OpenRocket

penRocket is a program that you can use to design and simulate a rocket before you actually build it. The program doesn't just give you the ability to build your rocket from scratch, which many people are sure to enjoy, but will also simulate your results with a virtual launch and give you the details on what it thinks would happen. OpenRocket has a very specific audience in mind - an audience with unconditional love for rockets, and a lot - a whole lot - of patience.