Upward Bound: Space Towers

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When it comes to dreaming up giant buildings and projects, we often say “The Sky’s the limit”, as we’ll see today, the sky is most certainly not the limit. So today we will be looking at Space Towers, Space Fountains, and Buoyancy-assisted launches. These are always popular concepts, and today we will see they have some advantages, but a lot of their popularity comes from a misconception about space and orbits. We tend to use ‘up in orbit’ as synonymous with being in space, but they are not the same thing. Just flying straight up in the air a couple hundred kilometers is not being in orbit and won’t keep you up there. You could jump up and wave at everyone on the International Space Station but only for an instant before you fell back down, and you’d only see them as a blur as they sped by at about 8 kilometers per second. They are in orbit, and you, unfortunately, are not. So simply building a giant tower up into space doesn’t get you into orbit, and launching from up high, from on top of a mountain or an airplane, does help, but not a lot. That said, it has a lot of other advantages that shouldn’t be ignored. For instance, there is a reason why we build cellphone and radio towers very high, it gives them a far larger coverage area, and up above the clouds you can also use solar power a lot more reliably. Before we get into the notion of active support structures, I thought we’d spend some time on buoyant structures instead. People often wonder about using blimps to lift spaceships up. Now as we have discussed before, it does help to be higher up when launching, but it mostly doesn’t help much. The advantage of the Launch Loop we discussed last episode isn’t that it lets you take off at high altitudes, but rather that it lets you accelerate against something while at high altitudes and speeds. Towing a rocket up on an airplane or blimp then releasing it for launch does have some advantages, so does launching from the top of a mountain, but they’re not usually substantial enough to justify the hassle. Partially that’s because buoyancy is limited, not by the total mass and density of the lifting gas and the object it is lifting, but the environment around it. A helium balloon filled on the Moon will fall to the ground for the same reason a ship can float on the ocean but will fall and smash if you push it off the side of a building. It’s all about relative density, that’s why a steel ship which is mostly full of air can float on water, even though steel is much denser than water, and why even a solid block of steel can float on mercury, the substance, not the planet, since mercury is denser than steel. Now the higher up you go the thinner the atmosphere gets. But it never actually reaches zero. So imagine for the moment if instead of suspending a rocket from a balloon you instead suspend a section of platform. You could have a maglev track down the middle and solar panels and batteries be attached to the sides. If you then lined up another balloon and another platform, and so on in series, until you had a long track, you would essentially have a mass driver, something we discussed earlier in the series. We also discussed, in the mass driver’s episode, how those have big long vacuum tunnels the vehicle races down and how we need the top to exit high in the atmosphere where the air is thin. You could hold up that end with balloons. We’ve gotten those as high as 50 kilometers, and could probably do higher with some modern materials, but they were not holding up any weight. Now a balloon loosely follows the Square-Cube Law, make it twice as wide and it has eight times the lifting force, two-cubed, but only four times the surface area, two-squared. If you didn’t need to make the balloon’s surface material thicker to make it bigger, you could make a balloon quite huge. A normal rubber balloon’s size is based on the external pressure pushing it to contract, plus the force of that rubber pulling itself to contract, minus the internal pressure of your lifting gas, trying to stretch it. We can increase that pressure by heating that gas, or by adding more gas particles, like when you inflate one, but we need it to float so adding more gas isn’t ideal. A rigid hollow structure that is airtight could have no lifting gas in it, be a vacuum, like the tubes from the mass drivers. Indeed they do have a buoyant force lifting them it is just assumed to be a lot smaller than the sheer weight of all those magnets and air tight walls. But if we imagined some very strong airtight material, that was strong and airtight even when only the thickness of aluminum foil, we could build some truly monstrous floating objects, even at high altitudes. Indeed at higher altitudes you don’t need the material as strong, since the air pressure outside pushing on it goes down. A hollow steel sphere a millimeter thick and 100 meters across, empty of all air, would mass in at a quarter of a million kilograms. That’s a volume of just over half a million cubic meters, or 630,000 kilograms of air at normal pressure and temperature. That would give you about 380,000 kilograms of lifting mass. However, even if it were lifting nothing at all, it would stop rising and float in place when air got down from about 1.2 kilograms a cubic meter to about half a kilogram a cubic meter. That would happen at about 8 or 9 kilometers up, which is high but not much higher than mountains, indeed that’s how tall Mount Everest is. If we doubled that sphere, 100 meter radius instead of diameter, but could keep the same thickness of