Try this : Squeeze Rocket

This week, we will make a simple model rocket and launcher. This rocket will be powered by compressed air. You can also find instructions for making a chemical-powered rocket on the Scientriffic site.

You will need:

• A plastic drink bottle. Try to get one with a small opening.

• To make a straw thinner, cut along one side, slide one edge under the other, then tape along the join.

  Several straws. If you can, get them in two different sizes, so one can fit inside the other.
• Blu-tack
• Paper
• Sticky tape
• Scissors

What to do:

To make your rocket and launcher:

1. If all your straws are the same size, you will need to make one of them thinner, so it will fit inside the others.
   1. Using a pair of scissors, cut all the way down one side of a straw.
   2. Slide one side of the cut under the other, so you get a thinner straw. Make it thin enough that it can slide up the middle of the other straws, even with a bit of tape around it.
   3. Put some tape along the cut to hold it in place.
2. Place the thin straw partway into the bottle and seal around it with the blu-tack. This will be the launcher. Make sure that the straw is not squeezed closed by the blu-tack.
3. Cut one of the thicker straws in half.
4. Seal one end by pushing a lump of blu-tack into it. This will be your rocket.
5. You can make a couple of rockets. Try attaching different shapes made of paper to the straw see how it affects how the rocket flies. You could try:
   • Wings.
   • A paper loop with the rocket stuck to one side.
   • Attaching a piece of thread to make a tail like a kite.

To fire your rocket:

1. Slide the rocket over the straw sticking out of the launcher.
2. Aim the rocket up. Do not aim it at another person or animal.
3. Squeeze the bottle hard.

You should find the rocket fires off and travel several metres

What's going on

The air around us is pushing in on us all the time. This is called atmospheric pressure. We don't normally notice this pressure, because it pushes in equally from all directions.

When you squeeze on the bottle, you compress the air inside it. This increases the air pressure in the bottle, including the air inside the rocket. Since there is a pressure difference between the inside and the outside, the rocket is subjected to a force, which pushes it off the end of the launcher. Once the rocket has left, the air keeps coming out of the opening until the air pressure inside and out is the same.

This is different to how a rocket like the H-IIA works. Those rockets work using Newton's Third Law of Motion, which says that if something pushes on something else, the other object pushes back just as hard. In a normal rocket fuel is burned in a combustion chamber, which is open at one end. As the fuel burns, it produces hot gasses, which rush out the open end of the chamber. Since the gasses are being pushed in one direction by the rocket, the rocket is pushed in the opposite direction with equal force.

While a rocket is moving through the atmosphere, it needs to keep facing in the direction it is travelling, or else the air resistance will twist it around and send it off course. To keep them facing the same way, rockets are carefully balanced and often have fins that help keep them facing the same direction. You can try experimenting with your rockets to see which design give the best results when flying through the air.

When rocket like the H-IIA pushes hot gasses out of one end (blue), it feels an equal force in the opposite direction (green)