Space Elevator

[Sisyphus]

so why fix it to the ground at all?

 

Many reasons:

 

1) So you don't have to find a way to rendevouz with a dangling cable in order to use it.

 

2) So the "launch" site doesn't move all around the world, across dozens of countries, etc. If it's attached to the ground, you can keep it somewhere convenient and safe.

 

3)In order to supply power. It requires a huge amount of energy to lift a payload out of the atmosphere. If you can't get that energy from the ground, you have to take it with you in some storable form, which in turn drastically increases the size of the "elevator car," which is exactly why rockets are so expensive and inefficient. If it's attached to the ground, you just power it with electricity generated by a power plant on the ground.

 

4) And even more importantly, an unattached cable needs to be in equilibrium - centrifugal force on the counterweight from inertia needs to equal centripetal force from gravity and cable tension. That means you can't add or remove payload without doing something like attaching rockets to the counterweight, which makes the whole thing, if not useless, than much more complicated and inefficient than it needs to be. If it's attached to the ground, it's much simpler, because there's plenty of tension to spare, and it can support the weight of the car.

[SkepticLance]

Sisyphus is correct about the need to ground tether the cable.

 

Just a comment about the other end. I suggested 78,000 kms length to avoid the need for a counterweight. With that much length, the cable itself acts as the counterweight. The advantage of that is that there is no obstacle to pass when accelerating a space vehicle along the length of the cable for launch into space. Also, due to leverage effects, the total mass needs to be much less. Of course the vehicle would have to let go of the cable some thousands of kms before the end to avoid repercussions from Newtons third.

 

To me, the space elevator offers two advantage.

 

1. Lifting out of the gravity well.

 

2. Acceleration without the need for reaction mass to launch to a destination, such as the moon or Mars. With magnetic levitation methods, the vehicle could accelerate sufficiently along the 78,000 kms to need no further boost to reach Mars.

 

It is also true that a high degree of acceleration is need to avoid radiation. If the vehicle carries people and moves slowly along the elevator's length, they will all be dead from the concentrated radiation in the Van Allen Belts long before they reach orbit.

 

Thus, advanced magnetic levitation with strong acceleration is needed.

[Sisyphus]

It is also true that a high degree of acceleration is need to avoid radiation. If the vehicle carries people and moves slowly along the elevator's length, they will all be dead from the concentrated radiation in the Van Allen Belts long before they reach orbit.

 

Maybe that could be dealt with by generating a deflective magnetic field around the vehicle while it's passing through the Van Allen's. I don't know how strong it would have to be, and hence whether it's feasible, but it might be easier than other options, such as just going really fast, or carrying heavy material shieding.

[Rocket Man]

1) So you don't have to find a way to rendevouz with a dangling cable in order to use it.

rendezvous between boats is common place. i did say dip the car in the water.

 

 

2) So the "launch" site doesn't move all around the world, across dozens of countries, etc. If it's attached to the ground, you can keep it somewhere convenient and safe.

 

consider the normal ground speed: nil.

if the counter weights are mobile on the cable you can put the whole assembly into faster or slower orbitals just by moving mass by mechanical means. you can place the end of the cable anywhere on the equator with pinpoint accuracy. it doesn't wander around

 

 

3)In order to supply power. It requires a huge amount of energy to lift a payload out of the atmosphere. If you can't get that energy from the ground, you have to take it with you in some storable form, which in turn drastically increases the size of the "elevator car," which is exactly why rockets are so expensive and inefficient. If it's attached to the ground, you just power it with electricity generated by a power plant on the ground.

rockets are inefficient because they are reaction drives. they use lots of energy to accelerate against a few tonnes of fuel. E=MV^2

this accelerates payload against a rather large counterweight to get the required momentum

 

 

4) And even more importantly, an unattached cable needs to be in equilibrium - centrifugal force on the counterweight from inertia needs to equal centripetal force from gravity and cable tension. That means you can't add or remove payload without doing something like attaching rockets to the counterweight, which makes the whole thing, if not useless, than much more complicated and inefficient than it needs to be. If it's attached to the ground, it's much simpler, because there's plenty of tension to spare, and it can support the weight of the car.

 

the car will be in equilibrium with the water ie: floating. if it floats, you have a very convenient fudge factor in terms of tension. move the assenbly up, you have more aparrant weight. move it down, you have less due to bouyancy.

if you add payload to the car, it moves down in the water, this does not have to be countered by the orbital sections.

when you want to put some muscle in to it to get it moving up, you shift the masses along the cable.

 

 

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

1mm of lead isn't a tremendous amount.

