Yes, that's right - a rocket that can fly AND still function as a bottle to drink out of!
Note - this isn't a beginners project! I reccomend you either have experience in model rocketry, or have some mechanical ability. I will be using some funny model rocketry terms (wadding! that's a funny word) If you don't want the rocket to actually fly, this is a great project for anyone.
Lets get started!
Step 1: Step 1: Drink Coca Cola
While you're at it, peel off the label of one of the bottles.
Step 2: Step Two: Prepare Lower Bottle
Next, take the cap of the bottle and drill a hole in the middle (second picture). I recommend using 18mm diameter motors for the best performance, the rocket shown has smaller 13mm motors. If you want your rocket to be a static display model, you can skip this step and the few coming after.
Step 3: Step 3: Engine Mounting
(If you do intend your rocket to fly, this step is highly recommended because rockets need engines)
Anyways, cut a balsa circle so that it will fit tightly inside the bottle. Drill a hole in the middle to accommodate the proper size engine tube you are using (13mm shown, 18mm recommended).
Grab an engine tube and cut it down to about 3 to 4 inches, whatever you see fit. Glue one end of the tube into the balsa circle you cut out. Wood glue works the best. Note I used a small strip of masking tape to make the fit a little tighter.
After drying, insert your engine tube and balsa circle contraption into the bottle. Push it far enough so that the other end of the engine tube sticks out of the mouth of the bottle. Screw on the cap with the hole you drilled in the previous step. Be very careful, as you don't want to distort the engine tube too much. Apply a small amount of glue on the outside to keep the engine tube stuck to the cap. Don't get glue inside the tube!
While glue is drying, you can draw some freehand fins and cut them out in balsa. Make them whatever shape and size you like, as long as its sensible (more than two fins, less then 24 fins, etc).
Step 4: Step Four: Upper Bottle
Remember how in the previous step you cut out a balsa circle and drilled a hole in it? Basically do the same thing here, but make the balsa circle small enough so that you can glue it to the bottom of the upper bottle (that gets confusing, just look at the picture).
Drill a hole in it to fit a dowel. Use a dowel with a diameter smaller than the engine you are using. Cut the dowel so if you fit the upper bottle and lower bottle together, the dowel would stick about half an inch into the engine tube. Glue the dowel into the balsa circle.
You need the dowel to seal the upper part of the engine tube, so use some tape (a lot of tape in my case) and wrap it around the end of the dowel, just enough so that it fits snugly into the engine tube. Also you can use another balsa circle, but I was getting tired of making balsa circles so I went with tape instead. Balsa squares or rhombi are not recommended.
Step 5: Step 5: Putting It All Together
First, get some elastic shock cord and do a tri-fold paper mount to mount it to the balsa circle in the lower bottle.
Take the other end of the cord and tie it to the dowel on the upper bottle.
Get your parachute shroud lines and tie it to the dowel as well. Use wood glue to secure the shock cord and the parachute shroud lines to the dowel.
Step 6: Step Six: Preparing for Flight
Next, before you go outside to launch the rocket, fill the upper bottle with Coca Cola to drink on your way out.
Now go outside and launch it! You'll need a lanch pad and electronic ignitor for model rocketry.