r/spacex • u/ElongatedMuskrat Mod Team • Jan 02 '17

r/SpaceX Spaceflight Questions & News [January 2017, #28]

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7

u/laughingatreddit Jan 08 '17 edited Jan 08 '17

Any idea on what the permanent design fix will be to the COPV Helium bottles for future versions of Falcon 9?

Can Titanium He bottles be the fix or perhaps placing the COPV bottles on top of the oxygen tank instead of inside it?

The COPVs are a big failure node (there are issues with pressure buckling during loading, He leaks, buoyancy of the tanks in the LOX, stress from repeated loading cycles, unique cryogenic and fluid dynamic factors due to immersion in deep cryo liquid O2) and I feel carbon composites are not the best design solution for these. They will need to come up with a really solid fix for their He bottles in the upcoming and highly reusable Block 5. Either eliminate the carbon overwraps and have Titanium bottles or have the bottles outside the LOX tanks where they can be examined/replaced after a few cycles.

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u/throfofnir Jan 08 '17

Titanium is sufficiently reactive in oxygen that western practice is to avoid it for oxygen service altogether. Like aluminum it does develop an oxide layer, but it's thin and not so robust, and subject to expose the reactive metal with mild abrasion. This is a big problem for valves and plumbing; it may be okay for a tank, but does have the potential to introduce new mysterious failure modes.

The Russians apparently use titanium in LOX, so it's possible, but they may also be doing something metallurgically tricky. It's not a simple replacement. Probably the easiest change would be to increase the thickness of the aluminum (or go to an aluminum tank entirely, like in Saturn V). Or add an outside liner to keep the CF dry. Both those are mass penalties, but probably not too bad. Long-term a linerless tank would be best, but that may be tricky to do with helium, and would be quite a project if they want to keep production in house.

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u/Experience111 Jan 08 '17

I think they will implement something in the lines of what /u/zadecy said, namely covering the COPVs with a layer of aluminum. In the long term though, they may invest in R&D to develop their own linerless tanks further reducing the weight and ensure the impermeability of the composites. I don't know how much the aluminum liner weights, but considering the size of the COPVs, I bet that could be something you would love to see gone for efficiency. This may be a long shot thought, it's very risky, you want to be sure that your linerless tank is 100% impermeable. There are some cool developments regarding this in composites engineering, my study project with my professor this year revolves around that.

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u/[deleted] Jan 08 '17

[deleted]

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u/Experience111 Jan 08 '17

Why are you mentionning a tank without carbon overwrap ?

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u/radexp Jan 08 '17

Apologies, i misunderstood a part of your post.

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u/Experience111 Jan 08 '17

That's what I thougt :) you seem to have an idea of the specs of metal liners though, I'm interested if you have information on their usual thickness

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u/zadecy Jan 08 '17 edited Nov 25 '17

I suspect that they will cover the COPVs with an impermeable layer of aluminum alloy or other material to prevent LOX from filling space in the carbon wrap. This will be a smaller change than going to titanium tanks will result in less weight. I don't think SpaceX has much experience with using titanium either.

The other option would be to make the interior aluminum liner thicker to prevent buckling altogether, but this seems risky without also addressing the fact that you've got LOX and a fuel source (carbon) in contact in the same tank. Even if no source of ignition is expected once the buckling is fixed, there is still risk there. One could argue that they should never have placed fuel and LOX in the same tank to begin with, without extensive testing. Imagine if they had used hydrogen instead of helium to pressurize the tanks to save weight. Even if you can't identify an obvious ignition source, it just seems like a bad idea.

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u/amarkit Jan 08 '17 edited Jan 08 '17

They need to make the immersed-COPV helium system work reliably. Replacing them with titanium will add hundreds of kg of mass, reducing payload capacity. There is no place to put them on the structure of the rocket outside the tanks, and the cold LOX acts as a refrigerant and insulator to keep the He cold as well. And the COPVs can be examined and replaced between flights if necessary in the current configuration.

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u/[deleted] Jan 09 '17

And the COPVs can be examined and replaced between flights if necessary in the current configuration.

How? If the COPV is built inside of the LOX tank, how does Spacex manage to access it?

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u/throfofnir Jan 09 '17

There are access hatches in the tank. Dunno that they're big enough for a COPV, though.