This is a very simple rocket. It uses baking soda and vinegar as fuel, I know sounds lame but its fun and easy. This is a good project for introducing kids to Rocketry ideas such as Newton's "Every action has an equal and opposite reaction" meaning shoot something out the back and it will fly.
Step 1: Materials
1 Film Canister - The kind where the lid snaps in, they are usually translucent white.
1 Plastic Syringe - Ask at your local drugstore for an Oral Syringe.
Baking Soda - Doesn't matter how much.
Vinegar - Again doesn't matter how much.
Paper - Plain printer paper, or construction paper if you like colour.
Tape - Regular scotch tape.
Scissors - They only need to be able to cut paper.
Step 2: Constucting the Rocket
Now cut a circle about 8 cm in diameter out of your paper, make a radial cut to the center of the circle. take one edge and slide it over the other edge, keep sliding it and it should start to form a cone, do this this until the base of the cone is the same size as the top of the film canister. Tape the cone all over and this will be the rockets nose.
Now cut 3 2cm x 20cm strips of paper and fold them until you have 3 2cm x 2cm squares, tape the squares so they will not unfold then cut them to what ever fin design you think looks coolest as these fins contribute little to the stabilization and flight. Tape them on to the Film Canister so the top is the closed end. tape on the nose cone now as well.
There you go, you have a rocket.
Step 3: Launching Thje Rocket
First you need to put in your Baking Soda, fill the Film Canister 1/4 - 3/4 full with Baking Soda, if you fill it less than 1/4 full it will not generate enough gas to blow the cap, if it is filled to much there will not be enough room for the vinegar to mix fully. The more baking soda that you put in the more unreacted sludge that will shoot out the back. Snap the lid back on tightly.
WARNING: This is a very messy part wear goggles if your worried and wear a t-shirt that you don't like as it may get splattered, i don't think the sludge will stain or harm anything though. If you don't like getting messy, get someone else to hold it.
Now take the Syringe and fill it with 5ml - 8ml of vinegar and stick the tip of the Syringe into the hole in the lid, don't worry the reaction wont start till the plunger is pushed.
Hold the rocket by the Syringe at arms length pointing downwards and shoot the vinegar into the rocket, quickly point the rocket skyward and make sure to hold the Syringe Plunger in or else the pressure will just shoot it back instead of shooting the rocket forward. The reaction can take a few moments so just keep it pointing away from you.
Step 4: Safety
Everybody Loves Safety!
Mostly its just common sense, don't point it at your eye, don't eat the sludge, don't shoot the cat.
Step 5: Possible Improvements
Here are some ways that one could improve the rockets performance.
Using a different fuel, such as Eno ("Effervescing Powder" the stuff you put in water when you have an upset stomach) or a crushed Alkaseltzer tablet. These fuels do not leave behind any sludge and they create a lot more CO2 gas.
Using a different material for the fins and nose cone, with stiffer materials you would get better aerodynamics as the fins wont flop around after a couple of launches.