The city - a modern, sterile 'utopia' (with the undercurrents of government control and suppression, of course), that can be current tech, or future tech so long as the day-to-day lives are in current tech. Lots of surveillance, propoganda, etc., but people seem to go about happy for the most part. A typical trope. Most buildings are ground-plus-two (G+2) stories maximum, except a few civic and institutional buildings are up to G+5. Isolated population of about a million "happy" denizens.

The Trophy - The city in my story is commemorating an event. My design is a sturdy rectangle measuring 150m x 150m and is about 10m thick (I can be flexible on this). I don't want to have any protrusions off of it, and I want it to appear to be solid black stone. The problem is that it is also is about 350m above the city.

It can float around a bit, but I don't want it to have a big air bag or anything like that and I want it to be silent to the people on the ground.

Now I'm at the point in the story where it needs to be explained with a minimum of hand-waving. Can you explain how my urban trophy hover around for years above a city?

EDIT: It should always appear to be a floating stone for decades or centuries; no-one must know it is a mechanism (e.g. - nobody can go near it once it's aloft).

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    $\begingroup$ I don't think this is Idea Generation, which is what the close voters seem to be getting at. The word "explain" is often a trigger word for a question like that, but here, it is simply a matter of phrasing. Changing the question to "How can my urban trophy float" preserves the intent but better communicates that this is not Idea Generation. $\endgroup$ – HDE 226868 Aug 18 '15 at 18:33
  • $\begingroup$ Since the citizens cannot see the top, does it matter what is above the Trophy? $\endgroup$ – user11426 Aug 18 '15 at 18:58
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    $\begingroup$ I don't understand the close votes. $\endgroup$ – Mikey Aug 18 '15 at 19:04
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    $\begingroup$ @HDE226868 I have adjusted the question phrasing accordingly. $\endgroup$ – Mikey Aug 18 '15 at 19:05
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    $\begingroup$ @HDE226868 I agree that this isn't idea generation, which is why I voted to close as too broad. I felt there wasn't enough information for there to be a most-valid answer. I didn't have the time when I voted to explain this; my apologies. $\endgroup$ – Frostfyre Aug 18 '15 at 20:57

Make it a big air bag... with a thin black stone veneer.

It's like an exoskeleton for your zeppelin. It will need to also have a lightweight internal structure, the stone veneer wouldn't be enough to keep it from buckling. Since no one can see the top of it you can keep the station keeping propellers there. By placing them on the top, and only needing to ramp them up when it's windy, the sound should be difficult to locate (as well as hear at all) when it's windy. Also, why not, paint them black too. The motors will need to be electric and powered via solar panels on the top of the device.

Stone veneer is about 6.10 kg / m2, so covering the bottom and sides would weight about 164,782 kilograms. A gas bag of 150m x 150m x 10m filled with hydrogen will lift 270,450 kilograms. So you have more than 100,000 kg left over for an internal structure, the shell for holding in the gas, and maneuvering equipment. If you find that is not enough, you can switch to flexible stone veneer for a lighter weight stone appearance.

If this must be maintenance free then some special equipment must be installed. Any rain caught by the Trophy will need to be run through electrolysis in order to regenerate any lost hydrogen. The rate of replenishment depends on the effusion through the shell of the Trophy, since you're designing this, you can decide what rate is appropriate. The whole Trophy being black should help by heating it, adding to its lift.

Such electronic control systems are rather simple and easy to add redundantly. It wouldn't be impossible to built such systems to last for decades. It is, at least, not infeasible to imagine such a thing in a science fiction setting.

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  • $\begingroup$ I sincerely apologize, I added one caveat (as shown). I hope it doesn't affect your answer - I know it's very rude. +1 for the helpful math. $\endgroup$ – Mikey Aug 18 '15 at 19:20
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    $\begingroup$ @Mikey Ok, I've added a bit to account for being maintenance free. $\endgroup$ – Samuel Aug 18 '15 at 19:31
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    $\begingroup$ Wouldn't it just float away? Not knocking your answer, it seems like adding placeholding engines would break the question's requirements and make this nearly impossible. $\endgroup$ – Dan Smolinske Aug 18 '15 at 19:47
  • $\begingroup$ @DanSmolinske The OP said "something discreet can be placed on top, but CANNOT be seen". Hovering shouldn't be too difficult. $\endgroup$ – Samuel Aug 18 '15 at 19:49
  • $\begingroup$ This doesn't seem feasible. Solar panels at peak sunlight produce about 200W/m2. We only get about 8-10 hours of sunlight (not at peak), so call that a generous average of 100W/m2. Electrolysis of water takes 237-280kJ/mol. That's about 250kJ of energy for 22L of Hydrogen. The entire solar panel output would be able to replace .09% of your Hydrogen bag per second. And that doesn't leave any extra power for anything else. Not to mention snow covering the panels or hail breaking them. What about a weeks worth of cloudy days? $\endgroup$ – Martin_xs6 Aug 18 '15 at 20:41

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