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SpaceX Capsule Explosion



 
 
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  #41  
Old July 21st 19, 07:58 PM posted to sci.space.policy
Niklas Holsti
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Default SpaceX Capsule Explosion

On 19-07-21 16:57 , Jeff Findley wrote:
In article ,
lid says...

On 19-07-20 15:40 , Jeff Findley wrote:
In article ,
lid says...

On 19-07-19 14:06 , Jeff Findley wrote:
In article , says...

I couldn't find a definitive reference. But the abort system is simply
not needed for a cargo flight. It could be that the parts will still be
on the Dragon 2, but the abort system's propellant tanks and high
pressure helium tanks simply won't be filled. That would also save a
lot of mass while minimizing the modifications needed to turn a crew
Dragon 2 into a cargo Dragon 2.

They might even use propulsive landing for cargo flights.

No, they won't. NASA has definitively nixed that idea. NASA does not
want irreplaceable returning cargo (e.g. EMU suits) splattered all over
a concrete landing pad when the inevitable "oopsie" happens.


...
... NASA told SpaceX that they would not allow propulsive landing
tests using returning Dragon 2 cargo vessels. It would have been
prohibitively expensive for SpaceX to fully test propulsive landings "on
their own dime".

You see, SpaceX wanted to test propulsive Dragon 2 landings on
operational cargo missions just like they tested Falcon 9 first stage
landings on operational missions.


Ah, I didn't know that. Yes, I can understand why NASA did not agree to
such "operational tests".

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  #42  
Old July 21st 19, 08:19 PM posted to sci.space.policy
Niklas Holsti
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Default SpaceX Capsule Explosion

On 19-07-21 16:48 , Jeff Findley wrote:
In article ,
says...

On 2019-07-20 10:16, Jeff Findley wrote:

Scott Manley's video showed springs pushing valve down (to illustrate
the backflow prevention). Is this how the real valve would function?

Yes.


systems than I do) speculate is that (relatively warm) gaseous NTO snuck
past the check valve and then re-condensed to liquid in the (relatively
cold) helium plumbing.



Shoudln't a check valve prevent passange in wrong direction on either
gas or liquid? It's not like there is a floater that rises up to close
valve is liquid rises up, is it?


You really can't rely on a check valve for this. That's why designs
also include (usually redundant) isolation valves. No, I don't know why
NASA "has always done it this way" if an isolation valve would have been
better.


With apologies to those who also follow the "arocket" list, I note that
on that list, Lars Osborne recently pointed out that the SpaceX
statement actually says "a leaking component" but does _not_ say that it
was the check valve that leaked.

Osborne continues as follows: "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."

While the fix involves replacing the check valves with burst disks, this
still does not strictly imply that the unexpected leak occurred in a
check valve.

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  #43  
Old July 22nd 19, 03:21 PM posted to sci.space.policy
David Spain
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Default SpaceX Capsule Explosion

On 7/20/2019 10:16 AM, Jeff Findley wrote:
In article ,
says...

[snip]

It is correct to state that the root cause is the failure of that
valve to prevent the liquid from flowing past it towards the He2
tank?


We don't know for sure, since SpaceX has not provided enough details.
But I find it possible that liquid leaked past the check valve (again,
no details, so we can speculate all we want).

That said, what others online (who know more about these sorts of
systems than I do) speculate is that (relatively warm) gaseous NTO snuck
past the check valve and then re-condensed to liquid in the (relatively
cold) helium plumbing. If that is the case, it's not a failure of the
check valve to prevent liquid from leaking past. That actually seems
more likely, IMHO.

Jeff


I hope I don't get in trouble posting snippets from Arocket to here, but
a decade or so back in time this likely would have been posted here as
well and Arocket is a "semi" public mailing list anyway...

This is a message from John Schilling who appears to have been in the
"trenches" so to speak when it come to dealing with hypergolic fuels and
he adds a point to the discussion over there I hadn't considered.

Dave


On 7/15/2019 10:03 PM, Henry Vanderbilt wrote:
On 7/15/2019 7:56 PM, Henry Spencer wrote:

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.

Note that the propellants don't have to condense as liquids to do bad 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


 




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