How big is the penalty for completely separate pressurization systems.....
On Mon, Jul 15, 2019 at 5:30 PM Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
wrote:
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> 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
