What Pushes a Rocket

Throw Back, Move Forward

Stand on a skateboard and hurl a basketball backwards, and you roll forwards. A rocket does the same: every second its engines blast tonnes of gas backwards at a few kilometres a second. The gas is pushed back, so the rocket is pushed forward.

This is Newton’s third law: push on something and it pushes back on you just as hard. It works just as well in space, so a rocket needs no air.

Saturn V, schematic · to scale; exhaust simplified

The Rocket Equation

In 1903 the Russian schoolteacher Konstantin Tsiolkovsky worked out how fast a rocket can go. It depends on two things: how fast the gas comes out, and how much of the rocket is fuel.

The catch is that more fuel makes the rocket heavier, with more to push. Extra fuel buys speed only slowly: compare a rocket that is half fuel with one that is nine-tenths fuel. The second carries nine times as much fuel for every kilogram of rocket, yet goes only a little more than three times as fast.

Dropping It in Stages

The answer is staging. Build the rocket in sections, and when one runs out of fuel, drop its empty tanks and engines. What is left is much lighter, so the next stage can go much faster.

Saturn V, which took the Apollo astronauts to the Moon, had three stages and weighed nearly 3 million kilograms at launch. Together, ideally, they could add about 12 kilometres a second. Built as one big stage, the same fuel would give only about 8 — not enough to reach orbit.

Saturn V stage masses: NASA

How Fast Is Fast Enough

To circle the Earth you need about 7.8 kilometres a second. A rocket also has to fight gravity and air on the way up, so in practice it needs about 9.5.

That is why a rocket starts to lean soon after lift-off, climbing and speeding up at the same time, flatter and flatter, until it is speeding along almost parallel to the horizon.