What is LOX?

A rocket needs oxygen far faster than it could ever scoop from the air, and it leaves the atmosphere within minutes, so it brings its own in a tank alongside the fuel.

It also needs a surprising amount of it. Burning kerosene properly takes more oxygen than fuel by weight, which is why the oxygen tank on a Falcon 9 is the larger of the two. Most of what looks like a fuel tank is not fuel.

Falcon 9 chills its LOX further still, to roughly 66 K — about −207 °C, well below the point where it merely becomes liquid. Colder liquid is denser, so more mass fits in the same tank, and more propellant in the same structure is performance gained for nothing.

That is why fuelling begins only 35 minutes before launch: densified oxygen starts warming the moment it is aboard, so it cannot be loaded hours in advance and a long hold in the countdown is expensive.

The cold has to be handled carefully at the other end too. Pouring liquid oxygen into a warm engine would crack the metal, so a little is trickled through all nine engines beforehand to chill them down gently — the engine chill step you hear called out before ignition.