Turns out, all that soda wasn't just to keep me happily carbonated. Instead, it was sprung on me that I would be responsible for constructing some kind of launcher for soda bottle water rockets. Now it turns out that there are tons of web sites and plans for the rockets and the launchers, but mostly these sites are mostly by people with way too much time on their hands, who have created extensive launch systems and constant redesigns of their rockets. Having neither time, nor much hardware at my disposal, I instead needed to think more like MacGyver. What I needed to build was the world's simplest two liter bottle water rocket launcher.
So at camp, the first thing I did was finish drinking a 2L bottle of soda, and then go looking around to see what parts I had at my disposal. Luckily the camp has a fair collection of random lumber, electrical and plumbing supplies; so after passing over boxes of PVC (hey, I'm here to weld, remember?), I collected up an assortment of various kinds of metal pipe. There was some electrical conduit that was small enough, but joining it to anything was going to be a pain. There was some copper pipe, but I suck at brazing, and I didn't see any fittings. Finally I found some black pipe (which wasn't black anymore), that it just happened to fit inside the 2L bottle (very snuggly in fact). There were various lengths, with threads on each end, and a couple of elbows. I also found an odd collection of adaptors that actually hooked up an air-compressor fitting to a 2" pipe. After taking most of the adaptors apart, I had a perfectly good air fitting to half inch pipe adaptor for the other end of my pipe.
What you see is what you get for the most part. The pipe sticks up into the bottle (hopefully a little above the water line), the pipe goes down to the elbow, to another short section of pipe (which was chosen only because it was it already attached to the elbow), which then connects to the adaptor where the air fitting is screwed in. The air compressor hose is attached to this.
Did it work? Could you have any doubts? Of course it did. We'd shove the bottles on there, hook up the air hose (to a el-cheapo compressor putting out about 40lbs of pressure while leaking air through several holes, bent lines, and rotted seals), the rocket would fill up with pressure and then zip it was gone. This kept thirty kids busy for close to an hour; and threw most of these rockets up in the air over 100'. Interesting, a few times, the rocket would stay stuck on the launch pad, but just the slightest tap to the side of the rocket would be enough to send it off. We tried some smaller water bottles that had about the same size neck, but they were like 0.3mm larger and so would not stick. However we did get a fair altitude by having the kids hold onto the neck of the bottle while pressurizing (do not try this at home unless you are as crazy as these kids were).
Oh, and if you study the larger picture close enough, you will notice a flat metal plate at the base of the pipe where it sets on the stand. That plate is there because I had lots of scrap metal with me, a drill, a cutting torch, and a welder. (I was here to weld, remember?) Otherwise it is probably completely superfluous, though it made a nice stop for the bottles, and actually helped provide a pretty good seal plate for starters. If you have a welder, go ahead and put one on. Otherwise, I wouldn't worry about it.