SpaceX Falcon 9 - AMOS-6 - (Pad Failure) - DISCUSSION THREAD (2)

[yokem55]

What are the odds of them pointing a tiger team at developing an autogenous pressurization system for the F9/FH LOX tanks? Just to retire the cLOX v He issue once and for all. Implement as "v1.2.1"

Maybe for Raptor based second stage...but unless you can figure out how to autogenous pressurize RP-1 you are going to need the He anyway.

If they need more reliable copv's it might be worth it to switch to a titanium liner for them in the short term. It would be expensive (which I'm guessing is a big part of the choice to use aluminum as the liner material), but they would get the first stage bottles back and it could hold them over until a raptor based autogenously pressed upper stage could be flown.

[HMXHMX]

What are the odds of them pointing a tiger team at developing an autogenous pressurization system for the F9/FH LOX tanks? Just to retire the cLOX v He issue once and for all. Implement as "v1.2.1"

Maybe for Raptor based second stage...but unless you can figure out how to autogenous pressurize RP-1 you are going to need the He anyway.

If they need more reliable copv's it might be worth it to switch to a titanium liner for them in the short term. It would be expensive (which I'm guessing is a big part of the choice to use aluminum as the liner material), but they would get the first stage bottles back and it could hold them over until a raptor based autogenously pressed upper stage could be flown.

Not submerged in lox.  Ti is not compatible. Stainless would serve and is what the vendor uses for similar COPVs  sold to other customers.

[jongoff]

Yeah, they'd have to move them outside the tank. There are ways they could do it more safely, but almost all of them will impact performance. And unfortunately SpaceX has always seemed a bit obsessed with performance.

~Jon

"more safely"

I don't think that's a fair statement. Unless you know something the rest of us don't about cryogenic COPVs, there's no reason to think the tank-within-a-tank method isn't inherently safe. This could be a manufacturing defect. It could be a programming error of SCADA controls on the GSE. It could a discovery of a new inhibit. Until we know, statements like that are premature.

The fact that there are many failure modes for internally mounted tanks that lead to LOX tank failure, and potential common bulkhead failure, suggests to me that the design isn't "inherently" safe. Maybe something you can tame and make "acceptably" safe, but that's not a system I'd call inherently safe.

~Jon

People keep saying COPV.  The SpaceX statement doesn't say "COPV".  It says "high pressure Helium system".  There is way more to this system than just the tanks (and the struts that support them.)  There is lots of piping, valves, regulators, and a supply system from the TEL.

IMO the only part of the high pressure helium system that has the energetic potential to pop the upper stage in less than 1/10th of a second is a pressurized COPV letting go.

I could also see the potential for other failure areas that could create a fast moving piece of shrapnel that penetrated the common bulkhead. Most people think COPV failure means "tank overpressure", but the SIVB had a test stand failure due to a Titanium GHe bottle's girth weld failing that propelled one of the hemispheres up through the bottom tank fast enough to penetrate the common bulkhead.

Still agree that with ~11MJ of energy each (2kg TNT equivalent), the COPVs were storing the one source of high energy in the stages. So even if it wasn't a COPV itself failing (I think that is a likely but not definite possibility), the COPV stored enough energy to propel shrapnel fast enough to cause other badness even before an overpressure event could take place.

Sorry if that's rambly,

~Jon

[jongoff]

They still want to fly again in November which sounds optimistic.

Yeah. I would estimate 6 to 9 months if it's a quality control issue, 9 to 12 month if they have to redesign something or fabricate a long lead item.

I would say November ... 2017.

I think that's pessimistic. SpaceX does tend to bounce back from things like this faster than people expect. I wouldn't be surprised if they didn't launch again till next year, but I also wouldn't be completely surprised if they had their return to flight before Christmas.

~Jon

What are the odds of them pointing a tiger team at developing an autogenous pressurization system for the F9/FH LOX tanks? Just to retire the cLOX v He issue once and for all. Implement as "v1.2.1"

You usually need both propellants to be volatile to do autogenous pressurization. Once they switch to Raptor, they could make it work, but they'll still need another pressurant for the Kerosene tanks while they're still flying with Merlins.

~Jon

[jongoff]

Yeah, they'd have to move them outside the tank. There are ways they could do it more safely, but almost all of them will impact performance. And unfortunately SpaceX has always seemed a bit obsessed with performance.

~Jon

Think there's a chance their customers will make that decision for them? I'm not sure how much the fact that the failure mode being different from CRS-7 matters - in fact it may make matters worse. Now there are 2 demonstrated ways that helium bottles in the LOX tanks are troublesome. How many more ways are there? Do we want to find them all by trial and error?

I doubt their customers will force a design change on them. While I personally don't like storing the equivalence of small bombs inside my LOX tanks, it may very well be a "tamable" problem.

