I quite like these. They're pretty well put together, with a nice flow between the sections, for the most part. My biggest grievance is the green. There's just a bit too much of it. If it was a less saturated shade of green it might be better. You might also want to try using the DST-swnc sprites to add details and greebles between the large, smooth green sections, or on top of them. Just a suggestion.
![Smile :)](./images/smilies/icon_smile.gif)
Anyway, keep doing what you're doing, practicing, and you'll improve. You're already pretty solid.
Edit: With those engine glows, if you put a smaller, white glow over the top of them, with additive blending, it'll look much for engine-glowy.