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MOTION · B1 · 004 · Action and reaction

A rocket doesn't push against anything. It throws something.

Newton's third law: every push comes with an equal push back. The engine shoves gas out the back; the gas shoves the engine forward. No air or ground is needed.

Try itThrow gas, get pushed

This is a Space Shuttle main engine (RS-25) with its real numbers. Push = gas thrown per second × how fast it leaves. Then add air around the nozzle and see what it does.

Gas pushes engine–
Air pushes back–
Net push–
Share of push in space–

NumbersHow big is it?

RS-25 data at 109% power. Gas speed here is the effective exhaust speed from the engine's specific impulse in vacuum. The sim treats outside air as pressure on the nozzle exit (2.3 m wide) and leaves out smaller effects.

514 kg/s

Propellant an RS-25 burns and throws out every second.

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≈ 4.4 km/s

Speed of its exhaust in vacuum (specific impulse 452 s × 9.81 m/s²). About 13 times the speed of sound in air.

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+22%

Push in vacuum vs at sea level: 2,279 kN vs 1,860 kN. Air pressing on the nozzle exit takes push away.

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HowWhat happens, step by step

  1. The engine throws gas out the back.

    Burning fuel makes hot gas at high pressure, and the nozzle shoots it out at thousands of metres per second.

  2. The gas pushes back on the engine.

    To throw the gas backwards, the engine has to push on it. The gas pushes on the engine just as hard, forwards.

  3. Nothing outside is needed.

    The push happens between the engine and its own gas. Air outside only presses on the nozzle and costs a little push.

Same on a skateboard: throw a heavy ball forward and you roll backwards. You pushed the ball, the ball pushed you.

Check3 quick questions

Bonus

That's also why a garden hose pushes back in your hand when you open it up: it throws water forward, the water pushes the hose back. In the sim, add the air back and watch the push drop.