What is thrust?

Thrust comes from throwing mass backwards, so it depends on two things: how much mass leaves per second, and how fast it leaves. Nothing outside the rocket is pushed against, which is why a rocket works better in space, not worse.

To lift off, thrust has to exceed weight. Nine Merlins together produce 7,600 kN against a fully fuelled Falcon 9 weighing 549,000 kg — so the rocket leaves the pad slowly, at only a little more push than weight. That is deliberate. Excess thrust at liftoff is wasted on fighting air resistance low down.

The same engine pushes harder once the air is gone. A Merlin makes 845 kN at sea level and 981 kN in vacuum — the atmosphere is pushing back on the exhaust the whole way up, and stops when it runs out.

It is measured in newtons. One newton is roughly the weight of an apple in your hand; 845 kN is 845,000 of them.

A separate number worth knowing is the engine’s own thrust-to-weight ratio. A Merlin weighs 470 kg dry and produces 845 kN, a ratio of about 184 — the engine can lift 184 times its own weight. Almost nothing else built by people comes close.