Megastructures 03 - Launch Loops, Mass Drivers, and Space Fountains

## Transcript

Today we’ll be finishing out our look at Megastructures designed to get people into space by looking at a few more concepts. Our main focus will be on the Lofstrom Loop and Star Tram but we will also have some miscellaneous systems near the end of the episode like the Space Fountain. After that we will be moving on to talk about rotating habitats.

Where the first few episodes dealt with megastructures for getting you off a planet, the next few will be looking at where people would live if not on planets. We won’t really be looking at technology dealing with planets in this series after today, except in terms of constructing completely artificial planets. We will look at more planet based technologies in the upcoming standalone video on terraforming, and we’ll continue to look at such things throughout the series on Habitable Planets. These vary so much from planet type to planet type though that it’s easier to look at them there individually. Our next video in that series on Rogue Planets in the Interstellar Void, will be coming after that Terraforming video and serve as a lead in to another standalone video on Interstellar Colonization. Both of those two videos, Terraforming and Interstellar Colonization, will be summary videos of concepts and technology and so will be on the longer side like the original Megastructures video was or the video on Comprehensive Solutions to the Fermi Paradox. If you want a reminder when those videos will be coming out, subscribe to the Channel by clicking on the logo in the bottom right corner, and you may want to turn on the closed captions for the video while you’re down there. All right, let’s get started.

The object that’s been drifting around the background during the introduction is a Lofstrom Loop. It is one of the more popular Launch Loop concepts. It is essentially a long runway or railroad track hovering 50 miles or 80 kilometers over the ocean, about 1,250 miles or 2000 kilometers long. That’s high enough up to get all the air out of the way causing drag on our accelerating ship but still low enough to enjoy Earth’s protection from space debris. You would usually situate one of these things on the equator though you don’t have to, and those lines running off at angles are tethers acting as guy wires to help stabilize it. What keeps it afloat is essentially the same concept as the orbital ring we discussed in episode 1, though it’s a bit trickier since it isn’t a big circle in actual orbit.

Down on the ground, connected to some power plants, are loops where matter is accelerated very quickly and hurled up the shaft to the track, running along the track down to the next shaft, which descends into the loop at the other end, which turns it around and throws it back a twin track running in parallel. These form the track the ship runs along like a maglev train, but which it can just keep accelerating on the whole way way because there’s not much air in the way to cause drag. The matter is running down a rotor, with a stator sheath around it, and that can be superconducting or regular magnets. The design predates superconductors. The matter running down the rotor can either be one long continuous ribbon of material, like a bike chain or wire, or it can be a stream of ball bearings. The rotor itself is an evacuated vacuum tunnel. Normally the sheath would be too.

Same concept as an orbital ring but shorter and you can build it on the ground and when you power up it slowly floats up. If a chunk breaks the rest of the apparatus falls down, slowed by parachutes. These land on the ocean and float there until you have a new section installed and turn it back on. Conceptually easy, though it is something of an engineering nightmare to build and keep stable. I’ve seen price tags as low as thirty billion dollars for this, but that’s probably being terribly optimistic. Like the orbital ring though, or sky hooks, there’s no super-science involved here. The science is solid, the engineering is a little less so, but it should be doable.

There are a lot of valid engineering problems with these, though I’ve heard a lot of silly ones too. Like how the loop contains as much energy as a nuclear bomb… which is true… but the nature of such a catastrophic failure would be nothing like a nuclear blast, even ignoring that a nuclear blast spread over a few hundred thousand square miles of empty ocean, minus all the radiation, would not even cause as much damage as a mild thunderstorm.

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There are some other launch loop designs but Lofstrom’s is the best known and they all basically parallel the design in concept. One could make a Launch Loop shorter in two ways. The first is to have a slower final speed, which could be done if ship flying off was getting hooked by a Skyhook at the end of the track. The thing about distance for accelerating objects is that the length of your track, for constant acceleration starting from being stationary, is that it rises with the square of the velocity. So if you want to end going twice as fast your loop needs to be four times longer. Three times as fast, three squared, nine times longer, ten times as fast, ten squared, a hundred times longer. So could use one of these with a skyhook to cut it down to a quarter as long, maybe shorter

The other way you can get a shorter track is with a higher rate of acceleration. The Lofstrom loop calls for a 3-g acceleration, which is uncomfortable for people to endure but not too bad and they only feel it for a few minutes. You could scale that acceleration down to 1-g, normal Earth gravity, but then your track has to be longer. A track length for the same final velocity scales inverse to acceleration. Double your acceleration, half the distance of track length, triple it, a third the distance, a hundred times the acceleration, a hundredth of the distance… of course if you did that, the people inside would be feeling 300 gees of force, which is more than enough to kill you. Anything much over 10 gees is a bad idea even for a short time, for people. Bullets routinely undergo tens of thousands of gees in the barrel of a gun but bullets have no hearts, lungs, brains, etc. Even fairly sophisticated electronics can be built to withstand high-gees, which is why we can get away with putting specially designed electronic fuses on artillery shells.

So very short, high acceleration tracks are possible but only for dumb matter or specially designed equipment and we tend to refer to these as Mass Drivers, though you will often hear them called a rail gun, coil gun, electromagnetic catapult, space gun, and so on. There’s no rigorous definition for these that excludes lower acceleration versions safe for people to be in, but we also don’t call a maglev bullet train a mass driver and it’s the same concept.

Star Tram, our next space launcher, is often called a mass driver, especially the generation one version which is for cargo-only. The Generation One Star Tram design slams a cargo pod down an 80 mile long track at a modest 30 gees. It starts off inside the atmosphere and has the ship fly down a tunnel from which the air is evacuated. It exits the tunnel which ends on a mountain peak and which air is kept out by a plasma window.

