And if that check valve was already leaking, some sort of grit or
contamination as the original cause is a pretty good bet.
Guessing at "what were they thinking" here... The SuperDraco thrusters
were originally going to be used routinely to land the capsule. Hence,
design them for easy turnaround - check valves to keep propellant out of
the He press system, rather than burst discs that'd need replacing after
every flight. Now, of course, the SuperDracos will only be used for a
presumably rare abort, so burst discs are tolerable.
As for titanium rather than, say, stainless, presumably that's to save mass.
On a pressure-fed thruster system that is to be routinely used, possibly
pressurized more than once per flight, any thoughts on how to avoid such
issues? Off the top of my head, doing sensitive fuel/oxidizer detection
on the He side of the press system during servicing is one possibility.
If you find traces, inspect the check valves. Maybe pull vac on the He
system to detect any leaky check valves. Maybe combine the two, doing
propellant detection on the evacuated stream as you pull the He system
down to vac.
Or, in-flight, propellant detection upstream of the He check valves - or
even simple liquid detection - could be enough for a no-go warning. Or
a "slow press only, if you MUST use these thrusters" warning.
Henry
On 7/15/2019 3:58 PM, Ben Brockert wrote:
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 <mailto:hvanderbilt@xxxxxxxxxxxxxx>> wrote:
On 7/15/2019 3:13 PM, Henry Vanderbilt wrote:
On 7/15/2019 1:05 PM, George Herbert wrote:<Emily Litella> Never mind! I went on to read the SpaceX piece,
*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?
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
