Submitted by Sorin61 t3_z5v1h4 in technology
danielravennest t1_ixyqy6k wrote
Reply to comment by Gingerbread-Cake in Space Elevators Are Less Sci-Fi Than You Think by Sorin61
The classic space elevator - tied to the ground and stretching beyond synchronous orbit - became obsolete in 1986 when Hans Moravec invented a better one, the orbital skyhook.
This is a rotating cable in orbit that picks you up at the low point and throws you higher at the high point. It can be 50 times shorter, and built from today's carbon fiber. Transit time is much shorter, around 15 minutes.
The cable knocks off about half the energy to reach orbit. The rest is supplied by a single-stage rocket similar to the Falcon 9 first stage that flies and lands today.
The economic problem, rather than technical one, is there isn't enough traffic to space to justify a 1250 km cable in orbit. Nobody would build a bridge or airport to use once a week, which is about how often we launch rockets. A skyhook is similar - expensive to build, but cheap to use once you have it. So you need lots of traffic to justify building it.
YaAbsolyutnoNikto t1_ixz09ih wrote
Whilst I follow what you’re saying, what about induced demand here?
No wonder we launch so few rockets - they are freaking expensive.
If we could bring the costs down, more initiatives would follow.
Not building it because there’s not enough demand is like not building train tracks and trains because “nobody uses them” (no shit, they don’t exist).
danielravennest t1_ixz0u5w wrote
Demand for transportation already existed before railroads. It was handled by horses and wagons on land, and sailing ships on the water.
What I am hoping is the SpaceX Starship will induce demand by lowering cost to orbit. Once the demand exists, people will look for ways to satisfy it even cheaper.
SIGMA920 t1_ixzzkkw wrote
> What I am hoping is the SpaceX Starship will induce demand by lowering cost to orbit. Once the demand exists, people will look for ways to satisfy it even cheaper.
Isn't it already doing that?
4onen t1_iy01vof wrote
The SpaceX Falcon 9 and Falcon Heavy are. The SpaceX Starship has yet to successfully fly a stacked configuration.
SIGMA920 t1_iy02k9d wrote
Ah, even so costs are going down so that's hopeful.
[deleted] t1_iy02n00 wrote
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SBBurzmali t1_iy1sglg wrote
Not really, there's lots of talk about prices going down, "One tenth the price by 202X" and all that, but launch prices are still pretty much the same as they have been.
lookmeat t1_iy1t7lg wrote
Most rockets make it to LEO, no need for more. We'll probably still have Leo rockets and then use skyhooks to move into upper orbit where moving by rockets. The thing is we rarely need to do this right now. Maybe once Artemis puts a lunar station, and there's incentive to mine or something in the moon, it might make sense to bake a skyhook bridge from LEO into Lunar orbit. But that is going to take a while, we might see it in our lifetime, but not anytime soon.
ahfoo t1_ixysf3e wrote
There are dozens of alternatives. The skyhook is a cool concept but you also can have a "virtual" space elevator which is simply a beam of concentrated energy such as laser or microwave beam that a vehicle "climbs" by utilizing the energy in the beam. In this case the elevator is not a physical cable but a beam of energy.
This virtual space elevator concept happens to be a perfect fit for beamed energy from geosynchronous orbit which, in turn, would be much easier to put in place from a lunar base than to launch from the Earth. So you start with a lunar base, make a beamed energy station in GEO from orbiting slabs of lunar bedrock and then you have your virtual space elevator at the same time.
There are other approaches as well and they can all be mixed together. It's beyond doubt that going into orbit will be as common as crossing the ocean in a jetliner today. It will be something that ordinary people will do on vacation just as they might fly from the US to Europe today. When you think about it, crossing an ocean is an astounding thing to do but it's no big deal and going into orbit or the moon and beyond will be the same in time. Like everybody else, I wish that time was a little bit closer than it seems to be though.
arcosapphire t1_ixz37yi wrote
Can you explain the propulsion involved in the virtual elevator?
Getting energy is all well and good, but that doesn't let you apply force. The only non-propulsive method we have for space travel is the light sail, but the force involved is way too small for getting to orbit, so it can't be that.
There are two unconventional ways we have to apply non-chemical energy sources to propulsion: nuclear engines and ion thrusters. Nuclear engines are right out because they don't involve an external energy source. So that leaves us with ion thrusters, which still need propellant (often something like xenon). They can use external power for the energy to accelerate the propellant, but their thrust levels are also extremely low (too low for getting to orbit) and they still need propellant.
So...what alternative mechanism are you proposing? Since no existing method will work for this.
Exsanguinatus t1_ixzmyms wrote
There's actually a laser lift mechanism I've seen employed by small scale test craft. It involves firing a laser at the reflective bottom of a specially shaped craft. I believe it actually ignited the air under it causing propulsion, but it's been a long time since I saw it.
The proof of concept actually worked but the power increase needed for anything at scale was quite high. Also, the laser was not cohesive enough after atmospheric scattering to provide lift to a significant height during the PoC phase.
I'm not 100% sure if this is what the previous post is referring to, and it's possible that the technology has advanced since I first heard of it which was at least a decade back.
a-priori t1_ixzhulv wrote
Another one are plasma jets, which use microwaves to turn compressed gasses into plasma and extract thrust from that. These can be air breathing in the atmosphere and use stored gasses in vacuum.
There’s a prototype microwave plasma jet that’s proposed as an alternative for airplanes because it has similar power density (factoring in battery weight) to turbojets.
https://aip.scitation.org/doi/10.1063/5.0005814
If you could power these by beamed power, and switch from air breathing to stored gasses when in a vacuum, then it could be a viable lifting engine.
quettil t1_iy13t8q wrote
> Can you explain the propulsion involved in the virtual elevator?
Fire up particles. Particles bounce of payload. Newton's third law of motion means the payload goes up. It's called a space fountain.
arcosapphire t1_iy144mt wrote
Except they were saying the beam came from above (lunar or GEO).
bannacct56 t1_ixz2wrq wrote
The problem with carbon fiber is not the strength, it's strong enough but we do not yet know how to braid glue, stick these pieces together so they do not unravel. A
4onen t1_iy02jyw wrote
Partially true. If we could manufacture 100km single-crystal graphene (e.g. carbon fiber but sheets) then that would be sufficiently strong, and to prevent unravelling you could literally use something as weak and heavy as scotch tape to hold different ribbons of the graphene together. More ribbons, more tape, bigger and stronger cable.
The problem is the single-crystal manufacturing, which we can't do. Without it, we need a high-tensile-strength glue to hold the strands we can manufacture together. No such glue exists, especially not one that could connect via polar and/or van-der-walls bonds to the graphene.
Ergo, no space elevator, yet.
dagbiker t1_ixzc0nh wrote
It also wouldn't be a total disaster if it broke. If a tension wire breaks at the speeds and velocities it would need to travel half the wire would hit the earth with enough energy to be as powerful as a bomb.
Look at videos of people trying to move their cars using rope that snaps. It's not good. And that's assuming the counterweight doesn't fall back to earth.
If the sky hook fails it will wodnt be good. But at least it won't devistate as much of the world.
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