Hanging a 2000 kilometer long railroad in the sky, suspended 80 kilometers in the air,
might seem rather fanciful, but as we will see today, it turns out to be a pretty cheap
and practical option for space travel.
Whenever we see a rocket launch, it is going straight upwards, which often seems a bit
confusing since the velocity needed to orbit the Earth is side-to-side, not upward.
Of course rockets do need to get up a ways, so that they don't crash into a mountain
or anything.
After rockets rise a bit and clear most of the atmosphere they fall over on their side
in a gravity turn and accelerate laterally instead.
On places like the Moon, where there's virtually no atmosphere, rockets don't need to take
off straight up, they can go straight down a runway.
With no air to generate lift they will stick to that runway until their speed reaches orbital
velocities.
That is after all what orbital velocity is, the speed necessary to move sideways faster
than gravity pulls you down, so the curvature of the planet, the horizon, slopes away as
fast as you fall down toward it.
Now it would be nice to have a runway on Earth up above the atmosphere so you could do the
same, but this still has limited advantages.
The nice thing about a runway is that it gives you something to push off, instead of tossing
matter out the back of your spaceship to do your pushing.
You still need energy to do that pushing, so a very good runway would provide you that
energy too.
The conceptually simplest approach is an electric motor spinning wheels that are running over
the top of a long heavy gauge wire that is conducting electricity.
The vehicle has a metal brush on the bottom that brushes along that wire permitting the
electricity to flow from it to the electric motor.
Such a magical runway, up above the atmosphere, would allow you to drive an electric
car up to orbital speeds and right into orbit.
Now our topic for today, the launch loop, is a little more sophisticated than
this, but the basic concept is the same... suspend a runway in the upper atmosphere that
provides a spaceship something to push against and the energy to do that pushing, so that
it doesn't need onboard fuel.
Not very complicated conceptually, but this runway does need to be thousands
of kilometers long and a hundred kilometers above the ground, something which is more
easily said than done.
So is transferring power to that vehicle, or pushing against that runway, when the vehicle
is moving thousands of meters a second.
You can't exactly rely on rubber wheels or metal brushes at those speeds, the friction
would vaporize them in an instant.
We get around the contact issue by avoiding it, and use magnetic levitation,
just as with MagLev Bullet Trains.
But our solution to height is something called Active Support, and it's going to form the
basis for today's launch system as well as the ones we'll be discussing in the next
two episodes of this series.
In this respect, they essentially form a connected trilogy and will finish off the main portion
of this series.
I won't say close it out, because we'll probably come back, but these three episodes
cover the big ticket items that form the basis for a truly high-capacity launch system.
We will be looking at the simplest forms, ones that we could build now for fairly
reasonable prices and use for relatively normal launches, and also the scaled up versions
capable of lifting megatons of freight and passengers into space which include all three
systems working in tandem as a hybrid form.
Now every episode in this series is supposed to be essentially self-contained,
so I will discuss active support in each, but this episode will feature the more detailed
explanation and I recommend watching them in the following order: Launch Loops, then
Space Towers, then Orbital Rings.
Active support is our method for overcoming the limits nature imposes on compressive or
tensile strength.
We discussed those earlier in the series, but the short form is that while it is possible
to build giant pylons to hold a runway or railroad track aloft a hundred kilometers,
it's not particularly practical.
And we definitely do want that runway or railroad track.
Fundamentally, every system we've discussed has either involved making rockets more powerful
or cheaper, or getting around the rocket equation by letting us supply energy to a ship, and
provide something for it to push off or with, without having to carry that energy and propellant
along.
Let's start by reminding ourselves what passive support is.
Most structures are passively supported by their various load-bearing members.
Weight rests on them and compresses them, and so long as you don't put too much weight
on them, all is well.
How much is too much depends on the material and its compressive strength.
You can also go the reverse way and pull on things, placing them under tension, like suspending
yourself from a rope.
Too much weight on that rope, too much tension, and it will break.
How much is too much depends on the material and its tensile strength.
Very long or tall things have to carry not only the weight of whatever you want to hold
up, or hang up, but also their own weight pushing on lower levels or pulling on higher
ones.
