Here's the setup. You have a gigantic mildly futuristic city, where construction is vertical rather than horizontal. Need a new city block? Add another few floors. Disregard for the moment any physical problems this may present.
To move throughout such a city, we've constructed public transportation VTOLs - capable of going from the ground level to the topmost floors in a reasonably short amount of time. Aerial buses.
I would like these VTOLs to eject steam when they take off. And I mean a lot of steam. Picture what it looks like when a Rocket ship takes off. Smoke explodes outwards, covering everything. I want my VTOL to do that, though on a smaller scale obviously. It should eject enough steam to cover the surrounding area, and the steam should be relatively thick.
What kind of engine could achieve such an effect? I don't care how the steam is formed (by-product, actually steam powered, etc.), it just needs to be ejected quickly and copiously.
Note: My first thought was of a nuclear engine, since I've heard they form steam as a byproduct. This presents the problem of shielding however, not to mention what might happen in the case of a mid-air collision.