For anyone into serious orbital mechanics but doesn’t want to pay for STK or GMAT’s learning curve: (Windows Astrodynamics Simulation Program, v4) is still out there and usable on 64-bit systems.
WASP IV (Version 4) is a steady-state gas turbine engine performance prediction code. It allows engineers to model compressor maps, turbine characteristics, bleed flows, and off-design performance. The code was never intended as commercial software; it was developed for government and contractor use.