What I love about this method is that it includes a way to take any cork and make it fit a bottle using the magic of microwaves!
This project was originally inspired by the howtoons about bottle rockets. My friend Will Macfarlane and I played with it a lot but found it tough to do with kids ages 6 - 8: the inner tube stopper would come unwrapped and kids had trouble putting it back together, let alone making it on their own. We got into microwaving everything in sight one week, including everything in Star's microwaving instructable, and realized that corks expand when microwaved.
This led to a new way to make bottle rockets! We've used this at Camp Kaleidoscope since '07.
Step 1: Preparing the Cork.
We think this works because water in the air inside the cork gets hot and expands, causing the cork to expand. The whole thing cools really quickly -- I've never had to do anything more than put it in the bottle, leave it for a minute, and then take it out.
For soda bottle size, I like 20 oz. or 2 Liters: they have the same opening size, so an opening made for either one will work on both.
We've found that microwaving for 25 seconds worked well, and a lot more would burst our corks. The exact time will depend on the power of your microwave.
You don't always have to microwave the cork -- sometimes the cork will fit naturally into the bottle. What's a nice about this method is that it's not a problem if it doesn't. Rubber stoppers from chemical suppliers will work too, but this is a scarcer item. I think size #4 rubber stopper fits 20 oz/2-L bottles.
Step 2: Cutting Up the Valve.
You can get lots of inner tubes from bike shops. They get a lot from replacing tubes in flat tires.
For this to work really well, cut the valve like the last two pics in this step. The rubber is tapered so that it's easy to push the valve into the stopper (once the cork has a hole in the middle) but hard to pull it out. Also, the tapered end is a bit wider than the rest of the valve, helping keep the valve inside the cork.
Step 3: Drilling.
For best results, make a guide hole by first drilling a hole with a bit around 1/8" size.
Then drill a hole with a 7/32 or 1/4" bit. I prefer a 7/32" bit because it's a bit smaller and should make a tighter fit, but both have worked fine. The hole will fill up a little with the surrounding cork filling up the new space, so make sure to drill all the way through twice to clear away all of the cork.
Step 4: Getting the Valve in the Cork.
Then press down on the cork really hard with the valve against a table. It should press fit in with a bit of effort.
The pictures are helpful here.
I think that if this doesn't work, putting hot glue along the shaft of the valve (along the long side) and then inserting it in would help. I haven't found anything but press-fit to be necessary.
And voila! You have a bottle rocket adapter!
Step 5: Time to Launch Rockets!
From here you can play with how much water to put in, make a stand for the bottles, put fins and nosecones on, strapped stuffed animals on and fire them off, etc.