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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.

enter image description here

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.

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    $\begingroup$ Three of the engines on the picture you posted do in fact emit steam as a combustion product. Of course, it is too hot to be easily recognized as such, but it forms a significant component of that smoke cloud. $\endgroup$ – Mike L. Sep 9 '15 at 2:23
  • $\begingroup$ Which three engines are you referring to? I would assume you mean those of the shuttle, but I was under the impression those did not fire during takeoff to conserve fuel. $\endgroup$ – Thomas Myron Sep 9 '15 at 4:10
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    $\begingroup$ The main engines (i.e. the three on the shuttle itself) are running at 100% of rated power at launch, throttling up to 104.5% after liftoff. $\endgroup$ – Steve Bird Sep 9 '15 at 5:39
  • $\begingroup$ It is as @SteveBird says. Having those engines and not using them at liftoff would be a humongous waste. $\endgroup$ – Mike L. Sep 9 '15 at 5:47
  • $\begingroup$ @Thomas Myron: In fact, IIRC it's just the other way around: the strap-on solid boosters are used because the main engines don't have enough thrust to lift the shuttle with a full fuel tank. $\endgroup$ – jamesqf Jan 22 '17 at 3:43
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There are few plausible ways to form clouds of steam in a VTOL, mostly because of the amount of energy that needs to be generated by the engine(s) in order to take off and land. The exhaust will be quite energetic, and the steam (if any) will condense quite some time and distance away from the launch pad.

There are a very few work arounds that will provide a similar effect.

Firstly, you could have the municipal pads cooled and protected by a stream of water that constantly passes over them (or is flowing under the pad but is accessible via a flame trench). The hot exhaust will boil some of the water and it will flash into steam, but being cooler and less energetic than the actual exhaust, will tend to condense much closer to the scene of the action. While this is good for the pad, most designers will probably object since the steam will cause damage and heighten the possibility of corrosion both on the VTOL craft itself and metal structures surrounding the launch pad (including reinforcing beams in the pad itself). Taking off and landing in a cloud of steam can also cause problems for human pilots, and even microwave and laser landing aids could be "blinded" at a critical moment.

The other way to produce steam would be the cooling system of the vehicle. This would require an open cycle evaporative cooling system where the coolant is allowed to circulate and is then vented to eject the excess heat from the vehicle. This is problematic since the weight and centre of gravity of the vehicle will be changing constantly, as well as providing a very hard limit to the amount of time you can have power (once you are out of coolant, you will have to shut down or throttle back to idle to preserve the engines). Once again, the clouds of steam being ejected from an evaporative cooling system will have effects on the craft, the pad and nearby structures, as well as flight safety. IF you are not using distilled water, there are also environmental effects as well (boiling glycol is not something you want to introduce to the local environment, and undistilled water will clog the engine cooling system with lime and metal scale deposits, reducing the efficiency over time).

If you want to impress the locals, have take offs and landings trigger a light show or similar display around the pad. The people will think that is exciting (for a while) and the aircraft and local structures will not have to deal with the dangers of steam, corrosion or limited visibility.

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  • $\begingroup$ Thanks for the answer, it was very clear, if a bit disappointing :). Hmm... what I really need is a cloud of something that obscures the craft as it takes off, and will not harm anyone standing in it. I'm going for the dramatic effect here (though that was not the intent of the designers), and I figured steam would be nice. Isn't there some way the craft could take off like a Harrier (e.g. primarily using air) and supplement that take off with suppressed water turned to steam? Or alternatively, could a multiple-rotor helicopter whip up enough standing water to create such a cloud? $\endgroup$ – Thomas Myron Sep 10 '15 at 22:56
  • $\begingroup$ Harriers use water injection for extra power, and the steam is not visible in the engine exhaust. If you want to obscure the take off and landing, the downwash can whip up large clouds of dust or sand in the desert, but once again, pilots are going to be very reluctant to fly in that sort of condition. $\endgroup$ – Thucydides Sep 11 '15 at 15:41
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Since a rocket engine that burns hydrogen and oxygen together to produce H₂O doesn’t produce enough steam for your purposes, you want anything water-cooled. It’s implausible that steam power would produce enough energy to justify carrying the weight of that much water, but maybe the launch itself generates enough heat that they keep a big pool of water around the first stage to dissipate it, and once the ship is launched, either it needs much less energy to maneuver or it’s in space, so it can just radiate all the heat it wants.

A specific example might be a mass driver powered by something that generates a lot of heat, so they built the launch tube under a lake.

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  • $\begingroup$ Won't work. See the picture in the question? See the distinct lack of steam plumes from the Shuttle's main engines? The huge clouds of steam are from the SRBs boiling off the water being used to protect the launch pad. The exhaust from the Shuttle engines disperses before it cools enough to form steam clouds. $\endgroup$ – Mark Sep 9 '15 at 2:35
  • $\begingroup$ Edited with another suggestion. $\endgroup$ – Davislor Sep 9 '15 at 2:45
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    $\begingroup$ "See the distinct lack of steam plumes" Since steam is an invisible gas, you won't be able to see it. What you can see are the clouds of water vapor that appear as the steam cools. Which may happen at some distance from the engine nozzle. $\endgroup$ – Steve Bird Sep 9 '15 at 5:41
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Easy. Power with steam.

Imagine what happens inside a steam engine which has stopped its motion for a limited time. The machinery cools off, and steam condenses inside the machinery. A significant part of its working volume is now full of liquid water instead of steam, which at the very least changes the compression ratio, and at worst could cause a hydraulic lock, since water is incompressible.

The procedure, then, is to open blowdown valves just prior to starting motion. The water is blown out using dry steam, in an impressive cloud. If blowdown is done by a simple automatic mechanism, it has no conceivable idea how much liquid water is present, so it's going to blow for a worst-case scenario.

The hot, dry steam may also re-boil some of the water.

Now, temperature and steam pressure are proportional. It's bad to reduce steam pressure to zero on every cycle, because that also means a significant thermal shock for the machine. So there's logic to keeping the machine mostly pressurized at idle. Water's boiling point is greatly raised at pressure, so that liquid water we're ejecting is at 150-400 degrees C. When it escapes to atmosphere, it will spectacularly flash to steam in a classic BLEVE, adding a great deal more steam cloud.

Dry steam doesn't cause any visible smoke (in fact boilermen wave a broomstick ahead of them as they traverse boiler spaces, so they can detect jets of leaking dry steam capable of cutting the broomstick in half). But saturated steam (no hotter than boiling point), upon mixing with cool air, will rapidly condense to water -- and that creates the visible cloud.

Won't be any trouble justifying a big steam cloud every time the engine starts.

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