As we discussed in the Space Elevator episode, we can increase this distance by either lowering
gravity, like building on the Moon or Mars, or by tapering the load-bearing element, making
it wider at the bottom for compressive strength or wider at the top for tensile.
No matter what though, you begin to hit practical limits.
These materials are relying on the binding forces either holding molecules together or
keeping them apart.
We often discuss super materials capable of stretching thousands of kilometers into the
air, like carbon nanotubes, but even then it would require pushing those boundaries
quite a bit to allow us to make a space elevator, let alone what we want, which is more extreme.
Not just holding a smaller tower or spaceship up, but holding up an entire city, or even
hanging a planet or black hole over a city.
Active Support does let us do such things.
So what is Active Support?
Exactly what it sounds like, you are actively pushing upwards on something to keep it from
falling.
What keeps a rocket from falling down when it's a hundred meters above the pad?
A big stream of superheated propellant shooting out the back, countering gravity's pull.
We have as many ways to provide active support as we have ways of pushing on things, but
the key trick is doing it in as close-looped a way as possible.
Short of a perpetual motion machine you can't close it completely, and you will need to
add energy, which seems to be a turnoff for many folks, but keep in mind that every structure
requires energy and resources to maintain.
Incidentally that closed-loop part is why today's structure is called a launch 'loop'.
Let's begin with the simplest active support system we can make.
A fan or heating vent pointed upward with a sheet of paper floating on it.
That is active support.
Shut off the air and the paper falls.
You could also do this with a firehose, aimed upward to push on the bottom of a plastic
plate.
If the stream of water is perfectly aimed and the water pressure maintained, it can
hold the plate aloft indefinitely.
If we felt like making that plate out of bullet-proof armor, you could actively support it by having
a squad of soldiers firing machine guns at it from below.
Basically this is how the Space Towers work, which we will discuss in the next episode.
We often call these dynamic structures, in part because we can actually change their
height.
The launch loop is one such dynamic structure, which takes a slightly different approach
to active support.
Simply put, we turn on the support and a giant runway lifts up from the ocean.
There are actually a few types of Launch Loops but the best known version, which itself has
a few variations, is the one we usually call the Lofstrom Loop, proposed by Keith Lofstrom
way back in 1981.
He has improved upon the design and explanation since then and I will include his 2009 write
up on it in the episode description.
He's quite detailed but keeps to pretty simple language and it explains it well, so
I will use his example of the concept.
Imagine a stream of water from a hose pointed at an angle into the sky.
The Stream forms a parabolic arc, each particle is rising and falling on a ballistic trajectory,
forming a parabola controlled by the speed and angle the water comes out at.
Much as an artillery shell's ballistic track can exceed the height of any building we've
ever made, this stream of water, if fast enough, could form a big arc kilometers high, and
since we are constantly adding more water this arch will remain.
You could keep things up at the top of it by letting water bounce off the bottom of
them.
Of course anything you had up there would move in the direction the water was going,
but we could make a second stream of water running backwards, forming a second arch pushing
the opposite way, and we could put a plate on top of both so it didn't drift either
way.
Now if we had a big basin at either end, we could catch the water falling from one stream
and pump it back up in the other stream.
Now you are probably thinking of a few problems with this concept.
First the water isn't going to stay in a nice neat stream and land in that basin, second
that plate might not drift forward or back but it will probably spin, and third, it takes
a lot of energy to shoot water.
We can fix these though, and we won't be using water for the loop anyway, but imagine
we wrapped a thin hose around the stream to keep the water in, and now that plate up top
is resting on the two hoses and so not spinning.
We could also make the water go in a circuit, conserving power.
You still need to supply power, it's not a perpetual motion machine, but we can save
a lot of energy that way.
Also, we can slowly turn the pressure down, then off, and lower the hoses to the ground
when not in use.
That's saves a lot of power too, and illustrates another reason why we call it a dynamic structure,
since we can change its shape as well as adjust the elevation.
You might be wondering how this qualifies as a flat runway or track, since it is clearly
a parabola, not a straight line.
