I saw an article about vacuum powered ping pong ball canons and thought it might be fun to replace the ping pong ball with a rocket. In a vacuum canon a ping pong ball is placed in the end of a PVC pipe, the ends are sealed with foil and the air is evacuated with a pump. The end closest to the ball is opened and atmospheric pressure accelerates the ball out the other end. With good technique the ball can hit a bit below mach 1. I thought with a rocket I might be able to break mach 1. I haven't seen this done with a rocket and I'm not sure how NAR would feel about it, but it could be fun.