Also in the article: "In the short term, this entails changing the COPV configuration to allow warmer temperature helium to be loaded, as well as returning helium loading operations to a prior flight proven configuration based on operations used in over 700 successful COPV loads. In the long term, SpaceX will implement design changes to the COPVs to prevent buckles altogether, which will allow for faster loading operations. ”This likely means that not everyone involved in the investigation is satisfied that returning to the slower, less-chilled loading cycle will eliminate all failure modes for the COPV; but there have been enough successful bottles to continue launching with the existing system until a redesign can be implemented. If I were wearing SpaceX shoes, I would make the same choice, but it is not without risk -and Yes, there could be a common bottle failure mode underlying both the pad mission failures: Hence, the redesign.
Quote from: dglow on 01/03/2017 02:01 amIs it possible that some of the COPV issues discovered with AMOS also contributed to CRS-7?From the article:"Specifically, the investigation team concluded the failure was likely due to the accumulation of oxygen between the COPV liner and overwrap in a void or a buckle in the liner, leading to ignition and the subsequent failure of the COPV,” added SpaceX." Assuming this is very carefully worded, they suspect either a void or a buckle. So the challenge is to avoid rapid pressure changes where expanding O2 could rip apart the case in a shear zone, like a phone book. Also in the article: "In the short term, this entails changing the COPV configuration to allow warmer temperature helium to be loaded, as well as returning helium loading operations to a prior flight proven configuration based on operations used in over 700 successful COPV loads. In the long term, SpaceX will implement design changes to the COPVs to prevent buckles altogether, which will allow for faster loading operations. ”This likely means that not everyone involved in the investigation is satisfied that returning to the slower, less-chilled loading cycle will eliminate all failure modes for the COPV; but there have been enough successful bottles to continue launching with the existing system until a redesign can be implemented. If I were wearing SpaceX shoes, I would make the same choice, but it is not without risk -and Yes, there could be a common bottle failure mode underlying both the pad mission failures: Hence, the redesign.
Is it possible that some of the COPV issues discovered with AMOS also contributed to CRS-7?
Agreed but do they have the room in the S2 to mount the bottles externally? Would appear to be a better and safer solution but only if they don't have to stretch the stack any more. Quote from: Steven Pietrobon on 01/03/2017 08:44 amA way to avoid the SOX problem alltogether is to use liquid Helium in an external bottle, like what Arianespace do for Ariane 5. That should also be a much lighter solution as well as providing more propellant volume. I'm surprised SpaceX did not choose to do this. I think any plan that loads Helium at below the freezing point of LOX is just asking for trouble.
A way to avoid the SOX problem alltogether is to use liquid Helium in an external bottle, like what Arianespace do for Ariane 5. That should also be a much lighter solution as well as providing more propellant volume. I'm surprised SpaceX did not choose to do this. I think any plan that loads Helium at below the freezing point of LOX is just asking for trouble.
The CTE of the liner and the overwrap are grossly mismatched. Aluminum shrinks away from the overwrap as temperatures drop. Steel liners would have less of a problem, but I understand Elon decided he didn't want to use them due to cost and weight.
Quote from: HMXHMX on 01/02/2017 10:24 pmThe CTE of the liner and the overwrap are grossly mismatched. Aluminum shrinks away from the overwrap as temperatures drop. Steel liners would have less of a problem, but I understand Elon decided he didn't want to use them due to cost and weight.I suppose uneven contraction could result in buckling due to axial forces overwhelming the weaken radial strength if the overwrap pressure relaxed more in that direction.However, it seems pretty odd to me that this wasn't previously discovered since I'd think that simulations and/or testing would have set operating envelope parameters for the tanks that would have included that effect.
Quote from: RDoc on 01/03/2017 04:16 pmQuote from: HMXHMX on 01/02/2017 10:24 pmThe CTE of the liner and the overwrap are grossly mismatched. Aluminum shrinks away from the overwrap as temperatures drop. Steel liners would have less of a problem, but I understand Elon decided he didn't want to use them due to cost and weight.I suppose uneven contraction could result in buckling due to axial forces overwhelming the weaken radial strength if the overwrap pressure relaxed more in that direction.However, it seems pretty odd to me that this wasn't previously discovered since I'd think that simulations and/or testing would have set operating envelope parameters for the tanks that would have included that effect.I'm pretty sure that Musk stated (though I can't find the exact quote) that the temperatures reached were outside of the range for which the tank was characterized. Presumably that's why they did not find this in testing or simulation.
I hope Elon eventually dumps the COPV tanks.
There's a LOT of weight bringing a titanium tank all the way to orbit. Not LEO, but GTO. And it may itself have problems.What about shrink-wrapping the COPV in a fluoropolymyer thermoplastic? Totally seal off the fibers from oxygen intrusion. Shouldn't add much mass.
