SpaceX’s Crew Dragon spacecraft static fire anomaly - THREAD 3

[ZChris13]

B) what leak around the flanges? Have you ever designed / tested a burst disc?

I spent over a year running high-pressure solid propellant combustion experiments in a shock tube, using burst disks to trigger the shock tube. So, yes.

As you know, replacing the burst disk requires that you have a bolted interface with O-rings, instead of welded butt joints like you have with check valves.

Bolted interface with O-rings is simply the easiest solution to replacing sections of pipe, not the only. If you're a masochist or the design calls for it, there's no reason you can't weld it in.

I'm talking about the seal between the flange and the burst disk, not the seal between the flange and the pressurant line. Yes, you can/would weld the flange to the line. But you can't weld the flanges to each other, because you have to be able to take out the old burst disk and install a new one.

So you still have a bolted interface that clamps the flanges to either side of the burst disk. That interface can leak, because someone installed the burst disk incorrectly, or scratched the mating surface, or whatever.

Now every time you replace the burst disk, you have to do a leak check. I'm not saying it's a major problem, but it is now a potential leak area that you have to check every time. As opposed to a welded-in-place valve body that you check once for integrity after welding and never have to leak-check again.

Also remember that we're talking about high-pressure helium, which is extraordinarily good at finding tiny leak paths.

This is offered by Zookdisk as "custom welded assemblies" and is nothing but welds. Having only worked with sensible burst disks that go in union joints or flange joints, I can't really comment on just how stupid of an idea this is, but it's out there.
Also included is the pdf on it.

I'm going to be straight here, I just woke up and this is me trying to save face.

[Kabloona]

This is offered by Zookdisk as "custom welded assemblies" and is nothing but welds. Having only worked with sensible burst disks that go in union joints or flange joints, I can't really comment on just how stupid of an idea this is, but it's out there.
Also included is the pdf on it.

I'm going to be straight here, I just woke up and this is me trying to save face.

Thanks for being refreshingly candid.

Let's agree that an entirely welded-shut one-shot-use burst disk assembly may have some use somewhere in the universe, but no one knows where, and probably (well, definitely) not on Dragon.

Now have some coffee. And get back to work. 

[dondar]

A question for propulsion experts:

Lots of spacecraft have used check valves to prevent upstream migration of NTO.  (Here's an an example from Galileo, that in turn refers to an earlier example from Viking.  "Check valves were provided to minimize MMH and NTO vapor migration upstream of the propellant tanks."

And for as long as they have been used, these valves have been known to leak (note the above sentence says minimize, not eliminate).  Presumably, once pressurized, all these other systems also pushed NTO through the check valve, too.   But this alone never seems to have caused a serious problem - it was only when MMH and NTO met upstream of valve.

So why did this cause a problems when similar systems did not?   I can think of four hypotheses:
   - Old check valves did not use titanium.
   - They did use titanium.  No problem occurs at 20 bar, but the reaction is much worse at 160 bar.
   - More NTO was leaked (by whatever mechanism) than in previous examples.
   - Previous accident reports missed this problem.  The accident reports I have seen tie themselves in knots figuring out how MMH and NTO could meet in the pressurization lines.  Maybe there was in fact no MMH leak, and the this particular problem has been happening all along, but was wrongly thought to be due to MMH and NTO mixing.

3000 Psi. This is the He pressure used to charge super dracos. It was reported that the delta pressure between He pressurizing system and the fuel line was around 2400 Psi.
 this is typical discharge pressure of the decent (starting level though) air guns. If you plug in actual energies all questions will disappear. SpaceX was as clear as they have to.

EDIT: there is no need to "remove" "install" etc. new burst discs.
Dragon 2 crafts will be used only ones, i.e. only ONE TIME as passenger vehicles. Superdracos and corresponding plumbing most probably will be removed afterwards in order to free space and weight for cargo missions. Even otherwise the burst discs will be used only in superdracos plumbing and are irrelevant in normal operations. They will be used only in the case of abort mission which would automatically write vehicle off and make it unsuitable for further missions.

[ZChris13]

This is offered by Zookdisk as "custom welded assemblies" and is nothing but welds. Having only worked with sensible burst disks that go in union joints or flange joints, I can't really comment on just how stupid of an idea this is, but it's out there.
Also included is the pdf on it.