steel on the outside, we’d need 4 times the steel to cover it, since it has 4 times the surface area, and we would mass in around a million kilograms, but the volume went up eight-fold, so it will now stop floating up when the density of air around it reaches about half what the previous one did, about 14 kilometers up. Now as I mentioned, we’ve gotten balloons a lot higher, up to 50 kilometers, that was simply a quick math example of it, but we can imagine materials strong enough to let us build and float vacuum spheres of quite impressive size quite high in the sky, and suspend the end of a mass driver from them or even an entire mass driver that started up in the sky. Also using active support, which we’ll get to in a moment, we can make some very rigid spheres that might be able to float on places like Jupiter or Saturn, where we can’t rely on hydrogen or helium as lifting gas since that is what their atmospheres are made of. Our biggest problem making vacuum balloons is that you have to build rather sturdy, and thus heavy, to avoid the outside atmosphere crushing them like an empty beer can. But as we will see in a moment when we get into active support, we can make some impressively strong and light objects that way which could allow very strong but light containers. Especially circular or spherical ones, as we’ll see with the Orbital Ring next week. But a buoyant object doesn’t need to be a sphere, and indeed people have looked into making buoyant towers. One whose insides or walls contained either a lifting gas or vacuum. Ignoring that wind can cause serious problems for these, you can build some very tall towers. They don’t need to be connected to the ground either, you can have floating buildings, you just need to watch your weight. Floating condominiums probably would not look kindly on you buying a grandfather piano or two-tons steel safes. At the tops of such structures we can launch, but more importantly a vehicle can either run up the side of it picking up speed without burning fuel, or run along a track laid down like a bridge between several towers. However there are some real limitations on this, not least being that while you can go wider and get taller, you really have to worry about the wind, and this just isn’t a great approach for moving lots of mass quickly. When buoyancy isn’t an option, we still have Active Support available. We’ve talked about this before and in detail last episode, but let me review it briefly. In active support, unlike passive support, we are constantly pushing on something. The simple example would be a piece of paper floating in the air above an air vent, or holding something aloft on a stream of water from a fountain. We usually mean something magnetic though, since we can accelerate something electromagnetically, like a small iron ball bearing, and have it slow down or deflect from something magnetic above it without touching it and doing any damage or losing energy to friction. Needless to say this would work way better if you have superconductors, especially cheap mass produced warm temperature superconductors. From a physics and engineering standpoint, this is easy enough, but it can get quite energy expensive, and of course if you shut off the juice, things fall down. Your simplest active support structure, often called a dynamic structure, is basically a levitating pad, a simple form of Space Fountain. Tons of small mass drivers down on the ground firing pellets up at the bottom of some pad which are magnetically deflected down or away, imparting their momentum to the pad and keeping it up. This works better if there is a long vacuum cylinder up the whole way which has no air resistance inside it, and can also keep the pellets on course, but it isn’t absolutely necessary. Without those walls, you can also just raise your pad when you are using it and slow down the matter stream to let it descend gently when not, or just use parachutes and shut it off. We usually call this a space fountain. It also isn’t hard to rig this up to use that stream to generate electricity up on that pad. Same as before, with the floating pads we can lay down a track between them and shoot things down those up to orbital speeds. This isn’t as nice as a Launch Loop because it’s not a neat closed system, and so it uses a lot of energy. We might not care if we invented fusion and we can also make them more neat and closed systems. Beyond just getting into space cheaper, active support is very important for building things that are simply very large. The basis for a lot of the megastructures we’ve discussed and very tall buildings, is something I call an Atlas Pillar. In its simplest form, this is just a contained space fountain acting as a support pillar, and since we usually think of using these on things like Matrioshka Shell worlds, which is basically where you build a series of relatively thin shells around a planet, each essentially the surface of a new planet, each held up by pillars like these, I call them Atlas Pillars in reference to the Titan of Greek Mythology who held up the sky. That image of a single stream of matter holding things up bothers people as flimsy, but it is just a simplification of the concept. It wouldn’t be one long hollow cylinder, it might be composed of hundreds or thousands of individual segments stacked next to and on top of each other, themselves working in tandem with other pillars for lots of redundancy. A warm temperature superconductor could actually form a closed system, accelerating the particles or rotor up and draining that momentum on the way down to power accelerating it back