OpenTsiolkovsky is a free , specific usage rocket flight simulator that allow you calcurate rocket trajectry.

The main features include:

• Three-degree-of-freedom, Six-degree-of-freedom flight simulation with attitude control system(like TVC)
• Command line interface software
• NOT fully-featured. It's tool kit
• Now it corresponded three stage rocket without booster
• It corresponds only to the sub-orbital and the Low-earth-orbit
• Including multi purpose python script
• It depends on boost library
• Cross-platform(modern C++ based)

Read more about it on the Wiki

Input file format is following input file format

OpenTsiolkovsky is an Open Source project licensed under the MIT License