Next to launching them, making the water rockets is the best part. The raw material, soda/pop bottles, are probably within a few hundred feet, in your home or office recycling box, or sitting in a ditch on the side of the road. What better way to pre-recycle plastic bottles than as a water rocket?
Soft drink bottles are built incredibly well. Using a minimum of material they are still able to handle high pressure. We use them as fermentation containers when making Homemade Ginger Beer, containing the pressurized C02. I've seen accounts of folks testing them to well over a hundred pounds of pressure without reinforcement.
If you've built the launcher, hooked up the pump, and just want to see something fly you can first experiment with a stock bottle. You don't even need to add water, although you'll get more elevation if you do. Without fins or a tapered nose the rocket will be a little unstable but still able to reach forty feet or maybe higher. Start off with low pressure, around 20 or 30 pounds, that seems to work best.
By itself the bottle makes a pretty good rocket, but as the air and water are expelled it's center of gravity changes and it tends to tumble. Putting more pressure into the rocket doesn't mean you'll attain higher altitudes. What you need is to move the center of gravity back. The best technique for doing that is putting a few fins on the bottle.
Ok, I've cheated a little. Gluing fins onto a soda bottle can be a real pain. The neck area is curved so you need to cut fins to follow the shape. Glue them on, wait for them to dry, and then after a few bad landings the rocket gets smashed or the fins rip off. Also, be sure you don't use any gluing methods which might weaken the bottle plastic and cause it to rupture.
My solution was to make a modular rocket system. Using a 1 liter bottle I cut the bottom off and glue fins on. The surface is flat and easy to glue. Furthermore since the outer sleeve doesn't need to hold pressure you can use a hot glue gun and be done fast. As a bonus the setup tends to backslide, which is a term for when a rocket comes back to earth like a feather or piece of paper, instead of nose cone first like a lawn jart.
The other nice feature is that you can use a wide variety of soda bottles for pressure containers. Anything that the one liter bottle can slide over should work. The nozzle on the top, by the way, helps to even the weight out a bit and streamlines it a teeny bit.
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