How long would it take to travel up an orbital elevator?

[Ursa major]

Funding would be relative I guess. We don’t blink an eye at [...] fast train links under the channel to France these days.

The train link that lost so much money, you mean?

 

[chrispenycate]

There are a couple of inconveniences that are frequently overlooked.
Firstly, due to the law of conservation of momentum, each load that goes up slows the counterweight a tiny fraction, so you either need to send as much down as you are hauling up (optimally every time something went up there would be simultaneously an equivalent load descending) of asteroid minerals, or industrial products manufactured in orbit, where there are no pollution laws, or have a reaction drive, possibly an ion drive continuously making orbital corrections. That's one reason the counterweight has to be so massive; you wouldn't want it to wobble. (On the Earth's surface you've only got hurricanes and the occasional earthquake to worry about, but up near the free fall point tensions need exponents – large exponents – to describe them; they are frankly unimaginable in terrestrial terms.) And the problem only gets worse if you go to the outer end of the tower and are thrown off into space (hoping that the computer got it right and your destination and you will end up at the same point some time in the not too distant future.

I've put a rotating space habitat round the counterweight on mine. Yes, impressive bearings, aren't they?

Another slight problem; the ground station has to be dead on the equator, and if you look at a terrestrial globe… yes, not much of it is land, and not much of the land is renown for its geological or political stability This is going to be a permanent, irresistible target for terrorists, so security, both of airspace and passengers, will have to be of the order of secret military or industrial installations, not the half-organised chaos we're used to in present day airports. Especially if there is only one beanstalk, and it's laying golden eggs.

I think the counterweight will have to be an asteroid, nudged into place with nuclear explosions; my orion technique for getting it into orbit from Earth generates just too much pollution, and lifting enough mass off the moon with mass drivers just takes too long. This indicates a thriving space industry well before construction can start.

The view? I wonder if they'd even put windows in it – structural weaknesses. Of course, there will be observation ports and telescopes for rent at geostationary, but you're so far away by then that Earth is much less impressive than at LEO.

 

[MstrTal]

Before you can talk about the practicability of funding such a project one has to consider the practicality of building it. I know that before I said I didn't see how it could be done. While having read the linked wiki article and googled some others I can now somewhat understand the concept better there is still another consideration.

While I will never even remotely pretend to understand the math or engineering involved in such an undertaking I have worked construction before. I have built bridges and steel structures and while I understand that the materials involved would be much more advanced than rebar and girders I still can not see how such a structure would be built from the ground up.

Even if you built it in sections and then assembled like an erector set when completed there is no way you are building the bulk of such a structure in the atmosphere. All you have to do is look at the super structures of today's tallest buildings to see the feats of engineering and countermeasures that go into just keeping such buildings aloft. Honestly about the only way I can conceive of completing such a structure is to build the bulk of the superstructure in space and then attempt to insert it into the earths atmosphere and anchoring it to a pre-built receiving station. Of course you would only have the one attempt and it would come with a whole world of issues of its own. No pun intended.

 

[Venusian Broon]

I still can not see how such a structure would be built from the ground up...only way I can conceive of completing such a structure is to build the bulk of the superstructure in space and then attempt to insert it into the earths atmosphere and anchoring it to a pre-built receiving station. Of course you would only have the one attempt and it would come with a whole world of issues of its own. No pun intended.

erm, In one of my favourite Arthur C. Clarke books (although to be fair nothing much really happens in it), The Fountains of Paradise, which is about the construction of a space elevator onto Sri Lanka, this is precisely what happens - although they have more than one attempt, which is related to something in the story.

I also think he invents an extremely strong material so that it is anchored in cables one at a time (it's been a while since I've read this, so I am ready to be corrected on this), rather than building the full 'tower' and lowering it down. So they build it up piece by piece (a bit like the threaded wires they use to build the suspension cables of bridges.)

 

[chrispenycate]

Actually, you build out, up and down, from the middle, your counterweight, stable in geostationary orbit. Since both directions are effectively "down", one towards the planet and the other thrown out by – yes, I know 'centrifugal force' is a term that shouldn't be used, being just an effect of inertia, but it's a force, and it's in the opposite sense to the planet which is the centre of our localised system, so what better phrase? – they don't have to be rigid to stay straight. The trick is to stop the thing starting to rotate.