But keep in mind that a track a couple thousand kilometers long and staying at the same altitude
above Earth is not a straight line either, anymore than Earth is flat.
If we are firing artillery shells or water streams in parabolas, by shooting them faster
and faster, you eventually reach a speed where it turns into a circle instead, since it is
actually now in orbit.
This is exactly how early, pre-rocketry designs for spaceflight worked, build a cannon so
big that it no longer shooting parabolic trajectories, since the range is so large you can no longer
treat the Earth as flat.
That's the concept, and we would make very big hoses and lay down a platform over them
with a way to transfer power to the spaceship we are launching, an electrified track for
instance.
Now the actual Lofstrom Loop doesn't use water, though it is typically over an ocean
since that's a nice safe empty place to build one.
Instead it uses a stream of iron running down a vacuum tube.
This can be a series of connected metal balls or links, like a big bike chain, of magnetically
sensitive material.
However Lofstrom's loop is simply one long hollow wire, or hollow iron cylinder, called
the rotor.
It is only 5 centimeters wide, and with the walls .25 centimeters thick, and presumably
several thousand kilometers long.
That is very flexible by the way, it is not a rigid iron rod, just a metal wire that's
rather thick and hollow.
You will use whatever the engineers decide is most practical, a big particle accelerator
works too, basically though it's a stream of matter running around inside a vacuum tube,
never touching the sides, pushed away by magnetics, so there is no friction.
It just keeps looping around, though you doubtless lose some energy, and with superconducting
magnets you would lose even less.
Lofstrom's loop is 2000 kilometers long and 80 kilometers high, it also masses several
kilograms per meter giving the whole loop a mass of several thousand tons, the rotor
alone comes in at about 16,000.
Which is actually quite light for something as long as an interstate highway.
Needless to say you can scale it up but it's big enough for a 5000 kilogram vehicle in
this design.
Lofstrom works through virtually every conceivable detail and scenario in his paper, which again
is attached in the episode description, down to the power requirements and individual components
for this size of loop, so I'll stick with that for specifics.
Again it can be scaled up, and I'd assume loosely linearly, double launch mass, double
loop mass, and power and financial costs.
More on those in a bit.
So why 2000 kilometers long and 80 kilometers high?
For the former, that's just how long you need to accelerate at 3 gees to reach orbital
velocity, same as the mass drivers we discussed before and you can make it longer if you want
to accelerate slower or make it shorter if you can handle more acceleration.
Like the mass driver this can be combined with a sky hook to let you go shorter on the
track.
Why 80 kilometers?
There is a good reason but it can be lower or higher.
So low in fact you could use this as a bridge between islands.
However, 80 kilometers is high enough to be over most of the atmosphere and avoid serious
drag effects on the vehicle, but still low enough that many meteors will be vaporized
before hitting the Loop, and probably more importantly there are no stable orbits for
space trash to follow and collide with your Loop.
There's not much air there to drag on our spaceship, but it's still enough to cause
a fast decay for anything orbiting at that height.
The launch vehicle itself rides on top of magnets and is just like a maglev bullet train,
but with no air to interfere, or at least very little.
It takes momentum from the rotor, which needs to be replaced, but needs no on board fuel.
This would be a space plane design, and carry only what fuel it needed for maneuvering in
orbit or going higher, though a longer loop could launch you into higher orbits.
You would land conventionally, using aerobraking like the shuttle did.
It is possible to land on such a loop too, where aerobraking isn't an option, but one
has no reason to build such a system where that's the case.
Unlike a lot of our other systems, the Launch Loop is mostly only useful for Earth.
It serves no purpose on the Moon, where there is no atmosphere to be above, and you would
only be elevating your track to avoid local geography... crater walls, hills, and other
possible obstructions.
That low gravity and low altitude allows normal supports to lift your track.
The launch loop is only necessary where you have an atmosphere.
One place it would be very handy is Venus.
You can for instance float a mass driver in Venus's very thick atmosphere, but not only
do you still need a wide thick vacuum tube, not a small one just big enough for the 5
centimeter wide rotor, but you also can only float stuff using buoyancy as high as your
lifting gas permits.
