Warning: These rockets accelerate quickly. Stand well back from the rocket and make everybody in the surrounding area aware that you are launching it.
You will need
Plastic drink bottle with a ‘pop top’ nozzle
Vinegar
Sodium bicarbonate (baking soda)
Polystyrene cup
10cm x 10cm Stiff card
Scissors
Sticky tape
Sheet of card or thick paper
Broom (just in case!)
What to do
Attach the rim of the polystyrene cup to the stiff card with about four pieces of tape.
Cut the bottom from the polystyrene cup so the top of the bottle can sit tightly inside of it. This will be your launch pad.
Pour vinegar into the bottle until it is 1/3 full.
Now, here is the tricky part. Make sure the nozzle on the pop-top lid is closed.
Turn the lid upside down and fill it with sodium bicarbonate.
Crouch near the launch pad and quickly empty the sodium bicarbonate into the bottle. Screw the lid on fast – it will be bubbling over the sides, but that’s ok. Just get that lid on quick!
Put the bottle into the launch pad with the nozzle facing down.
Stand well back. Watch your rocket fly!
Try designing your rocket with fins. Do they make it fly straighter?
Note: If for some reason it fails to launch after several minutes, use the broom to push the launch pad over onto its side so it won’t go off in your face. Carefully pick up the rocket, aim the nozzle away from yourself (and other people!) and pop the top to release any pressure.
What’s happening?
Inside the closed bottle, vinegar (also called acetic acid) and sodium bicarbonate react together to produce carbon dioxide. Because carbon dioxide is a gas, it takes up more space than the vinegar and sodium bicarbonate did on their own – its molecules zip around inside the bottle, crashing against the walls and bumping against each other. As more and more gas is made, it has less and less room to move.
The pop-top lid is stuck into place with friction, which is why you need to push it hard to get it open. As soon as the force of the gas is greater than the force of that friction, the nozzle will pop open, releasing the pressure.
It helps to think of the gas as a spring being pushed down. Once it can’t be pushed any further, it will resist and push back. Between the gas and the bottle’s lid is a layer of vinegar and sodium bicarbonate that hasn’t yet reacted. Liquids cannot be squeezed, so it has nowhere to go but out the tiny nozzle. All of that liquid has to move pretty fast to get out through the small hole, which means it speeds up rather quickly.
The laws of motion state that for every action, there is an opposite and equal reaction. This is another way of saying if you push against something, it will create a force that resists you. Since the liquid is accelerating down, it pushes up against the bottle, sending it up into the sky. As long as there is liquid and gas pushing out through the nozzle, the rocket will continue to fly up.
For a great description of the vinegar and sodium bicarbonate reaction, see the miniature version of this activity, Canister Rockets.
Applications
The only real difference between your rocket and a real one is the type of chemicals used.
What you’ve made is essentially a type of jet engine. There are many ways to create thrust, but most rely on the same principle; a chemical reaction inside a closed space produces gas that escapes through a small hole, or nozzle. In jet planes, this reaction is between a liquid hydrocarbon fuel (such as kerosene) and oxygen. Rockets, however, don’t rely on the oxygen in the atmosphere. Instead, they carry their own chemicals to react with the fuel.
Some fuels, such as gunpowder and those used in the space
shuttle's boosters, are solid. Others, like that in the space shuttle's
main engine, are liquid. The choice of fuel often comes down to something called ‘specific impulse’.
In driving your car, you want to get as far as possible on the least amount of petrol. Rockets are the same – more fuel just makes the rocket heavier. Specific impulse refers to the amount of acceleration every unit of propellant provides. Liquid fuel usually has a higher specific impulse than solid fuel, which is why it’s used to send rockets into space. However, solid fuel is easier to store and handle, which is why it’s often used by the military.
