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ahfoo t1_ixysf3e wrote

Reply to comment by danielravennest in Space Elevators Are Less Sci-Fi Than You Think by Sorin61

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.

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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.

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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.

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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.

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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.

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arcosapphire t1_iy144mt wrote

Except they were saying the beam came from above (lunar or GEO).

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