up again. This is a lot like a perpetual motion machine, but those usually supply energy too, this is just 99-100% efficiently recycling it. Of course it would doubtless either need maintenance or have a working life after which it needed replacement. One using normal superconductors will need coolant supplied constantly, and one not using superconductors will be radiating a lot heat lost in the process and need a steady supply of power. Needless to say it’s a good idea to build in batteries and coolant tanks as backups. They can break and be sabotaged, but so can any pillar or support, thus, you build for redundancy. If you want to build a long runway in space like the Lofstrom Loop, but held up by these instead, and you need one pillar able to hold up a hundred tons every 2 kilometers, you build each pillar to hold 200 tons and put them every kilometer. That sort of thing. Since you now have a support structure that can handle ridiculous forces, you can build some very tall buildings, things that dwarf modern skyscrapers, or even go up past the sky so they are scraping the stars not the sky. These Starscrapers are also cheaper to build in one key respect. The most expensive thing about skyscrapers is doing all that construction high up in the air, building from the bottom up. You actually can build active support structures from the top down, you start on the ground, build a level, levitate it up, build the next level, levitate those up, and so on. It's sort of like those new buildings that we can make with large 3D printers, only backwards, the building just rises up as you go, and hypothetically you could keep adding more height to the thing as it grows. I remember a couple months back some architect proposed something called the Analemma Tower, one hanging down from a space elevator to just off the ground. It would be hard to forget, over a dozen people messaged me the article and the mods on the channels Facebook Group had to keep deleting redundant postings. Very popular, beautiful artwork too, but from an engineering perspective it’s terribly flawed. You’re hanging a giant building from the end of a space elevator, tens of thousands of kilometers long, instead of rooting it to the ground a few kilometers below. Not an entirely new concept either, I usually have heard these referred to as chandelier cities. Of course, if you want a moving city you probably do want to go the hanging route, or just make it buoyant, but if you specifically want a city with no visible connection to the ground for some reason, even though it presumably needs connections for transport of people and supplies, you would just use some thin atlas pillars. Remember the rotor for the Lofstrom Loop, the thin metal wire that provided all the support to it, is only 5 centimeters wide, wrist-width, and hollow. You can’t see that unless you are pretty close and we can make things harder to see. Paint it blue or white, or give it a changing color surface to do the whole chameleon effect. You’d want a lot of these and they’d need wider feet to support the ‘legs’, but you could have it ‘walk’ like Baba Yaga’s hut. Floating or walking cites always seems a really popular notion, like humanoid battlemechs, everyone likes them but they’ve got no real practical purpose. But not everything has to, we wouldn’t have art if it did. You could have a building suspended along a space elevator, tethered to the ground or not, but you can have more weight higher up anyway so you might as well build a very long and tall building on the whole length rather than just the end. Of course, you can build these starscrapers arbitrarily high which gives us an alternative to a classic space elevator. A Space Elevator is tens of thousands of kilometers long, with only the last hundred or so kilometers in the atmosphere, but there’s a lot of concerns where wind and sabotage are concerned there, so you might prefer a more robust space tower rising up above the atmosphere which connects to a space elevator at its top. Again though, there are not many limits on how tall an active support structure can be, so you can remove the space elevator and its tensile strength issues entirely from the equation. An Active Support Space Elevator can be of any height, and doesn’t require precision manufacture of super-materials like graphene. It works anywhere, even on Super-Earths with far more gravity than here. You can build them wide enough to run trains up and include multiple redundant active support structures. Once you get over the atmosphere you can get rid of all the weight of making the fountain airtight to make it a vacuum too, and like the space elevator as you get further from Earth gravity weakens too, both of which make higher sections easier to build and support. Another neat application of Space Towers is with Skyhooks. We’ve discussed before how on airless places like the moon a rotating skyhook could swing right down and snatch something off the ground. That doesn’t work on Earth because you’ve got kilometers of air the hooks need to swing down through at hypersonic velocities, burning up huge amounts of energy even if the hook doesn’t disintegrate. At the top of a space tower though, one rising up just above the atmosphere, you could have a cargo pad suitable for direct pickup with either the rotating skyhook or cardiorotovator design we discussed in that episode. We also discussed buoyancy earlier on, and the notion of holding up a launch track using these dynamic structures as support pylons, but we can use active support for buoyancy too. Those vacuum balloons I mentioned earlier can be constructed