Designing a portable space elevator

Question

So, I have been trying to come up with ideas for space launch without rockets for this sci-if project, but then something occurred to me: rather than have craft jumping off and onto a planet’s surface, why not have them remain in orbit and instead have the occupants go up and down?

Here’s the idea: the ship contains a sealed landing-capsule or pod. This pod is attached to an incredibly long cable of carbon-nanotubes which is coiled up most of the time, and lowered down to a planet’s surface to pick up/drop off passengers. It basically functions as a portable space elevator. It will naturally take up a lot of space while coiled up, but since this ship spends its life in orbit, it can afford to be chunky.

Are there any major or unsolvable flaws with this design?

Answer 1

You want that space elevator? You got a space elevator! Don't listen to people who think that today's science is relevant

I've been having a number of discussions with people lately who are dead set on turning this Stack into physics-lite. Who cares if we can't do it today? What arrogance, that we think that we know the end-all of science today — but it's a common (and mistaken) belief.

• Authors in days past who dealt with incredibly long tether/rope structures used the technobabble "monofilament" to describe something that doesn't require 30% of the volume of the ship. You're using the technobabble "carbon nanotubes" to do the same thing. Good for you! In the long run, how much volume or mass the tether takes on your ship is irrelevant to your story (unless you, the author, make it so).¹

So, you've a ship, you've a long ropey-looking-thing. And you want to shuttle people and stuff up and down it. (You probably have an on-carriage or on-ship power source to make that happen — which also has nothing to do with your question, if anybody's wondering). What are the problems you need to overcome and how can you overcome them?

1. Weather. No matter how you design your tether, it's going to move around in the weather. In fact, the lighter your rationalize that tether to be, the more the weather will affect it. Wind frequently moves in different directions and at different speeds at different altitudes. Your intrepid crew will obviously look for the calmest spot nearest their target location — but the odds of anything being perfect (for long...) are pretty long.

2. Electricity. Carbon nanotubes are electrically conductive. All carbon is to one extent or another. Not only are you dealing with static electricity in the atmosphere, you're dealing with the varying electrical charge in the ground plane (the Earth's surface is not universally the same voltage potential, and it changes), any generated electricity due to the passage of the carriage and (and this is really fun), a piezo effect due to the mass of the tether in the gravity well stretching the tether. And heaven help us if the tether gets struck by lightning (it will eventually). All that electricity needs to go somewhere. You could absorb some of it on your ship, but it's impractical to assume your ship can absorb an infinite amount of electricity. Where does the rest go when your ship hits its limit?

3. Slack. It's a bit unrealistic to believe your ship can hold itself precisely over one spot at a single altitude all of the time. Even geosynchronous satellites have thrusters to make adjustments now and again. This means that the amount of tether touching the ground (or not...) will vary over time. That's actually bad, I mean, what happens if the daughter of the Preeminent and Benign Monarch of Skazzlewhoop! happens to have her ankle tangled in that nearly weightless and incredibly thin tether? When she finally hits the ground, the troops can start looking for her foot. No, you don't want that. You need the trade deal.

4. Others have mentioned the motion of the tether due to the difference in angular momentum. I'll let those answers deal with that.

So... there's some problems. What to do about it?

Obviously, you need some kind of base

Now, in a Clarkean Magic world, this base would self-anchor to bedrock in a manner that can be released without too much cost at a later date. It also needs a bit of mass by itself. But what it really needs are:

• A way to adjust the slack in the tether, not only to help compensate for the inevitable imperfections of orbit, but to dampen vibration due to the weather. This is actually really useful as it provides a mechanism for feedback to the ship for its own stabilizing adjustments. After all, it would cost too much to build a base that could deal with any amount of slack... and you need a way to avoid the problem of the tether ripping loose. Yeah... feedback is your friend.

• And a way to quickly and safely disperse what could periodically be a sizeable amount of electricity. Some of that can be absorbed by the ship. Some can be used to help power the carriage. Some can be used to power the tether base itself. The rest needs to shunt to ground.

Considering the natural size required for this base, which you want to gather back up to the ship and reuse, it's reasonable to assume it could be used for temporary storage and quarters while people are on the ground.

How you engineer this base is up to your imagination. Remember footnote #1.

