In article ,
says...

On 2019-07-20 08:54, Jeff Findley wrote:

No, SpaceX has already said the NTO got into the helium line during
ground (re)processing of the system.


Would this have involved purging the tanks and refilling them with new
hydrazine? Or did the processing just clean up the outside and fired the
engines in the same state thay came in at landing?

They have not provided details other than what I said above.

Scott Manley's video showed springs pushing valve down (to illustrate
the backflow prevention). Is this how the real valve would function?

Yes.

Or are we talking about a motor controlled valve that only opens valve
when pressure on He2 side is greater than pressure on the fuel/oxydizer
side?

No.

It is correct to state that the root cause is the failure of that
valve to prevent the liquid from flowing past it towards the He2
tank?

We don't know for sure, since SpaceX has not provided enough details.
But I find it possible that liquid leaked past the check valve (again,
no details, so we can speculate all we want).

That said, what others online (who know more about these sorts of
systems than I do) speculate is that (relatively warm) gaseous NTO snuck
past the check valve and then re-condensed to liquid in the (relatively
cold) helium plumbing. If that is the case, it's not a failure of the
check valve to prevent liquid from leaking past. That actually seems
more likely, IMHO.

Jeff
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