Space Elevator Issues

[GregBurch]

I think building a space elevator would be technically possible within the next 100 years. But the prospect doesn't excite me, because of some fundamental issues that I'm not sure elevator enthusiasts address satisfactorily.

The main issue to me is that a space elevator by necessity must be situated on the equator, and must pass through the most valuable near-earth space. Collision with a space elevator would be a global disaster, (as depicted in Kim Stanley Robinson's Mars books).

I've seen discussion of "plucking" the tether so that it oscillates back and forth, allowing things to pass by its "straight up" path. But that seems like it could work for 1) only a very few (at most) 2) very well-characterized objects.

The bottom line is that an elevator seems to me to be an "all-or-nothing" solution that requires a kind of global social and political control over space access that just doesn't seem either feasible or desirable.

I'd be curious to know what others on the forum think of this problem.

GB

 

[tblaxland]

IMHO, a collapse of the elevator would not be quite so dramatic as depicted by Kim Stanley Robinson (it was a great scene though!). Firstly, the location of a break would affect how much would fall to Earth. Lets assume a break occurs at say 300km altitude due to a LEO satellite collision. Most of the cable would rise harmlessly away from Earth (some issues for those on the cable, I guess). Some of the cable would burn up on reentry and the part that doesn't should not hit the Earth very hard due to low density of the material. High density materials would not be practical for building the elevator out off in the first place. An elevator would probably fall harder on Mars due to a thinner atmosphere providing less drag but it would not be as bad as KSR's Mars books.

Higher cuts would not be more catastrophic because any of the cable at high altitudes will fall fast enough to burn up.

I agree that collision avoidance is a real barrier.

 

[Urwumpe]

More important as collision avoidance will also be vibration avoidance - the tether will want to oscillate by tiny gravity changes and aim for its existing resonance frequency. Managing these oscillations will be a really hard task, maybe even too hard too also plan for avoiding satellites. Also, the position of the space elevator will be another problem - it will never be a straight line, but have many tiny changing curves.

 

[simonpro]

Space Elevator around earth = Non-starter.
Space elevator on the moon is another story, though.

 

[GregBurch]

Space Elevator around earth = Non-starter.
Space elevator on the moon is another story, though.

That's an interesting idea. I've never even thought of a lunar elevator. What would that look like -- wouldn't it have to be VERY tall due to the Moon's low rotation rate?

 

[Urwumpe]

That's an interesting idea. I've never even thought of a lunar elevator. What would that look like -- wouldn't it have to be VERY tall due to the Moon's low rotation rate?

Not really, i think the low rotation rate is offset by the lower required speed. Also, the weak stability boundary is quite close in technical terms on the moon, which means you could place a spacecraft almost into it and would require only weak thrusters for getting anywhere.

EDIT: What about launch loops, BTW? Compared to space elevators, this concept sounds at least realizable.

http://en.wikipedia.org/wiki/Launch_loop

 

[thomasantony]

The biggest problem with space elevator is avoiding collisions. Cable breaks won't be as catastrophic as Robinson depicts in his novels. Most cable designs are made so that they would burn up in the atmosphere. Also, most proposed cables are extremely light (1kg/km) and flat, so that they will be slowed down by air resistance and settle down on the ground similar to a piece of paper falling down.

http://en.wikipedia.org/wiki/Space_elevator#Cut_up_to_about_25.2C000_km

Of course the station at the top would end up in an unstable orbit, but I guess it should have thrusters in case of an emergency like this.

Another option would be an orbital ring. It can be made kinda like geostationary at LEO altitudes also.

http://en.wikipedia.org/wiki/Orbital_ring

The only problem is, we should get someone like Branson or Musk interested in it enough to fund research and subsequent construction. But it would require funding in excess of their current net worths .

~
Thomas

 

[Piet Barber]

http://en.wikipedia.org/wiki/Launch_loop

Now THAT would be an awesome add-on for Orbiter.

 

[Urwumpe]

Now THAT would be an awesome add-on for Orbiter.

