[icetank]

The issue of the line falling back to earth is solved by putting the base of the elevator on water. If the top part of the elevator was cut of you could even detonate charges along the line to make sure all pieces fall into water.

[0xEF]

Are we to assume they would be falling straight down? Because I'm pretty sure that's wrong. I'm not a physicist, though, and am happy to be corrected because every time the Space Elevator comes up, I want to know what happens when catastrophic failure occurs and how we'd mitigate that.

[speed_spread]

It would become a giant whip falling faster and faster round the equator.

[devilsdata]

https://en.wikipedia.org/wiki/Space_elevator

> Above GEO, the centrifugal force is stronger than gravity, causing objects attached to the cable there to pull upward on it. [...] On the cable below geostationary orbit, downward gravity would be greater than the upward centrifugal force, so the apparent gravity would pull objects attached to the cable downward.

So, without defensive countermeasures, the Space Elevator would indeed whip around the Earth.

But honestly, if I were designing such a thing, it would have break points, and maybe even a whinch at the base, to pull the line in. I'd also build it over water, and not over a population centre.

But I'm only a software engineer– it's likely a lot more challenging than this.

[LorenPechtel]

If you blow the cable apart at a few important points the mass that falls either hits fairly near downrange from the tether, or does not hit at all. Have a group of range safety officers for the charges and a law that when on duty they are expected to shoot any politician that contacts them. (I'm thinking of Fukushima. We need to vent the reactor or it will blow! You can't vent until the city has been evacuated. The reactor didn't listen to the politicians.)

[mr_toad]

A space elevator on Earth would be over 35,000 kilometres long. Depending where it broke it could wrap halfway around the Earth.

[floxy]

Let's say the cable gets detached really close to the geosynchronous tether point (that's the worst case, right?). How much of the cable will burn up in the atmosphere? What is the density of the carbon nano-tube / graphene ribbons? And what is their terminal velocity? Has anyone proposed dimension of a graphene ribbon tether? Like it is 300 mm wide by 0.01 mm thick?

[kqr]

At first I was confused by this because the Kármán line is less than a percent of Earth's circumference up, but then I realised we're probably talking a geostationary anchor or something, which is very nearly a circumference up.

[tsimionescu]

Yes, you need a geostationary orbit for the tether, otherwise it would either fall down or spiral away with time.

[gambiting]

Could you make it over double the length needed, so if it ever broke it would be pulled away from Earth and float into space and not crash into the Earth?

[tsimionescu]

No. You'd need an even more magical material that can witstand at least double the tensile strength (since the parts that go above and below the GEO anchor would be pulling with about the same force in opposite directions). And if you destroyed the GEO point anchor, the cable would just split in two - everything that's below GEO would fall, everything that's above would float away.

[tsimionescu]

I don't understand why you think that where you put the base of a 35000km cable makes a difference for where the rest of it would fall. I also don't understand why you think that a 35000 km long cable falling in the ocean from space would cause any less damage to the planet than it falling down on solid ground, or at least why the difference would be significant.

[JumpCrisscross]

> why you think that a 35000 km long cable falling in the ocean from space would cause any less damage to the planet than it falling down on solid ground

They’re not obviously wrong.

A lot of the cable is moving at escape and orbital velocities. Tensile strength is all that holds it together.

If, as the cable fails, you sever the parts above from below around escape velocity, you’ll significantly reduce the length of cable that will ever hit the surface.

[cfraenkel]

Orbital and escape velocities??? The elevator is sitting over a stationary spot... it's moving at earth's rotational velocity. Only the portion above the GEO anchor is moving at orbital velocity.

[LorenPechtel]

But it has altitude. The stuff that's low down doesn't have a lot of orbital velocity, blow the cable and it falls nearby. And the stuff far away has enough velocity that it goes into a very eccentric orbit rather than hitting the atmosphere.

Just because it's moving below circular orbital speed doesn't mean the periapsis is in the atmosphere.

[kakacik]

What would be the wnergy delivered by 35k km of ultra strong thick cable falling down with possibly supersonic speed? A small bit not much, but such length adds up.

[pfdietz]

There's also the issue of the vehicle on the space elevator falling back to Earth if it detaches from the space elevator (accidentally or deliberately in case of malfunction that stops it from moving up). This means each vehicle will need rockets on it. At low altitude, the rockets are fired to keep the vehicle from reentering the atmosphere too fast at a steep angle, killing the passengers. At high altitude, the rockets fire to raise the perigee enough that the vehicle misses the atmosphere entirely (or enters at a very shallow survivable angle). There's a cross over point that dictates the delta V the rocket must be able to deliver. which if I vaguely recall correctly is greater than 4 km/s.

Pure payload capsules with no passengers wouldn't need this.

The argument for space elevators is that there's a pretty strong limit on how much payload can be launched by rockets due to injection of water into the upper atmosphere. Starship could arguably reach this limit with plausible projected growth rates in traffic.

[LorenPechtel]

Ouch, never thought of the re-entry angle problem. Straight down is bad.

[actionfromafar]

Why is the water bad?

[pfdietz]

In the stratosphere it both contributes to IR opacity, increasing global warming, and can provide ice surfaces on which ozone destruction is amplified. The stratosphere is normally extremely dry, so even small inputs can have an effect that would be invisible in the much moister troposphere.

[actionfromafar]

Aha, so in the stratosphere we should use oxygen / solid carbon boosters?

[pfdietz]

Their Isp is very low, unfortunately, because the molecular weight of the combustion gas is too high. Ditto for oxygen/carbon monoxide.

Maybe the Isp could be increased by mixing in some helium, but helium is very expensive.

[LargoLasskhyfv]

Methalox should suffice for practical and technical reasons?

Compensate the slight loss of ISP by using aerospiked rotating detonation engines...

[tonyhart7]

heavy as fuck

[actionfromafar]

It would like, rain down hard?