[aardvarkr]

Failures point out flaws to be fixed but also stop you from gathering more data through the rest of the experiment. The earlier SN5 had a huge list of failures but at least landed in one piece which meant they could extract data from their equipment, tear down systems to understand how they handled the stresses and strains of the launch, and then they could launch it again to see how their fixes worked.

Thankfully this failure was at the very end instead of, say, twenty seconds into flight or exploding on the launchpad. This way they got to test every subsystem which is phenomenal and about as good of a result as they could have expected. However, the landing system didn’t trigger and they’ll never know exactly why. The engines ate themselves alive and it would have been great to study how it performed in that scenario but now they’re in pieces. The header tank failed to provide enough pressure and it would have been great to test it further. Now they have to comb through the pieces and piece things together like a detective. They learned A TON but it’s definitely better to get it home in one piece.

[Kliment]

As I understand it the landing legs didn't deploy because velocity was too high and angle was wrong. All this stuff gets reported by telemetry because they know exactly how likely it is that a first attempt flight will fail catastrophically (very). So these test rockets are heavily instrumented and networked. Watching it the first time I also thought one of the engines had failed but then that engine had a successful relight and another did not, and the fuel mix ratio appeared to be wrong on one of the engines. This is all consistent with loss of propellant (there was a quite significant leak after the first fire in engine bay) and not consistent with "engines ate themselves alive".

Your greater point about it being better to land than not land stands, but these are experimental rockets. They're expected to fail catastrophically and you plan accordingly.

[nine_k]

The video shows us the wreck site at the end. The rocket is mostly one piece though damaged by hitting the ground too fast and from a (relatively minor) explosion.

It must be possible to find most pieces, and certainly check the engines, even deformed, for overburn signs.

[grey-area]

Bright green flames at the end is a giveaway of engine-rich combustion.

[mhandley]

Indeed, which in turn is an espected outcome if the engine is running oxygen rich, and hence much hotter than the design point. Pretty likely the low fuel tank pressure was the cause of this, and I'm sure SpaceX will have the telemetry data to know for sure if this was the case, irrespective of what pieces remain after the landing.