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So... this would obviously cost a ton of money initially, but long-term, I believe the savings both environmentally and financially would be returned many times over.

If a building can already be made to reach the clouds (as below), and that is a tall, slim building, then it would theoretically be possible to do the same thing but with a huge, wide building (i.e. an airport). Now, I'm not saying it could reach 30,000ft (or could it?!), but even if an airport could be built (complete with runway) at 10,000ft, the massive savings in takeoff would be returned on the initial outlay.

Aircraft would simply roll off the runway and would already be at cruise, or only have a small, gentle climb to cruise. It would probably look like an aircraft carrier, just... really high.

Passengers, staff and parts could all be transported up and down via elevators (yes, they would need oxygen masks between the airport and the aircraft and the airport would need its own oxygen supply and pressurization).

Is this possible? If yes, why hasn't it been done?

Building into the clouds

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    $\begingroup$ I'm good reopening this. While the proposal is wildly unrealistic (even at 10k, let alone 30k), the reasons why it is are a good discussion that touches at least as much on aviation as on Civil Engineering. You can still have good discussions about how "this" is an unworkable plan; I think this question is better than it's getting credit for. $\endgroup$ – Ralph J Aug 21 at 17:57
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    $\begingroup$ @RalphJ But this stack isnt about civil engineering. Even civil engineering for airports isnt really about aviation. We have a list of topics in the help center which I'm open to changing but perhaps this is a better discussion for a meta post than the comments under a question. $\endgroup$ – Jamiec Aug 22 at 7:54
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    $\begingroup$ In actual fact this question hasn't made it to HNQ, but as has been explained to you in the past @Cloud, an answer can be good (upvoted) even on a poor question (downvoted). $\endgroup$ – Jamiec Aug 22 at 10:09
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    $\begingroup$ @Jamiec My point was that this question is sufficiently about aviation to be worthwhile: why aren't high runways a thing? (Clearly, they aren't - nobody has built them as described, but why aren't the advantages of being immediately at higher altitude persuasive?) Sure, there are C.E. aspects (and Physics, and probably others too) to the question, and some answers, but at heart I see it as being about aviation & why things are "this" way rather than "that" way. JM2C, and I'm good with letting the voting process work. $\endgroup$ – Ralph J Aug 22 at 15:33
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    $\begingroup$ While this isn't a good question, it can be edited to be aviation-related. Just remove the building problem part, and assume it's a given. $\endgroup$ – Therac Aug 23 at 13:50
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The Burj Khalifa in Dubai is currently the tallest building in the world. It is 828m (2717ft) tall and cost about 1.5 billion US dollars to build. Putting all technical and logistical difficulties aside, let us assume one can build a similar structure to 10,000ft, so about a factor 4 taller. Since building cost probably scales with volume, this would make our new building considerably more expensive:

$$ 4^3 \times \$1.5 \, \mathrm{billion} = \$96 \, \mathrm{billion} $$

And that is just one small tip at 10,000ft. A whole runway would require a much larger structure. The runway itself would have to be considerably longer at these altitudes, because the air is thinner. This would easily put the total cost in the trillion dollar range.

How much fuel is actually saved by this? This answer says a Boeing 737-800 requires about 2300kg of fuel for takeoff and climb. Even a very generous assumption of saving 2t of fuel per takeoff from this airport would only save $1200 at current jet fuel prices per takeoff. London Heathrow had about 475000 movements in 2018 (takeoffs and landings). Assuming similar traffic, it would take

$$ \frac{\$1 \, \mathrm{trillion}}{\$1200} \times \frac{2 \, \mathrm{yr}}{475 \, 000} \approx 3500 \, \mathrm{yr} $$

to pay for the investment (and this is a very generous calculation).

Going to 30,000ft would make this even more expensive by at least a factor $ 3^3 = 27 $.