[Sisyphus]

Hehe. I didn't say it was impossible. I just said it was drastically inferior. Which it is.

[egenius]

in a news paper article i read about this topic it said about having it connected to the earth. if your going to power it with electricity it needs to be connected to the earth so that we can get power to it.

 

The article i read said there was a possibility it would be in the sea out of all the flight paths.

 

thank you "Sisyphus" for putting on about powering the elevator.

[Rocket Man]

Hehe. I didn't say it was impossible. I just said it was drastically inferior. Which it is.

 

how is it inferior? the cable *must* originate from space. why not leave it there. you'll use just the same equipment to make the cable as to service it. you need to bring the end down slowly and accurately into the base station, you can't put any tension on it initially anyway.

 

locking it down puts it under threat from wind shear and all manner of debris. the only complications arise in power source and more equipment to make use of mobile weights.

 

edit: most carbon nano tubes are highly conductive, more so than silver. it doesn't take much to modify the cable to pass a current from the top end down. you can put a lot of solar collectors in space before structural constraints come into play.

[SkepticLance]

The thought of an untethered space elevator ribbon, 36,000 km long (to the central geostationary point) fills me with trepidation.

 

What you end up with is a 36,000 km long stock whip. Any movement anywhere on the ribbon ends up with a wave passing up and down that ribbon, just like the wave that runs down a whip when you crack it.

 

An untethered cable would snap at the end with more energy than a billion stock whips.

[Rocket Man]

e=mv^2

as the wave travels down a stock whip it loses mass on the wave so the velocity of the whip increases and you hear a small sonic boom at the tip.

where did all the energy come from? any movement on the cable runs up and down with the energy of what you initially did to it. by the way, isn't a stock whip firmly in the hand of the stockman? if you've ever cracked a whip you'd know that the handle is stationary when the tip cracks.

 

i'd be more concerned if the cable were tied to the ground without dampenners. the energy would have no where to go but straight back up the cable. the force would be lateral on a vertical cable so think about the vectors. that force could shred anything without slack, especially the cable.

if the cable sheared part way up, you'd have the elasticity of the cable flicking the base station like an enormous rubber band.

 

at least if its sitting in water the energy from the initial wave has somewhere to go. water is a remarkable dampenner.

[SkepticLance]

If a tethered cable is held taut by centrifugal action, the only waves able to move on that cable will be of small amplitude. Like a violin string held tight. Pluck it and you get a hum, not a whip. Leave that cable untethered at the bottom and waves can reach substantial amplitude.

[Rocket Man]

amplitude is different to energy. a high amplitude wave on a cable of low tension is easy to stop and will probably stop dead when it reaches something as light as a car.

 

when you tighten a cable, even the wind will make it ring. when the wind makes a cable ring, you get constructive harmonics. the oscillations at the bottom get sent back up the cable if it's locked down and the energy must be dissapated at the top where maintenence is difficult.

 

if you leave the cable under low tension, it won't respond to the higher frequencies which are more destructive

for a rendezvous you dip one end in the sea and you're suddenly connected to the most powerful damping mechanisim available. pull it back out of the atmosphere and you remove the source of most of the oscillations.

[CPL.Luke]

meh, you could get just as destructive oscillations from high amplitude waves, the energy would be the same and you wouldstill have constructive interference, albeit with far more amoshperic drag.

 

as for leaving the cable untethered that would lead to a base end that flys around wildly at even the slightest gust, I would not want to be on such a cable, also the point of the elevator is to reduce energy costs, making it so that it has to be raised and lowered constantly would lead to the whole thing being useless.

 

remember in order for this thing to make dollars and cents sense it wuold have to bring the cost of launching down by a factor of 10-100

[Rocket Man]

the motion of the counter weights is primarily magnetic. the energy is likely to come from a solar array which would be feasble with the launch capacity of a space elevator.

your point on the base flapping around is valid. but i'm surprised no one mentioned the lateral velocity that needs to be imparted on the car.

on a tethered lift, the energy can come from the rotation of the planet. the far weight can swing and you can send cars up on the forward swing to slow it.

you've still got a problem with that though, it would take months for the elevator to go through one full oscillation.

so you still have a logistical problem with the launch capacity.

an untehered cable needs a small booster facing west on any upward moving car, which can drastically increase the lift capactiy of any style of elevator.

 

i was mainly concerned about the stresses on the cable in the wind but if it's on the equator then it's in the doldrums where you wouldn't need to disconnect it.

[JohnF]

You could reduce the amount of energy you need by having two elevator cars connected to each other; one going up with one going down. Since there would be easy access to space you would soon start asteroid mining which means as you sent payload up you would send an equal payload down.