Air power!

Because every physics equation must be about POWER! (not really Rob. . .) (actually the pressure force acts on the rocket over a fixed distance of the launch tube and so does a fixed amount of work -enough to accelerate the rocket to about 40 m/s (140 kph). It's quite tricky to see this using the wmv file here, but in the original avi file, you can measure the distance travelled between consecutive frames at 30fps and calculate the speed...)

and another clip from further off so that the rocket is in at least 3 consecutive frames (it's fast!)...

and here's the modified version we made next