Key concepts
Propulsion and thrust

From National Science Education Standards: Motion and forces

Have you ever wondered how a space shuttle launches all the way into outer space? It takes a lot of energy to make such a heavy object (4.5 million pounds at liftoff) go from standing still to blasting off toward space at more than 17,000 miles per hour—in just minutes!

For real space launches rocket scientists figured out special fuel to make enough energy to get a heavy shuttle off the ground. You, too, can use the same principle (but without dangerous rocket fuel) to propel a balloon rocket across the room.

Complex chemical formulas aside, rocket fuel is based on a simple idea: create enough power to push an object forward. This movement works in part because the power created by burning fuel is focused in a single direction. By controlling the direction that force goes, you can create thrust. During a space shuttle launch, the power is focused down, forcing the shuttle to move in the opposite direction.

In this activity we are working with air instead of rocket fuel, but we use the same idea of force in one direction moving an object in the opposite direction. When you blow up a balloon, you force extra air into it, creating higher air pressure inside the balloon than outside of it. Given the chance, the air molecules will move to a lower-pressure environment—which is why, if you let go of a balloon's opening without tying it off, air you added will rush out again.

If you were to pop a full balloon, the air from inside goes in all directions, distributing the force so that none of it is that strong in any one direction. But if you allow the air to exit through only one small hole, the force will be strong enough to propel the lightweight balloon in the opposite direction.

•    Balloon (Long ones work best, but a round one will do, too.)
•    Piece of string at least 10 feet long
•    Plastic straw
•    Tape
•    Two chairs or sturdy door handles about 10 feet apart (with clear space in between)
•    Balloons of other shapes and sizes (optional)
•    Other thin materials that can work as a guide wire, such as fishing line, ribbon or twine (optional)
•    Stopwatch or clock that indicates seconds (optional)

•    Tie one end of the string to a chair, handle or other steady object.
•    Thread the string through the plastic straw.
•    Making sure the string is taut, tie it to another chair or handle at least 10 feet away, keeping it at the same height so there is no upward or downward slope, and making sure the area around the string is clear.


•    Blow up your balloon (this is the part that’s like filling a rocket engine with fuel) and pinch the opening with your fingers to keep the air inside. (Don't tie it off.)
•    While you are pinching the end of the balloon, secure it onto the bottom of the plastic straw with a few pieces of tape.
•    Pull the full balloon with the straw to one end of the string, so that its opening faces in the opposite direction from the clear line of string ahead of the balloon.
•    What do you think is going to happen when you let go of the balloon opening?
•    Let go of the balloon, then release its opening.
•    What happened when you let go of the end of the balloon?
•    Which direction—and how far—did it go?
•    Try it again with the balloon only half inflated. How fast and how far do you think it will go?
•    What are other ways you could get a balloon to go faster or slower—or longer or shorter distances?
•    Extra: Try other sizes and shapes of balloons. How do they perform?
•    Extra: Try using other types of thin materials as your line, such as ribbon, twine or fishing line. Do these make a difference in balloon rocket speed or distance?

Read on for observations, results and more resources.

Observations and results

How far did your balloon rocket travel?

Even though you did not fill the balloon with rocket fuel, it was able to blast off just like a space shuttle. In fact, the balloon is so light that all it needs is a jet of air to create enough thrust to get it to move through space.

Of course, if you blow up a balloon and let go of the end without sending it along a string, it might zip around in several directions, but it probably won't travel as far in any one direction. Keeping it moving along a string focuses the power from the balloon's air in a single direction, generating more concentrated thrust and helping it travel as far as possible in one direction.

If you have balloons of other sizes or shapes (or even if you just try filling one with more or less air), which ones travel farthest or fastest? You can also try setting up two parallel strings and having balloon races!

What other forms of transportation use thrust? Have you ever seen a hovercraft? It uses a giant fan mounted on the back of the boat, which helps it move forward using only air. Although the fan is powered by fuel, it generates air thrust just like your balloon rocket. Some animals even use the principle of propulsion to get around. Squids, octopuses and jellyfishes, for example, can fill part of their flexible bodies with water and force it out through a smaller opening, propelling them through—and even out of—the water. Humans have studied these natural forms of propulsion to get ideas for our vehicles and technology. Scientists were able to take this simple principle and combine it with knowledge of chemical reactions to create much stronger boosts—like those found in rockets.

Share your balloon rocket observations and results! Leave a comment below or share your photos and feedback on Scientific American's Facebook page.

Remove the straw from the balloon if you plan to reuse the balloon (but be careful for weak spots from the tape). Take down the string and reuse it for something else.

More to explore
"Can You Explain How Jet Propulsion Engines Work?" from Scientific American
"Can a Squid Fly out of Water?" from Scientific American
The Space Place site for kids from NASA
"Beginner's Guide to Rockets, Rocket Propulsion Activity" from NASA
Move It!: Motion, Forces and You by Adrienne Mason, ages 4–8
365 More Simple Science Experiments with Everyday Materials by Judy Breckenridge, Anthony D. Fredericks and Louis V. Loeschnig, ages 9–12

Up next…
Talk through a String Telephone

What you'll need
•    Two large paper cups (disposable plastic cups will also work)
•    Two paperclips or toothpicks
•    Length of cotton string or fishing line approximately 10 to 30 feet long
•    Quiet area