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_{¹ One difference between writing fiction and non-fiction (aka, novels and textbooks) is the use of metaphor. Metaphor allows the author to substitute simple ideas for complex issues that aren't relevant to the story. I was president of a micro-publisher for ten years, and I know from experience that a good story can be ruined by the distraction of too much detail. If you don't believe me, go read Dostoyevsky's Crime and Punishment.Wow, that book was a tedious read. Anyway, something isn't "realistic" simply because it's explained in terms of today's science or assumed in terms of today's mathematics. It must also be something your reader can relate to, something which allows them to become a part of the world your building. So, please don't get offended if you're using "carbon nanotubes" in a sincere effort to be "realistic." Understand, instead, what you're doing as an author: you're using a metaphor to substitute for something complex that would be a distraction to explain in detail. In its vulgar form, you're using technobabble. That word isn't as evil as you think. Today's science is yesterday's nonsensical dream.}

Answer 2

The Pod Can't be Lowered

The fact that a space elevator is tethered at both ends is fundamental. When you descend, you need to transfer your kinetic energy into something else (as per conservation of energy). In a rocket, this is exhaust (de-orbit burn) and aerobraking. In a space elevator, your kinetic energy goes into the rotational speed of the Earth.

In your case, unspooling the cable does nothing. Both your ship and the pod are in orbit, unspooling the cable doesn't change that. You still need to somehow get rid of your pod's speed so that it can get down.

You can pull the pod off the surface (subject to everyone else's caveats), but you still need to get the speed from somewhere. Either your ship in space will slow it's orbit to speed the pod's or it will need to use thrusters to maintain speed.

TLDR; Space elevators trade speed with the Earth. No connection, no speed trading.

Answer 3

At present, it is far beyond our abilities.

It absolutely only works over the equator of the planet. Otherwise the lower end is moving by at huge speed relative to the ground.

You want your ship to be in geo-stationary orbit, or the equivalent, for the planet you are approaching. For Earth that's about 35 thousand km. (Corrected this value after seeing another answer.) Again, otherwise the lower end is moving relative to the surface at a huge speed.

You will need a very long rope.

You need to do something to balance. As you lower something you need to extend something upward as well. Otherwise you screw up your orbit. The upward thing does not have to be as long, but needs to equal the lever-arm, like a teeter-totter. So if you want the lower one 1000 times as long the upper needs to be 1000 times the mass. It's actually a little more complicated than that due to tides and orbits and such, but as a back-of-the-keyboard calc, it's roughly that. Extend 1000 kg 25 km up to let 1 kg go down 25 thousand km.

You might be able to accomplish it by having the ship move upwards enough to balance the rope going down. It would be a delicate thing with odd forces changing over time.

You might possibly be able to jim-jam this a little with assists from the rocket engines on your ship. Keep the ship in geo-stationary orbit at a lower altitude by constantly thrusting to stay there. This would be even more delicate. It would require the engines be on whenever the rope end was in the atmosphere. Since your passengers want to go up or down at least 1000 km or so, this means probably hours. If you can run your engines for hours, maybe just land the ship.

And finally, your rope needs to be stronger than any material we have currently got the ability to make by a large factor. The longest cable car rides we have are about 8 km. (Hmm... I think in several spans, not a single span.) If we could do longer we would. You need more than 3,000 times as long. Possibly carbon nanotubes would do it, but that is not certain. And we can only make those at very short lengths currently, not thousands-of-km as you would need. We can do stuff sub-millimeter if I recall. We also do not know how to make rope out of them as yet.

This strength requirement also applies to whatever you use to lower and retrieve your rope. Plus the motors to do that will need to be very powerful to lift this 25 thousand km cable. And whatever power source you have for them will need to be honking big as well.

All in all, with materials and capabilities we have now and are likely to have in the next few years, it is not practical. Much easier just to land the ship.

Answer 4

I think you are forgetting that, while at the equator the earth surface travels at about 460 m/s, an orbiting spaceship will travel at around 8 km/s. As aptfully illustrated by Randall Munroe, that cable would be a safety hazard

Having a loose end cable match the changing speed as it moves up and down the atmosphere would require having some sort of propulsion available to deliver the needed momentum.

Therefore your idea to get rid of a rocket would end up needing a rocket, or something equivalent, to match the delta v.

Answer 5

You get a classic answer to a classic question. Space elevators aren't possible for a variety of different reasons, the major one here relates to tensile strength of materials.

However space elevators are an accepted trope, and hence will be accepted if you don't try to over justify it. Monofilament or monomolecular line is the limit of what you should say about its construction, anything else should be handwaved and nobody will question it.

Remember that we don't talk about how things work in the real world unless they go wrong, we just use them. As such, to talk about what its made of and how it works would be inappropriate to anything other than an engineering lecture scenario.

Answer 6

The question brings to mind An old ocean research ship, FLIP, FLoating Instrument Platform.

Yes the ship would be huge. but instead of a giant spool of carbon nanocarbon you have useable ship area. Just rotate the ship on its center axis and dip the end into the atmosphere perhaps just enough to lower a flexible space elevator to the surface.

Answer 7

An interesting idea, except that what you are proposing is NOT an 'elevator', it is a 'sky crane'. Methinks the pod would need some form of thrusters in order to steer what is essentially a suspended load. Going down is not really a physics problem, it is an engineering problem - strength of materials. A very big parachute or umbrella would help, especially in an emergency.