Alternative technique – build the thing out by the orbit of the moon, then nudge it complete, tower and all, with its centre of gravity in geostationary orbit. Of course, when the end of the tower hits atmosphere it's not going to be synchronous with Earth's surface, so wind resistance is going to give some interesting stresses.

I like orbital towers (oh, you'd noticed).

 

[MstrTal]

Interesting.

So what your saying is it would have to be built in space regardless.
I may not know math and engineering but it's good to see that I have some common sense.

 

[Galacticdefender]

Five days sounds quite comfortable, and not over wasteful. After all, you want to reclaim as much of the kinetic energy as you can,
Actually, since the point of highest velocity is deeper into denser atmosphere the trip down should take longer, not less time. I suppose we could cut the trip shorter by building a tube onto the side of the tower and evacuating the air; it's air friction that's burning us up like a meteorite), but unless the tube itself is diamond, or something like that with an inherently high tensile strength, this is going to complicate construction considerably (and it never looked easy).

Uh, who says the elevator can't be heat sheilded? If you just let it fall, it would get down in about the same time as a space shuttle (or, if its not heat shielded, it could slow while it is first making contact with the upper atmosphere, then speed up once its at a reasonable speed). Just have some means to slow it down near the ground, like a big "tunnel" of magnetic braking rings for the last half-mile or so down.

It's not the propulsion system that counts; with sufficient available energy (and yes, I suspect lossless superconducting cables will be essential over the tens of thousands of kilometres involved) a Laithwaite linear accelerator will do the job fine. Think of it as a vertical bullet train And I really wouldn't want to aim a laser that close to the tower; can you imagine the disaster if you succeeded in breaking it? (If you think of it as a cable it could wrap itself clear round the equator and eat its own tail, multi-megatonnes coming in at meteorite speed). And laser power can not be recuperated on slowing down, unlike your coilgun equivalent.

It would be as simple as having an emergency shut off if the laser goes out of alignment. And energy isn't really much of a problem in my universe, because everyone fuses helium 3.

Geostationary is essential ( and I don't think the shuttle goes up that high), otherwise the counterweight is wrapping its tether round the Earth. Of course, you could get off earlier – put a request stop half way up – but this would not be in orbit, and you would start to fall toward the planet unless you added some serious delta v sideways.

Yeah, after some reading on wikipedia I realized that low orbit was not at a high enough altitude.

I've used the model with the counterweight in geostationary orbit, and built the tower outward at the same time as inward. Less maximum tension in the structure, and you don't need to fly in an extremely unwieldy structure (though yes, you need a bigger counterweight). This means end structure is ±36,000 km from the surface. If you can average 1,000 km/hr, that's 36 hours, and your maximum speed is twice that; serious energy losses and heating if there is appreciable atmosphere; much better to take it more gently. But acceleration is comfortable, nothing like the ten or twelve gees you get in a rocket.

As I said earlier, if you could even get to one tenth the speed of a space shuttle, it would only take about 85 minutes to get into low orbit, plus a bit of time to get into high orbit. I think an Orbital elevator could get going faster than 1000 kph when it is in space and there is no atmosphere to cause friction. If I recall correctly, we have some aircraft now that can go that fast.

Or, I could have absolutely no clue what I'm talking about.

And I can't just ignore how long it takes, because I very much want to describe the trip up, at least in part.

 

[Bowler1]

The train link that lost so much money, you mean?

Yes my cute little, bear, the very train link. Anything with a link to France was always going to hit your wallet!

You could also say the M25 is a money pit, and it is, but without it London would be a nightmare. Our beanstalk to the heavens may not lay any golden eggs, but I'd like to think lots of Jack the lads would be zooming up and down knocking out hooky gear!

No income tax, no vat, no money back, no guarantee.....

 

[Galacticdefender]

There are a couple of inconveniences that are frequently overlooked.
Firstly, due to the law of conservation of momentum, each load that goes up slows the counterweight a tiny fraction, so you either need to send as much down as you are hauling up (optimally every time something went up there would be simultaneously an equivalent load descending)

Easily solved by having two strands, sending up one pod then sending down another at the same rate.

