# Balloon rockets bring spaceflight physics home

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Joe Skipper/Reuters
In a rocket, the fuel is burned in the combustion chamber, where it rapidly expands into gas that is forced out of the engine nozzle, pushing the rocket forward.

A humble balloon may seem to share little in common with a multimillion-dollar rocket, but they both rely on the same laws of physics for propulsion. Both rockets and balloons use the principles explained by Newton’s third law of motion, which states that every reaction produces an equal and opposite reaction.

In a rocket, the fuel is burned in the combustion chamber, where it rapidly expands into gas that is forced out of the engine nozzle, pushing the rocket forward. When an inflated but untied balloon is released, the stretched latex pushes against the air inside, forcing it out the opening and sending the balloon flying.

Modern rockets use gimbaled nozzles and adjustable fins steered by sophisticated onboard computers. For a balloon rocket, you can use a simpler guidance system: about 20 feet of string, a drinking straw, and tape.

## Why We Wrote This

Rocket science is often used as shorthand for something nearly impossible to comprehend. This experiment uses ordinary household objects to demonstrate basic principles of space flight.

To make a balloon rocket at home, you’ll need these materials, along with a latex balloon. (Note to parents: Latex balloons present a choking hazard to young children.

Tie one end of the string to a doorknob, or other support, and run the string through the straw. Then, pull the string taut and tie it to another support.

Blow up the balloon, but don’t tie it off. Pinching the end, tape the balloon to the straw, with the opening aligned with the string, facing in the opposite direction of the balloon’s flight path. Release the balloon and watch it go!

Rockets typically have multiple stages. When the first stage runs out of fuel, it separates from the rocket and falls back to Earth, allowing the lighter second stage to continue without carrying the weight of the first stage.

Congratulations! You’ve now performed basic rocket science, for a fraction of the cost of the real thing, even when you account for inflation.

This column first appeared in the Nov. 20, 2017 edition of The Christian Science Monitor Weekly magazine as part of the Monitor's occasional Science at Home series.

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