Rockets (and jet engines) work much like a balloon filled with air.
If you fill a balloon with air and hold the neck closed, the pressure inside the balloon is slightly higher than the surrounding atmosphere. However, there is no net force on the balloon in any direction because the internal pressure on the balloon is equal in all directions.
If you release the neck of the balloon, it acts like a hole, with no surface area for the internal pressure to act on. There is now an imbalanced force on the balloon, and the internal pressure on the front of the balloon is greater than the internal pressure on the back of the balloon.
This results in a net force acting forward on the balloon—thrust. The balloon flies forward under the influence of the thrust, and the air coming out of the back of the balloon is the equal and opposite reaction to the thrust.
When we think of rockets (or jet engines) we rarely think of balloons. Instead, we think of the big rockets that carry satellites, supplies, or people into space. However, balloons and rockets are very similar. The only significant difference is the way the pressurized gas is produced. With rockets, the gas is produced by burning propellants that can be solid or liquid in form or a combination of the two.
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.