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Comment by ChuckMcM | Hacker News Reader

Two things I learned from the report, one was that the helium tanks are actually inside the oxygen tanks. All the rockets that I can recall seeing the insides of, had the helium tanks as a sort of 'ring of pearls' around the end of the tank. Clearly from a volume perspective it doesn't matter if they are inside or outside, and from a thermal perspective it means the LOX can keep the helium cool (or vice versa I guess).

And the second is that their wrapping method allows for the creation of voids between the inner aluminum tank and the carbon fiber wrap. I can certainly imagine how that could happen with carbon fiber prepreg (it's a prewoven fabric which maximizes strength by pre-laying out the various ways the fiber goes through it) but if you were to spool on straight up fiber that would make it easier to be conformal.

I wonder if other people do the 'tanks within tanks' architecture and if so how they avoid voids and buckles in their overwrap.

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In general it seems the voids and buckles problem is only seen at very very low temperatures. The AMOS-6 failure occured because the LOX was loaded too quickly after loading the helium tanks, without allowing the He to come up in temperature.

SpaceX was pushing the loading speed to expand the flight window. The update (and Ms. Shotwell a few months ago) indicate the short term fix for this is to slow down the propellant loading to allow the He to warm up a bit before LOX arrives... Which is a long way to say that it's likely other companies have not pushed the envelope far enough to encounter this voids and buckles problem.

The update also says they will eventually adjust their COPV construction to eliminate this problem and return to rapid propellant loading, but I doubt we'll hear anything about HOW they improve it. A not insignificant amount of SpaceX's "secret sauce" is in their composits manufacturing (case in point the enormous ITS LOX tank recently tested.)

Helium has a much lower boiling point (~4K vs ~90K) than oxygen, and so at the same temperature would have to be at significantly higher pressure to stay liquid. I'd expect that putting the helium tank inside the oxygen tank would allow the helium tank to be of lighter construction, since it would only have to withstand the difference in pressure between the liquid helium and liquid oxygen. Scaling up the oxygen tank to compensate for lost volume would be lighter due to the squared/cubed law.

I imagine the reason it's not done this way traditionally is that it'd be a pain to fabricate.

I think this might have something to do with it. SpaceX being SpaceX, they have a significant advantage in the agility of their fabrication process(es).

I'm not aware of any modern US rockets (other than SpaceX) that install COPV tanks inside LO2 fuel tanks. SpaceX is certainly getting a performance gain from the design at the risk of these kind of thermal related failure modes. I hope they figure out how mitigate these failure modes as a COPV exploding inside a LO2 tank is scary! Especially once they start flying humans on-board in 2018 or 2019.

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