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fitzroy95 t1_ixy5muh wrote

Space elevators are largely a matter of engineering nowadays.

Yes, the materials aren't there yet, but they are getting closer every year.

Climbers are largely solved, basic design is largely solved, materials for the cable is the sole remaining barrier. And thats a not insignificant barrier atm, but its getting closer every year

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TheImmortalLS t1_ixy6xy9 wrote

If a material strong enough to hold existed, we would have other tech instead. Such strong materials can improve compression ratios, improve engine efficiency via less moving weight, tank warfare would be dramatically different, etc.

Space elevators are built along the premise there is a material strong enough to anchor; everything else in its design is trivial. We are probably 100+ years away from a material, maybe 30 if a world war breaks out soon

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sector3011 t1_ixy7cmq wrote

It still doesn't solve the problem of elevators getting hit by debris be it space rock or junk. There is no way to avoid this since the elevator is stationary.

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dgriffith t1_ixyh54v wrote

They can be stationary but flexy.

If you can build a cable for a 40Mm long space elevator, then bending said cable sufficiently to avoid space debris by a few kilometres should also be doable.

You'd end up with waves travelling up and down the cable as it oscillates due to natural resonance anyway, you could probably augment or dampen that resonance as needed to ensure the cable avoids the big stuff along its entire length. These waves would be basically unnoticeable to cars riding the cable due to the sheer scale of things.

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NinjaFenrir77 t1_ixyagh5 wrote

That’s a problem, but not one that will prevent space elevators from being built/used. The chances of this happening are relatively small, and the damage being much less impactful than the impact of having an operational space elevator.

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TomSwirly t1_ixyg485 wrote

> The chances of this happening are relatively small,

Can we see the math?

> and the damage being much less impactful than the impact of having an operational space elevator.

Having a 35,000km structure collapse onto the Earth sounds pretty darn "impactful".

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NinjaFenrir77 t1_ixyib4o wrote

There’s not a lot of math to show until we start talking specifics of the elevator and know the materials we are working with. In general, space is pretty empty.

Are you taking about the orbiting station? Because that won’t crash into Earth, it will fly away (slowly) because it is orbiting faster than it should to have a stable orbit in order to balance out the elevator cable. A few thrusters can help it stay in a stable orbit if the cable breaks.

The cable itself isn’t a huge threat. It could potentially kill some people if it landed on them, but the scenario of it whipping into the Earth at supersonic speeds isn’t going to happen due to wind resistance and basic safety precautions.

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Harabeck t1_ixz6vua wrote

> Having a 35,000km structure collapse onto the Earth sounds pretty darn "impactful".

I might damage your roof, and it'd be a huge pain in the ass, but it's not like it'd be a tower that crushes everything beneath it.

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TheImmortalLS t1_ixz9z4v wrote

It’s the same probability as ISS getting hit, +/- a bit since the iss can go up and down. Maybe a space elevator can dodge a bit too.

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drysart t1_iy1p5nu wrote

Of all the practical problems facing a space elevator, dealing with debris is by far the easiest of them.

You don't run one cable from the ground up to orbit; you run several of them parallel to each other, spaced far enough apart from each other that no piece of debris could sever more than a certain amount of them at once. And at regular intervals down the cables, they'd be linked to each other such that if any subset of cables gets severed, the remaining cables would continue to hold the entire structure upright.

How many cables you'd need and how far apart they'd need to be would need to decided upon by dedicated research into the nature of the debris problem -- how much debris, how big it can be, etc. And then you just engineer in redundancy for the unavoidable failures to reduce their impact into being a bothersome maintenance task to repair/replace severed cables rather than a complete catastrophic disaster collapse.

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jherico t1_ixy6u3z wrote

> Yes, the materials aren't there yet, but they are getting closer every year.

The Moon is getting further away every year too but I'm not going to invest in any company whose business plan involves solving the "missing moon" problem.

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TomSwirly t1_ixyesgy wrote

> Space elevators are largely a matter of engineering nowadays.

We are talking about a structure that is over 35,000km tall.

The tallest structure to date is less than 1km tall.

In the last 200 years, the tallest structure height has increased by less than 3% per year, on average, so we would expect to be able to build a 35,000km structure in about 400 years.

EDIT: I was wrong - the center of gravity of the structure has to be 35,000km tall. That means that the structure has to be higher than that.

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IvorTheEngine t1_ixyl204 wrote

That's a false equivalence though, as towers are compressive structures (that buckle before the material fails), while a space elevator could be in tension. A 1km tower is hard, but we regularly hang cables 1km down to the seabed, or for suspension bridges.

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danielravennest t1_ixyspk5 wrote

There is a figure of merit for materials, which is the breaking strength divided by the weight per meter. This gives the maximum length/height at which it fails under its own weight.

Best available carbon fiber is 385 km scale height. But nobody engineers a structure at the failing point. With a reasonable 2.4 factor of safety, you get a 160 km vertical cable as the maximum dangle.

Towers need epoxy to stabilize the fibers in compression, so the maximum erection height is less than that.

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IvorTheEngine t1_iy0wqzt wrote

The next logical step in most discussions about space elevators is that you can taper them.

However, you're right, it's still not practical with even the best current materials, even if it's hair-thin at the ground, it would need to be impossibly large at the orbital end.

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danielravennest t1_iy3gpkl wrote

This is why I mention the Skyhook elsewhere in these comments. It tapers from the center outwards rather than top to bottom. Also the stress ramps from center to tip linearly. So compared to a stationary vertical cable the total stress is 1/4 as much.

For example, a 587 km radius (twice that in total length) skyhook moving 2400 m/s at the tip at 1 g acceleration sees 294 g-km of total stress. A safe stress I showed above is 160 g-km, so the cable needs to taper by 6.27x in area from center to tip. This is fairly reasonable.

To avoid atmospheric drag, you place the tip when vertical at 200 km, and thus the center at 787 km. Orbit velocity is 7464 m/s at that height. The tip moves 2400 m/s counter to orbit at the low point, or 5064 m/s. The equator moves 465 m/s, so the tip moves 4599 m/s relative to the ground. That's the speed a rocket needs to match velocity. That can easily be done with a single stage.

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IvorTheEngine t1_iy3jal4 wrote

I love the idea of a skyhook - they seemed like an idea that could only work on paper, until Space-X started landing boosters on barges.

I can see how you could visit it, then it drops you off when you go home, but if you use it to launch things into orbit, doesn't it lose energy?

I've seen proposals where it flings things to the moon or mars, and recovers energy by catching incoming mining products - but could you use it just to put things in orbit?

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danielravennest t1_iy3l1yy wrote

If traffic to a Skyhook is "unbalanced" (more mass going in one direction than the other) the orbit will change. You can make it up over time with electric propulsion. Since electric propulsion is at least ten time more efficient than rocket fuel, you still come out ahead.

In Earth orbit you can potentially react against the magnetic field by running a current through the ionosphere. That uses almost no fuels, just a little gas to make plasma contact.

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