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Comment by onychomys | Hacker News Reader

So Musk was arguing that the reason it didn't explode as it should have is that he just built it too darn strong? I'd have thought they would have tested the bomb on some big hunk of steel just like the actual tank to make sure that it worked right. They test all the other parts of a rocket, surely they do the termination system too.

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The charges worked fine. They blew holes in the tanks as planned. The issue is that this didn't cause immediate structural failure as intended. But structural failure of the rocket while flying through upper atmosphere at supersonic speed is not something that you can test on the ground.

^ THIS ^

This is the whole of Rocket Science. You can't test for some conditions, you can only test IN them by actually launching a rocket.

I think of it like diving into a pool. There is no way to practice diving into a pool before actually diving in. You learn to dive by diving.

I read that the destruction mechanism is explosives that rips open the fuel tank, internal to the rocket, but apparently then requires atmospheric pressure to collapse the rocket shell, and at the altitude it was initially at the pressure was insufficient to do that.

If this is really the mechanism then I suppose tough to test (at least expensive to have large enough low-pressure test chamber to blow stuff up in), but it does seem odd they wouldn't just have explosive charges to destroy the rocket shell as well.

The "rocket shell" is the "fuel tank". They blew a big hole in the side of thing, which usually is sufficient to destroy any rocket in seconds. Turns out this one it took a while for the damage to cause complete failure.

Interesting! Do we know how soon after it started tumbling they fired the self-destruct ?

Here's a video of the flight, where you can see the charges activate and venting start at about 15:12, the linked time: https://youtu.be/NCYSVmSPM7E?t=663

You can decide how long that is before and after, though I'll note that it may be complicated to determine what's not nominal: Starship has an unconventional staging maneuver that's basically a spin.

I'll also note that "they" didn't send anything; the FTS is autonomous, and will trigger on violation of its conditions, which is probably (1) departure of the instantaneous impact point (where the rocket will hit if all thrust stops now) from a safe ground corridor or (2) indications of disintegration of the vehicle. "The vehicle is doing something weird" is not a condition for flight termination.

His point had nothing to do with the termination system.

The rocket tumbled for four complete revolutions. Most rockets will be destroyed by inertial and aerodynamic forces well before the first revolution (they are designed to withstand forces along the major axis, not significant side forces).

It was helpful from an aerodynamic force POV that the rocket was well over 100,000 feet before it started to tumble. Still, the inertial forces when the full stack started rotating must have been huge.

That's not exactly a direct response though to the question unless you're suggestion is that SpaceX's destruct mechanism consisted of expecting aerodynamic forces to destroy the rocket during a tumble.

I interpreted Musk's comment to reflect the fact that rockets generally will be destroyed if the tanks lose pressurization and/or the rocket tumbles. The (load bearing) tank walls usually cannot support major side loads. The Starship stack needs to be a lot stronger than "usual" rockets to withstand the rotational forces and aerodynamic forces involved in landing them.

Ref: https://headedforspace.com/why-and-how-rocket-fuel-tanks-are...

OTOH, most rockets are transported horizontally. (insert picture of Falcon9 going down a road on a flatbed here). This means that they are designed to withstand 1g side loads. SuperHeavy is always roughly vertical. Starship of course has to withstand insane loading horizontally during it's crazy descent and flip stage, but SuperHeavy stays vertical.

The tanks are held in supporting cradles when horizontal so that their weight is distributed properly. Sometimes the tanks must be pressurized when transported horizontally or they would be damaged.

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