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SpaceX's Starship Mk1 just blew off its bulkhead in a pressurization accident

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Re: SpaceX's Starship Mk1 just blew off its bulkhead in a pressurization accident

#71

Post by NamTaf »

Oh dear! It looks like someone accidentally divided by 2 for the cylindrical hoop stresses, forgetting that they're not the same equation as for spherical stresses. :)

Less facetiously, a thin-walled cylinder's stress is a function of pressure, radius and thickness with the equation:

sigma = P*r/t

whereas for a sphere (including the spherical end caps on a cylindrical pressure vessel), it follows the equation:

sigma = Pr/(2t)

To me, it looked like it failed around the join between the spherical end cap and the cylindrical body, where this transition between stress regimes occurs, which is a common weak spot in the above analysis. This is a canonical early engineering statics problem.

It's worth noting, though, that all of the above is almost certainly a gross simplification and likely has very little bearing on the actual problem. It's fun to simplify and hypothesise, though! Also, unplanned failures make for great viewing :)

Re: SpaceX's Starship Mk1 just blew off its bulkhead in a pressurization accident

#72

Post by londons_explore »

Aren't pressure tests done with water?

That way, since water is non-compressible, if it fails, you just get a small hole or buldge somewhere rather than the catastrophic damage seen here.

Perhaps they already tested with water and now needed to test with the real fuel/oxygen to get the temperature correct?

Re: SpaceX's Starship Mk1 just blew off its bulkhead in a pressurization accident

#74

Post by CydeWeys »

saagarjha wrote: post #64
ethbro wrote: post #62

Earlier quoted context omitted.

But the primary cost of lifting fuel (if you have a reusable rocket) is fuel. Leaving us back at cheap. But we're essentially arguing whether max payload per launch or payload efficiency to orbit is more important.

With exponential compounding, I wouldn’t be so sure that it’s still cheap…

Yeah, and with much higher required dv and more required stages this would be a huge problem. But we only need two stages to orbit when launching from Earth, so it's really not that big of a deal, and it turns out that optimizing for reusability ends up being a lot cheaper than optimizing for minimum fuel use. For example, it only costs $200k for a fuel load of Falcon 9 fuel. If you got rid of reusability you'd need less fuel and could save tens of thousands of dollars ... but at the cost of needing to build a new $54M spacecraft with each launch. Clearly that trade-off isn't worth it.

So yes, fuel is cheap.

Re: SpaceX's Starship Mk1 just blew off its bulkhead in a pressurization accident

#75

Post by WalterBright »

zionic wrote: post #14
andrewflnr wrote: post #11

If you go back a couple hours in the linked live feed you can find the event pretty easily. Look for the big plume in the preview when you hover over the progress bar. About 1:50 ago at this working writing. I guess it would have been around 1540 EST.

Looks like the welds holding the bulkhead on failed. For those who aren't keeping up with this the failure was on the Mk1 prototype. Mk2 has been build nearly in parallel with Mk1 and will take over early flight testing (20km hop). Elon has tweeted that this team will jump to Mk3 instead of trying to repair Mk1.

Airliners rarely use welds because they're very hard to make with consistent high quality and are also very hard to inspect. Instead, they're riveted together.

Re: SpaceX's Starship Mk1 just blew off its bulkhead in a pressurization accident

#77

Post by gpm »

walrus01 wrote: post #70
DennisP wrote: post #28

Earlier quoted context omitted.

I think SpaceX has proven they know how to design rockets. But they are newcomers to building rockets from welded steel.

It is my understanding that the pressure vessels are carbon fiber composites. The skin is steel.

The skin is the pressure vessel, double walls are a waste of valuable mass.

They gave up on carbon fiber awhile ago.

*I think they still use small composite overwrapped pressure vessels for fluids they need less of than CH4 and O2, just referring to giving up on carbon for the main tanks.

Re: SpaceX's Starship Mk1 just blew off its bulkhead in a pressurization accident

#79

Post by bdamm »

mattrp wrote: post #53
Davesjoshin wrote: post #30

Earlier quoted context omitted.

Yea, there is something about all the attention on this thing that stresses me out. Its a prototype, right? There is going to be things that appear bad, but its really just a treasure chest of data.

That’s one way to get the neighbors to move!

Also, by paying the neighbors 3x their property value to buy out their homes.

Re: SpaceX's Starship Mk1 just blew off its bulkhead in a pressurization accident

#80

Post by taneq »

NamTaf wrote: post #71

Oh dear! It looks like someone accidentally divided by 2 for the cylindrical hoop stresses, forgetting that they're not the same equation as for spherical stresses. :) Less facetiously, a thin-walled cylinder's stress is a function of pressure, radius and thickness with the equation: sigma = P*r/t whereas for a sphere (including the spherical end caps on a cylindrical pressure vessel), it follows the equation: sigma…

That's why you take your maximum design stress and double it, right? /s
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