# What would be the practical considerations for a floating city always at day?

Suppose this world suddenly has night creatures that only appeared at night and disappear at day. With the world's civilization at risk of total destruction by randomly appearing night creatures, they build a floating city using the local unobtanium that allows it to float and they have it fly counter to the Earth's rotation so that it is always in the day light.

What would be their considerations in designing this city such as self sufficiency, weight, aerodynamics, and dealing with constant sunlight?

Assume they have modern or near future technology with the exception of floating unobtanium. Night creatures disappear and appear out of thin air but only at night rather than just shadows.

• A better idea would be to have two cities at either pole and then a seasonal migration via high-speed aircraft every half-year. – Hugh Mar 1 '15 at 3:50
• If you flew to the pole and then flew up you might be able to get constant daylight that way. I've posted a question about that to confirm: worldbuilding.stackexchange.com/questions/11141/… – Tim B Mar 1 '15 at 14:49
• Turns out you would need to be too high, so that doesn't work :( – Tim B Mar 1 '15 at 16:15
• Kim Stanley Robinson had people living on Mercury, and to stay in the habitable zone they were on a huge train that moved with the sunrise. Presumably the rails were made of unobtanium. – RedSonja Jan 29 '16 at 9:55
• This reminds me of Minecraft, somehow. – beppe9000 Dec 21 '16 at 21:51

This is not going to work.

You have to fly very fast to stay in the daylight. Over 1,000 miles per hour (1600 km per hour) at the equator. A city is anything but aerodynamic, actually it's about as aerodynamic as a brick, and will fly approximately as well. With modern tech and real life physics, you're simply not going to get one flying at over mach one for any sustained period. Further north or south, before getting past the arctic circle where the sun will set no matter how fast you go, the city would have to travel half as fast. Let's explore that.

Powering flight

Unfortunately, the city would have significant difficulty just to create enough power to maintain that speed. Drag increases by velocity squared. The power to overcome drag, at high velocity, increases by velocity cubed. It's quite difficult to find any data on the cross-sectional area of a city, but I think a square kilometer is a very conservative guess.

$$P_d = F_d * v = {1\over2}\rho v^3AC_d$$

To fly at 800 km per hour (222 m/s), the city needs to provide 14 gigawatts in effective propulsion$^1$. That's two very large nuclear power plants running all the time if it's perfectly efficient and there is never a head wind.

Of course, the city would be ripped apart by the winds before it made it a day. If anyone reading this thinks 'golly, it would be windy' and then thinks using wind turbines to harvest the energy is pretty clever, please slap yourself. That's like generating power from a wind turbine with fans that you're powering.

Alternative

A much better idea would be to spend six months near the north pole for its summer and six months in the south pole for its summer. Then all you need to worry about is the flight between them every six months. Unfortunately, since the city still can't fly fast enough there will be some night time to deal with.

If the unobtanium can provide variable lift and you are able to make the city airtight, you can essentially create a city space station that dives into the atmosphere occasionally to refresh air and water. But this doesn't appear to be the solution you're looking for.

Sustaining the people

As for sustaining the city, just make the floating platform large enough to grow food. Basically, if the city brings enough land with it, it can support itself. It'll still be in the weather, so it'll have rain and shine, and can even float over to better weather. There should be lots of recycling.

Weight

The weight is magically cancelled by the unobtanium.

$1$: I assumed an air density of 1.225 kg/m^3 and a drag coefficient of a brick, 2.1 the same as a rectangular box. If you want the cross section of the city to be something other than a rectangular box, refer to drag coefficients. Multiply 6.66 GW by the coefficient for the shape the city will present to the wind (still assuming a 1000m^2 cross sectional area).

• "The ships hung in the sky in much the same way that bricks don't" – apaul Mar 1 '15 at 2:47
• Ah, I should have worked that HHGTTG quote in there. – Samuel Mar 1 '15 at 2:49
• About the drag bit, what if the city were shaped (more?)longer and slender and the buildings designed more compactly, although i suppose at that point it would look more like a very large airship than a city – Ryan P Mar 1 '15 at 3:07
• Please do not assume that people will build a huge floating city with unobtainium and nuclear powered jets and then shape it like a brick. Given the power required for flight a level of aerodynamics similar to a car would be more reasonable. Nobody will spend that much resources on flying structure without trying to reduce the drag. Also (not your fault, I know) if it floats with unobtainium there is no reasonable way to assume altitude and air density. But there is no reason to assume an altitude low enough to collide with the ground... – Ville Niemi Mar 1 '15 at 10:40
• @VilleNiemi Ha, well living that high means you have to seal the city and supply oxygen. You're arguing for a spaceship now, which is clearly a better option than a floating city, but I think outside what the OP wanted. – Samuel Mar 1 '15 at 17:31

The answer will vary based on latitude since the earth's rotational velocity is a function a distance from the axis of rotation, e.g., 1000 mph / 1600 kph at the equator, 0 at the poles. Since the equator is the worst case, I will use that. Since mach number is temperature dependent, mach 1.3 is just an approximate reference value for your flying city.

