Rockets

A rocket is the only machine that can carry us away from Earth. Aeroplanes need air under their wings and oxygen for their engines, and space has neither. A rocket brings everything itself: fuel and the oxygen to burn it. But what does it actually push against when there is nothing up there?

The recoil principle

The answer: a rocket does not push against anything at all. It throws hot gas backwards at enormous speed, and that pushes the rocket itself forwards. This is the recoil principle. You know it from an inflated balloon you let go: the air streams out the back, the balloon shoots forwards. No surrounding air is needed for this, quite the opposite: in a vacuum the rocket flies even better, because no air slows it down.

Fuel to carry the fuel

Why are rockets so huge? Because they are almost all fuel, often about 90 percent. The reason is a vicious circle: a rocket has to carry its own fuel. To go faster you need more fuel. But more fuel makes the rocket heavier, so it needs even more fuel just to carry the extra fuel. The payload, the satellite or spacecraft at the top, ends up being only a tiny part of the whole rocket.

Stages: dropping dead weight

An empty tank is dead weight. That is why rockets are built in stages: once the first stage has burned out, it is dropped, and a smaller second stage takes over. Without the heavy empty tanks the rest picks up speed far more easily. Reaching orbit around Earth takes about 28000 km/h, and to this day no single stage rocket has managed that. From lift off to that speed only takes around eight to nine minutes, by the way.

Landing instead of throwing away

For decades almost every rocket was disposable: the stages fell into the sea, and everything was built new for the next launch. Today reuse is routine. First stages turn around after separation, brake with their own engines and land upright on landing pads or ships. Some boosters fly more than twenty times. That makes launches far cheaper. After all, flying would be unaffordable too if every plane were scrapped after a single flight.