Oh, Lord, I’m sorry! 
If that’s the worst thing that happens to both of us all day, it’ll have been an outstanding day. Cheers!
My guess: the rust/paint issue.
The alloy of stainless steel (301 Stainless, I believe) they are using has the property of getting much stronger when it is very cold. Since the rocket needs the most structural strength when it is loaded with cryogenic propellant, an engineer at SpaceX did the math and realized that this alloy of Stainless would be superior to carbon fiber, which up to that point was assumed to be the direction they needed to go in. He showed the math to Musk, who agreed with him and made a call and scrapped their composite program including many millions invested in giant mandrels to spin the carbon fiber into a huge fuselage.
Carbon Fiber has a lot of drawbacks for rockets, especially reusable ones. Primarily, it’s not happy with heat and rockets get a lot of heat loading. It’s also hard to inspect and repair. Going to stainless not only simplified and lowered the cost of construction of the rocket, but also of the thermal protection system which doesn’t need to be as insulating of the skin as it would on a carbon fiber rocket. It turns out that building the rocket out of 301 Stainless had a lot of positive synergies.
As an example of how strong that rocket is when cold, the rocket stayed in one piece while tumbling at supersonic speed. When they fired the flight termination system, it didn’t destroy the rocket - it just blew big holes in the tanks, and in the video you can see the crygenic fuel spraying out of them. It wasn’t until the tanks were substantially drained that the rocket came apart. Not because it warmed up necessarily, but because tank pressurization gives the structure strength and when it lost it it was finally weak enough to come apart.
I can’t tell if you’re talking about the booster or the second stage, but the booster was tumbling when second stage separation would have been imminent and substantially empty. Though as @Dr.Strangelove indicated above, with so many engines lost the booster wouldn’t have been dry, but hardly full of propellant.
Have a look at a video of the flight termination. You can see it go off, and three large holes appear in the rocket with fuel spraying out of them. You can see that go on for some time before the rocket comes apart.
I see very very low detail video of the vehicle venting like crazy for 46 seconds starting at T3:14, an indication the whole thing is coming apart. Then it blows up at T4:00. You’re saying the venting shows the destruct signal had been sent and that caused the venting? I mean, how could you know?
Also 46s to destruct the vehicle seems like a very very serious safety issue if this were the case, and should prevent a return to flight in my utterly amateur opinion.
Go back to about 3:01, where they say ‘beginning the flip for stage separation’. There is no sign of venting from Starship or Superheavy. after a rotation, you see a bit of a fireball, and a hole appears in Starship and another in Superheavy, with gas streaming out of them.
For corroboration, have a look at where the charges are on Starship in this article:
Right in the middle of Starship. If you look at 3:34 in the video you can clearly see the fuel streams coming out of the rocket, and they are right in the middle of Starship and in the middle of SH. It looks like there are only two - I originally added the smoke from the engines, but in fact it looks like there are two holes in the rocket, and they are right where the FTS charges were.
That’s a good point. If I am right about when the FTS fired, the rocket survived for a full minute afterwards. That would render the FTS a bit useless or the first few thousand feet of flight as a means for avoiding pad destruction.
You can see a hole? Or do you just see venting, same as me? This all seems like wild speculation.
And if the flight termination system is unable to actually terminate the flight, it appears incompetently designed. What happens if Starship with dry tanks is heading for Houston? Nothing? ETA, I see you acknowledged this point, not sure if you edited or I missed it the first time.
I don’t know enough about their FTS to say how it’s supposed to work, but that’s not the only mitigation strategy they have. Starship would never be in a ballistic trajectory toward Houston in the first place. The current F9 boosters aim for the ocean for most of their flight, and only when they get close to the ground do they perform a diversion burn. If something goes wrong, they continue on and hit the water. No chance of overshooting and hitting Disney World.
FTS works by detonating an ordinance that should make a sizable split in the tanks. This pressure release is usually pretty violent and rapidly kills the engines. If the engines continued to burn for 40+ seconds after Booster’s FTS had fired, then yeah, that seems like a pretty big issue.
I’m dubious about the FTS failing to properly explode the rocket. This isn’t SpaceX’s first rodeo. But let’s see.
According to FAA docs, the test didn’t require a destructive termination system:
During a suborbital or orbital launch, the launch vehicle would be equipped with either a thrust termination or a destructive flight termination system, or both. In the event the vehicle varied from the planned trajectory, the applicable system would be initiated, and the vehicle would break up.
It could be that they took a two-stage approach: initiate tank venting initially, and if that doesn’t do the trick, engage the explosives.
The vehicle had a huge launch corridor here and was never in danger of leaving it. So they may have just done everything as slowly as possible to get max data.
Jet: “a rapid stream of liquid or gas forced out of a small opening”.
“This blasted debris up into the jets, damaging and disabling 8 of them.”
Rocket engines are not called jets. It’s a minor error and one I’d not bother correcting if some random person made it, especially since the wider definition of “jet engine” could include rockets. But if someone is making factual claims about a subject and demonstrating a lack of even the most rudimentary knowledge about it, I see no problem in calling that out.
Okay, that part is wrong. But the part about jets firing into the pad is technically correct. Which is the best kind of correct.
There weren’t 32 of them, either. 33 or 30, depending on how you want to count it. Again, a minor error, but if someone is stating things with ABSOLUTE CERTAINTY and lots of CAPITAL LETTERS, I’m gonna nitpick.
This seems like the most likely thing to me - blow the tanks and see what happens, let the ship do what it will for a bit and collect telemetry, then blow the big charges. That was a pretty authoritative bang at the end.
I think they only have the two termination charges. At leadt, I haven’t heard anything about a third, bigger charge. The big boom at the end I think is the rocket finally breaking apart, leaving a massive cloud of fuel and oxidizer.
The tanks already have vents to handle overpressure. They can open these up without explosives. That would result in thrust termination and probably structural collapse, depending on the circumstances. Maybe they did that, waited a bit, and then fired the charges when it was still coming down in one piece.