Also, not clear from your question whether you will be the only population on earth, so I will assume you are not as this adds some issues to consider.

Elevation? High altitude means lower air pressure (reduced drag, etc.) but means you have to pressurize your city. If you needed to supply a pressure of 1/2 atmosphere, you have a static load over 1000 lbs / sq ft or about 5000 kg / sq meter. Not very high pressure for a tank, but building a pressure vessel the size of a city will have challenges. Extra complications if you want to live in a clear dome. But overall, higher is probably better in most respects. Radiation is not once of those. High elevations means you will encounter a lot more radiation. Anything over 10,000 meters is too high over a long term unless you add radiation shielding.

Air turbulence. You will have to completely seal your city from the environment. At Mach 1.3, everything your city would be ripped to shreds by air turbulence (ripping up trees, buildings, etc.) if not encased. You need a strong container to resist not only air forces, but also deal with things like running into a flock a geese. They are low grade canonballs under your conditions.

Drag? Does your unobtanium magically hold in in place relative to the sun, or you do have to supply the power needed to propel the city against the wind. Since you are flying a city at greater than mach 1, you will need power and lots of it (billions of watts, maybe trillions) to counteract drag. Since you exceed mach 1, there is not much you can really do about it, i.e., streamling only gets you so far because you are compressing the atmosphere to punch through it at high speed.

Cooling your city. Remember that due to air compression, you are going to encounter high temperatures from the compressed air. You also have to get rid of the heat from generating all the power to counteract drag. So you need to run the biggest air conditioner in the world 24x7. You cannot use the ambient air to cool your city since it will heat up by around 100 C when you attempt to use it this way.

Noisy neighbor, you will be the noisiest neighbor in existence. The sonic boom that you generate will be huge, and you will overfly each point on your flight path once per day. Nobody is going to like you.

Economic logistics. Unless their is also a different kind of unobtanium, everything you do will cost more as the difficulty of moving everything to and from your city will be quite difficult. In fact, no jet on the planet could safely land on your city today unless you build an entirely unique airfield that brings air to rest relative to your city and still be an open air field. Perhaps if your city is large enough, your could design a leeward hanger deck that is open air large enough support the necessary air traffic. Remember that your service craft will also have to have mach flight -- unlike any existing cargo planes.

Equipment maintenance. You will have to power your city's flight with the equivalent of fighter jet engines and a lot of them. Maintenace on such systems is very high, lots of downtime, inspection, replacement parts., etc. So you have to have lots of extra capacity, the ability to redo the parts from scratch (i.e., smelting, etc.) and trained personal pulling maintenance.

Self-sufficiency If you are the total population you will of course have to be self sufficient. A lot like a colony on mars, except you have a few advantages, mostly freely available air and water (i.e. collect the rain, assuming you are not too high or condense it out of the air). However you will presumably need to replenish supplies for energy, etc. so the logistics statements already mentioned still apply though they would be less frequently needed

Construction. Building all of this while under attack from your night creatures won't be a picnic either. Under the most optimistic assumptions, this city would be incredibly complicated, expensive and time consuming to construct initially. Much worse than going to the moon.

24 hour sunlight would be way down on the list of your problems. Hang some curtains.

Honestly the tech to fly a city through the sky would probably be sufficient to simply put a city at a Lagrange point between the sun and the earth, or maybe even better, put a giant array of mirrors in space to reflect sun at the city perpetually. You would have he issue that the surface would never get a chance to cool down though. Mercury has sections that are pretty much in constant sunlight, and they can hit ~800F (430C). It would be less on the earth due to us bing farther away and having an atmosphere, but I doubt it would be comfortable.

• Maybe the amount of light needed to prevent the night creatures to form is much lower than daylight. Then only relatively little light would need to be redirected to the night side of earth. I guess the light of the full moon doesn't suffice, but maybe ten times as much is already sufficient? – celtschk Aug 23 '15 at 9:46

Have a fence so people don't fall off.

I'd have it hover over a sizeable ocean or just build it to float on this ocean. Are the night creatures able to swim or use tools?

Building mammoth walls instead? Depends on the abilities of these creatures

• I would assume that Ryan already thought of this. Can you also discuss some of the other considerations mentioned (self sufficiency, weight, etc.)? – HDE 226868 Mar 20 '15 at 0:12

Pressurize your city and go all the way up to space. The L1 point is a stable orbit around the Sun, between the Sun and the Earth, where their gravities cancel each other out (beacause it's much closer to Earth). The Sun would always be "above" as far as we can say that Earth is "below". Once you're there, the energy to maintain position is just for fine-tuning.

Your issues are now those of a very large space station. Plenty of material to look up. Rotate to generate artificial gravity, hydroponics, extreme recycling, etc.

You might not even need the unobtanium after all, just lots and lots of resources. Today's technology could be enough. But I don't think today's resources, even if used up just for that, could lift up the mass to build a self-sustaining city of millions. More like a couple thousand. Except, perhaps, if that handy unobtanium you're not using anymore could lift stuff up to space on the cheap?