# What is the feasibility of a space port?

Let me explain what I mean by space port. I'm playing with the idea that mankind has finally built the space elevator. After the first space elevator was built, the other space elevators were far more easily created.

My idea was that the airport still exists, but the idea was extended to be a spaceport. By that I mean that a space elevator is installed at the airports, and, as an alternative means of transport, one can climb into a sort of terminal floating in space. From there, you board a specifally designed plane to mostly to take you to another terminal floating above another airport (aka spaceport) somewhere else in the world.

• You're in space, so there is little to no air friction. It means you can accelerate and require very little fuel to sustain yourself.
• The planes are maintained and, aside from extraordinary circumstances, are left in orbit above the planet earth, requiring little additional maintenance costs.
• Solar energy can be more quickly gathered, reducing the amount of actual fuel even further.
• You can conceivably go very fast and still have it be economically viable because, again, you're in space.

• Space debris. Though I think I can explain this away that probes were launched in the year 2085 which have slowly, but steadily matched the orbits of space debris and sent them to earth to burn up, ultimately causing near elimination of space debris by the year 2096.
• Climbing to the height might be time-consuming in of itself.

Is there anything I'm not considering? It is clearly not economically viable today, but assuming space elevators exist, and assuming technology improves, could this ever be eventually economically viable? Would there be other possible changes in the culture resulting from the existence of such a form of travel that I'm not considering?

Again, consider that this isn't supposed to replace planes, as it would almost certainly be cheaper to fly by plane. People would be willing to pay more for a potentially faster, turbulent-free, ride where they can see the stars out their window and the earth down below. By economically viable, I mean that the cost of a ticket traveling from spaceport to spaceport couldn't cost much more than twice the cost of a first class ticket for the same distance.

Thanks!

• The geostationary orbit is 35,786 km above sea level. This is more than the distance between any two points on Earth. (And you need to go twice the distance, one time up and one time down.) Sep 17 '18 at 21:05
• +1 — idea is clear, every needed detail is there, there are no doubts what you are asking. Sep 18 '18 at 15:50

The space elevator will never replace regional airports, if it replaces any at all

You've already pointed out that the transit time up an elevator and down another will add time. You're also taking a longer arc since you're so much further away from the center of the sphere. Your total transit distance is therefore much, much greater than just taking a traditional plane. Your space planes are limited in their speed due to the acceleration limits for people (remember, your passengers aren't astronauts in perfect health but senior citizens, children, overweight folks, people with heart conditions, etc.) Consequently, the time will regularly be greater than just taking a regular plane.

To make matters worse, you've taken airports and turned them into space elevator hubs. That's unsafe. Weather conditions move planes around (I've been in planes that had to land rotated left to face into the wind as they landed). My point is, safety boards would never allow the combination because the risk of a plane hitting the space elevator are simply too high. They'd be built where all air traffic could be diverted well around them. AKA, a long way away from any airport.

And to wrap up, even if your longest international flights could be done faster with the space elevator, your local flights would never be supplanted. Today there are 17,678 commercial (commercial! not including private) airports in the world. There would never be a reason to have so many space elevators. Thus, there will be planes — a lot of them.

# It costs too much to in space

### Elevator energy costs

How much energy does it take to travel up the space elevator? Specific energy to 350 km altitude is about 3 MJ per kg, just to overcome gravitational potential energy. Assume an electric motor with 100% (!) efficiency; the current cost of electricity is about 100 USD per MWh, so you are looking at 300 USD per kilogram for a one hour journey to space.

