Download Gravity Engine Free Unity.
Gravity Engine provides a complete toolkit for your space game physics.
– add stars, planets, and spaceships that interact via gravitational N-body or “on-rails”
– design orbits by shape and see their paths in the editor and scene
– manual orbit changes with clickable handles
– orbit transfers/rendezvous from any orbit to any orbit
– rocket engines with staging
– model of Earth atmosphere
– solar system builder: add any object in the JPL database
– use real-world units (km/AU, kg)
– full support for particles