Rocketing into Orbit
To reach Earth orbit, a rocket must accelerate to about 8 kilometers (5 miles) per second—about 25 times faster than the cruising speed of a passenger jet. To escape Earth’s gravity, it must travel even faster. To accelerate even a small payload (the object being sent into space) to such speeds takes a huge amount of energy, which rockets carry in the form of propellants.
Most of a Rocket Is Propellant
A rocket needs lots of propellant, which consists of fuel and the oxygen (or other oxidizer) needed to burn the fuel. Since it flies in airless space, a rocket must carry its own oxidizer, which weighs far more than the fuel.
The rocket must initially lift not only its payload, but also the much greater weight of its propellant. The propellant needed to launch a payload into Earth orbit is usually at least 20 times more massive than the payload itself.
Reaching Orbit—One Stage at a Time
Most of the mass of a rocket before launch is propellant. Much of the rest—supporting structure, tanks, pumps, engines, and more—is useless once the fuel has burned. To avoid having to carry all that excess weight into space, rockets often have several stages, or sections, each of which drops away after use.