A launch loop https://en.wikipedia.org/wiki/Launch_loop is a proposed way to put lots of things into space at lower unit cost than rockets (only need to pay the energy cost in electricity for each payload) without needing very strong materials (the exoatmospheric-height structure is supported by electrodynamics, not the strength of materials).

Postulating a setting where the technology and economics are up to designing such a thing and manufacturing all the components, how would you go about building it? In a half-built loop, the rotor is not circulating, so no support. The obvious answer is static scaffolding, but if you could build static structures that high, you could just let that suffice for the final structure and avoid all the trouble and expense of the circulating rotor.

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    $\begingroup$ I don't think I've ever seen this idea proposed to be built at altitude. Turning it on is what raises it to it's design height. There are plenty of other drawbacks but most proposals assume this is doable at current technology levels. The "static scaffolding" is definitely beyond current capabilities and you're right that if we could do that it would make sense to avoid the rotor altogether. $\endgroup$ – ben Apr 24 '19 at 0:01
  • $\begingroup$ However you like, given that you're assuming it can be built within the story, the story is where you answer how it's done because your storyline dictates what technique(s) are available and preferable. $\endgroup$ – Ash Apr 24 '19 at 12:35
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    $\begingroup$ There's a key sentence in that article: When at rest, the loop is at ground level. You don't build it 80km up; you build it on the ground and it rises up when activated. $\endgroup$ – elemtilas Apr 24 '19 at 13:47

It is built and raised in one of 'a few' ways, but both use the same force that makes it work:

Built "In Full" on the ground, then slowly powered up. The forces that you're using to keep it up are what raise it, and the end stations down on earth move closer together. [Building over an ocean is probably handy to avoid clearly too much space...]

Built "Live, in segments", where a core station is put in place, and a small scale loop is started. This smaller loop is then slowly expanded by adding more and more segments until it is grown to the final size.

  • Segments could be added in different ways, such as being brought up as wedge shaped pieces that fit in/outside the ring they're destined for. Somewhat like tunnel boring machines laying pre-cast concrete segments that fit through the tunnel already laid. [In short, adding links to a bicycle chain while the chain is being flung through the sky... No one said sci-fi engineering was easy or graceful.]
  • "Hoberman sphere" like expanding structures are also another option for the segments. https://en.wikipedia.org/wiki/Hoberman_sphere But in ring form. Connect the edges of a contracted ring to the inside of two existing segments at a joint, then expand the new segment as the previous joint is released.

A third potential option would be to just build the entire structure in a "Hoberman sphere" like nature, where it is compacted for initial on-the-ground form, and is then telescoped/expanded during power up.

Neither option is really all that easy, but the "Live, in segments" design is a solid demonstration of how to conduct maintenance and repair. This is not a structure you want to power down and lower back to earth every other month to change bearings...

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    $\begingroup$ I am super curious as to how the "segments" method would work. I'm imagining adding portions to a loop that's already in motion but how would one even begin to do so? I must be missing something here. $\endgroup$ – ben Apr 24 '19 at 2:53
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    $\begingroup$ @ben I might need to go digging around on youtube for a video. I'm not sure I have enough coffee in the house to help me gracefully explain how that would work through text alone, but the above edit might help. $\endgroup$ – TheLuckless Apr 24 '19 at 3:29
  • $\begingroup$ Fair enough, that definitely does help explain it. I'll leave the implementation of that to minds much smarter than mine. $\endgroup$ – ben Apr 24 '19 at 4:41

You could consider using alternative megastructure/launch system engineering methods to raise your launch loop if you can't build and raise it in place under its own steam.

Given the technology required to build a launch loop, one such bootstrapping system would be the space fountain which uses magnetically accelerated pellets to impart force to the top of a tower via deflector coils, so it isn't entirely unrelated to launch loops. The fountains on their own are handy for space launch purposes, but unlike a launch loop don't provide any speed boost, only a helping hand up in the atmosphere. Fountains have the advantage that extending and contracting them is much simpler as the pellets are connected together into a single giant rotor. Build a series of fountains along the length of your loop to help push the structure into place whilst you're getting it ready, or to provide maintenance later in its operating life.

An alternative would be the colossal magnetic levitation system proposed for the Star Tram gen 2. This is intended to be a launch system in its own right, so you'd need to decide what advantages the launch loop brings over the startram (and there are one or two, and in the limit, launch loops start looking like orbital rings which are all sorts of interesting and useful). Basically you use a whole bunch of giant superconducting cables to generate magnetic fields that repel each other, and some dumb tethers to hold the levitated structure in the right altitude and angle. Astonishingly, you could apparently get the business end the rquired 22km up this way. Scaling to 80km and the weight of a whole launch loop may be rather difficult, but might be enough for assembly and partial raising.

To be honest, I think a Star Tram type system is much easier than a launch loop, and somewhat more straightforward to make and a lot easier to maintain, even if it isn't as cool or powerful. YMMV, of course.

The obvious answer is static scaffolding, but if you could build static structures that high, you could just let that suffice for the final structure and avoid all the trouble and expense of the circulating rotor.

Scaffolding of any kind is going to have some serious limits... even aside from the issue of ultimate material strength and whether you could build a static structure 80km tall, it would require a colossal amount of materials and have a vastly greater environmental impact than a dynamic structure (it would affect weather patterns like a mountain range, and you might need to find an asteroid to provide you with enough metal at a reasonable price and minimal environmental damage). It may actually be easier to build a launch loop, and that's not something you get to say very often.

Whether dynamic (like a fountain) or static, a tower also only gets you up, not out. You still need to achieve orbital speed, and a launch loop provides a means to help you with that in a way that also supports its own weight, killing two birds with one stone.

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