Hauling it back up, now there is the rub. But if you really ponder it, lifting a load 50 meters up on an Earth based crane takes the same effort as raising it the same 50 meters on a sky crane. Helicopters can lift very large loads, so your spaceship should easily be able to lift the same load, only a lot further. The added problem is that the cable would have to be able to lift itself plus the load.

But the job definitely gets easier the higher the load is lifted.

There is, however, the minor problem of imparting enough velocity to match the orbiting ship. It's like that sky crane helicopter that still has to move the load somewhere else. Horizontal as well as vertical acceleration is required.

So here is the catch.

Most people think of dangling the entire cable from the space platform. Instead, break the distance up into sections, and tie each section to some form of platform like a dirigible or balloon. The cable does not need to suspend itself over the entire distance, just the distance between the intermediary supporting platforms. These platforms would be sent down from the main ship, and maybe even discarded when the job is done.

Answer 8

All of the current answers seem to assume orbit around Earth or an Earth-type world. They give excellent reasons of why the space elevator is impractical for that purpose.

However, if you assume lower mass worlds, such as Mars or the Moon, then the cable could be made of modern materials, or a much smaller amount of super-material.

It is still going to be a HUGE coil of material that will be very costly in delta-v when the ship travels to new locations.

Answer 9

Part One: A technologically Possible Aeronautical Version.

There was actually a somewhat similar, though of course almost infinitely smaller, device.

The spy gondola, spy basket, observation car or sub-cloud car (German: Spähgondel or Spähkorb) is a crewed vessel that an airship hiding in cloud cover could lower several hundred metres1 to a point below the clouds in order to inconspicuously observe the ground and help navigate the airship. It was a byproduct of Peilgondel development (a gondola to weight an airship's radio-locating antenna). They were used almost exclusively by the Germans in the First World War on their military airships.

LZ26's basket was lowered from the airship on a specially constructed tether 1,000 metres (3,300 ft) long;[4] other airships may have used one approximately 750 metres (2,460 ft) long.[5] The tether was high grade steel with a brass core insulated with rubber to act as the telephone cable.[4]

https://en.wikipedia.org/wiki/Spy_basket

Obviously a low flying Zeppelin hovering over one spot could theoretically have lowered its spy basket to the ground and used it as an elevator for people or objects.

And possibly some science fiction stories featuring large aircraft capable of hovering could have the aircraft dropping an "elevator" to the ground to transfer people and goods. Such stories should have a reason why the entire aircraft doesn't land, but instead uses a much smaller device as an "elevator".

A Large enough aircraft could have several kilometers or miles of cable to lower the smaller craft with. Experts in material strength should be able to say how long a cable could get to hold a vehicle of a specific weight under a specific surface gravity without breaking.

Part two: Use in Astronautics.

And I have thought of a possible use in astronautics.

A large spaceship visit a moon or asteroid that is large enough to be gravitationally rounded and thus appear spherical when seen from space and have proportionally low relief. Several spacecraft land on the surface and are used as a ready made base for some scientific, industrial, mining, military, or purposes. The large spacecraft stays in orbit around the world, to avoid using up too much delta v and not having enough for the trip back home.

The large orbiting spaceship has an eccentric orbit allowing it to get rather low over the surface of the world - the precise lower limit may depend on the altitude at which the extremely thin atmosphere of the world be dense enough to have significant drag on the spaceship.

When someone (or some objects) from the orbiting spaceship is sent down to the planet the orbiting spaceship lowers a much smaller spacecraft down the cable to full length as it approaches the lowest point in its orbit, which is always close to the location of the base. At the lowest point the smaller spacecraft detached from the capable and uses its rockets to match speed with the rotation of the world and then travels to the base location and land.

When someone or something has to be sent to the orbiting spaceship one of the small spacecraft is launched on a suborbital trajectory which will take it to the region where the end of the cable will be approaching. So the spacecraft will rendezvous and attach to the end of the cable at the lowest point in the orbiting spaceship's orbit. Then the orbiting spaceship will winch up the cable to draw the small spacecraft to it.

And I guess that such a procedure would take less fuel than sending the small spacecraft between the surface and the orbiting spaceship using their rockets all the way. And quite possibly all the fuel for the small spacecraft will have to be brought at great expense all the way from Earth, so saving fuel may be a big deal.

Thus I have managed to think of an astronautical use for a cable of technologically plausible length.

Part Three: Future Technology Might Seem Like Magic to US.

You might want to look at the Sliding Scale of Science Fiction Hardness.

https://tvtropes.org/pmwiki/pmwiki.php/SlidingScale/MohsScaleOfScienceFictionHardness

The lower the hardness score you desire for your story, the less you need to justify to your readers the technological marvels in your story. You just assume that over centuries or millennia of scientific and technological evolution and revolution, technology has been developed which seems like magic to people in the early 21st centuries.

