Quote from: ellindsey on 09/23/2016 05:54 pmSpaceX has taken some steps in their design to make sure that a single engine failure does not bring down the entire vehicle. The engines have firewalls separating them from each other and the rest of the rocket, there are blowout panels to direct pressure outward, and automatic shutoff valves to isolate dead engines. They've had one first stage engine failure that did not result in a mission failure due to this precaution. They have however now had two mission failures related to the second stage helium system. Would it be at all possible for them to make design changes so that a single COPV failure would not being down the rocket? I expect it would take a major redesign of the LOX tanks to do so, but two failures with similar causes might mean that has to be done.I'm not sure how you can do that without abandoning the "LOX-immersed COPV" concept entirely. These are pretty good sized tanks, charged with a very lightweight gas at several thousand psi. If one of those fails, chances are high that it will do so spectacularly. If it's inside the tank, there's not much you can do.
SpaceX has taken some steps in their design to make sure that a single engine failure does not bring down the entire vehicle. The engines have firewalls separating them from each other and the rest of the rocket, there are blowout panels to direct pressure outward, and automatic shutoff valves to isolate dead engines. They've had one first stage engine failure that did not result in a mission failure due to this precaution. They have however now had two mission failures related to the second stage helium system. Would it be at all possible for them to make design changes so that a single COPV failure would not being down the rocket? I expect it would take a major redesign of the LOX tanks to do so, but two failures with similar causes might mean that has to be done.
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
Some have argued if the He system failed spectacularly enough to cause this resulting high speed failure of the second stage, COPVs must have ruptured violently. But I also understand that the He system / COPVs shouldn't have been fully charged yet, so perhaps a COPV failure, while still potentially catastrophic, might have taken long enough to occur to be easily detected in telemetry as rising pressure (and possibly even mitigated partially by LOX venting, though potentially still leading to failure just in a slower fashion)... and thus a COPV failure would seem unlikely to be directly the cause, as they perhaps weren't under significant pressure yet?How are COPVs loaded with He? Do you have to force feed / expose the system to a high pressure of He into the whole He system and the COPVs fill until they're in equilibrium with the pressure you've chosen, or do you gradually raise the pressure of the entire He system / pump in He raising the pressure via the pump until you reach capacity? A bad analogy being a fuel cell car vs regular gas car, with the fuel cell car the hydrogen station has to pressurize the tank with high pressure hydrogen, then afterwards the fueling system must 'recharge' the pressure in the hydrogen station before the next tank can be filled. A regular gas car merely pumps it in at regular pressure until it's full, instead of giving it many PSI of pressure all at once.I'm wondering if the supply lines to the COPV to pressurize them (which might include the same lines used to provide pressure to the LOX tank and whatever else uses He from the COPVs, or might be dedicated to just filling/draining He) had a failure somewhere along the line (such as the previously suggested fitting failure). The wording of the SpaceX update is vague enough that one could even make an argument it might have been part of the He filling system which could include equipment on the TEL, the umbilicals, or even the mating point between the umbilicals and the stage, in addition to the plumbing / COPVs inside the stage.
Also my internet research reveals that COPVs are usually tested to a standard called 'Leak before Burst" Unknown if SpaceX tests to this standard.
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.
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?
It would probably depend upon whether Space X can conclusively determine the exact cause of the failure. If they can, then probably the COPVs can stay where they are. If, on the other hand, it’s a bit nebulous as to what happened or multiple issues are identified, then maybe a larger redesign is warranted — or needed to keep key customers happy — that includes relocating the COPVs. Time will tell.
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.
Quote from: Kabloona on 09/24/2016 12:52 amIn 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.This is outside by area of expertise, so bear with me here... "Empty" He tanks aren't actually empty, right? They're not a vacuum, they are just not pressurized or loaded with He yet. So wouldn't part of the process of pressurizing them be to open a valve to release the atmospheric gasses as the He filled?
The He tanks contain high pressure gas, not liquid. So if they burst, it's not going to be as violent
Hmmm...interesting.https://forum.nasaspaceflight.com/index.php?topic=30981.msg1577821#msg1577821
Quote from: ccicchitelli on 09/24/2016 01:40 amThat 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.Here, I'll step way out of my engineering discipline for a fun moment...The He tanks contain high pressure gas, not liquid. So if they burst, it's not going to be as violent as a liquified gas cylinder in which the pressure remains constant until all of the liquid changes state, thus propelling tank shrapnel in all directions. It would be a quick "crack" and a release of gas with immediate decaying energy. If you purposefully position the He tanks outside the LOX and perhaps shield the LOX tank dome a bit then perhaps it's possible to survive a failed COPV event.Now, rocket plumbers, pile on and correct me!
Quote from: wolfpack on 09/24/2016 01:52 amThe 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.
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"
