[steve86]

Yep, but nine-cylinder radials still aren’t turboprops or jets.

[PSYCHO-FREEP]

Those vapor reactions are coming off the tip of the props, not from the engines and are caused by tips of the props moving faster than the speed of sound. It is a common phenomenon that when aircraft pull high “G” maneuvers, they also generate a vapor trail off the wing vortices's for the same reason; Due to air compression in both circumstances.

Take high moisture content cold air at high altitude, super heat it and then dump it back into subzero air at high velocity, and you have what you claim is a rocket contrail. Only this time, it is purely and simply a jet airliner cruising from West to East. It's very normal and boring physics.