a lot thinner and lighter by using active support to make the skin around them stronger. We picture active support as pretty big, but recall that the active support element from the Launch Loop was just a wire 5 centimeters wide and hollow, and that’s intended to be strong enough to support a 5 ton ship and stretch thousands of kilometers. You can use that exact same principle, scaled down, say a wire a few millimeters wide and hollow, to craft a series of rings tilted at angles to each other to form a spherical shell for instance, and we will actually discuss using a scaled up version of that concept to create artificial planets briefly next episode. But this allows much larger and safer buoyant structures for Earth and for places like Venus or Jupiter. Safety is a pretty big concern with an active-support structure, since if you turn off the energy the thing falls down. Now in the past we’ve mentioned that you can put explosive bolts on large objects to break it into smaller segments, then let parachutes gently carry them down, and that works here as well, but with a couple notes. First, if you go that route you also need to give everything a lateral shove too, otherwise your segments start smacking into each other and higher segments landing on top of lower ones. Though space towers do not have to go straight up either, you can build at angles just fine. Indeed you might build such structure like the vertices of a pyramid instead of a tower or as large skinny upside-down U. We also often think of these as considerably more massive objects too, not wispy structures like a space elevator or launch loop, these are buildings, so the parachute approach gets a bit trickier. This is why I wanted to discuss the second point, which is that it’s nonsense to regard active support as somehow more fragile than passive support. I’ve already said you’d use multiple redundant atlas pillars in any construct, but I suspect many folks think “Oh, well, someone could still shut the power off for all of those”. And they could, they could also go blow up all the support pillars on a conventional bridge or skyscraper. People get cautious about safety on any new technology, which is a good thing, but there is such a thing as being overly cautious and active support is just as safe as passive support. More so, since it allows stronger structures. As to cost, that’s really hard to estimate. On the one hand you get a huge cost savings on vertical structures since you can build each level low and push it up then work on the next, which makes it cheaper than traditional tall building methods. On the other hand, you do have to provide a constant power flow and quite a lot. If you’ve got cheap superconductors, especially room temperature ones, this isn’t a big deal. Nor if we’ve got very cheap electricity from fusion. Having both would make this approach viable enough I’d expect to see it incorporated into regular architecture. This has nothing to do with launch costs though. It is the nature of an active support structure that you always have access to energy when on the thing, unlike a space elevator where you either need to beam in power or add a lot of weight running thousands of kilometers of wire, usually very wide wire too. So for launching we have no need to carry onboard fuel, the vehicle climbing the tower has all the energy it needs and something to push off, eliminating the need for propellant. This allows very low launch costs compared to most systems we’ve looked at, dollars per kilograms, not thousands or hundreds, but then you’ve got to factor in the cost to keep it up there. Space Towers are not really our most attractive launch system, in terms of active support the Launch Loop from last episode or the Orbital Ring from the next are more attractive, but they can be quite advantageous and are far better, if you’ve got the energy, than most modern options. Though they are useful for defense purposes, where spilling a lot of matter into low orbit, that will then fall down, can be handy. It’s one way to clear space garbage from orbit for instance, since whatever you spray up with a space fountain will fall down as will anything that wanders into it. But their real value lies in how they can be used for construction of very large objects. So while I don’t expect them to play much of a role in getting into space, I actually expect us to see them used a lot more in the future than any other system we have looked at. Active support isn’t absolutely necessary for technologically advanced civilizations but it forms the keystone, in a very literal way, to building a lot of the truly impressive structures of the future that we can only dream of now. We will also see they are quite valuable, though not absolutely necessary, additions to concept of the Orbital Ring, a truly huge launch structure that wraps around an entire planet, and our topic for next week. These structures allows the lifting of massive amounts of cargo into orbit, and at dirt cheap prices, but can also be used to help us go beyond orbit into interplanetary space as well. For alerts when that and other episodes come out, make sure to subscribe to the channel, and if you enjoyed this episode, hit the like button and share it with others. Until Next Time, Thanks for Watching, and Have a Great Week!
Info
Channel: Isaac Arthur
Views: 145,168
Rating: 4.9394455 out of 5
Keywords: space tower, space, space fountain, megastructure, launch assist, space exploration, airship, buoyant structure
Id: 5QLOAQmZbZs
Channel Id: undefined
Length: 24min 1sec (1441 seconds)
Published: Thu Jun 15 2017
Reddit Comments

