How To Make
How to Make a Water Bottle Rocket
A water bottle rocket consists of
an intact two-liter soda bottle at the bottom to hold water and air under pressure,
optionally, a second soda bottle for greater length and stability,
weight at the top, and
fins at the bottom.
The 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.
water bottle(s) - two-liter, not scratched. The bottle opening must be large enough to fit on the launcher (every now and then you get one that doesn't).
manila folder(s) - preferably recycled
PlayDoh or home-made alternative (one-bottle rocket, optional for two-bottle rocket)
box cutter or knife (two-bottle rocket only)
kitty litter (two-bottle rocket only)
Cut narrow end off one bottle (two-bottle rocket)
For a two-bottle rocket, the first step is to cut one end off one bottle, as shown here. One bottle must remain intact; it is the "firing chamber" that will hold water and pressurized air, and be attached to the launcher.
You should cut off the narrow end (with the cap), keeping the rounded end. This is more aerodynamic (!!), safer, and durable; here's why.
Start the cut with box cutter, and finish with a scissors (for a nice clean cut, so the bottles can be taped together cleanly and straight.
You 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 rocket
For 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
For a one-bottle rocket, I use playdoh or clay. Mold about a half-cup of playdoh into the ridges in the original bottom of the rocket to form a rounded end, then cover with duct tape, as shown here.
If you don't have enough playdoh, the rocket will not be stable, and will go any which way. Add more.
I tried kitty litter inside a one-bottle rocket - it was a dismal failure. To launch the rockets we need to tip them upside down and pour in water. That sometimes loosened the kitty litter enough to break free at launch, showering anyone too close (that would be me) with wet kitty litter.
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.
I use recycled manila folders, but you can substitute cardboard, styrofoam, paper, etc.
Cut a manila folder in half where it is normally folded. Fold each half in two the long way and trim off tabs to make straight sides; see top half of picture above.
With the paper folded, cut off the ends diagonally, like a parallelogram; discard the ends (at right and left, in the bottom half of the picture above). Optionally cut through the middle of what is left, parallel to the end cuts, to make two shorter fins instead of one long one.
Fold tabs on the sides of each fin opposite the fold; these tabs are for attaching to the rocket.
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.
Dec 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.
You'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 :-)
I use one from http://www.ez-launch.com, I paid $50 in 2011. I like it. I previously used a similar one made by his brother-in-law, now retired, but the ez-launch one is even better. (The picture here is the older one.)
Pound the stakes firmly into the ground (you don't want the launcher tipping toward the person who pulls the rope). Attach a bicycle pump.
Tip the rocket upside down, fill the intact bottom chamber one-third to one-half full of water (you need to leave room for air), quickly tip the rocket onto the launch tube (the gray tube in the center) and push down until the bottom of the rocket goes over an O-ring on the launcher. You should be able to push in the copper U-shaped wire in through holes in the side of the launcher, and the wire should sit above a flat lip of the bottle; the wire will hold the bottle down until you are ready to launch.
While wearing safety goggles, pump air in using the pump. this bubbles through the water in the intact chamber. When you launch this will push the water out the bottom to provide propulsion. I put in 80 psi, using a floor pump with a pressure gauge. This is a lot of work, a floor pump helps.
Countdown - 10, 9, 8, 7, 6, 5, 4, 3, 2, 1... To launch, jerk the U-shaped wire out with an attached rope. It's off!
I don't bother, though sometimes kids in my groups get creative. Put it on straight!
I'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.
Can 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.