Here's the relevant section of an image I produced recently. I'm using Ray Dream Studio 5.5, but I think the cloud functions are still the same. The exhaust is an elongated cloud sphere with the following settings: Color: (dark gray) Container: Sphere Quantity of Clouds: 1.50 Cloud Size: 0.80 Density: 0.20 Edge Falloff Size: 50% Quality: 300% The jets I used were F86 Sabrejets (the jet preferred by the Japanese for fighting giant monsters due to their ability to carry a massive payload of harmless, inaccurate bottlerockets) The F86 was a greasy, smokey beast so you might lighten up on the gray for your sleek modern jet. That's a great model, by the way, and the streaking on the surface adds a lot to the image.