 

[chrispenycate]

Uh, who says the elevator can't be heat sheilded? If you just let it fall, it would get down in about the same time as a space shuttle (or, if its not heat shielded, it could slow while it is first making contact with the upper atmosphere, then speed up once its at a reasonable speed). Just have some means to slow it down near the ground, like a big "tunnel" of magnetic braking rings for the last half-mile or so down.

Oh, heat shield it, fine, no worries. I just don't want to have to pull the thing out of service after every trip to replace the ablative tiles; just unload, bring in the next lot, and off it goes again. Maximum of an hour's turnaround. This is supposed to be a civilised means of transport, smooth enough that you don't spill the port.

And all that multi-thousand degree plasma right next to the tower? Sounds like a bad idea to me; even if the structure doesn't degrade, lots of other bits are going to wear out faster.

How big are your fusion generators? It might be more efficient to mount a couple on the 'train', rather than use superconducting cables. Less elegant than using the energy from the falling unit to power the rising one, and gives further cooling problems, but means a lot less non-structural weight in the tower.

So, you're into a double speed curve; the first 120 km we take nice and slow, not even breaking the sound barrier so as not to annoy the neighbours, then, when outside almost all of the atmosphere, accelerate at 5 metres/second/second until we're halfway there (or a bit more) where we swap ceiling for floor and start slowing down just as fast. 120 km at an average of 600 km/hr = half an hour then put your foot down for half an hour to bring you up to about 36,000 km/hr, at which point you flip and start slowing - not braking, using the kinetic energy to generate electricity that you store, or use for something. Less than two hours door to door, not recommended for heart patients and I hope all your bearings are friction free. Theoretically you could use higher deceleration rates, as the remains of Earth's gravity is in the opposite direction, so keep speeding up longer. Means a lot more coils along the outside of the tower than my first diagram, and I'm not sure I want to ride in it, but it gives you your express service and since you're reclaiming a high percentage of the energy, not too expensively.

Do you have any idea how long it's been since I last did differential equations?

 

[Galacticdefender]

Oh, heat shield it, fine, no worries. I just don't want to have to pull the thing out of service after every trip to replace the ablative tiles; just unload, bring in the next lot, and off it goes again. Maximum of an hour's turnaround. This is supposed to be a civilised means of transport, smooth enough that you don't spill the port.

And all that multi-thousand degree plasma right next to the tower? Sounds like a bad idea to me; even if the structure doesn't degrade, lots of other bits are going to wear out faster.

Well laser propulsion even today is considered a possible means of propulsion for an orbital elevator, so by the year 4150 I think they would've figured out how to make it safe.

How big are your fusion generators? It might be more efficient to mount a couple on the 'train', rather than use superconducting cables. Less elegant than using the energy from the falling unit to power the rising one, and gives further cooling problems, but means a lot less non-structural weight in the tower.

Uh, saying this you assumed the laser propulsion wasn't being used. But that's how I'm going to go ahead and write it, so actually the fusion generators are on the ground, as well as on the anchor station. All they need to do is power the lasers. The "train" or "pod" is pretty much free on the strand, though magnetically stabilized. Speaking of magnetism, couldn't that be used to get a pod up an orbital elevator as well? Might be an idea for a different story.

So, you're into a double speed curve; the first 120 km we take nice and slow, not even breaking the sound barrier so as not to annoy the neighbours, then, when outside almost all of the atmosphere, accelerate at 5 metres/second/second until we're halfway there (or a bit more) where we swap ceiling for floor and start slowing down just as fast. 120 km at an average of 600 km/hr = half an hour then put your foot down for half an hour to bring you up to about 36,000 km/hr, at which point you flip and start slowing - not braking, using the kinetic energy to generate electricity that you store, or use for something. Less than two hours door to door, not recommended for heart patients and I hope all your bearings are friction free. Theoretically you could use higher deceleration rates, as the remains of Earth's gravity is in the opposite direction, so keep speeding up longer. Means a lot more coils along the outside of the tower than my first diagram, and I'm not sure I want to ride in it, but it gives you your express service and since you're reclaiming a high percentage of the energy, not too expensively.

