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Space Elevator

neal.fun

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Re: Space Elevator

#361

Post by LorenPechtel »

Pxtl wrote: post #176
messe wrote: post #158

Earlier quoted context omitted.

> it's just hard to go up Going up is the comparatively easy part, it's not exactly rocket science. Going fast enough sideways so you stay up there is the tricky bit.

Which is another part of why a space elevator is nifty - by definition it extends out to a distance where you are going fast-enough-sideways. Now, I have no idea how practical it is to build one (Angela Collier has a video saying it's kinda ridiculous), but it's a cool idea. https://www.youtube.com/watch?v=Z5aHMB4Tje4 Also since rockets have moved away from hydrolox, it would be nice to have a greener launching syste…

I've worked out one that "can" be built (it's beyond ridiculous, though.)

Build towers around the equator. Build a ring around the equator on those towers. Build a ring inside the ring, maglev supports. Evacuate the ring, spin the inner ring above orbital velocity. The objective is to generate as much outward force as the weight of the entire assembly including the towers.

Do it again, on top of the first one. Keep doing it until you reach synchronous orbit. If you want to go higher the inner ring is not moving, exerting downward force countering the outward force of the rest of it being above orbital velocity.

Forces:

1) Compression on the towers. Note that this goes to zero as tower height goes to zero.

2) The outward force on the ring exists across the whole ring, but the downward force of supporting the towers only exists where there is a tower. Your ring needs to be stiff enough to counter this. But, again, note that this force goes to zero as the space between towers goes to zero.

3) Maintaining a very hard vacuum in the structure.

We know how to do #3, the others must have answers. Thus it can be built.

Nothing else is feasible with current technology, the taper of the cables is highly dependent on the strength of the material (in tension, an elevator is based on always pulling out as the upper end isn't hooked to anything.) You need much better than anything we can currently do before the taper blows up so badly you can't build it.

Re: Space Elevator

#362

Post by notepad0x90 »

Why isn't there an elevator to the highest altitude where an elevator could be built, and then spacecraft can propel from there. It should still reduce fuel costs and the cost to construct space craft?

Would powering the cable permanently, with a power station at the bottom and a constant feed of water for super-heated steam thrusters work? Just throwing scifi at it. I'm just curious why it has to be one component or why the weight can't be supported by propulsion. I'm guessing the TL;DR is the numbers just don't work out?

Re: Space Elevator

#363

Post by LorenPechtel »

raducu wrote: post #194
Pxtl wrote: post #176

Earlier quoted context omitted.

Which is another part of why a space elevator is nifty - by definition it extends out to a distance where you are going fast-enough-sideways. Now, I have no idea how practical it is to build one (Angela Collier has a video saying it's kinda ridiculous), but it's a cool idea. https://www.youtube.com/watch?v=Z5aHMB4Tje4 Also since rockets have moved away from hydrolox, it would be nice to have a greener launching syste…

> Angela Collier has a video saying it's kinda ridiculous. There are other concepts like space fountains, orbital rings and sky hooks that seem more doable -- especially the sky hook seems close to do-able, especially on the Moon.

Fountain--while it could be done in a perfect world I do not believe it would be feasible and the whole thing is so vulnerable to disruption. Dragon's Egg got it very wrong, if one of your fountain bits misbehaves it misbehaves very badly. They're moving way above orbital velocity if they hit something that energy is promptly liberated. The Cheela couldn't catch those rings, they had to be moving at high relativistic speeds and would have hit with something akin to antimatter level force.

Orbital rings--only if you have elevators. Remember, the Ringworld is unstable. So is every other planetary ring.

I do not recall numbers on hooks so I will not address them.

The Moon has a whole different set of problems. There is no synchronous orbit, elevators must go above synchronous orbit, so the normal version can't exist. Nor can anything stand up to be yanked around by the Earth.

But there are two cases that avoid the yanking problem: pointed towards and pointed away from Earth. Current cables are good enough for a useful Earth-pointing cable. The free end dips below synchronous orbit, but it's moving very slowly. You do what people think rockets do--go up. It takes a lot less energy to catch the cable than it does to even reach orbit.

How to have such a cable in an environment with geosynchronous satellites is another matter...

