A balloon rocket is a balloon taped to a straw that zips along a string when you let go. It is one of the cheapest ways to show a child that air is real stuff that can push. Below: build the rocket, then a balloon-powered car, then turn both into a fair test with real numbers.
Our air pressure guide has a quick hallway launch. Here we explain the push and collect data. Plan one evening to build and one to test.
A push or a pull3.
Say it: “What is doing the pushing here, your hand or the air?”When one thing pushes on another, the second thing pushes back just as hard, the opposite way14.
Say it: “The air goes back. Which way does the balloon go?”The rubbing between two surfaces that slows things down. DK lists it with push and pull as a playground force3.
Say it: “Will the car roll farther on the rug or on the floor?”The rod a wheel turns around3.
Say it: “What happens if the axle rubs against the box?”The one thing you change on purpose in a test2.
Say it: “What is the one thing we are changing this time?”How to build a balloon rocket on a string
Isabelle and Zinn's STEPS to STEM, a science curriculum for upper elementary and middle school, sets this exact challenge for student teams1:
What design will make a Balloon Rocket travel the farthest distance possible?1
The book leaves the design to the team. The string-and-straw version below is our adaptation, sized for ages 5-10 and a home hallway.
The balloon rocket experiment, hallway edition
- Long balloons and round balloons
- 2 drinking straws
- Smooth string or fishing line, the length of your hallway
- Masking tape
- 2 chairs
- Tape measure, or a child's own steps
- Adult supervision the whole time: balloons and broken pieces are a choking hazard.
- Thread the string through one straw. Tie each end to a chair and pull the string tight.
- An adult blows up the balloon and pinches the neck shut. Do not tie it.
- Tape the balloon to the straw with two strips, the neck pointing at the start chair.
- Ask: "Which way will it go?" Then release.
- Mark the stop with tape. Measure in steps (ages 5-7) or feet (ages 8-10).
Why it moves: the balloon rocket science experiment explained
Start with what the balloon holds. STEPS to STEM links its air activities to a science standard that says gases are made of particles too small to see, moving freely around, and that this explains the inflation and shape of a balloon1. A stretched balloon squeezes that air. Open the neck and the air rushes out.
Now the push. NASA's Glenn Research Center states Newton's third law this way: whenever one object pushes on another, the second object pushes back with an equal and opposite force4. NASA's own example is a jet engine: hot gases flow out the back, and a thrust force is produced in the opposite direction4. A balloon does the same thing with plain air.
The balloon pushes the air backward. The air pushes the balloon forward. Same push, opposite directions.
The balloon moves because the air pushes on the wall or the floor behind it.
Nothing behind it is needed. The push is between the balloon and the air leaving it, the same action and reaction NASA describes for a jet engine4.
Bigger always goes farther.
Maybe, maybe not. A big balloon holds more air but is also heavier and wider, and the standard reminds us more mass needs more force for the same change in motion1. Test it instead of guessing.
How to build a balloon powered car from cardboard and bottle caps
Same push, new job: now the air drives wheels. DK's 1000 Words STEM labels axle and wheel on its machines pages3, so your child is building a real machine part.
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01Body
Cut a rectangle of stiff cardboard, about the size of a paperback book. Your child decorates it.
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02Bearings
Tape two straws across the bottom, one near each end, perfectly parallel. The axles turn inside these.
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03Wheels
An adult pokes a hole in the center of four plastic bottle caps. Off-center holes make the car wobble.
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04Axles
Slide a wooden skewer through each straw, push a cap onto each end, and snip off the sharp tips. Spin each wheel: it should turn freely.
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05Engine
Slip the neck of a balloon over a third straw and seal it with tape or a small rubber band. Tape this straw on top of the car so it sticks out past the back edge.
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06Launch
An adult blows up the balloon through the straw, pinches it, sets the car down and lets go.
