I thought this little mechanism would be fun for my kids as well as the Cub Scouts I work with. I thought I'd give my kids a shot at it first.
So here is a 3 liter soda bottle. A 2 liter bottle would probably work better. It is attached to some PVC pipe (1/2 inch size) with tape (you can choose your favorite duct tape brand/color).
I'd say this length of PVC is about four feet, give or take.
The PVC is connected to a 90 degree angle piece of gray PVC which you have to go to the Electrical aisle in the hardware store to find. The 90 degree piece doesn't have wide ends, so we had to get two pieces of the connecter pieces, also gray. The 90 degree piece is also 1/2 inch size. We haven't glued anything, the pieces are snug enough, which is good, that way I can dismantle the thing for storage.
Going up from the 90 degree gray PVC piece, is a short length of 1/2 inch white PVC pipe that goes into a white PVC cross piece. A longer length of 1/2 inch white PVC pipe continues up from the cross piece. On the sides of the cross piece are two gray 45 degree angle pieces. The ends that go into the cross pieces are plugged with some red pieces that say "5x" on them, which you can't see here and I can't undo it, so you'll just have to take my word for it. I don't know what that 5x means, but it was either that or kid's modeling clay, and since my husband knows more about this stuff than I do, I yielded to his better judgment. Anyway, coming out of the gray angle pieces are two more short lengths (about two feet each) of 1/2 inch white PVC pipe. We angled those pipes away from the vertical pieces to make kind of a tripod that would keep it steady.
If you measured the short vertical piece of PVC and the taller vertical piece of PVC, you'd get about four feet, I think. It's not exactly rocket science here. Oh, wait a minute, I guess it is. (sheepish grin)
So, for the "rockets" we pried apart the staples in a magazine, pulled out a two page section and rolled it up. I placed it over the PVC to check that it could slide on and off easily but still fit well on the PVC pipe.
I cut up a plastic grocery bag and stuffed it into one end of the paper tube and taped it down. I've seen some of these rockets with paper fins taped to the open end but we were going to launch these inside so we didn't bother. If we were to do this outside the fins might actually help stabilize the rocket's "flight".
With the rocket on the launcher, we are now ready for a test.
Crap. The video is not going to work. Oh well. Make one yourself and take it outside and let your kids have at it.