How a Rocket Engine Works: Newton's Third Law, De Laval Nozzles & Staging

Newton's Third Law is the whole story

Every action has an equal and opposite reaction. A rocket engine burns fuel to create incredibly hot, pressurised gas, and that gas rushes out of a nozzle at the bottom of the rocket — Newton's Third Law slingshots the rocket in the opposite direction. Crucially, the rocket isn't pushing against the surrounding air: the reaction is between the rocket and its own escaping exhaust, which is why rockets work perfectly in the vacuum of space. Blow up a balloon and let it go untied — the air rushes out the back and the balloon flies forward. That's exactly how a rocket works, just with hot gas at far higher pressure. A Saturn V nozzle blasted exhaust out at almost 3 km/s, about nine times the speed of sound.

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Why a rocket is mostly fuel

To escape Earth's gravity a rocket needs to reach about 11.2 km/s — roughly 40,000 km/h. The trouble is that the heavier the rocket, the more fuel it needs, but more fuel makes it heavier still, demanding even more fuel. Konstantin Tsiolkovsky described this "tyranny of the rocket equation" in 1903. The practical result: a typical rocket is about 90% fuel by mass at launch. The Saturn V weighed 2,970,000 kg on the pad, yet the Apollo capsule that actually reached the Moon and back was only about 28,800 kg — roughly 1% of the total.

Staging: shedding dead weight mid-flight

Engineers get around the fuel problem with staging: instead of one giant rocket, several smaller rockets are stacked on top of each other. The first stage fires and lifts the whole vehicle; once its fuel is spent it is jettisoned, instantly making the rocket lighter. The second stage then fires without having to drag along the heavy, empty first stage, making it far more efficient — and a third stage, if present, pushes the payload the rest of the way into orbit or beyond. It's the same logic as throwing away an empty water bottle before running a race. SpaceX's Falcon 9 takes this further, landing its first stage back on a drone ship so it can be reused, dramatically cutting launch costs.

The de Laval nozzle

The nozzle shape itself matters enormously. A de Laval nozzle narrows to a throat and then flares back out, accelerating hot combustion gas from subsonic to supersonic speed as it expands through the diverging section. Chamber pressure, throat area and the nozzle's area ratio (exit area ÷ throat area) together set how efficiently thermal energy converts into thrust, and how well the exhaust plume matches ambient pressure at a given altitude — a nozzle tuned for sea level is under-expanded in vacuum, and vice versa. Real engines like the Saturn V (35,100 kN of thrust) and SpaceX's Starship (~74,000 kN) push enormous mass flow through nozzles engineered around exactly this trade-off.

Frequently asked questions

Why does a rocket work in the vacuum of space if there's no air to push against?

A rocket doesn't push against the air — it throws its own hot exhaust gas backward, and by Newton's Third Law the escaping gas pushes the rocket forward with an equal and opposite force. The reaction is entirely between the rocket and its own propellant, exactly like a balloon flying forward as air rushes out the back, so it works perfectly in vacuum.

Why is a rocket about 90% fuel by mass at launch?

Escaping Earth's gravity requires reaching roughly 11.2 km/s, and the heavier the rocket the more fuel is needed — but more fuel adds mass, requiring still more fuel. This "tyranny of the rocket equation", described by Konstantin Tsiolkovsky in 1903, means a typical rocket launches with about 90% of its mass as propellant and only a small fraction as structure, engines and payload.

What is rocket staging and why does it help?

Staging stacks several smaller rockets on top of each other instead of building one giant one. Each stage fires and, once its fuel is spent, is jettisoned — the rocket sheds dead weight so the next stage doesn't have to carry empty tanks. SpaceX's Falcon 9 goes further, landing and reusing its first stage to cut launch costs.

Try it live

Everything above runs in your browser — open Rocket Engine — de Laval Nozzle & Thrust, pick a propellant combination, and drag chamber pressure, throat area and area ratio to see thrust and specific impulse respond live. Nothing is installed, nothing is uploaded.

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