- Axles do not rub on the cardboard
- Straw nozzle points straight back, not down
- Sharp skewer tips are cut off
- Bottle caps and balloon scraps counted and put away after (small parts)
Turn it into a balloon powered car science fair project
A launch is a demo. A fair test is an experiment. Harland's STEM Student Research Handbook, written for high school researchers, gives the frame: the independent variable is what you change on purpose, the dependent variable is what you measure, and everything else stays constant2. In kid words: I change, I measure, I keep the same.
Harland also stresses replication, running each setting more than once, because it reduces the effect of measurement error and random differences2. For projects you design, the book recommends between 4 and 10 entities in each group2. For a car, that means 4 to 10 runs per setting.
| I change (one thing) | I measure | I keep the same |
|---|---|---|
| Rocket: long balloon vs round balloon | Distance along the string | Same breaths of air, same string, same straw |
| Car: floor vs rug vs tile | Distance rolled | Same car, same balloon, same air |
| Car: 0, 2, 4 coins taped on | Distance rolled | Same surface, same air |
| Car: short vs long nozzle straw | Distance rolled | Same car, same surface, same air |
Measuring "the same air" is the hard part. Count breaths, or wrap a string around the full balloon and blow up to the same mark each time. Harland calls practice runs pretrials: test your method before real data collection so the procedure works2. Do two practice launches that do not count.
| Setting | Run 1 | Run 2 | Run 3 | Run 4 | Middle value |
|---|---|---|---|---|---|
| EXAMPLE: floor | 11 ft | 13 ft | 12 ft | 12 ft | 12 ft |
| ____ | ____ | ____ | ____ | ____ | ____ |
| ____ | ____ | ____ | ____ | ____ | ____ |
DK's Experiments pages give the words for the poster: question, prediction, method, results, graph, conclusion3. A bar graph with one bar per setting is enough for a first science fair.
Can: Predict which way it goes, measure in steps or tape marks, and compare two settings.
Try: Floor vs rug with the car. Line up a strip of paper for each run and see which pile is longer.
Can: Name the variable, keep the rest constant, repeat runs and find the middle value.
Try: Coins on the car, 4 runs each, a bar graph and one sentence about action and reaction.
More balloon science activities for a second evening
- Two-stage rocket. Tape a second balloon to the string and ask: does it go farther than one? Why might it not?
- Uphill string. Raise one chair end. Does the rocket still reach the top?
- Steering the car. Bend the nozzle straw slightly left. Predict the path before launch.
Frequently asked questions
How does a balloon rocket work?
The stretched balloon pushes air out of its neck. By Newton's third law, the air pushes back on the balloon with an equal and opposite force, so the balloon moves the other way4.
What is the variable in a balloon rocket experiment?
Whatever single thing you choose to change, such as balloon shape or string slope. That is the independent variable; distance is what you measure, and everything else stays the same2.
Is a balloon rocket science project good for a science fair?
Yes, if it is a fair test rather than a demo. Change one thing, repeat each setting 4 to 10 times, and show the results in a table and a graph23. STEPS to STEM uses it as a team challenge in an upper elementary and middle school curriculum1.
Why won't my balloon powered car move?
Usually friction: an axle rubbing the cardboard, an off-center wheel, or a nozzle pointing down. Spin each wheel by hand and fix the one that sticks.
What other science experiments with a balloon can kids try at home?
Start with the air-pressure experiments in our guide below, then try the two-stage rocket or the steering challenge above. Keep the same safety rule each time: an adult inflates balloons and collects broken pieces5.
Keep exploring
- Air is not nothing Four kitchen experiments that show air takes up space and pushes.
- Ramps, cars and your child's first fair test Another car test with one change at a time.
- Science fair board and the scientific method How to lay out the poster once the data is in.
If you want a kit for this
A cardboard balloon car works fine from a cereal box. If your child wants a bigger stock of sturdy parts and connectors to keep building, this is the kit we make for ages 5-7.
Build it with / Ages 5-7STEM Cardboard Construction KitKid-safe tools turn any box into a machine.$42.99GoP Toys sells the kit mentioned above. None of the sources cited tested our products.