A rocket is simply a chamber filled with pressurized gas. A small opening called a nozzle allows the air to escape, causing thrust that propels the rocket. You can demonstrate this by building a DIY rocket car with simple materials you probably have at home.
If you fill a balloon and hold the end of the balloon 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 is equal in all directions.
Balloons store potential energy when they are filled with air. When the air is released, the potential energy is converted into kinetic energy.
Watch our video to see a balloon rocket in action!
If you release the neck of the balloon, there will be an imbalanced force on the balloon, and the internal pressure on the front of the balloon is greater than the internal air pressure on the back of the balloon. This action will result in a force acting forward on the balloon—thrust. The balloon will then fly forward, and the air coming out of the back of the balloon is the equal and opposite reaction to the thrust.
Sir Isaac Newton laid the foundation for the modern science of rocketry near the end of the 17th century. Newton's Laws of Motion are essential to rocket flight: 'Objects at rest will stay at rest and objects in motion will stay in motion in a straight line unless acted upon by an unbalanced force.' In other words, the forces pushing a rocket up must be stronger than the force of gravity pulling it down. And 'For every action, there is always an opposite and equal reaction.' When an action takes place, like gases escaping from the rocket, a reaction follows - the rocket rises in the air. Newton's third law of motion states 'all forces between two objects exist in equal magnitude and opposite direction,' therefore the amount of thrust exerted by the air leaving the balloon would be equal to the force of direction the balloon travels.
The principles of rocketry apply to more than flying rockets. With this home science project, you can make a rocket car that is powered by pressurized gas (the air in a balloon!).
Adult supervision is recommended.
How to Make a Balloon Rocket Car
What You Need:
- 16-20 oz. plastic water bottle
- Drinking straws
- Wooden skewers
- 4 plastic bottle caps
- Your own balloon
- Duct tape or masking tape
- Nail, hammer, knife, scissors
Or buy our Balloon & Rubber Band Race Car Kit
What You Do:
The water bottle forms the chassis, or body, of your balloon racer. You can start by mounting the wheels on this body.
- Stretch out a large balloon by blowing it up and then letting the air out of it a few times. Next, make a nozzle. The size of the nozzle is very important. If it is too small, the air can't escape with enough force to propel the car forward. If it is too big, the air will escape too fast and the car won't go very far. Create the nozzle by taping four drinking straws together. Insert the straws into the mouth of the balloon and seal the opening by wrapping a strip of duct tape around it several times.
- To mount the balloon/nozzle on the car, use a knife to cut two perpendicular slits (to make an X) in the top of the car about 4' back from the mouth of the bottle, as shown in the illustration. Thread the nozzle through this opening and out through the mouth of the bottle. Leave about an inch of the nozzle sticking out of the mouth.
- Find a hard surface, like a long table, linoleum floor, or sidewalk. Blow up the balloon through the straws at the mouth of the bottle without the air escaping. Pinch the base of the balloon to prevent the air from escaping too soon. Set the car down, let go of the balloon, and watch it go!
What Happened to Your Balloon-Powered Car:
The air in the balloon is gas under pressure. The air pushes against the balloon, causing it to expand, but the balloon is also pushing back on the air. The pressure of the balloon pushes the air right out through the nozzle, which creates thrust that propels the car forward.
Keep track of how long the rocket car travels and how far it goes. Try it several times, then try changing the design to see if you can get it to go farther or faster.
How will it work if you only use three straws for the nozzle? What if you use a bigger or smaller balloon? Does the car go farther on linoleum or the sidewalk? Why do you think this might be? Will the car go farther if you start it at the top of a ramp?
Decorate your car and have races with siblings or friends. Try to figure out why one car goes faster or farther than another, and keep experimenting to make your design better!
Film Canister Rocket Science Experiment
What You Need:
- Clear film canister (the kind where the cap fits inside the canister, rather than over the outside. See if a local photography shop has any extras they can give you. )
- Alka-Seltzer tablets
- Baking soda
- A piece of paper
- Cardstock or poster board
What You Do:
- The water bottle forms the chassis, or body, of your balloon car. You can start by mounting the film canister. This is the rocket engine. Make a body for it out of the piece of paper. Line up the open bottom of the canister slightly below the bottom (short) edge of the paper, then tape the longer edge to the canister and start rolling to make a tube. Tape the tube closed.
- Cut out four triangular fins and one nose cone from the cardstock. (To make the nose cone, cut out a circle, then make a slit from the outer edge to the center. Overlap the paper at the slit and tape it to create a cone shape.) Tape the fins and nose cone to your rocket.
- Turn the rocket upside-down and fill the canister 1/4 full with water. Add half of an Alka-Seltzer tablet, snap the lid on, turn the rocket over, and back away. 3-2-1 blast off!
What Happens When Your Car Moves:
The Alka-Seltzer reacts with water to produce carbon dioxide. When enough carbon dioxide is produced to create pressure on the inside of the canister, it will force the lid to pop off so the gas can escape.
As the gas escapes the rocket is propelled upward.
Try to measure how high your rocket goes compared to a nearby fence or a tree. Try it several times; do you get the same results each time? Does the rocket go higher if you add more or less water? Why do you think this is? What happens if you change the design of your fins or nose cone?
Now try it again with a different type of fuel.
When vinegar and baking soda are mixed together, they produce carbon dioxide. Experiment to see how much baking soda and vinegar will launch the rocket the highest.
(Some tips: You can either pack the baking soda in the lid with a damp thumb so that the reaction doesn't start until you turn the rocket over, or you can wrap it in some tissue paper or toilet paper to delay the reaction starting and give you time to set the rocket right-side-up.)