Ha, best answer I've been given all thread long. But why would coils be needed on the outside of the tower? (Coils for what?) I guess everyone would be strapped in tightly though . How many G's would this produce? It wouldn't be as bad as a space shuttle flight, would it?

 

[chrispenycate]

Laser (or preferably, in this case maser) launcher's for free capsules, best into LEO (but no reason why it couldn't go further.

Coils? You know how a linear accelerator (railgun) works? Polyphase power (just ordinary three phase in the one I built back in university days) is run through coils separated on a rigid support, generating a continuously advancing magnetic field. Anything conductive (and not merely ferromagnetics; Heinlein got it wrong in "The moon's a harsh mistress") will attempt to stay in the middle of the field, due to eddy currents if it slips. By changing the spacing on the coils you can change the stable velocity of the conductor – in my case, logarithmically increasing the spacing the length of a two metre plastic drainpipe took ball bearings and small screws slightly over twice the speed of sound. I'd intended the coils to be wide spaced on the main continuous run bit (one every kilometre? Two?), and closer together at the ends where the main delta v is taking place. Cruising speed took too long for you, so I went onto continuous variable acceleration, and needed to cram the coils closer together (increasing mass and stresses in the tower structure; and cost) – possibly one every ten metres or so?

Rather than run the polyphase power up our superconducting cables (and I really need superconducting for runs of tens of thousands of kilometres; for the coils, too. If you can't invent me room temperature superconductors you're going to have to pump liquid helium around) I'm planning to send up DC, and every single coil has a control box calculating phase angle and a fibre optic link (maybe with boosters) handling overall control; that way you can either do your strap in and grit your teeth method (but yes, it is smoother than the shuttle, so you can actually grit your teeth) or do the ambulance run getting heart patients up to microgee where their circulatory system is under less stress in a dignified few days. In your 'I'd watch the in flight movie but my eyeballs are squashed' express you don't get out of your seat for a pee.

You could have another generator on the counterweight, you know. I'm worried about power failure on something travelling far faster than a meteorite. You can't just put the brakes on; if you shorted out the coils to bring it to as rapid a halt as possible your passengers would be reduced to meat paste, so I suggest not putting buffers at the top, leaving the rail aimed off into space… you might even be able to catch up with one of them and save people if you were really lucky, and had a ship in the right place.

You can't use laser propulsion next to the tower; what you'd get would be laser powering. Which means converting energy from one form to another, and that's never lossless. And the bottom hundred kilometres or so are in atmosphere, defocusing and absorbing your power. It's an alternative solution to the 'loss of power in tens of thousands of kilometres of cable' problem, and eliminates the 'what it the interaction betewwn all that current flowing and the Earth's magnetic field' question, but I'm not convinced.

 

[RJM Corbet]

... the physics of it escape me ... Wouldn't the earths rotation, winds, gravity, the van allen belts and all sorts of other things let alone the earths rotation around the sun cause it to whip around and crash back into the earth? ...

Think of a lead weight on the end of a string swinging around your head: the counterweight attached to a graphene cable, very light but incredibly strong, keeps the cable taut against the earth's rotation. That's the concept. With the development of graphene/fullerine technology it is being investigated by NASA etc, as a serious possibility ...

 

[psychotick]

Hi all,

It's late and I'm tired, so this may not make as much sense in the morning as it does to me right now. But why get hung up on a 36,000 km journey to find a nice point at which a counter weight will spin holding up your elevator. That seems like way too much trouble. And can you imaginehow many rockets it would take to get all that weight up there? Better just to go to say a high earth orbit satellite level, say 200 kms, and use fuel burns to keep the top end in orbit, and it'd be a hell of a lot easier to build.

So here's my sleep deprived suggestion for how to build one say in the next ten years. I think it's actually realistic, just not the way everyone seems to envision one.

So start with a shuttle launch (ok that's a slight bummer there, but never the less, we could unretire one of them for one flight - and no landing by the way).

The shuttle carries with it a four hundred km long loop of microfiliment cable. You know, one of those super strong super light filaments we're always being told exists in research labs.