Plasma Windows, which are basically like a force field on the end of the launch tube to keep the air out, are a pretty neat if new and energy intensive concept. You can look them up, but for our purposes they are a problem only in that they need a lot of energy and the energy rises with the diameter. Double the diameter, double the power needed. One about ten meters or a bit over 30 feet in diameter would need a constant power supply of about 8 megawatts which isn’t horrendous but is nothing to sneeze at either.

Big difference, and advantage, over the Loftsrom loop, is that it runs at local ground level up the side of a mountain rather than starting off above the atmosphere. It is simply an evacuated tube you can run down without air slowing you down. This often called a Vacuum Train, or VacTrain, and your acceleration can be made lower by making the track longer. VacTrains have long been discussed as a way of doing hypersonic travel at low energy costs, and the HyperLoop design Elon Musk has proposed is a partial VacTrain, though it actually runs on an air cushion like an Air Hockey table instead of via MagLev. Star Tram’s Generation 2 Design is basically a tilted Lofstrom Loop, in that’s it very long and cargo runs at only 2 or 3 gees along it.

The issue with these sort of launchers is really just about how to keep the top part up, where the air is thinner, from falling down. Early designs for more modest ones had them held up by hot air balloons for instance. The issue is that not only does air slow you down for accelerating but when you exit a vacuum tube into normal air, or even fairly thin air at mountain peaks, it is a bit like slamming into a wall at those kinds of speeds. Same as jumping off a diving board into a pool compared with climbing down a ladder into one.

The paths for these don’t necessarily need to be straight lines either but straight is generally preferable, adding curvature adds in engineer complications that probably aren’t worth it in most cases. StarTram is a very attractive system though, especially used in combination with a space plane with its own engines and a skyhook at the end. It requires some precision timing, and some propulsion to allow you to maneuver, but if you miss the skyhook you can always fly off to land at a normal airport and try again. The StarTram Generation 1.5 system embraces this idea, calling for a three gee acceleration down a track about 200 miles long ending on a mountain peak where the ship exits and links up to a skyhook.

Based on current engineering that system would reduce fuel costs for getting into orbit to about 1% of normal and is something we have the technology to build and could probably do so for a price tag in the upper tens of billions to lower hundreds of billions. Hypothetical as little as NASA’s annual budget but more likely you’d be talking about a decade long project that would cost about NASA’s annual budget to maintain each year.

If I had to guess as to a Space Launch system we’d be likely to employ in the not too distant future, excluding any major breakthroughs that changed the game entirely, then I would bet on something along these lines, probably a shorter and slightly higher acceleration tunnel rising up to a mountain peak then linking up to a skyhook.

One big thing against it is that this stuff is always best done on or near the equator and west to east, and there aren’t a lot of mountains in the United States on the East Coast and you really do want to exit over ocean or at least uninhabited land if you can. Conveniently you do tend to have higher mountains near equators which is not entirely coincidental but I won’t go into now. Unfortunately none of countries on Earth with the sorts of industrial and technological muscle to be looking at building such a thing happen to have a convenient east coast mountain near the equator making it a less desirable option.

Now there’s a number of other ideas that kicked around that are very similar to Launch Loops, Skyhooks, Space Elevators, or Orbital Rings and we’ve already gone long for this video but I want to give a quick honorable mention to the Space Fountain, as it’s probably the only commonly named megastructure that circulates such conversations, and because the concept gets used in a lot other, bigger megastructures.

Space Fountains are neat ideas, essentially a vertical structure held aloft by firing high speed projectiles straight up which get slowed down along the way, not just from gravity but from magnets at the end or along the path, transferring their momentum into the structure to keep it aloft. Energy lost to gravity gets regenerated on the fall back down, though some have suggested that you might not even want to have the matter come back down initially and just use it as construction material to build the tower higher or build other things up there.

It’s a launch loop in the idea that holds it up, and an orbital ring in that it’s a high structure that remains right over the same place. They’re not very good for space launches unless you build them thousands of miles high, in which case they are space elevator that doesn’t rely on tensile strength, but they are ideal for holding up structures that are far taller than you can build from conventional materials. That means you can use them to hold structures aloft, like pylons for a bridge, and you can build a lot more redundancy into both them and whatever they are holding up as a result.

So space fountains could, for instance, be used to hold up a VacTrain tunnel and be spaced close enough together to give it redundancy if one fails or is attacked and blown up. They could be included as part of an orbital ring too as a failsafe, and they have the advantage that you can add segments to increase height as you go, growing them from the ground floor up. They also allow you to skip on symmetry so you could use one to build a curved or helical launch loop for instance.

We will be seeing these a lot further down the road when we talk about certain megastructures like the Matryoshka Shellworld. Like the orbital ring, space fountains can be used to do things conventional construction materials could never allow. Their obvious disadvantage is that they require a constant power supply to replace lost energy, how much would depend on how efficient the machinery is. Theoretically one could be 100% efficient and be a closed system, same as an orbital ring, but in practice nothing is every 100% efficient. You can also play this trick with lasers or microwaves by firing them from the ground to bounce off the underside of a station with a reflective bottom. Same concept. And if power is cut the thing just falls down slowed by parachutes.

From here on out we’ll be looking at stuff bigger and further ahead in time, genuine megastructures. Don’t forget to subscribe to the channel if you want alerts when those come out and like this video and try some of the other’s on the channel in the meantime. As always, thanks for watching, and have a great day!

1. ↑ So does the combined sunlight hitting a farm in 1 day.
2. ↑ "New" = 1995
3. ↑ It's a geology, erosion, glacier thing.
4. ↑ This was the original means conceived for doing a Space Elevator, before Carbon Nanotubes.