The Space Elevator

[onejayhawk]

Right, and when you attach a 1000 lb car to climb up the ribbon, that will be 1000 lbs of force pulling down on the satellite. It would force it downwards over time, unless there was a mechanism by which the force (keeping the satellite in space) was 'recharged'. This outwards pull is maintained by having the bulk of the mass outside of geosynchronous orbit but tethered to Earth to keep it from flying away. A mass going at 'geosynchronous' velocities, but higher that GEO orbit would fly off into space.

As soon as the majority mass falls below GEO, it would be going too slowly to stay up.

A ton or two would not greatly effect the angular moment. Still, orbital adjustment would be an important issue. Rocket jets are very inefficient. There are various forms of drives using solar power. In fact, the Earth's magnetic field could be used as a generator or a motor, depending on whether you want to go up or down.

Also, things would be coming down as well as going up.

J

 

[Thorgalaeg]

Dont know if it has been commented yet, but what about the skyhook?

It is not as cool as the real space elevator and you need a plane to board it, but it seems much more feasible.

 

[El_Machinae]

A ton or two would not greatly effect the angular moment. Still, orbital adjustment would be an important issue. Rocket jets are very inefficient. There are various forms of drives using solar power. In fact, the Earth's magnetic field could be used as a generator or a motor, depending on whether you want to go up or down.

Also, things would be coming down as well as going up.

J

Things coming down only really add to the force pulling down the center of gravity. And,while a ton or two would not affect angular moment much, there'd be a goal of putting on as much weight as possible.

Seriously, centrifugal force keeps the tether taut. It's what allows rebound of the counter-weight to prevent it from being pulled down by the elevator cars.

 

[Silurian]

You also have to consider wind. While in general the mass of air is moving with the planet, there's still winds pushing on the tether all the time. That would mean both that a high strength tether would be needed, and the masses on both ends would have to be great enough so that the wind couldn't move them much.

As I pointed out in the other thread.

If a 30,000km long cable was deflected 1km side ways 10km up it would be stretched 50m assuming the ends are fixed.

A stain of 0.000166% is unlikely to require a very large wind loading on the cable.

The mass required to hold the bottom of the cable in position would be relativilly managable but unless the cable has a Modulus of Elasticity many orders of magnitude greater than existing carbon fibre it will stretch due to the wind and other forces.

 

[CavLancer]

Dont know if it has been commented yet, but what about the skyhook?

It is not as cool as the real space elevator and you need a plane to board it, but it seems much more feasible.

I'd never heard of this before, thanks!

 

[peter grimes]

I read a thing on Ars a while back about a scheme to recover a Space Shuttle crew by sending up a second shuttle to rendezvous.

The shuttles would orient 90° to each other, cargo bays facing each other. The shuttles would be 14' apart. That means their orbits differed by 14'. The small difference was enough to result in the shuttles drifting apart, necessitating constant attitude and orbital adjustments during the rendezvous.

So it may be the case that the cargo would draw "that much" angular momentum, but it still must be accounted for.

 

[hobbsyoyo]

Oh so now you finally want to talk about the Columbia rescue article, eh?

 

[onejayhawk]

I'd never heard of this before, thanks!

The Skyhook is interesting in part because it would have pseudo gravity at each end. Also the orbital adjustments could be done using the electric motor dynamics mentioned above. The short version is that the Earth provides a rotating magnetic field. Apply current to a conductor in that field and you get elctromotive force, which can be used to keep the orbit from decaying. The sticky problem is radiation shielding.

The mass required to hold the bottom of the cable in position would be relativilly managable but unless the cable has a Modulus of Elasticity many orders of magnitude greater than existing carbon fibre it will stretch due to the wind and other forces.

That is a significant issue. Stretching/contracting of 0.01% is 3.5 km over the length of the ribbon.

Concerning weather, Our calculations show that the cable should survive wind speeds up to 72 m/s

That converts to 161 mph.

http://www.niac.usra.edu/files/studies/final_report/521Edwards.pdf page 13

J

 

[CavLancer]

Radiation shielding wouldn't be any worse than that encountered by astronauts going to Mars or on the space station, yes? It would be less than a Mars ship as it would still be within Earth's magnetic shield, what remains of it anyway. Or does the hook and magnetism somehow attract radiation to the spacecraft?

If that's not the case then could a space in the interior be built for astronauts to go in radiation events?

Lots of questions Jay, sorry. It seems you know your subject.

 

[El_Machinae]

The radiation will only be half that experienced in deeper space, because the Earth will block half of it.

 

[CavLancer]

Thanks!

 

[onejayhawk]

Radiation shielding wouldn't be any worse than that encountered by astronauts going to Mars or on the space station, yes? It would be less than a Mars ship as it would still be within Earth's magnetic shield, what remains of it anyway. Or does the hook and magnetism somehow attract radiation to the spacecraft?

