MegaStructures 02 - Skyhooks

## Transcript

Last time we covered Space Elevators and Orbital Rings, two very nice ways of hauling massive amounts of cargo into space at a fraction of the current cost per pound. Now, both of those were essentially introductory pieces so we could move on to even bigger and cooler things. We wanted to look at how we’d get up in space in the first place to build all this cool stuff and move people from Earth to them. But both had a pair of weaknesses. We haven’t got any materials truly strong enough to build a space elevator and an orbital ring is very expensive to build. So before we move on we’re going to take a look at a couple other types of megastructures that can help get you into space.

The first of those is the Skyhook, something you’ve probably never heard of but in truth are very familiar with. A classic space elevator is a type of Skyhook, called a Synchronous Skyhook. This was before we found ultra-strong materials that made space elevators plausible and it was just one type of skyhook.

A skyhook is a pretty simple concept. A cable that instead of running from the surface of Earth up past geosynchronous orbit instead hangs from a high orbit down to just over atmosphere. Ships then fly up to it, get hooked, and can climb up higher. What’s more, while the bottom of the hook moves relative to the ground, it moves slower than objects in orbit at that altitude normally do, so the ship doesn’t need to fly as fast to hook up.

# K

1
2

m

v
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{\displaystyle K=\frac{1}{2}mv^2}

But how does this work? In a nutshell it works off how rates of orbit vary with altitude. The further you are from Earth, the slower you orbit, both in real speed and in how long your orbits take. To illustrate this let’s take three glowing satellites at different altitudes. One, the purple one, is just over the atmosphere. One, red, many hundreds of miles higher up, and the third, blue, about twice as far up as that. As you can see the purple light orbits faster than the red one which orbits faster than the blue one. For a skyhook its top and bottom cover a very long distance, hundreds or even thousands of miles. As long as your material can support. Since it is occupying all those orbits but is rigid, it will not orbit as quickly as objects at the lower altitude or as slowly as objects in the upper altitude but somewhere in between instead. As a result of this the bottom of the skyhook is moving much slower than normal for that altitude, and the top is moving much faster than normal. Since kinetic energy rises with the square of velocity, and drag forces also grow as you get faster, you save a lot of fuel by hooking up with the bottom of a skyhook.

Now the one I’ve been showing you is called a non-rotating skyhook, and is the visually simplest form but not generally considered the best one. Most skyhooks designs call for skyhook to rotate. Rotating skyhooks spin around their center of mass even as they orbit the Earth, and do so backwards. Normally spaceships launch west to east to gain a little extra speed from the earth’s own rotation. It’s not a lot, but it’s not tiny either, especially when you don’t need to go as fast because you’re linking up to a skyhook. By spinning the skyhook backwards we can play on that trick even more. The tip is moving very fast, a few thousand miles an hour in the opposite direction the thing is orbiting, so it is still zipping around the Earth but much slower compared to even a non-rotating hook, making the speed our launch vehicle needs to hook up even slower.

And that’s the skyhook. Not nearly as cheap as a space elevator or orbital ring in energy cost per pound put in orbit but much cheaper than normal space launch. If you remember the last video when I mentioned the main cost of an orbital ring was getting it up there in the first place, something like a skyhook could cut those cost down to a fraction. But you still need a hypersonic launch vehicle to get to the hook, and while we can build those it’s possible one might want to a skyhook in tandem with a maglev launch system like a Lofstrom Loop or StarTram. Types of ground-based megastructures we’ll be looking at in our next video in the series. Before we look at those however we’ll be jumping back to our other series on Habitable Planets and looking at Earth-like worlds orbiting as the moons of gas giants.

That’s it for today, if you enjoyed the video hit the like button and don’t forget to subscribe to the channel if you want to be alerted for future videos, or click the video links on the screen to watch one of the other series. As always, questions and comments are welcome, as are requests for which megastructures you think I should cover next. Thanks for watching, and have a Great Day!