Now it reaches a nice geostat orbit two hundred kms up above say Cape Kennedy, and adds a one ounce weight to one part of the cable, fires it downwards with just enough force to get it locked into a re-entry flight, and when the weight of the sinker is enough, slowly lowers it to the ground. Yes I know wind's going to be a ******* and the thing will fly in all directions, but if the weight has a lowjack in it, that could be dealt with.

Now we've got a shuttle in orbit with a two hundred km long loop of cable hanging from it. We secure the bottom end to a sort of very loose pulley - obviously there has to be a lot of slack in the system to allow for wind and for the shuttle not being able to maintain a perfect orbit, and the top part at the shuttle end is also on a very loose pulley.

In essence we've just made a cable car.

Now start turning the pulley wheels, with on the upwards run say maybe five pound bags of goodies every km or so. The clever part about this is, that to launch say a ton of payload into orbit I think you need roughly a thousand tons of rocket fuel. To slowly winch one up, far far less.

Now in these payloads going up, you need to allow enough of them to be carrying fuel to keep the shuttle flying / orbiting. And the rest is bonus stuff that you can use to start building a massive space station. One that can maintain its own orbit. And then as you build it, you can slowly add more cables or thicker microfilaments, and little by little get to a cable car system strong enough to lift people into orbit.

And as for time, say if it travels at fifty km perhour, then a four hour ride?

Building an actual elevator with tubes and lift shafts and what ever else seems an incredibly difficult task. But why do you need to do that if all you want to do is get people and equipment into orbit?

So is this simple genius? Or the delusions of sleep deprivation?

Cheers, Greg.

 

[psychotick]

P.S.

If NASA decides to go with this I'm going to demand that it's called the Psychotickavator!

Cheers, Greg.

 

[chrispenycate]

Now it reaches a nice geostat orbit two hundred kms up above say Cape Kennedy, and adds a one ounce weight to one part of the cable, fires it downwards with just enough force to get it locked into a re-entry flight, and when the weight of the sinker is enough, slowly lowers it to the ground.

Geostationary orbit = 35786 kilometres above the Earth's surface. And it has to be over the equator. Bring your anchor point lower down and it will either move relative to the Earth's surface (quite rapidly, at the height you're suggesting) or you'll have to give it continuous lift, like a hovering helicopter. Lots of lift; it'll probably be able to stay there for about five minutes before fuel runs out. Certainly not long enough to lower your tether to Earth.

Of course, you could give up on all this stationary lark, and just have the end of the skyhook higher than any in-path mountains and hook onto it with a plane. Bit of a jolt, mind, and since the aircraft would be of comparable mass to the shuttle, the centre of gravity of the pair would remain at the same height, quite possibly within the atmosphere. Inconvenient.

No, there are reasons for that orbit, and that the counterweight be at least two orders of magnitude more massive than the heaviest load sent up. That's why satellite TV mushrooms can always point to one point in the sky, don't need to swivel round like astronomical telescopes. A particular distance from the centre of a particular planet defines a particular speed for an orbit to be stable (yes, I can show you the maths, but you don't really want me to, do you?). For this speed to give the same angular velocity as the Earth's speed of rotation, there is only one solution to the equation. Maybe a couple of minutes off precise 0° for the ground station might be acceptable (although that's an awful lot of force pulling the counterweight off line) but the main damping mass must be vertically over some spot on the equator. No choices, no alternatives, there.

 

[psychotick]

Hi,

Thanks Chris, and may I just say - Bugger! It seemed like such a brilliant idea last night.

Ok so 36,000 kms seems a little long even to me, no matter the strength of the micro filament. But you can still do this by going lower and maintaining a geo synchronous orbit, and burning fuel to maintain your altitude. Any idea how low you could go before the amount of fuel you would have to burn to maintain your altitude would become prohibitive?

Cheers, Greg.

 

[James Coote]

Maybe this is stupid, but why not have a water pipe up the central shaft of the elevator. You pump the water up to the station and then fill the elevator cars with it to act as counter weights on the way back down.

