[mabbo]

If you put a spark inside a full gas/petrol can, not much happens. Not enough oxygen. Half full? Still not such a big deal.

But empty the can, leaving behind just the fumes and the oxygen, and you have yourself a bomb.

My understanding is that the rocket here is a similar problem. It's a giant tube of explosive material. I wouldn't want to tap it with a hammer. And they've got it falling to earth at supersonic speeds, then calmly setting itself down on a barge at sea in 3m waves.

[mrfusion]

It fills with helium as it empties out the fuel.

[mabbo]

In that case, I have no idea why it exploded. :)