Rocket Propulsion

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.

Credit: National Air and Space Museum, Smithsonian Institution

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.

Ask an Explainer

What’s the difference between liquid and solid-fuel rockets?

There are two main types of rockets: liquid-fuel and solid-fuel. Liquid-fuel rockets consist of a fuel and oxygen (or other oxidizer) in liquid state. They are combined in a combustion chamber and ignited. The fuel flow to the engine can be controlled, the amount of thrust produced can be regulated and the engine can be turned off or on as needed. ... more

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Watch a Video

Thrust: Rocket Engines

Explore how rocket engines generate thrust.

Did You Know?

The Bell X-1, the D-558-II and the X-15 were rocket-powered airplanes used for high-altitude and high-speed testing. Even though they only flew in the atmosphere, the best jets of their day could not provide enough thrust, so they required rocket propulsion. To offset the rocket’s lack of endurance, these craft were initially carried aloft on a "mother" ship and then released.

Pop Quiz

Which works better in a vacuum?

A) Propeller

B) Rocket

C) Jet

D) None of the above

Rocket. As a propeller spins, it creates a difference in air pressure between its forward and rear surfaces. But take away the air and you take away the pressure difference that makes the propeller work. In a vacuum it continues to spin freely but creates no thrust. And of course, a jet engine requires outside air to burn fuel. Without a continuous flow of air streaming in through the intake, the jet engine creates no thrust. To produce thrust in the vacuum of space you need a rocket, which carries its own oxidizer to burn fuel and does not rely on air pressure to produce thrust

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Rocket Propulsion