Here on Earth we've gotten balloons 50 kilometers high but that is not 80 kilometers and they
were nothing but balloon.
We weren't hanging heavy masses on them either.
However with a Launch Loop on Venus you can put the lower part of the structure down where
even normal air is a good lifting gas and let the active support lift the higher portions
up to where the atmosphere is too thin for buoyancy alone.
You can use this trick with gas giants too, but it requires some extra effort, they
tend to be quite thin and composed of hydrogen and helium, our two preferred lifting gases,
so you have to use things like vacuum balloons, which we'll discuss next episode, or more
active support structures like the Orbital Ring.
So how safe is the loop?
More or less the same safety factor as a mass driver, but that rotor, the big long metal
wire running around the loop, is carrying a ton of energy.
Actually its carrying about a megaton of energy, in terms of explosive yield.
So essentially there's a decent sized nuke's worth of juice running around that loop, but
even if the thing blew apart, that energy is 80 kilometers up and spread over 2000 kilometers,
and it not radioactive.
For that matter the rotor is traveling at orbital velocities, so a lot of it would fly
up into orbit or even beyond, since the default speed of the rotor exceeds Earth's escape
velocity.
The track itself would just fall, and we could
use explosives to break it into chunks and let parachutes bring it down.
Having the rotor slice its way out of the conduit is going to severely damage it too,
so breaking it up with explosive bolts isn't a big deal since you will be needing to replace
lots of conduit anyway.
Also, it's a good idea to design it to float on water as well.
That's for a critical failure, you can also lower it by just shutting off the
power, it will actually take quite some time to descend that way.
It's usually assumed to run on a few hundred megawatts, replacing lost power, and it takes
days to raise or lower that way, though you can add more power to raise it faster.
You normally wouldn't build these near cities though, and oceans are considered
preferable to land.
This lets you put it down near the equator and it can float on the surface while you're
doing repairs or replacing a segment.
A scaled up version could include a lot more redundancies, additional loops and so on,
but it would also have more energy in it so you might hesitate to put it near a city as
well.
But these can be any length or angle you want, so you could actually run it between two cities
and have a very fast bullet train between them.
They also normally have tethers coming off the sides to help stabilize it like guy
wires.
A very scaled up version of this could actually use those tethers as entry ramps from other
places, if you were building over land.
These are the sorts of things you would need to plan for, and Lofstrom goes into just
about every safety factor and concern I can think of in his paper so I will refer you
to that, again linked in the episode description.
There are quite a few though, it is a bit of engineering nightmare with only our current
technology, but it does appear doable.
As to cost, here's the fun bit.
Again Lofstrom was very detailed with this, down to tables for individual parts, and he
gives an estimate including research costs to build the loop of just 2 billion dollars,
and an average launch cost of $3 per kilogram.
It actually beats the space elevator.
He also only uses components and technology we already have, and again we don't need
mass produced carbon nanotubes like for the space elevator.
All of which sounds excellent but it does have some problems.
First off, being that long and relatively flimsy, you do have to worry about weather
a lot more, especially down at the loops and inclines running up.
It also runs very hot, though that issue is eliminated if you have superconductors in
play, and the price to run it would drop a bit too.
You are stealing your launch energy from the rotor and have to put that back, and if you
want to be running constant launches you need two big power plants at either side of it.
Personally, I love the Launch Loop, especially when combined with skyhooks so
that you can lower the track length and energies involved.
Fundamentally it is a lot like the mass driver but you get to skip on the big expensive vacuum
tunnel as wide as your ship in favor of one about as wide as your wrist and avoid the
issue of trying to hold the tunnel high up in the air.
Of course holding much heavier objects like that tunnel stationary at high altitudes would
be handy too, and we will be examining those methods in our next episode, "Space Towers."
We will look at using Active Support for that as well and also take some time to discuss
buoyancy options like balloons or even towers made of buoyant segments.
After that we will finally move onto the Orbital Ring and explain a lot of the hybrid options
we can use with that to create the kind of massive orbital infrastructure a civilization
needs if it wants to be moving millions of people and millions of tons of cargo back
and forth from Earth to orbit on a daily basis.
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!
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