How Rockets Work: Newton's Third Law & the Tsiolkovsky Equation

Newton's Third Law is the whole story

A rocket doesn't push against the air — there's no air in space. Instead it throws hot, pressurised gas out of a nozzle at the bottom, and Isaac Newton's Third Law of Motion — every action has an equal and opposite reaction — pushes the rocket forward with exactly the same force the gas is thrown backward with. Blow up a balloon and let it go untied: the air rushes out one end and the balloon flies the other way. That's precisely how a rocket works, just with much hotter gas at much higher pressure. Because the reaction is between the rocket and its own exhaust rather than the surrounding air, it works perfectly in the vacuum of space. A Saturn V engine nozzle expelled gas at nearly 3 km/s — about nine times the speed of sound.

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The tyranny of the rocket equation

Reaching orbit demands a huge change in velocity — roughly 9.4 km/s of delta-v once gravity and drag losses are counted, or 11.2 km/s to escape Earth entirely. In 1903 the Russian engineer Konstantin Tsiolkovsky worked out exactly how much fuel that costs:

Δv = v_e · ln(m₀/m_f)
v_e = I_sp · g₀   (effective exhaust velocity)
m₀ = fully fuelled mass,  m_f = dry mass after burn

Because delta-v scales with the logarithm of the mass ratio, every extra bit of speed costs disproportionately more propellant: heavier fuel load needs still more fuel just to lift the fuel. The practical result is that a typical rocket sits on the pad at roughly 90% fuel by mass — the Saturn V weighed 2,970,000 kg at launch, yet the Apollo capsule that actually reached the Moon and back was only about 28,800 kg, roughly 1% of the total.

Why staging is unavoidable

A single stage needs a mass ratio around 23 to reach orbit, which is close to impossible to build since tanks and engines themselves weigh a lot. Staging solves this by stacking several rockets on top of each other: the first stage fires and lifts the whole vehicle, then — once its fuel is spent — it drops away, instantly making everything above it lighter. The second stage fires next, no longer hauling the dead weight of empty tanks, so it starts with a fresh, favourable mass ratio. Total delta-v becomes the sum of each stage's contribution, which comfortably beats what any single stage could deliver. It's the same logic as finishing a bottle of water and throwing away the bottle before running a race, rather than carrying the empty weight the whole way. SpaceX's Falcon 9 goes further still, landing its spent first stage on a drone ship so it can fly again, which is what has driven launch costs down so sharply.

Real rockets, real numbers

The Saturn V (NASA, 1967–1973) remains the most powerful rocket ever flown, sending 12 astronauts to the Moon with a launch thrust of 35,100 kN. Falcon 9 was the first rocket to routinely land and reuse its first stage, becoming the workhorse of modern satellite launch. Starship is the largest and most powerful rocket ever built, generating roughly 74,000 kN of thrust and designed with Mars in mind. Ariane 6 remains Europe's primary heavy-lift vehicle for science and commercial payloads. Every one of them is, at heart, still solving the same equation Tsiolkovsky wrote down in 1903.

Frequently asked questions

Why does a rocket work in the vacuum of space, with no air to push against?

A rocket doesn't push against the surrounding air at all — the reaction is between the rocket and its own escaping exhaust gas. Newton's Third Law says every action has an equal and opposite reaction: the engine throws hot gas backward, and that gas pushes the rocket forward with exactly the same force. Since no air is involved, the effect works perfectly in a vacuum, which is why rockets are the only way to manoeuvre in space.

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

Escaping Earth's gravity needs a huge change in velocity, and the Tsiolkovsky rocket equation shows delta-v scales with the natural logarithm of the mass ratio between fuelled and empty mass. Because that relationship is logarithmic, each extra bit of speed costs disproportionately more propellant. The result is that a typical launch vehicle is roughly 90% fuel, with payload, structure and engines making up only a small fraction of the total mass.

Why do rockets use multiple stages instead of one big tank?

Staging lets a rocket drop its empty, spent tanks and engines mid-flight instead of hauling that dead weight all the way to orbit. Each new stage starts with a fresh, favourable mass ratio, so total delta-v becomes the sum of every stage's contribution rather than being capped by one impractical single-stage vehicle. Reusable rockets like Falcon 9 take this further by landing and reflying the first stage.

Try it live

Everything above runs in your browser — open Rocket Launch (Tsiolkovsky) and build your own vehicle from Isp, propellant fraction, stage count and thrust, then watch whether it earns enough delta-v to reach orbit. Nothing is installed, nothing is uploaded.

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How Rockets Work: Newton's Third Law and the Tsiolkovsky Equation