At that price, going to space is obviously not worthwhile. The tyranny of gravitational potential energy cannot be changed, and we are already assuming a 100% efficient electric motor, so the only thing that can be improved is the price of electricity. The price would have to drop to somewhere in the 1 USD per MWh range in order for a 100 kg person + luggage trip to space to only cost $100; that could be competitive with air flight. And that only considers the cost of energy to transfer someone to orbit. Consider also the energy required to move the elevator car, capital costs for the space elevator, desire for profit, etc. Also, if energy costs are really that low, how much cheaper will airline flights be? ### Launch from a space elevator is possibly costly Without knowing the specifics of where each space elevator is located, this is hard to say, launching from a space elevator into an orbit that takes you to another space elevator could be very costly in terms of the rocket burn necessary. The delta-V budget of setting yourself into the correct LEO orbit to reach another space elevator could by up to 7 km/s. That kind of burn in space currently requires a chemical rocket; even if you only needed a 1 km/s transfer orbit due to the orientation of the stations, you would still need a rocket burn. Sure rockets might get cheaper in the future, but again, how much cheaper would airplanes also have gotten in the meanwhile? • You might have skipped a couple decimal places in your math there, 1MWh = 3600MJ not 1 MJ, so it'd be$100 per tonne to LEO. Numbers look ok, just the wrong order of magnitude. Sep 17 '18 at 22:47

A space elevator is currently seen as extending from geosynchronous orbit to a point on the equator. Other orbits are possible, but don't have a fixed corresponding position on Earth. Therefore, the ports in question would all have to be on the equator, which makes them not all that useful even if the idea works.

• Not entirely correct. You could "just exit" the elevator at another level. The problem is that you'd be way too slow for the low orbit and you'd either go down ballistically or enter an elliptical orbit, depending on how high your exit point is. You wouldn't gain anything from that, unless you accelerate your spacecraft. Sep 18 '18 at 12:47
• @Infrisios That looks like a good comment for another answer. Sep 18 '18 at 17:09
• Yeah maybe, but then I'd feel forced to do the maths for orbital plane rotations and the likes and I'm too lazy to do that right now. Kingledion already approached those issues closely enough. Sep 19 '18 at 7:23

Space elevators for ground-to-ground transport: Sensible

Space elevators for ground-to-rocket-to-ground transport, Less likely.

In theory the energy costs of actually going up the space elevator can be completely ignored. First remember that in space solar energy is VERY abundant, to the point that you simply won't have energy scarcity after inventing the space elevator, second remember that in theory all the energy you spend going up the elevator can be re-claimed going back down (though in practice it'll probably be closer to 60% or so, maybe 90% if you've got really good magnets.). Space debris itself shouldn't be a concern, if you are building space elevators then you have a significant space industry, that will clean up space debris for you.

I also don't think that JBH's concern about planes colliding with space elevators is too problematic. The space elevator would probably be quite far away from the actual run-way (air-ports being very big remember) possibly even on a separate site a mile or so down the road. The space elevators would likely be very well lit near the surface of the earth to prevent this and it's likely that air-craft control technology will improve in the future.

Such a system has other benefits too, it cuts down on the amount of run-way you need, which allows for more premium real-estate in the middle of major cities, It reduces noise pollution (and regular pollution if your planes aren't beam or battery powered, which is still a very real possibility.)

The first problem you have is that your p̶l̶a̶n̶e̶ rocket is going to have to be-rocket powered (ion thrusters taking too long and no other reaction mass being availible in space, possibly you could just fire it out of a mass driver in the space port, but people tend to ask for a refund when you say "then we fire you out of the cannon"). Rocket power is dangerous, inefficient and very costly, and you need to slow down when you reach the other space port.

It's also not going to be much faster than a normal plane if you go all the way into geostationary orbit, go to a different space elevator then go all the way back down and also be comfortable for the majority of passengers. in order to actually make the most of the vacuum of space you need to be accelerating a lot, more than anyone wants to have to endure. For example getting to geo-stationary orbit at 1-G of acceleration (which would feel like 2-G to the passengers at first, due to earths gravity. That's already a bit much for many to be handling for prolonged peroids, and likely more than you want to do). Would take over an hour (once you factor in slowing back down.) Going a little bit of the way up and then going back down is more feasible (since it would avoid having a really long orbital period, which would take ages to get anywhere with, and could put you in the elliptical orbit needed to intercept the other spaceport), but it's still the sort of thing you'd only do to go from Mexico-city to Tokyo. When you factor in the time spent in orbit (which could vary wildly depending on how much accelerate, where the other space elevator is etc...). In short while you probably COULD shave a few hours off flight times (perhaps even a dozen on some of the longest flights.) you'd have to spend a lot more on sick bags if you did.

