Step 1: Concept
Step 2: Materials
a plastic soda bottle. ( You can play around with sizes, we found 2l and 1l bottle worked best)
a bicycle tube with a schrader valve
a bicycle pump
silicone glue (only need if problems occur)
a sunny day at an large open space, and clothes to get wet in!
Step 3: Constuction
Cut the valve out of the bicycle tube. leave a "collar" of rubber around the base of the valve, the same size as the inner diameter of the bottle lid. This collar will form the seal in the lid, preventing leakage before release, and will avoid the use of any sealing glue in the lid.
push the valve through the bottle cap hole, so that the valve sticks out the top.
If you find the rubber collar is not sealing properly, then you can use the silicone glue to seal, however if you have cut right then this should not be necessary
Step 4: Firing
We found that a little less than a quarter full of water and masses of air produced impressive results. Put in as much air as possible. We pumped to 170 pump strokes (until the pump could not do anymore, or we ran out of muscle)
Launching the rocket takes practice. Hold the bottle cap in one hand and spin the bottle off, leaving the cap in you hand and the bottle in the air .If you spin badly, the water and air will leak out before it even launches. We found one initial small twist to loosen the lid, then one big twist to release it works best.
Check the attached vid for a compilation of launches. A slow Mo camera shows some good action!