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