Much to everyone's relief. The SpaceX Starship SN10 successfully completed its flight and landed.....
And did it have anything to do with the fire near the Raptor engines?
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EDIT: Elon has revealed in multiple tweets (one, two, three, four) what went wrong, and although this doesn't tell us exactly what led to the explosion directly, it gives some insight to what went wrong. I will be updating this answer later...
The proper answer to questions like, "What went wrong with SpaceX's [insert mission here]?" is:
Unless SpaceX tells us what went wrong, we won't know, and they haven't told us anything yet.
Elon or SpaceX will probably tweet/eventually talk about what went wrong, so until then, all we can do is guess. That said though, one of the key events that led to the explosion seems to be that the landing legs didn't lock into their "deployed" position after being released. This is clearly visible in this video which compares a previous hopper to SN10's performance. Furthermore, from the video of the landing, it looks like the landing wasn't quite "soft" and there was an appreciable impact on the landing pad. In this picture you can see that there are no landing legs deployed and that the whole thing is leaning, which implies hull damage:
In my (not-a-rocket-engineer) opinion, this sequence could've led to the explosion:
An unlikely alternative (although I think Elon would've tweeted if this were the case), is that they could've determined that they can't make SN10 safe to approach because de-tanking systems were damaged or whatever, so they decided to detonate it via abort system instead of waiting hours for it to maybe explode.
Eric Berger, who writes extensively about space exploration for ArsTechnica and for other organizations, and sometimes appears on national TV wrote
SpaceX has yet to provide details about what happened and likely won't.
Eric has some speculations, based in part on "informed sources":
However, informed sources suggested the accident may have been caused by a leaking valve, likely methane fuel. It is notoriously difficult to operate fuel valves at cryogenic temperatures.
This of course is speculation. However, Eric's "informed sources" can be extremely well-informed. On the other hand, SpaceX can be notoriously tight-lipped with regard to technical details. Being tight-lipped is how SpaceX protects its intellectual property. (SpaceX tends to eschew patents.) The general public may never know why SN10 failed.
Something I noticed from a higher-resolution feed: in the half-second before the explosion, there was a very brief black plume that “broke out” from the area near the rear flap of the ship. Here are some screenshots in quick succession:
I would guess from this, that an internal structural member failed suddenly, possibly after having sustained damage from impact and weakened from the fires at the landing site. I don't know enough about the internal structure of the ship to guess any further, but I could imagine that if a significant structural member failed at this location, and with some remaining amount of fuel and O2, it might explain what we saw.
Alright, so I found an answer on YouTube.
This video explains it nicley: SpaceX Starship SN10 Test Flight
It starts to explain the reason for the explosion from this part: Reason for Explosion
Seemingly, the oxygen tank ruptured; According to Scott Manley, the oxygen tank was under pressure and it ruptured before any fire started which might have triggered the explosion
Until SpaceX announces the cause, the best we can do is guess.
Obviously something was damaged during the flight1 or landing, which allowed fuel to leak in the area under the skirt and find an ignition source (likely a hot engine component).
It bounced on landing and had a noticeable lean afterward, so it's reasonable to assume that the damage occurred during landing. It could be a tank dome or fuel line broke or was punctured. They've also had issues with the Raptors themselves, and it could be one of the engines had a leak.
But again, this is all guessing. We'll have to wait for official word from SpaceX, if they decide to give it.