If the pipe is always filled, the mass is evenly distributed along the length of the shaft, regardless of whether it is moving. If you need to add mass to the counterweight station, you just pump more water into an inflatable bag at the top

The only problem would be insulating it, or instead of water, you could use some liquid with a very low melting point, but boiling point higher than planet surface temperature

To get over the putting a large mass in geostationary orbit, you could have the skyhook idea attached to a LEO station, but make it a pipeline instead of an elevator. Then you could pump liquid plastics to manufacture new pipeline in orbit. Once you have a long enough pipeline, you pump up fuel to a rocket that pulls the end of the pipeline up to geostationary. It then hooks onto the inflatable bag and you start pumping up water/mass. Finally, you attach the skyhook end of the pipe to the ground and start building your elevator

Edit: Since I have far too much time on my hands, I made an illustration to go:

 

[chrispenycate]

Let's have a look at your ideas.

The first one, the water counterbalance. Despite what your common sense is telling you, up near the counterbalance, the water has no weight, so it does not tend to pull the lift bit upwards. For it to be able to apply a force we'd need to use just the lower bit of the tower with water, and have the piece of string going over a score of pulleys, gearing it down fiftyfold, so the lift capsule travelled fifty times the distance of the water, at a fiftieth of the speed. Since we are limited by atmospheric friction in the lower bit we've probably extended the trip to several months.

Not to mention the "piece of string" itself. The chief problem of constructing the tower is finding a substance that can support thirty six thousand kilometres of its own weight. Call that thirty thousand, and we'll build up a six thousand metre mountain to meet it. A braided steel cable would stretch like taffy near the geostationary point. (Actually, it could be built of steel cable, in a sort of inverted pyramid, but the size of the construction as you approached orbit would be something ridiculous, a cross section like the surface area of London). The weight of that length of fishing line would be in the tens of thousands of tons (and wouldn't hold its own weight). Far better to have a rigid structure and force against it than attempt something flexible.

Now, our 'hosepipe into space' version. The rocket towing that multi-megaton hose is going to have to be something quite special;I suspect only an orion design would deliver enough power (it's all right, it's over the equator; who's going to worry about a bit of fallout?). And it's far easier to lift the entire hose into space, and drop one end. now, what are we going to make it out of? It has to be strong enough to support not only its own weight but that of the water inside it, and it has to stand a pressure… Well, ten metres of water equals an atmosphere of pressure, if I remember correctly, so a kilometre is a hundred atmospheres. Would it be safe to judge that with gravity decreasing with height we could estimate about half the height of the column equivalent at sea level? So we have a mere million eight hundred thousand atmospheres of pressure; quite some hose. And quite some pump driving it.

At the same time your added weight is slowing your satellite. You're going to need to accelerate an appreciable percentage of your water off as an engine keeping the tower in place. How much depends on how efficiently you can bring it up to what speed; boiling it as steam propulsion would lose lots, a long ion accelerator bringing it near light speed much less. I'm convinced that nudging a near-Earth asteroid into a stable orbit would involve a lot less work, even if the exchange risk is a powered dinosaur killer.

 

[Sapheron]

Out of interest, Chrispy (or any of you others with more of a handle on these sorts of things), how big an asteroid are you thinking of? I assume rather a large one. You mention a dinosaur killer there, but would one quite that large be necessary? Wouldn't just a few hundred metres across be ok? I mean, that would still weigh several orders of magnitude more than anything we'd possibly want to send up or down, and that weight ratio is the only issue, as I understand it. Obviously then, the bigger the asteroid the higher yield possible from the elevator, but the difficulty and potential consequences of using a larger asteroid scale up as well. If we drop something a few hundred metres across we might just have to look embarrassed when people mention those countries that no longer exist, but if its a dinosaur killing kilometres sort of thing we might be heading into post apocalypse territory.

As a second point, as I understand, the bigger things are the further apart they need to be. If Jupiter was where the Moon was we'd be in for quite a ride. If the Moon was only a few thousand kilometres away from Earth things would quickly go wrong. Satellites we put up are, in the scale of things, tiny, so this factor doesn't matter. If we put the anchor for an elevator up at, as you've said, approximately 36,000 km (a small, but perhaps important, percentage of the distance to the Moon), and it was a solid lump of rock in the order of hundreds to thousands of metres across, wouldn't this become a problem? If the Moon's gravity began to move the anchor, even slightly, I could see this causing huge problems. A little bit of compression, and the whole life buckles; a little extension and it will come apart. Neither is a good result.