In Invasion game mode, after the first round where the Elites take control of the ship, the Spartans will fall back to the garage where the warthog is located. Search inside the buildings for a rocket launcher. Take it outside of the garage where the Warthog is found. Look up in the sky. You should see a Phantom hovering the area. Shoot the Phantom with the rocket launcher. You should see a plasma turret fall down. This turret is usable.