What is stage separation?

Falcon 9 separates about 2 minutes 30 seconds after liftoff, four seconds after the first-stage engines cut off. At that moment it is roughly 68 km up and travelling at about 8,300 km/h.

There are no explosive bolts. Air-powered pushers shove the halves apart, which is gentler and cannot damage the engine that is about to light. Seven seconds later the upper stage ignites, into clean air rather than into a dying exhaust plume.

Staging is the single most effective trick in rocketry. A single stage carrying the maximum sensible amount of fuel still cannot reach orbit; two stages comfortably can. The reason is that the empty tank is not just useless, it is actively expensive — every remaining kilogram of propellant has to accelerate it too.

The numbers show how much is left to do. The first stage does the loud, heavy work and gets the stack to 8,300 km/h. The much smaller second stage then more than triples that, to about 27,000 km/h, on a fraction of the propellant — because it is no longer dragging an empty first stage along with it.

From here the two halves have completely different jobs. The top continues to orbit; the bottom turns around and flies home.