Water Bottle Rockets
Water bottle rockets are fun, and impressive. This picture shows the first 10 feet of a launch; it doesn't show the other 200 feet!
I've done water bottle rockets about 20 times now with groups of kids at different camps. It's a great group activity; each child or pair of children can make a rocket and we launch them together. Here are some pictures from a camp in Oct 2005.
Want to do it yourself? Read on!
I impress on kids the importance of safety. They start by thinking that bike-pump powered rockets are a joke; but I tell them about air-powered jackhammers, and about when I was a teen working on grandpa's farm, and an old tire we were pumping blew out; I was about 5 feet away, and just the air pressure alone hitting my chest was like being socked in the gut. If a bottle would bust in a way that released shards of plastic as shrapnel, and unprotected eyes were close, it would be bad.
When launching the rockets, I have the kids behind a rope 20 feet away, and they yank the launch rope from that distance. I'm right by the rocket pumping, but I wear safety goggles.
I talk about the importance of stability, which requires mass up front and fins in back, like an arrow. We want the rocket going up, not sideways and hitting someone.
The only rocket I've had burst was one I tried heating it to change the shape for better aerodynamics; that apparently made it brittle.
Update (2011) - make sure that people nearby are paying attention, and with the sun at their backs, so they are not looking into the sun if the rocket comes toward them.
Each child, or pair of children, can make their own rocket. Supplies are one or two 2-liter soda bottles, duct tape, manila folders to cut for fins, and clay, playdoh for weight up front, and markers for decoration.
Here's a quick overview. For a one-bottle rocket, turn the bottle upside down, stick playdoh on the new top, and tape fins near the bottom. For fins I cut and fold manila folders. I make a middle fold, then two folds for tabs for taping the fins to the rocket body. I leave a gap so the fins and rocket body form a hollow triangle when viewed from above. The gap makes the fin more rigid, and the inside of the fin doubles the fin surface area, doubling the effectiveness of the fin.
For a two-bottle rocket, cut off pointed end of one bottle, and duct tape the two bottles together.
You can make your own, or buy. I buy--while making the rockets is easy, making a good safe launcher is not.
I use one from http://www.ez-launch.com, ($50 in Jan 2016). I like it. (The one with 3 stakes is a pre-2011; with a single tent pole is newer.)
There are cheaper launchers, or you can make your own; for instructions google "water rocket launcher". However be sure to get one that
attaches firmly to the ground - I started off using a friend's home-made launcher, which had the nasty habit of sometimes tipping toward the person pulling the launch rope, and
has a launch tube, which is important for safety, to get the rocket going straight at the start. It also helps performance - you get good initial speed before losing any water. It may be hard to get the launch tube the right diameter for soda bottles if you make your own..
I'd like to thank Stephen Kaluzny, who introduced me to water bottle rockets.
The launcher should be firmly attached to the ground; pound in those stakes with a heavy hammer (and bring a pliers to get them out later). Otherwise the rocket could tip over when a kid yanks the launch rope, and head right in the direction of that kid.
I put a rope on the ground 20 feet away from the launcher, to keep other kids a safe distance away.
I attach a 25 foot rope to the firing pin, so that a kid can launch from a safe distance away.
The rope should not stretch! Update 2010 - I just had a safety incident. I used a new rope that was stretchy. It acted like a rubber band, and when a kid yanked the rope, it pulled the firing pin toward the bunch of kids there, hitting another kid in the face. As an extra precaution, keep the other kids 10 feet to the side of the kid yanking the rope. Plus, a stretchy rope makes it hard to yank the firing pin out.
Update 2011 - the new launcher does not have this safety issue. The rope remains attached to the launcher; there is no metal U-shaped firing pin that comes flying out. And it is easier to pull.
Tip the rocket upside down, and pour water into the "firing chamber" (the intact bottle, which is normally at the bottom of the rocket) until 1/3 to 1/2 full of water. There's a trade-off, you want a good balance of power from compressed air, and propulsion mass from water; too little of either is bad. (I've often wondered just what the optimum balance is; any science teachers out there who want to give this problem to your students? Please let me know what you discover.)
Quickly tip the rocket onto the launcher and down into position; position the firing pin. The kid who will launch the rocket (usually the kid who made this rocket) grabs the other end of the rope.
Pump to about 75 psi, and start the countdown--10, 9, 8, ... The kids will be happy to join in. Keep pumping to 80 psi, and stand back as the countdown nears zero.
Yank the firing pin out, and Blastoff!
A few tips:
Fill the pressure chamber (bottom bottle) about 1/3 full.
I pump to 80 psi.
If you are going to do more than a few launches, get a good pump (foot-powered or floor pump), with a pressure gauge.
Keep kids 20 feet away while launching; I use a rope on the ground to keep them from gradually inching forward.
The rope should not stretch. Keep other kids 10 feet away from the kid yanking the rope.
2-bottle system best - longer than one-bottle system, hence more stable. But one-bottle system works fine; over time I do this more.
For weight in the nose, use playdoh or clay on the outside, or kitty litter on the inside (of the front of a two-bottle rocket). Playdoh on the outside is softer than kitty litter on the inside, when it comes down.
Fins should be rigid.
Use duct tape for construction -- no hot material. Some glues will weaken the soda bottles. Avoid scratches.
Be sure that the inside of the soda bottle does not have sand or grit. That can prevent the rocket from taking off, and can damage the launch tube.
If the rocket gets stuck, what probably happened is that sand inside the bottle settled to the bottom of the water. Then when the rocket started to go up, the sand caught between the rocket and the launcher tube. Trying to pull the rocket off will just make things worse. Try depressurizing the rocket (detach the bike pump), free the launcher from the ground, tip launcher+bottle upside down, and try to push the bottle back to the original position. Shake to try to splash water up to wash sand off. Then try to pull the rocket off the launcher. If this fails, you may need to cut the rocket off the launcher. A utility knife and scissors can cut the soft plastic away, then use a hacksaw to cut the hard plastic at the neck of the bottle. This is a pain, but rare.
An occasional bottle has an opening too small to fit on the launch tube. Move on to the next bottle.
I have broken one tent stake for the newer model, by repeated pounding into hard ground. Bring an extra.
The kid should pull the string straight out, not upward (we have broken one newer launcher this way, breaking the tab the string attaches to).
Another useful site is https://ohio4h.org/rocketsaway.