How do rockets work? An explanation that survives being questioned
Rockets do not push against the air or the ground. They push against the stuff they throw out of the back — and that single idea explains almost everything else.
6 min read
There is a persistent idea that a rocket works by pushing against the air. It is easy to see why — you can feel a hairdryer push back on your hand. But if it were true, rockets would stop working in space, and they very much do not.
A rocket works by throwing mass away from itself, very fast. Throwing the mass one way pushes the rocket the other way. That is Newton’s third law, and it needs nothing outside the rocket to push against.
The skateboard test
Stand on a skateboard holding something heavy and throw it forwards. You roll backwards. You did not push on the ground or the air — only on the object, and it pushed back just as hard. Throw something heavier, or throw it faster, and you roll away quicker.
Those are the only two levers a rocket engineer has. Throw more mass per second, or throw it faster. Every design decision — the fuel, the pumps, the shape of the nozzle — is an attempt to increase one of those two numbers.
Why rockets carry oxygen
Burning needs fuel and oxygen. A car engine takes its oxygen from the air. A rocket cannot, partly because it climbs out of the air within a couple of minutes, and partly because it needs oxygen far faster than it could ever scoop it up.
So it carries both. A Falcon 9 carries refined kerosene as fuel and liquid oxygen — ordinary oxygen chilled until it becomes a pale blue liquid — as the thing that makes it burn. Two tanks, one purpose.
The narrow bit in the middle
Inside the engine, fuel and oxygen are forced together and set alight. That makes hot gas at very high pressure. The gas escapes through a waist called the throat, and then out through the flared bell.
Squeezing the gas through the throat is what makes it fast. The bell below then lets it expand and speed up further. A Merlin engine on a Falcon 9 produces 845 kN of thrust at sea level — enough by itself to lift about eighty-six cars — and there are nine of them at the bottom.
Why rockets are almost entirely fuel
A fuelled Falcon 9 weighs about 549,000 kg and can carry up to 22,800 kg to low Earth orbit. The cargo is roughly four percent of what leaves the pad. Everything else is propellant and the tanks to hold it.
The reason is unforgiving arithmetic. To go faster you need more propellant — but that propellant has weight, so you need more propellant to lift it, and so on. The relationship is logarithmic, which means doubling the fuel does not double the speed. It is why rockets are enormous and payloads are small.
And why they throw bits away
Once the lower tanks are empty they are just weight. So the rocket drops them. On a Falcon 9 the two halves separate about two and a half minutes after liftoff, and the lighter upper half carries on alone.
Staging is the single most effective trick in rocketry. In our Rocket Lab you can prove it to yourself: a single stage with the maximum fuel still cannot reach orbit, and two stages comfortably can.
Common questions
How do rockets work in space with no air?
A rocket does not push against air. It throws hot gas out of the back, and by Newton’s third law that gas pushes the rocket forward. This works better in a vacuum, not worse, because there is no air resisting the exhaust.
Why do rockets carry their own oxygen?
Burning needs oxygen, and a rocket leaves the atmosphere within minutes. It also needs oxygen far faster than it could collect from the air, so it carries liquid oxygen in its own tank alongside the fuel.
Why are rockets mostly fuel?
Extra propellant has weight, which itself needs propellant to lift, so the returns diminish logarithmically. On a Falcon 9 the payload is about four percent of the mass that leaves the launch pad.