spilling the beans - Boeing would've been furious if they had for the Starliner test failure.  And yet, from SpaceX's point of view, NASA has historically wanted them to keep quiet and not publicize possible findings until they've been through review.  Neither wants to speak first.

All speculation, of course.  Hopefully anyone with grievances has aired them by now.

[SWGlassPit]

This: it's in a DTIC memo from the 60s (page 9).  I don't know why it's "unexpected" for titanium to ignite when NTO is rammed into it at high pressure.

It's unexpected because SpaceX apparently didn't review the literature.

I'm honestly surprised their M&P folks did not even think of this possibility.  That's what the M&P discipline is for.

[gongora]

It's unexpected because SpaceX apparently didn't review the literature.

NASA review evidently also didn't catch the issue. I wonder if Orion and CST-100 use titanium check valves and if they use burst disks. I do know that check valves are known to sometimes leak, because I vaguely recall reading a description of the propulsion system of some historical spacecraft (perhaps Apollo) that used burst disks precisely for this reason.

Orion's abort system has nothing at all in common with Dragon (or Starliner).  It uses solid rocket motors.

[tp1024]

It's unexpected because SpaceX apparently didn't review the literature.

NASA review evidently also didn't catch the issue. I wonder if Orion and CST-100 use titanium check valves and if they use burst disks. I do know that check valves are known to sometimes leak, because I vaguely recall reading a description of the propulsion system of some historical spacecraft (perhaps Apollo) that used burst disks precisely for this reason.

Erm, I thinik you both missed that the the NASA document is talking abotu an impact of NTO on titanium, while there was no impact here at all. The slug of NTO was already in the line, at the valve, and pushing the NTO through the valve - while it was being activated - was sufficient to ignited the titanium. No impact required. That was new.

[Semmel]

This: it's in a DTIC memo from the 60s (page 9).  I don't know why it's "unexpected" for titanium to ignite when NTO is rammed into it at high pressure.

It's unexpected because SpaceX apparently didn't review the literature.

No, it's unexpected because there was no reason for a high velocity impact of n2o4 into titanium. Its not like the helium pressure line was supposed to hammer n2o4 into the check valve. Or any liquid for that matter. Thats why the check valve was there. And thats why its perfactly safe to use titanium.. if it was not leaking. Even water would have destroyed the valve, but without igniting it of course.

[SWGlassPit]

It's unexpected because SpaceX apparently didn't review the literature.

NASA review evidently also didn't catch the issue. I wonder if Orion and CST-100 use titanium check valves and if they use burst disks. I do know that check valves are known to sometimes leak, because I vaguely recall reading a description of the propulsion system of some historical spacecraft (perhaps Apollo) that used burst disks precisely for this reason.

Erm, I thinik you both missed that the the NASA document is talking abotu an impact of NTO on titanium, while there was no impact here at all. The slug of NTO was already in the line, at the valve, and pushing the NTO through the valve - while it was being activated - was sufficient to ignited the titanium. No impact required. That was new.

Driving a slug of liquid through a check valve at high pressure creates a hydraulic hammer effect that is plenty impact-like.

[FinalFrontier]

Good to know the final cause. Looks like a simpler fix as opposed to what it would have been had it been a COPV design flaw. Still not great that they missed the material issues and also not great that the pressurization system was designed in a way which could allow fluid to migrate into the gas side of the system. But all in all, not terrible to fix.

As far as delays probably not going to get a crew up until next year IMHO but that's not a big deal as long as it doesn't go too much further down the road.
Imo I am glad they published this as opposed to keeping it quiet or trying to have NASA be the ones to announce it. With that said however there should be some PR lessons learned from how this was handled initially.

[cuddihy]

This: it's in a DTIC memo from the 60s (page 9).  I don't know why it's "unexpected" for titanium to ignite when NTO is rammed into it at high pressure.

It's unexpected because SpaceX apparently didn't review the literature.

If you want to go down that route...NASA didn't either as NASA let DM-1 use this config.  Lets not go down that path in this thread please. It will just muddy it all up.

I am glad however that we learned in ~4 months what happened, that it seems easily fixable, and that lessons were learned and no lives lost to learn them.  Good day!

Theory: the “unexpected” was meant to refer to this event at high pressure, and the PR person re-ordered it to make it “clearer.” Money on the Dragon propulsion designers (at least some of them) understanding the materials risk but thinking it didn’t apply since the NTO should be on the other side of this valve located between the helium and NTO systems....just not whoever reviewed/approved the statement.

[FinalFrontier]

This: it's in a DTIC memo from the 60s (page 9).  I don't know why it's "unexpected" for titanium to ignite when NTO is rammed into it at high pressure.

It's unexpected because SpaceX apparently didn't review the literature.

No, it's unexpected because there was no reason for a high velocity impact of n2o4 into titanium. Its not like the helium pressure line was supposed to hammer n2o4 into the check valve. Or any liquid for that matter. Thats why the check valve was there. And thats why its perfactly safe to use titanium.. if it was not leaking. Even water would have destroyed the valve, but without igniting it of course.

Leaking NTO block valve is basically the root cause, this and faulty gas side design which allowed migration of NTO at that valve if it leaked.

[1]

This: it's in a DTIC memo from the 60s (page 9).  I don't know why it's "unexpected" for titanium to ignite when NTO is rammed into it at high pressure.

It's unexpected because SpaceX apparently didn't review the literature.

No, it's unexpected because there was no reason for a high velocity impact of n2o4 into titanium. Its not like the helium pressure line was supposed to hammer n2o4 into the check valve. Or any liquid for that matter. Thats why the check valve was there. And thats why its perfactly safe to use titanium.. if it was not leaking. Even water would have destroyed the valve, but without igniting it of course.

Leaking NTO block valve is basically the root cause, this and faulty gas side design which allowed migration of NTO at that valve if it leaked.

Agreed. This isn't M&P missing something that was known, this is underestimating the severity of (or failing to predict altogether) NTO leaking through the check valve. I suspect it wasn't an engineer that wrote the press release.