~Jon

I dunno. The engineer in me just doesn't like the idea of putting tanks pressurized to hundreds or thousands of pounds inside a tank that's built for 30.

That has nothing to do with it. If the tank fails, it doesn't matter if it's inside or outside the LOX tank. The only reason this is a discussion is because of the cryogenics of the LOX tank, not the pressure levels.

That's not entirely true. If you wanted to take the performance hit, you could make a high-pressure GHe system that could survive failure of any given bottle or pressurant line. I'm not saying that level of armoring and redundancy is the right decision (for instance Centaur has externally mounted bottles, but I don't know if it is designed in a way that could survive one of them letting go--knock on wood), but it *is* possible to design a pressurization system in a way that failure modes don't immediately lead to an earth-shattering kaboom. SpaceX made a design choice. It's one that some companies have made similarly, and some differently. I personally don't like it, but that doesn't mean it's wrong per se.

~Jon

[jongoff]

What are the odds of them pointing a tiger team at developing an autogenous pressurization system for the F9/FH LOX tanks? Just to retire the cLOX v He issue once and for all. Implement as "v1.2.1"

Maybe for Raptor based second stage...but unless you can figure out how to autogenous pressurize RP-1 you are going to need the He anyway.

If they need more reliable copv's it might be worth it to switch to a titanium liner for them in the short term. It would be expensive (which I'm guessing is a big part of the choice to use aluminum as the liner material), but they would get the first stage bottles back and it could hold them over until a raptor based autogenously pressed upper stage could be flown.

Have people done Titanium GHe spheres in LOX? I know they've done it in LH2, but I was under the impression that Titanium was very dangerous in LOX. Even scratching titanium in LOX can lead to an explosion (due to rapid titanium combustion). IIRC it had something to do with Titanium being highly exothermic when burned in LOX, combined with low thermalconductivity, and it not forming as good of an oxide layer as aluminum (or maybe it was that Titanium oxide is soluable in titanium). Can't remember all the details. I just remember researching it at Masten, because I wanted to find a way to do it, and coming away with a "maybe if I coat it just right with copper it wouldn't be suicidal".

~Jon

The He tanks contain high pressure gas, not liquid. So if they burst, it's not going to be as violent

If you're designing a factory system, for instance, sure this makes sense. But a leaking He supply in a rocket is a mission failure, so it's a moot point.

Maybe true, but a leaking He system is easier to root cause than an exploding one where the telemetry abruptly ends.

And there are ways to design a helium system with redundancy enough that it can survive a leak. Not at all sure if it's worth it or justified, but when people say "it can't be done" when it clearly can, it bugs this not-so-old curmudgeon.

~Jon

[LtWigglesworth]

What are the odds of them pointing a tiger team at developing an autogenous pressurization system for the F9/FH LOX tanks? Just to retire the cLOX v He issue once and for all. Implement as "v1.2.1"

Maybe for Raptor based second stage...but unless you can figure out how to autogenous pressurize RP-1 you are going to need the He anyway.

If they need more reliable copv's it might be worth it to switch to a titanium liner for them in the short term. It would be expensive (which I'm guessing is a big part of the choice to use aluminum as the liner material), but they would get the first stage bottles back and it could hold them over until a raptor based autogenously pressed upper stage could be flown.

Have people done Titanium GHe spheres in LOX? I know they've done it in LH2, but I was under the impression that Titanium was very dangerous in LOX. Even scratching titanium in LOX can lead to an explosion (due to rapid titanium combustion). IIRC it had something to do with Titanium being highly exothermic when burned in LOX, combined with low thermalconductivity, and it not forming as good of an oxide layer as aluminum (or maybe it was that Titanium oxide is soluable in titanium). Can't remember all the details. I just remember researching it at Masten, because I wanted to find a way to do it, and coming away with a "maybe if I coat it just right with copper it wouldn't be suicidal".

~Jon

I seem to remember reading that the Zenit 1st stage uses Ti tanks in LOX.

Of course, they could be coated with something.

[Dante80]

If a COPV bursts, it doesn't matter where it is situated or whether it is immersed or not. LoV is assured. The context for this is whether the specific way SpaceX manufactures and integrates them into the LOX tanks may cause a burst or not.

[guckyfan]

So the COPV but they rule out any connection with the former incident which is important.

That's assuming they were correct in their root cause for the incident last year.

Indeed it does. As I understand the discrepancies between SpaceX conclusions and NASA/FAA were not about the strut at fault. They agree on that. The discrepancy is they cannot completely rule out the strut was ok but there was a mistake in installation. This can indeed not completely ruled out when you don't have the strut. It is only supported by the fact that they have found defective struts in stock that would have caused this mishap.

So the failure being the strut and not the COPV stands for CRS-7. Which means the root causes are unrelated.

[cuddihy]

I wonder if any testing was done by SpaceX on potential for a brittle fracture-like failure mode for their COPVs when they went to subcooling.

