OpenRocket is an Open Source model rocket simulator, can used in designing and simulating rockets before actually building and flying them.
OpenRocket features a full six-degree-of-freedom simulation, realistic wind modeling, a multitude of different components including free-form fins and canted fins, clustering and staging.

OpenRocket Features:
General:
1. Fully cross-platform, written in Java
2. Fully documented simulation methods

User interface:
1. Easy-to-use user interface for rocket design
2. Zoomable schematic view of rocket from the side or rear
3. Rocket rotation around center axis
4. Real-time view of CG and CP position
5. Real-time flight altitude, velocity and acceleration information from a continuous simulation performed in the background

Design:
1. A multitude of available components to choose from
2. Trapezoidal, elliptical and free-form fins supported
3. Support for canted fins (roll stabilization)
4. Staging and clustering support
5. Automatic calculation of component mass and CG based on shape and density
6. Ability to override mass and CG of components or stages separately
7. Simulation and analysis
8. Full six degree of freedom simulation
9. Rocket stability computed using extended Barrowman method
10. Realistic wind modeling
11. Analysis of the effect of separate components on the stability, drag and roll characteristics of the rocket
12. Fully configurable plotting, with various preset configurations
13. Simulation listeners allowing custom-made code to interact with the rocket during flight simulation

Requirements:
* Java

The license of this software is Free, you can free download and free use this calculator software.