How to Make a Water Bottle Rocket
IntroductionA water bottle rocket consists ofThe weight and fins are for stability, like an arrow; for a rocket to be stable the center of mass (the balance point) should be ahead of the center of surface area. The rocket is not stable when first launched, when it has a lot of water in the bottom. But the water is pushed out quickly (in about the first 10 feet of flight) and after that the rocket is just coasting. When coasting, the weight up front pulls the rocket through the air, and the rocket corrects itself if it gets a bit crooked. Rockets can be one or two bottles. I usually have groups of kids make two-bottle rockets; it takes longer, so we have more time to talk, they work together more, and it is a longer build-up to the launch. One-bottle rockets are a bit lighter. Two-bottle rockets are more stable, and more impressive. One group of kids made a three-bottle rocket; it was really impressive, although it didn't fly as high. Materials
Cut narrow end off one bottle (two-bottle rocket)
BallastYou need weight up front, to pull the rocket through the air when it is coasting. I'm a fan of kitty litter, because it is cheap, and it's a great educational lesson for kids to see the use of unglamorous material as kitty litter in a rocket.Ballast two-bottle rocketFor a two-bottle rocket, I use clumping kitty litter in the top end. It is cheap, heavy, and when wetted will stay in place well. Tip the front chamber upside down, pour in about 1/2 inch of kitty litter, add enough water to wet the whole kitty litter (pour out any extra), add another 1/4 inch and soak again.If you start with too much kitty litter, the water hitting the top of the litter forms an impermeable barrier before water penetrates all the way through, leaving a layer of dry kitty litter, that could get shaken loose under acceleration. If you use too much kitty litter altogether, then the rocket hits hard when it comes down. Dry the inside walls of the bottle, and add duct tape to help hold the kitty litter in place. Fold a 9 inch piece of duct tape into a U shape with the sticky side out, place it into the bottle with the bottom of the U against the kitty litter, and press the sides of the U against the inside walls of the bottle. (This is tricky, and may not be necessary - the kitty litter seems to hold well by itself.) Optionally, you can add playdoh to the outside, as in a one-bottle rocket; this adds weight and makes the front more rounded for better aerodynamics, and is softer and hence safer when it comes down. You can also use just playdoh instead of kitty litter. Ballast one-bottle rocket
Tape together (two-bottle rocket)For a two-bottle rocket, tape the ends together. Line up the bottles as shown in the two-bottle picture above (but now with ballast in the front). Press the bottles together, and tape together with duct tape.Fins
Tape the fins to the rocket as shown in the "One-bottle rocket" picture above. Remove the label from the bottle first, so the tape sticks better; otherwise they rip off at speed. The tabs are folded out and taped to the bottle using a half-strip of duct tape; you may need to adjust this to get the fins straight. The fin's main fold is away from the rocket. There is air space between the sides of the fin; this doubles the surface area, and makes the fins more stable. Viewed from above, the sides of the fin form a V, closed off by the side of the rocket. Use three (or four) fins equally-spaced around the rocket. The fins are attached near the bottom of the rocket, with the bottom of the fin tabs at the bottom of the straight sides of the firing chamber. Smaller finsDec 2009 update-I now recommend smaller fins than shown in the pictures and instructions above.Cut a manila folder in half where it is normally folded. Fold each half in two the long way and cut again; fold in half the long way again, and trim off tabs to make straight sides. Then cut three fins instead of the two shown in the picture above. Another update-you can tape the fins at a bit of an angle so the rocket spins, for better stability. Too much of an angle, and the rocket spins a lot but doesn't go very high or fast. A good straight-fin rocket seems to fly higher, but let the kids experiment. I had previously recommended no spin, based on a theory I read on another site - that the spin would force the water to the perimeter, keeping it from exiting the bottom. We tried it, no problem. There is so much acceleration, and the water comes out so fast (all out after about 10 feet), that the water is forced out the bottom regardless of spin. DecorateYou're not going to miss this chance to decorate your rocket, are you? Go wild, rocketeer!Markers on the fins work well. Water-proof markers are best - to launch you'll need to pour water into the bottom of the rocket, and may spill. Decoration is also practical - it is a way to occupy early finishers when working with groups of kids :-) Launch
VariationsNose coneI don't bother, though sometimes kids in my groups get creative. Put it on straight!ParachuteI've never figured out how to make a good parachute, that opens up just when you want. I'd love to hear your ideas. One idea I've heard is to use create a parachute, and hide it under a nose cone that is loose on top of the rocket. As long as the rocket is going straight, the nose should stay on. When the rocket tips over at the top, the nose should fall off, exposing the parachute. That is the theory. I've heard from one person that it works great. Then I later heard from the same person that it failed :-( The rockets usually come down hard and smash in the nose, but can still be flown a few more times. With large groups of kids you don't want to send any rocket up more than a few times anyway, because pumping gets exhausting.Egg ChallengeCan you construct your rocket to give an egg a safe ride up and down? Without a parachute? Thanks to Sinclair (a student reading this site) for suggesting this challenge. I haven't tried it yet, I think you could do it. It would take a two-bottle rocket, with the egg well-cushioned in the middle. I've got some ideas about cushioning, but I'll let you get creative. If you try this, let me know what you did, and how it works. |
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