I'm going to be straight here, I just woke up and this is me trying to save face.

Thanks for being refreshingly candid.

Let's agree that an entirely welded-shut one-shot-use burst disk assembly may have some use somewhere in the universe, but no one knows where, and probably (well, definitely) not on Dragon.

Now have some coffee. And get back to work. 

There's two of my life guidelines at play here:
1. Always learn something whenever I open my stupid mouth and garbage comes out.
2. It is always possible to replace whatever fastening/joining contraption is being used with a weld.

[docmordrid]

When did the SuperDraco test appear on the schedule? I don't remember it being mentioned until weeks before the test.

Was it added after the Boeing Starliner incident, to see if Crew Dragon would also suffer a valve failure at high pressure?

[Kabloona]

EDIT: there is no need to "remove" "install" etc. new burst discs.
Dragon 2 crafts will be used only ones, i.e. only ONE TIME as passenger vehicles. Superdracos and corresponding plumbing most probably will be removed afterwards in order to free space and weight for cargo missions. Even otherwise the burst discs will be used only in superdracos plumbing and are irrelevant in normal operations. They will be used only in the case of abort mission which would automatically write vehicle off and make it unsuitable for further missions.

That could be true, assuming SpaceX doesn't do a full-up static fire of the abort propulsion system before every flight.

I don't know what their plans are, but since (1) abort propulsion is critical for crew survival if needed in flight, and (2) it can be tested before flight, analagous to F9 pre-flight static fires, and (3) SpaceX will likely now take extra pains to demonstrate before flight that the Dragon abort propulsion system isn't going to explode, my money is on betting that SpaceX will do a static fire of every Dragon abort propulsion system before flight.

[whitelancer64]

*snip*
Dragon 2 crafts will be used only ones, i.e. only ONE TIME as passenger vehicles. Superdracos and corresponding plumbing most probably will be removed afterwards in order to free space and weight for cargo missions. Even otherwise the burst discs will be used only in superdracos plumbing and are irrelevant in normal operations. They will be used only in the case of abort mission which would automatically write vehicle off and make it unsuitable for further missions.

Firstly, there is no change to the propulsion systems for the Cargo Dragon v2. SuperDracos are mounted external to the PV so removing them would not free up any space inside.

Secondly, there is nothing preventing SpaceX from using a Dragon v2 to fly people a second time, for example, if Bigelow buys a crew launch to their B330 space station. Or should the ISS be extended out to 2030, it's entirely possible NASA would give the OK to fly their astronauts on one again.

[guckyfan]

Will the burst disks be expended when the LES is activated ahead of launch or only when it would actually be fired?

[Kabloona]

Will the burst disks be expended when the LES is activated ahead of launch or only when it would actually be fired?

We know from the Dragon test failure that system pressurization occurs just before firing (IIRC, it was about 100 msec prior to firing). So the burst disk would remain intact until an abort is commanded and the helium valve is opened to pressurize the NTO tank.

[Star One]

Will the burst disks be expended when the LES is activated ahead of launch or only when it would actually be fired?

We know from the Dragon test failure that system pressurization occurs just before firing (IIRC, it was about 100 msec prior to firing). So the burst disk would remain intact until an abort is commanded and the helium valve is opened to pressurize the NTO tank.

Would they need inspecting after each flight even when not used?

[Kabloona]

Will the burst disks be expended when the LES is activated ahead of launch or only when it would actually be fired?

We know from the Dragon test failure that system pressurization occurs just before firing (IIRC, it was about 100 msec prior to firing). So the burst disk would remain intact until an abort is commanded and the helium valve is opened to pressurize the NTO tank.

Would they need inspecting after each flight even when not used?

They might want to take a look at the disks after the first few flights just to make sure there weren't any adverse effects from vibration, etc. But the disk environment should be pretty benign. No pressure (just 1 atm) on the helium side, and relatively modest vapor pressure on the NTO side.

I would think that after they do a few post-flight inspections and see no ill effects on the disks, they would feel confident enough to leave them alone after future flights.

[envy887]

Will the burst disks be expended when the LES is activated ahead of launch or only when it would actually be fired?

