I wanted to add to this discussion, that based on a careful re-reading
of the SpaceX statement, they do not identify the check valve as the
leaking culprit, they simply say 'a leaking component'. My intuition
is that they did have multi-use isolation valves, and those were what
leaked during the propellant loading, and of course the MON vapors
would migrate through the check valve.
Thanks,
Lars Osborne
On Fri, Jul 19, 2019 at 6:22 PM John Schilling
<john.schilling@xxxxxxxxxxxxxx> wrote:
On 7/15/2019 10:03 PM, Henry Vanderbilt wrote:
On 7/15/2019 7:56 PM, Henry Spencer wrote:Note that the propellants don't have to condense as liquids to do bad
The problem is that check valves don't reliably block slow reverse
flow of *gas*, and so a volatile propellant can seep up past the
check valve and condense in colder plumbing upstream. This is a known
problem, and has been for decades! In the case of N2O4, such seepage
can also corrode upstream components. (This is almost certainly what
really happened to Mars Observer, whose helium pressure regulators
were *not* rated for N2O4 exposure -- when the pressurization system
was activated, the corroded regulators failed to control the helium
flow, and the propellant tanks burst. Once this possibility was
noticed, the regulator failure was successfully duplicated in the
lab.) So just taking it slow on the pressurization is not sufficient.
The one time I was involved in an incident investigation where fuel
and oxidizer had met illicitly and noisily, I ended up spending some
considerable time testing out that
migrate-as-vapor-then-condense-in-a-bad-place possibility. FWIW, in
the real world it's quite difficult to make that happen.
Check valves are prone enough to just flat-out sticking and leaking
fluids that more exotic explanations aren't often needed.
things. Vapor-phase reaction of hydrazine and nitrogen tetroxide
produces a solid residue that precipitates on the nearest convenient
surface. Which will very often be just inside one of your check valves,
where the leaking vapor of one component has its first opportunity to
meet a pre-existing saturated vapor of the other. The precipitate is a
detonable high explosive, but you'll probably get a boring failure
before you ever see that exciting one - because the precipitate is also
either a gummy or hard crystalline solid depending on e.g. how
methylated your hydrazine was, but either one in the working parts of a
check valve will tend to make the valve stick. If the check valve
sticks closed, you probably can't pressurize your feed system and you
can't run your engines properly.
This was implicated in the failure, er, "anomalous behavior leading to
premature shutdown" of the main propulsion systems of the Juno and
Akatsuki space probes, and several other missions that I'm going to be
vague about. Common pressurization systems with check-valve isolation
are good for about a month with high confidence. At least with
traditional hypergols; if you come up with something new, you'll have
different reactions at different rates with different products. It's
still a fairly common scheme for communications satellites that are only
supposed to use their bipropellant systems for orbit transfer in the
first few weeks of the mission, but probably best to just not do that.
Also, and particularly for series-redundant check valves that get to
arguing about which one of them is supposed to be checking the flow,
they are prone to chatter among some flow conditions, which damages the
seats and produces leakage. Figure out what flow conditions do that,
and avoid them. Do NOT, NOT NOT NOT, use audible chatter of the check
valves as a ground-test diagnostic for "yep, the check valves are
unstuck and working". And don't ask me how I know that.
And don't trust them below -40 deg C, if that comes up (and it might in
a fast-flowing blowdown helium system). If the spec sheet says they are
qualified for that, it was probably twenty years ago with a grade of
teflon seat material that current production can't match.
John Schilling
john.schilling@xxxxxxxxxxxxxx
(661) 718-0955
