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Episode 7: Rocket Balloon

If you’ve ever wanted to boldly go to infinity and beyond, or to a galaxy far, far away, you can start by sending balloons across your living room in this week’s episode of Do Try This at Home.

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Read the full transcript of Episode 7: Balloon Rocket

Before you start

These experiments have not been specifically safety tested for home use but we believe them to be safe if the instructions are followed. Adult supervision or direction is recommended as appropriate. All experiments are carried out at your own risk.

What you’ll need

  • A balloon (any shape or size)
  • String (a few metres)
  • A straw (we’ve used a paper one)
  • Tape and scissors
  • Clothes peg (optional)

What to do

Before you start, decide how far you’d like your string to span, whether it’s all the way across the room or less. Choose a location so that everyone in your family will be able to see your rocket balloon fly!

  1. Cut the string to the right length, but don’t tie it yet!
  2. Cut your straw in half. 
  3. Thread the two bits of straw onto the string and then tie the string to two secure points somewhere in the room. When you tie it make sure the there’s a little bit of tension in the string.
  4. Cut two short lengths of tape and place the middle of them on each bit of straw. These will hold the balloon in place.
  5. Blow up the balloon, but don’t tie it off. Hold the end closed with one hand, or use a clothes peg, and stick it to your two bits of straw (get a helper for this bit if required).
  6. Start your countdown and let go when you reach zero. We have liftoff!

What to talk about

  • Which way will the balloon go when I let go?
  • In which direction does a real rocket start moving when it launches? What about the fuel?

What’s going on?

A rocket is a vehicle that carries everything it needs with it. Our rocket balloon carries air inside it and that’s what makes it go.

To inflate the balloon you have to blow pretty hard. This is because once you put some air inside, it starts pushing back against you.

Air is made up of particles called molecules, moving around in all directions and at very high speeds. They push against anything they bounce off.  When the balloon is inflated and closed off, there are loads and loads of air particles trapped inside. They’re whizzing about, crashing into each other and the insides of the balloon. All these tiny pushes add up to a force that is large enough to hold the balloon in its round shape. The air particles push equally to the left, right, up and down. Overall the forces cancel each other out, they are balanced forces and that’s why the balloon stays where it is.

When the end of the balloon is tied or closed shut, the forces inside the balloon cancel each other out.

When you let go, the air can’t push on the part of the balloon where the opening is. There’s nothing to push on. But on the opposite side, inside the balloon, the air is still pushing. The left and right forces aren’t balanced anymore, and it’s this unbalanced force – due to the air particles bouncing off the front the balloon – that sends it forwards.

As air is pushed out of the open end of the balloon, the forces are no longer balanced and this pushes it forwards.

Real rockets use fuel, but they work on the same principle called Newton’s third law of motion. You may have heard “for every action there must be a reaction”, which means that it’s impossible to move in any direction without pushing something else in the opposite direction. Our balloon rocket moves air backwards for it to move forwards. In a real rocket, the rocket must push fuel downwards (very quickly) to start moving upwards.

What next?

You could challenge your family to design the perfect rocket balloon. Here are a few top tips to get you started:

  • Try changing the shape of the balloon
  • Try using larger and smaller balloons
  • Can you change the way the air comes out? Maybe try taping an extra piece of straw into the opening
  • Can you change the direction the air comes out? Try taping the balloon so that the neck of the balloon isn’t totally lined up with your string.

Once you’ve perfected your living room design, can you find a way to launch your balloon straight up like a real rocket?

Did you know?

The Russian Soyuz programme is the longest running and most successful way of getting rockets into space. The programme has been going since 1967 with over 1680 successful launches!  


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