There's also another interesting cable situation. Cable on Mars? Iffy--and those two moons would be a major problem. But flip the problem over--put the cables on the moons. The low end dips into the atmosphere at aircraft-type speeds. The cables can toss to each other. The high end can capture/eject to Earth or the asteroid belt.

Re: Space Elevator

#364

Post by LorenPechtel »

0xEF wrote: post #83
icetank wrote: post #77

Earlier quoted context omitted.

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.

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.

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.)

Re: Space Elevator

#365

Post by LorenPechtel »

cfraenkel wrote: post #342
JumpCrisscross wrote: post #283

Earlier quoted context omitted.

> 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…

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.

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.

Re: Space Elevator

#366

Post by LorenPechtel »

pfdietz wrote: post #108
icetank wrote: post #77

Earlier quoted context omitted.

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.

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 roc…

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

Re: Space Elevator

#367

Post by LorenPechtel »

pbmonster wrote: post #135
Yizahi wrote: post #97

Earlier quoted context omitted.

Russia, China, Iran etc. are throwing a hissy fit whenever even a small weapons are deployed in the neighboring countries. USA too if we are being fair. They won't even wait for that opportunity.

I think they mostly throw a fit because medium range ballistic missiles allow practically no useful early warning. When the ICBMs go up, early warning radars notice them right away and you still have time to act. Leaders can make it to helicopters and basement bunkers, bomber squadrons can scramble, missile silos can already be empty when hit, road mobile ICBM launchers can still relocate. But with a large enough num…

Yup, it's not like a video game where you get instant notification/identification.

And it's why we have been so worried about Russian nukes--they have used liquid fueled birds, they can't be held ready to launch. Such birds are pretty much only useful for a first strike as they won't be able to launch them once incoming missiles are detected unless they're being held at launch ready (and they can't do that for too long.)

Re: Space Elevator

#368

Post by zwog »

messe wrote: post #158
jvanderbot wrote: post #143

Very cool. One thing I wish was better shown: space is close, it's just hard to go up. Our liveable breathable atmosphere is razor thin compared to the size of earth. In most cases, 100km is less than the distance between sizeable metropolitan areas. It's a day long bike ride. Air runs out less than a bus ride across town. A 15k jog/hike would put you in the stratosphere. Those jet aircraft that seem so high are clos…

> it's just hard to go up Going up is the comparatively easy part, it's not exactly rocket science. Going fast enough sideways so you stay up there is the tricky bit.

There's an XKCD for that: https://what-if.xkcd.com/58/

Re: Space Elevator

#369

Post by LorenPechtel »

pbmonster wrote: post #63
tsimionescu wrote: post #54

Earlier quoted context omitted.

The bigger problem I think is the elevator itself. Cutting it and letting it fall would be far more destructive than any weapon ever fired or even conceived.

Probably no easy task. Snipping off just the first few kilometers is not catastrophically destructive yet, and cutting it down further up would require multistage rocket designs, sophisticated steering/targeting and potentially significant yield (you'd need to cut unobtainium, after all...). If you can build a space elevator, you can defend against those. You better thoroughly inspect what cargo you put on the elevat…

Yeah, can't remember the author but it was about hunting a nuke on an elevator.

Re: Space Elevator

#370

Post by LorenPechtel »

adwn wrote: post #46
tempestn wrote: post #8

TIL it's estimated that over 48 tons of meteors hit the atmosphere every day. Regarding actual space elevators though, while they're not sci-fi to the extent of something like FTL travel - ie. they're technically not physically impossible - they're still pretty firmly in the realm of sci-fi. We don't have anything close to a cable that could sustain its own weight, let alone that of whatever is being elevated. Plus,…

Almost all discussions around space elevators focus on the cable itself, how to manufacture and deploy it, and completely forget about the issues that would arise afterwards: 1) How do you attach the climber to the cable without affecting its structural integrity? By squeezing it really hard? A material that's optimized for longitudinal tension strength is probably not very tolerant of lateral compression. 2) How do…

#1 is one of the things they typically get wrong in stories.

Climbing the cable is a nightmare, especially as it gets thicker as you go up. Thus do not climb the cable! Rather, when the cable is built a whole bunch of anchors are built into it. You are not climbing the cable, you are climbing a track on the side of the cable. The cable's job is to support the track plus any load on it.

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