OpenRocket - an Open Source model rocket simulator
OpenRocket is a free, fully featured model rocket simulator that allows you to design and simulate your rockets before actually building and flying them.
The main features include:
* Six-degree-of-freedom flight simulation
* Automatic design optimization
* Realtime simulated altitude, velocity and acceleration display
* Staging and clustering support
* Cross-platform (Java-based)
Read more and downloads:
http://openrocket.sourceforge.net