The higher the hardness score you desire for your story, the more you need to justify to your readers the technological marvels in your story. You need to decide how plausible various futuristic marvels seem to your or to scientists, and limit your use to the ones most plausible.

So if you decide that a low score is good enough for your story, you can use all the scientific and technological wonders you want to, such as orbit to ground cables. But you need to be careful to make their use logical and consistent. So if a particular technological marvel would solve all your protagonist's problems too easily, it can't exist in your setting, or maybe be too expensive or otherwise unavailable to them.

Heroes ignoring an available tool which would solve their problems with ease will annoy some readers.

Answer 10

Just a couple of considerations, really:

Geosynchronous orbit is really far away. Over 35k km. That's a heck of a long cable. Of course, you can maintain position over a given point on the surface at any altitude so long as you don't mind burning fuel to do it -- i.e if you want to hold position at 200,000 feet, no problem! Just provide appropriate constant thrust.

So pick your handwavium, is really what I'm saying. You can sit in geosynchronous orbit, with a 35km long unbreakable cable, or sit at a lower altitude and engage thrust to maintain position while using a much shorter unbreakable cable. (If your ships are ala Star Trek, then nearly endless free thrust is a given so that should be fine. Or even The Expanse, where thrust is basically so cheap that they rarely bother talking about the cost.)

Answer 11

You couldn't get it in a spool

All of the answers here provide solid reasons for this being impractical, but I'd like to cite one why the "taking it with you" plan has problems.

In order to hold a weight at the bottom, the cable has to get thicker as it goes up. The last estimate I saw suggested that the top of a carbon nanotube cable would have to be a kilometer in diameter to support a ton of load at the bottom. Not exactly something you can carry in a spool.

This Estimate says that the cable, in total, would need to weigh at least 3438 metric tons, and the counterweight would need to be at least that massive.

Answer 12

Depends on the Planet

Answers so far assume you mean Earth, but there are a lot of planets out there that are much easier to do this on... and if we assume this is a specific ship designed to colonize or explore a specific alien world, then just design that world around what makes your idea make since.

Your orbit will need to be geostationary, but this does NOT mean you need to be at an altitude of 35,000km. That just so happens to be the height of a Geostationary Orbital orbit here on Earth. This distance is determined by the ratio of a planet's gravity vs how quickly it spins at the equator. If you planet is less massive, then your orbital speed goes down because the force of gravity goes down... or if your planet spins faster, you could have an Earth like gravity and still a much shorter elevator.

So, if we assume an Earth like world (same general mass, density, and atmosphere) but make it spin about once every 80 minutes, then you could reach a Geostationary Orbit at an altitude of just 100km. Just high enough to stay out of the atmosphere, but low enough you don't need a crazy massive spool (relatively speaking).

Now this said, there is no way that this would be the standard issue setup for all your ships in any realistic setting. A lot of worlds spin slower than Earth too, so while Earth is just at the edge of theoretically able to support a space elevator, many planets literally can not under any circumstances have an elevator long enough for this to work. If you are trying to land on a tidally locked planet or moon, then your orbital distance would be the same as the distance to the parent planet or star... in which case, you'd be far more effected by the gravity of the parent body and not be able to orbit the smaller one.

Answer 13

Does the "space elevator" have to be made of physical material?

If not, make it out of some energy-based tether where the only physical object is the base that is actually touching the ground. That way, the "tether" itself is some kind of energy-based vector field handwavium that works as long as the platform has some kind of line-of-sight connection to the ship in orbit. From there, the astronauts just pull out some kind of device similar to a rope clamp that allows them to control their ascent/descent.

Alternatively, pull a page from Halo's book and use a bounded gravitational field.

Answer 14

This might be practical for visiting asteroids or very small planets. Instead of wasting propellant to stop a large craft to pick up one person, they might send a line and yank the person off the surface. It would have to be a long line if there is a large mismatch in velocity, but not the thousands of miles that a conventional space elevator needs.

Answer 15

A space elevator will be a gigantic undertaking. Still, could be better that landing and starting many many rockets. What the ship carries is not the elevator or a coiled up cable, but the factory required o build one. The factory is then parked in a synchronous orbit and will then slowly build the thether and a counterweight from asteroids.

Without doing any math, getting into low or geosynchronous orbit from earth takes about as much or not far less delta-v than getting from either anywhere else. Conversely, getting lots of mass from elsewhere in the system to either orbit will require lots of delta-v but possibly less than lifting all this stuff from ground.

So a ballpark estimate is, if you want to lift or land about as much mass as the space elevator cable itself will have, the delta-v budget could speak for the elevator.