If that's not the case then could a space in the interior be built for astronauts to go in radiation events?

Lots of questions Jay, sorry. It seems you know your subject.

My brother is a physicist. I'm the numbers guy.

The radiation is still far above safe levels for long term exposure. The key point is long term, as in years to decades, even generations.

This approach could be used for colony habitats. Using two tethered masses, you could avoid spinning things to produce artificial gravity. This is another reason why a lunar elevator is such a good idea. You can dig in on the moon and have gravity for free. In free fall, both are continual issues.

J

 

[CavLancer]

Something close to my heart is a colony ship. The radiation has always concerned me though. No point in heading for an Earth like planet for 10 (or however many) generations if when one gets there procreation is impossible. Alternately if when one gets there the ship is a cemetery. Plus, blowing off nukes behind the ship can't help. Whoa, that might be an interesting book. A colony ship sails into Earth orbit from another, alien, civilization, loaded with high tech, and all the aliens are dead from radiation. Imagine the war that would break out on that ship...could be a good read.

 

[uppi]

It's a satellite. The same thing that keeps all of them up.

This satellite has a tail that drags the ground. That's why you start at the top and run the ribbon down.

J

You can tell by a simple conservation of angular momentum argument, that the satellite will not stay up if you keep bringing things into orbit. So you need huge ion thrusters (or something similar with large exhaust velocities) to keep the counterweight in the orbit it needs to be.

Edit: or can you supply the angular momentum from earth through the cable? I am not sure, it gets quite complicated when the cable has to go around in loops around the earth (and you would need an even longer cable)

 

[onejayhawk]

You can tell by a simple conservation of angular momentum argument, that the satellite will not stay up if you keep bringing things into orbit. So you need huge ion thrusters (or something similar with large exhaust velocities) to keep the counterweight in the orbit it needs to be.

Edit: or can you supply the angular momentum from earth through the cable? I am not sure, it gets quite complicated when the cable has to go around in loops around the earth (and you would need an even longer cable)

A looping cable would not be stable. The amount of thrust needed to maintain orbit is a question I cannot answer. "Huge ion thrusters" may be excessive. As noted above, application of current through a conductor in a rotating magnetic field produces electromotive force, which is the basis of electric motors. Solar power is readily available. If practical, this would involve no thrusters. Also, the system can gain angular momentum by catching objects in orbit, such as incoming loads. There are a lot of metalurgical processes that are environmentally dangerous. Moving these off planet would be helpful on several levels. Providing regular downcoming loads is just one of them.

J

 

[uppi]

A looping cable would not be stable. The amount of thrust needed to maintain orbit is a question I cannot answer. "Huge ion thrusters" may be excessive. As noted above, application of current through a conductor in a rotating magnetic field produces electromotive force, which is the basis of electric motors. Solar power is readily available. If practical, this would involve no thrusters.

That wouldn't work for keeping the counterweight in orbit. To generate momentum in space you need to eject material. If that material comes from earth you need to eject it with enough velocity to gain significantly more momentum than you lose by transporting it there. With current technology there are only ion thrusters capable of doing that.

Also, the system can gain angular momentum by catching objects in orbit, such as incoming loads. There are a lot of metalurgical processes that are environmentally dangerous. Moving these off planet would be helpful on several levels. Providing regular downcoming loads is just one of them.

J

In principle true, but as there will be some drag and friction losses it would only work if there is more going down than going up. Until there are significant space mining operations bringing material back to earth, there will be a lot more things going up than down.

 

[onejayhawk]

That wouldn't work for keeping the counterweight in orbit. To generate momentum in space you need to eject material. If that material comes from earth you need to eject it with enough velocity to gain significantly more momentum than you lose by transporting it there. With current technology there are only ion thrusters capable of doing that.

Why? Electromotive force has no ejecta. It is also available continuously.

In principle true, but as there will be some drag and friction losses it would only work if there is more going down than going up. Until there are significant space mining operations bringing material back to earth, there will be a lot more things going up than down.

Inefficiency is like death and taxes. Your designs allow for it.

J

 

[CavLancer]

If they wish to know the parameters of inefficiency I'd be happy to contribute my services.

 

[uppi]

Why? Electromotive force has no ejecta.

And exactly that's why it doesn't work in space. You cannot just put wheels on your spaceship and expect it to drive anywhere. You need something to apply that force to. Newton's third law and all that.

 

[onejayhawk]

If they wish to know the parameters of inefficiency I'd be happy to contribute my services.

Talk to Cutlass.

And exactly that's why it doesn't work in space. You cannot just put wheels on your spaceship and expect it to drive anywhere. You need something to apply that force to. Newton's third law and all that.

Electromotive force is applied to the conductor. It need not be wrapped around a rotor. It can be used to excelerate a considerable length of ribbon. Or you can draw power off and decelerate.

J