Titanium is dramatically less compatible with oxidizers than aluminum is.
It's sensitive to impact with N2O4, especially when there's grit or filings
in the impact, and check valves are really effective at creating impacts
and finding grit.
On Mon, Jul 15, 2019 at 3:28 PM Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
wrote:
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