Hehe, that was also my second thought. :lol:

 

[zerofay32]

I had heard of a similar idea; an orbital ring that would be constructed in geostationary orbit. But it wasn't for travel, the ring was comprised of solar power cells and powerstations. The ring would collect energy from the sun and it would be sent to Earth in the form of microwaves to stations on the ground, then the stations would convert it to electrical energy.

I can't remember were I heard this though. Maybe one of my friends at Ohio State.

zerofay32

 

[Pilot7893]

Also, who would want to sit in an elevator for 2 months to get to a hotel in GEO orbit?

 

[Quick_Nick]

I know it isn't as useful but what about a polar elevator? If you put an elevator at a pole you could just require all satellites to stay away from a perfect 90 degree inclination and therefor avoid collisions. You could even set up many elevators at high latitudes and just restrict inclinations a little more. The only ways I see for a collision to happen then is either 1: Someone breaks the 'law' or 2: A satellite 'launched' from an elevator has a deteriorating orbit.(though with a high enough altitude it shouldn't hit too hard)

 

[tblaxland]

I know it isn't as useful but what about a polar elevator?

How would the polar elevator maintain its position over the pole? It can't orbit the earth because that means moving away from the pole. I have seen a proposal for a pole-sitter communications satellite that used a balance between Earth's gravity and a solar sail to maintain its position but the mass of an elevator would be too high for that system to work. Also, a non-orbiting space elevator does not have the one big advantage of an equatorial elevator, that is, acceleration of payloads to orbital velocity. All a polar elevator would do is get you above the atmosphere - you would still need a lot of delta-v.

 

[Urwumpe]

Also a polar orbit is not free of collisions. The planes will intersect over the poles.

 

[simonpro]

Polar space is about the place you're most likely to have a collision with something. Around the equator a satellite only passes over a point something like once every three days, but it will pass over the pole once every 92 minutes.

 

[Urwumpe]

Polar space is about the place you're most likely to have a collision with something. Around the equator a satellite only passes over a point something like once every three days, but it will pass over the pole once every 92 minutes.

Well, currently, the (near-)collisions take place pretty evenly spread over the globe, but you have a strong concentration in the congested region around 900 km altitude, where an astronaut could nearly cross the orbit by jumping from one military satellite to the next.

 

[Bullethead]

I was rereading Frederik Pohl's 30-year old "Heechee" series recently. One of the things he mentioned was a Lofstrom Loop. Lofstrom's changed the design since then, however.

In Pohl's older version, the thing was like a giant Ferris wheel. The cargo pods weren't directly attached to the cable but used magnetic grippers I guess, so there wasn't a sudden jerk to start with. Then the pod would ride up for 1/2 a circle and be released on top in LEO.

As shown in the link above, the idea has since evolved into a huge elevated tram line, with all the acceleration taking place pretty much parallel with the ground. This spreads it out over a few thousand kilometers of ground surface, but it can at least launch in 2 different directions as the cable goes back and forth between the ends instead of being in a big ring only going 1 way.

I've always been more partial to this idea than the "space elevator". I don't think the structure sticks up high enough to be a real problem with satellites. Besides, it just seems cooler than being stuck listening to elevator music for however long .

In Pohl's books, terrorists blow up one of the old models. It causes a big mess, but the damage was confined to the immediate area, thanks to having an artificial lake as a big heat sink.

In the link above about the new model, failure modes are discussed. The worst-case scenario doesn't seem that bad.

BTW, on the subject of unconventional launch mechanisms, have any of you all checked out the ZTL-Tether add-on that n0mad23 and tblaxland are working on? It's over in n0madik's add-on developer support threads in the add-on support and bugs section of this forum. This thing is :speakcool:. They've got it now so that it'll throw you to the moon perfectly. You MIGHT need to make a small MCC, like 5 seconds tops, but otherwise the only burn you have to do is braking into orbit.

Right now, they're kinda on hold as n0mad23 is moving across the country. But the next step is to make a slightly bigger and differently configured version in lunar orbit to CATCH your ship and set it down on the lunar surface. Or pick it up and throw it back to the one at Earth.