[AR] Re: What blew up Crew Dragon...

  • From: Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
  • To: arocket@xxxxxxxxxxxxx
  • Date: Mon, 15 Jul 2019 15:27:41 -0700

On 7/15/2019 3:13 PM, Henry Vanderbilt wrote:

On 7/15/2019 1:05 PM, George Herbert wrote:
        *Lars Osborne (@lars_0 <https://twitter.com/lars_0?s=11>)*
7/15/19, 12:51 PM <https://twitter.com/lars_0/status/1150855437765316608?s=11>
I work with hypergolic propellants now. I feel pretty satisfied with this explanation and think it has lessons for the industry.
What is missing is that the destruction of the check valve exposed bare Ti and accelerated the metal chips into the MON tank at high velocity. pic.twitter.com/09ud6OZFMv <https://t.co/09ud6OZFMv>


Lars posted this a bit ago as the SpaceX failure analysis went public.  Basically, slug of NTO migrated upstream past a He check valve, pushed back downstream on subsequent pressurization to fracture the check valve and throw Titanium metal fragments into the NTO tank...

Fix is replacing with burst discs.


Replacing He propellant pressurization check valves with burst discs?  I'm not following your thinking here...  Or are you saying, adding burst discs by the check valves?
<Emily Litella>  Never mind!  I went on to read the SpaceX piece, and their cure is to make the high-thrust escape rockets one-time activation by subbing burst discs for the check valves in the high-speed He press system, thus eliminating this particular failure probability. </Emily Litella>

Interesting that high-velocity NTO igniting the inside of the Ti check valve is described as a surprise, and something not seen before.  I'd assume Ti is like Al in inherently forming a surface oxide layer that makes it compatible with strong oxidizers, unless something is actively scrubbing off the oxide layer.  So, was it the check valve slamming open too hard with NTO present?  Or could high enough velocity NTO fluid perhaps directly scrub the Ti down to bare metal?

Henry


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