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arcosapphire t1_ixz37yi wrote

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

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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