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  • $\begingroup$ Plus certification cost, as aircraft are normally only certified for takeoff up to 8000ft or so, depending on specs, so each aircraft type requires (re-)certification for this single/unique „air“-port... $\endgroup$ – Cpt Reynolds Aug 29 at 21:11
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How long would it take to recoup the billions (tens of billions?) of dollars it would cost to build a tower even as little as 10,000 feet high? Millions of flights, if it saves a thousand dollars per flight -- and I strongly doubt the saving would be anything like that large.

Worse, if you could build to 30,000 feet (and for a tower a couple miles long, I don't doubt it's possible) passengers wouldn't just need oxygen masks to get from terminal to aircraft, they'd need a pressurized boarding ramp. Apron personnel would need full pressure suits, tugs and luggage tractors would need a complete redesign -- and operating costs of the terminals would likely increase by much more than the saving in fuel (not to mention the runway would have to be much longer, as stall speed at 30,000 is significantly higher -- in ground speed -- than near sea level.

Generally, airports are better near sea level, because the higher the altitude of the field, the faster the aircraft have to land, the longer it takes (in time, and moreso in distance) to accelerate to the increased rotation speed. Far from saving money, flying out of, say Denver as opposed to, say Miami costs more -- because the runways have to be longer (and if it's hot in Denver, they need to be longer yet).

tl;dr Being able to lift off slow and land slow are more important that the small saving in climbing through 10,000.

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  • $\begingroup$ Why would you need lift to takeoff? You're 30,00 feet in the air. Simply roll off the runway (in a stall condition) and use a few thousand feet to recover and gain airspeed. Landing would still need to be at high speeds, but takeoffs could be the same as ground takeoffs, speed-wise. $\endgroup$ – Cloud Aug 22 at 14:08
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    $\begingroup$ @Cloud Oh, that's going to go over well with passengers right after they see apron crew working in pressure suits. "But your tickets are almost three dollars cheaper if you fly from Superhigh!" $\endgroup$ – Zeiss Ikon Aug 22 at 14:22
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    $\begingroup$ For that matter, can you even start a high-bypass turbofan at 30,000 feet and -20 C? $\endgroup$ – Zeiss Ikon Aug 22 at 14:23
  • $\begingroup$ If you couldn't, what would be the point in engine restart checklists? (Which are run through at cruise, if that's when an engine fails) $\endgroup$ – Cloud Aug 22 at 17:18
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    $\begingroup$ All passengers would also need to wear pressurised suits with individual oxygen tanks for takeoff and landing, in case there's a need to evacuate during takeoff/landing. $\endgroup$ – Gavin Coates Aug 29 at 11:56
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The potential for savings is not there.

Most of the fuel spent on climbing is not wasted. It's recouped back in the descent, which is performed at very low thrust. An aircraft's combination of kinetic and potential energy is a perfect energy storage system.

The only major losses of energy added by the climb-descent cycle are those from the added friction drag of passing through denser air. These are fairly small compared to the overall flight's fuel consumption.

Starting at 30,000 ft, you still have a takeoff to perform. The airplane will need to accelerate, to a higher velocity than normal, to stay in the air and be able to perform a go-around if necessary. At the end of the trip, the plane still needs to bleed airspeed, although less of it.

Meanwhile, there's lot of reasons not to place an airport at 30,000 ft. High altitude airports are much more dangerous compared to normal ones. High winds, high approach speeds, limited runway length combine to that. Part or all of the money saved in fuel would be compensated for by the losses from the increased crash rate.

To compensate for the increased stall speeds, high-altitude airports need longer runways than usual. The currently second-tallest airport also features the longest public runway in the world. High airports also place restrictions on takeoff mass, which means you'll sometimes have to use heavier aircraft at reduced load (e.g. A330 instead of A321) to meet your range and capacity requirements, which wastes much more fuel than low-altitude climb does.

So even if the height itself cost nothing, you wouldn't want to build your airport as high as possible. For a combination of safety, efficiency and cost reasons, you'd still want it on the ground. The extra energy cost from drag would be more than compensated for by the ability to take off at full weight, meaning that routes can be served by smaller, and often with reduced takeoff thrust.

About the only serious reason to place the airport very high would be for noise abatement.

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