[jgoldader]

Also my internet research reveals that COPVs are usually tested to a standard called 'Leak before Burst"  Unknown if SpaceX tests to this standard.

"Leak before burst" is really a *design* principle that is later verified in qualification testing. But once the design has been qual tested, flight articles are obviously not tested to that level.

For acceptance testing of flight article COPV's, SpaceX probably tests to a factor of, say, 1.5 x Max Expected Operating Pressure (MEOP) or whatever the NASA factor of safety is for testing pressure vessels on man-rated vehicles (I forget exactly what the NASA requirement is, but 1.5 x MEOP sticks in my mind. Jim can correct me.)

In any event, SpaceX would have acceptance tested the flight COPV's at a pressure well above MEOP. So if a COPV failed at or below MEOP, there will be much head-scratching. If a COPV got over-pressurized, then the question is how.

Probably a stupid question, but IANARS.  Do such things as the COPVs have burst disks (like the helium tanks in the LEM had) to relieve pressure and prevent RUD in case of overpressurization?  Or do you just do all you can to prevent overpressurizing them in the first place?

[Rocket Science]

I could see a COPV failing in the "bottom radii" of the tank where maintaining the thickness of the material "could" be an issue. If such an event took place and pierced the common bulkhead at the tank wall, rapidly mixing LOX/RP-1, we "could" see the "effect" appear to happening on the outside of the vehicle...

[Jarnis]

Probably a stupid question, but IANARS.  Do such things as the COPVs have burst disks (like the helium tanks in the LEM had) to relieve pressure and prevent RUD in case of overpressurization?  Or do you just do all you can to prevent overpressurizing them in the first place?

I doubt this was overpressure. They obviously have sensors and a way to vent the tank.

Sounds more like hardware giving up way below expected pressure.

[spacenut]

How far along are they on the Raptor upper stage engine that the Air Force wanted?  If, big if, they could get a replacement upper stage with a Raptor metholox engine, they could eliminate the helium problems.  It would also give more capabilities to the F9 and FH. 

I would only wish Musk had deeper pockets than Bezos.  He probably would already have the BFR flying. 

Also, if they had the helium system outside the lox tank, would it have to be a bigger system?  Would they have to stretch the second stage to handle it?

Is the system on the second stage different than the first stage?  They haven't had a problem with the first stage and it is under more abusive conditions than the second stage from my view, by having to land also.

[Jim]

What are the odds of them pointing a tiger team at developing an autogenous pressurization system for the F9/FH LOX tanks? Just to retire the cLOX v He issue once and for all. Implement as "v1.2.1"

Would require an engine redesign and reduction of performance

Probably a stupid question, but IANARS.  Do such things as the COPVs have burst disks (like the helium tanks in the LEM had) to relieve pressure and prevent RUD in case of overpressurization?  Or do you just do all you can to prevent overpressurizing them in the first place?

I doubt this was overpressure. They obviously have sensors and a way to vent the tank.

Sounds more like hardware giving up way below expected pressure.

There is only one inlet/outlet

[Jim]

I'm only talking about a difference of tens of milliseconds (likely only two video frames).  The disturbance created by the breach is still going to have to propagate to whatever failure path created that cutoff.  It would only be perfectly simultaneous if the initial location of the breach coincided perfectly with either the boxes or the data/power lines.  Otherwise, it would take a small -- but finite -- amount of time for the failure to propagate far enough to kill the data stream.

Helium lines would run parallel to data and power harness in the cable tray on the outside of the vehicle.

[pippin]

- COPV's will be redesigned to be LOX hardened (option least adviced given that composites and LOX don't like each other very much)

There are other composites than just carbon based ones.
No idea whether bringing back old glass fiber composite technology back is easier than the other options but it's not like it has never been done before...

[Proponent]

The second paragraph of SpaceX's statement  begins with the words "The Accident Investigation Team (AIT)...," but the term AIT appears nowhere else in the statement.  It shows to go you that engineers absolutely love their TLA's (three-letter abbreviations).

[Echium]

- COPV's will be redesigned to be LOX hardened (option least adviced given that composites and LOX don't like each other very much)

There are other composites than just carbon based ones.
No idea whether bringing back old glass fiber composite technology back is easier than the other options but it's not like it has never been done before...

Specific tensile strength difference.  I.e.  a CF overwrap compared to a GF overwrap is significantly lighter for an equivalent burst performance.

[HMXHMX]

- COPV's will be redesigned to be LOX hardened (option least adviced given that composites and LOX don't like each other very much)

There are other composites than just carbon based ones.
No idea whether bringing back old glass fiber composite technology back is easier than the other options but it's not like it has never been done before...

Three gold stars to you!  Plus, using urethane as the matrix is much better than epoxy; I tested it with carbon fiber and LOX as early as 1998, and it is what SpaceX's winding vendor uses. It essentially eliminates microcracking.