While using space elevators for point to point travel through a plane would be inadvisable using the tethers of a space elevator or orbital ring as a sort of rail-way is a very similar concept and one that is much more feasible.

Essentially the idea is that instead of a single tether going to earth your space elevator splits into multiple smaller tethers near earth, this also adds a bit of useful redundancy. Someone can then simply take a high-tech cable car to the splitting point and then go back down any tether they choose. You would need to split your space elevators like this anyway if you want them tethered in Air-ports literally anywhere but the equator, which is useful if you want to go anywhere other than the eleven countries on the equator.

Orbital rings work even better for this since they by nature cover many points on the earth, they are also (theoretically) much easier to construct that space elevators, especially away from the equator. I would definitely recommend isaac arthurs "upwards bound" series if your interested in this sort of thing.https://www.youtube.com/watch?v=dc8_AuzeYKE&list=PLIIOUpOge0LsGJI_vni4xvfBQTuryTwlU&index=2

The biggest problem with this is the speed at which the elevator must orbit at the altitude at which you're talking.

Orbital speed is synchronized with the rotation of the Earth at a distance of just under 36,000km. The closer you are the the Earth the faster you have to go to keep from hitting the ground. The ISS must orbit at 7660m/s at its altitude of 400km, which is way above the Earth's rotational speed of 460m/s To remain above the same spot on the Earth, assuming the elevator is actually in orbit:

1) a large enough counter-balancing mass must extend quite some distance beyond 36,000km, and

2) the elevator must be orbiting directly above the Equator.

Number 1 creates quite an engineering problem aside from that of return-on-investment.

Number 2 necessitates that you would not be able to have a space elevator at the locations you wish. There doesn't seem to be a list of equatorial airports, but looking at Google Maps I see that there are some cities and large towns on or close to the Equator in many countries. Other than Quito and Macapa, most of them are not travel hubs and some you wouldn't even want to visit.

That's the reality check.

Having said all that, the space elevator is also an idea with which I am enamoured and have tried to think of ways in which one might make it work -- although certainly not in the near future!

When I was a kid some science writers and/or illustrators envisioned a revolving hub with three long cables. The end of the cable would be traveling 460m/s, the rotational speed of the Earth, and would appear to descend from the sky, pause for a brief moment above the ground, then return to space.

That's cool, but there are other problems. For one, you need to counterbalance the hub of the rotating elevator against the weight of the cargo. A movable counter-balance may be able to account for that. There will be drag against the line as it is entering and leaving the atmosphere, and you would have one spoke entering the atmosphere as another was leaving. Unfortunately the only thing you can do about that is to add energy to the spin, and there is going to be some deformation of the spokes.

You could calculate the orbital inclination and rotational speed of the three-spoke elevator so that it "walks" across the face of the Earth, lifting passengers from one location and depositing them elsewhere, but it would be far from timely or convenient. Each hub could have a spoke touching down once every 8 hours, but each individual spoke would only visit once every 24 hours. Because it's moving at a different rate as the Earth is spinning, however, it would take several days, probably, before the elevator would return to a specific spot. You would have to have multiple elevators in the same orbit.

A rotating elevator only rotates at 15 degrees per hour, which may or may not be a problem depending on the ultimate design.

Energy is still a really big problem. Keeping this mass rotating and in a precise orbit is going to be far from free.

In my experience, the best way to deal with something like this in a story or RPG adventure is simply not to deal with it. Once you start explaining it your readers/players will be unable to suspend disbelief once they realize that it simply won't work or is impractical.

Just hand-wave. Or better yet, just make it a part of your world's everyday culture: "Baldasaro knew that the ferry could get him to the space station in only four hours, but he couldn't use traceable credits and paying the ferry fee in cash strips would attract too much attention. He decided to purchase a one-way travel pass at a nondescript bodegita and take the 12-hour space elevator trip, hoping he wouldn't stick out among the commuters and budget vacation travelers. He knew the one-way pass would stick out, but it was the best way to stretch his cash strips, and by the time they discovered the purchase he would be three days out."

I hope you find a way to make it work. Space elevators are always cool. Good luck.