Some innovative people came up with cool variations. If you send pictures or video of yours, I will post here.
Erik Herman, Xraise Cornell
COMPRESSED AIR
Kids have a way of stomping on the end, breaking the bottle. Wouldn't it be great if we could pump up air resivior instead of stomping? My freind and collaborator Erik Herman, who does science outreach at Cornel University, came up with a brilliant mechanism.
From a safety point of view, it would be very difficult to accidentally launch it. The higher the pressure, the harder you have to push to launch it. From an education standpoint, it’s so visible and available for kids to figure out how it works. From an economic standpoint, ingenuity triumphed over money!
Illya Quandt of Nevada
Illya Quandt of Nevada developed an air rocket variation which is an alternative to the short, lashed-on legs that keep the rocket pointed into the sky. He makes legs from plastic pipe that is one size larger than the 1/2" pipe that actually conveys the air from the bottle to the rocket. Here is a picture, and if you want to know more about it, go to the bottom of this page. I have not tried it yet, but if it works for me I might rewrite the directions.
Guy Migneron of Canada
A launcher and rocket ready to fly, by Guy Migneron of Canada. As an alternative to white plumbing pipe Guy used 1/2" PVC electrical "conduit" pipe, which is usully gray and usually used to contain electrical wiring. As you can see from the picture above, an added bonus is that the 90 degree elbow has a more gentle curve. This probably results in less friction as the air rushes through the tube. You find it in the electrical section rather than the plumbing section.
Phil Weglarz
Phil Weglarz sent in a clever rocket variation with a parachute. He says, "Adding an inverted bottle cap to the tip allows the rocket to carry a payload with parachute, which deploys on its own on the way down. I used the standard army guy with chute and glued him onto a piece of cork that fits in the bottle cap. (here's a close-up of our "pilot" on the launch pad) "I have also made parachutes that attach directly to the rocket.(1:03 or so, among other science exhibits).
We had to dumb-down the launchers because the rockets were hitting the gym ceiling so hard that I was afraid they would make holes!.
More about Illya Quandt's Rocket Launcher Leg Design
Here is the picture: And here is a closer view of that picture. Notice the 1/2" pipe actually goes through the 3/4" cross intact. There is no need to block off the air from escaping.And here is just the legs.
And here is Illya's explanation:
This looks like a very smart design--I'd like to try it myself when I have some time.