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TomSwirly t1_ixyesgy wrote

Reply to comment by fitzroy95 in Space Elevators Are Less Sci-Fi Than You Think by Sorin61

> 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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