Vacuum spheres seem like they could be pretty cheap at large scale. A 1 km diameter steel sphere at 1 mm avg thickness would mass about 25,000 tons which costs about 7.5 million dollars at $300/ton. If we assume a dollar per square meter of fabrication cost, the total cost is around 11 million dollars. Not too bad for something with 600,000 tons of lifting power.

👍︎︎ 2 👤︎︎ u/lsparrish 📅︎︎ Jun 16 2017 🗫︎ replies

I'm a bit curious as to how these towers could handle earthquakes.

👍︎︎ 2 👤︎︎ u/Doobly_Baggo 📅︎︎ Jun 17 2017 🗫︎ replies

I'm not sold on the idea of vacuum spheres.

The mass of a regular pressure vessel scales linearly with the volume, so you aren't getting a huge benefit by making it huge. Also, external pressure is much, much harder to deal with than internal pressure, because your failure mode is buckling as opposed to bursting.

A 1mm shell on a huge sphere is going to buckle extremely easily, because a solid dense shell is very weak against buckling forces. For example, if I take a pop can, it takes a lot of pressure to burst it. However, a relatively small decrease in pressure will completely crumple the can.

Maybe I'm missing something, but it does not seem feasible to me. Here is some more theoretical math on using vacuum spheres for lifting. Again, it seems feasible until you factor in buckling forces.

👍︎︎ 2 👤︎︎ u/fjdkf 📅︎︎ Jun 17 2017 🗫︎ replies

Hotter gasses are are cooler, why not thermal gradients in balloons? The gasses conduct heat poorly, until it starts convecting. It probably starts convecting fairly soon? Still, costing some weight, could use barriers/baffles or simulation and little interventions to try suppress them.

This might make larger balloons more effective than the volume/surface area suggests. If P=kρT and T_e external temperature, g the gradient permitted, then ρ=P/kT=P/k(T_e + g(R-r)), mass of the gas m_g=∫4πr² P/k(T_e + g(R-r))dr .. of course less than with uniform temperature..(too tired/lazy for solution) edit: did the math(use substitution).. but it is not particularly enlightening.

And then respectively mass of the surface and internal baffling are proportional to ∝A=4πR² and ∝V=4πR³/3

👍︎︎ 1 👤︎︎ u/Jasper1984 📅︎︎ Jun 16 2017 🗫︎ replies
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