We know from the Dragon test failure that system pressurization occurs just before firing (IIRC, it was about 100 msec prior to firing). So the burst disk would remain intact until an abort is commanded and the helium valve is opened to pressurize the NTO tank.

Would they need inspecting after each flight even when not used?

They might want to take a look at the disks after the first few flights just to make sure there weren't any adverse effects from vibration, etc. But the disk environment should be pretty benign. No pressure (just 1 atm) on the helium side, and relatively modest vapor pressure on the NTO side.

I would think that after they do a few post-flight inspections and see no ill effects on the disks, they would feel confident enough to leave them alone after future flights.

I'd expect a preflight (or possibly postflight) test firing of the abort system on every Dragon, so the burst disks would need to be replaced after that.

[Star One]

Will the burst disks be expended when the LES is activated ahead of launch or only when it would actually be fired?

We know from the Dragon test failure that system pressurization occurs just before firing (IIRC, it was about 100 msec prior to firing). So the burst disk would remain intact until an abort is commanded and the helium valve is opened to pressurize the NTO tank.

Would they need inspecting after each flight even when not used?

They might want to take a look at the disks after the first few flights just to make sure there weren't any adverse effects from vibration, etc. But the disk environment should be pretty benign. No pressure (just 1 atm) on the helium side, and relatively modest vapor pressure on the NTO side.

I would think that after they do a few post-flight inspections and see no ill effects on the disks, they would feel confident enough to leave them alone after future flights.

I'd expect a preflight (or possibly postflight) test firing of the abort system on every Dragon, so the burst disks would need to be replaced after that.

Would that be something NASA would require of them? If it was does that mean on non-NASA flights they wouldn’t feel the need too?

[envy887]

I'd expect a preflight (or possibly postflight) test firing of the abort system on every Dragon, so the burst disks would need to be replaced after that.

Would that be something NASA would require of them? If it was does that mean on non-NASA flights they wouldn’t feel the need too?

SpaceX generally test fires everything they can, even for non-NASA customers. It's probably one of the main reasons for their high mission success rate.

[Ad_Astra7]

I'd expect a preflight (or possibly postflight) test firing of the abort system on every Dragon, so the burst disks would need to be replaced after that.

Would that be something NASA would require of them? If it was does that mean on non-NASA flights they wouldn’t feel the need too?

SpaceX generally test fires everything they can, even for non-NASA customers. It's probably one of the main reasons for their high mission success rate.

I bet AMOS-6 could give an earful on test fires and mission success....

[Lar]

I bet AMOS-6 could give an earful on test fires and mission success....

(fan) That would be the exception that proves the rule. Further, it is pretty clear that SpaceX learned a lesson about payloads and static firings.

(mod) DO NOT go down the AMOS-6 rathole. my comment above was intended to be all that needed to be said about that.

[envy887]

Dragon has also been highly successful to date.

[Ad_Astra7]

Dragon has also been highly successful to date.

Well this is a thread about Crew Dragon and the only Crew Dragon vehicle to fly is currently in pieces (albeit during a test that did identify an issue that can be resolved).

[envy887]

Dragon has also been highly successful to date.

Well this is a thread about Crew Dragon and the only Crew Dragon vehicle to fly is currently in pieces (albeit during a test that did identify an issue that can be resolved).

DM-1 was quite successful, largely because D2 builds on a lot of Cargo Dragon heritage.

And SpaceX isn't going to stop ground testing because one ground test found a problem. Rather the opposite, I should think. That's the whole point of ground testing.

[Ad_Astra7]

Dragon has also been highly successful to date.

Well this is a thread about Crew Dragon and the only Crew Dragon vehicle to fly is currently in pieces (albeit during a test that did identify an issue that can be resolved).

DM-1 was quite successful, largely because D2 builds on a lot of Cargo Dragon heritage.

And SpaceX isn't going to stop ground testing because one ground test found a problem. Rather the opposite, I should think. That's the whole point of ground testing.

Most things are successful until they are not.  Past performance does not guarantee future success.  Testing is always important.  All engineering firms do it.  SpaceX just seems to have more spectacular failures when they do it, which may be what draws everyone to their test campaigns.