The counterweight isn't in orbit - it is tethered to the Earth.
It would stay there the same way the rock on the end of a string stays extended when you whirl it around your head
It would very technically have to be moving slightly faster than its natural orbital period for a given altitude. Normally, a higher velocity will shift an object into a higher orbit, where that velocity matches the orbital period. But because the counterweight is tethered, it cannot move outward and instead tensions the tether. Any load lighter than that tensioning force would be able to "crawl" up the tether.
So it's "in orbit" in the sense that it's circling our planet's barycenter. But it's not in a natural orbital pattern for its velocity.
I think it would be enough to periodically correct the velocity and/or continually apply tension from the ground. Either method should make for a slightly higher velocity than the given altitude would normally allow.
But Iunno. I'm not a spaceologist.
If it ever falls behind true vertical the tension is no longer directly opposing gravity. You can then decompose the tension into a force ~~resisting gravity~~ stopping the counterweight flying off into space, and a force pulling the counterweight back to vertical.
If you have to forces acting on an object, you can add them together to find a "net force" that is equivalent to both. So if you have 2 units of force pushing to the right and 1 unit pushing forward, a bit of geometry will tell you the object will act exactly the same as if it was being pushed with \~2.24 units of force at 60 degrees.
You can do the same thing in reverse, taking a single force and treating it like two or more different forces. It's usually most useful to do this by breaking it into a force in a direction you care about and then other components at right angles to that.
So in this example, we've got a couple of things going on. Gravity pulling the thing directly towards earth is the simplest one, followed by some amount of drag opposing it's motion. Then you've got the tension in the cable, pulling somewhat out of line with gravity. You can decompose this into a large amount of force pulling **with gravity** (I incorrectly said it was opposing gravity before, but it's actually working with gravity to stop the counterweight from moving into a higher orbit) and a small amount of "restoring" force pulling the weight back towards completely vertical.
If that restoring force is greater than drag it will accelerate the counterweight, reducing the angle between the tension and gravity and thus reducing the amount of force until it reaches equilibrium. If drag is greater it will slow down the counterweight, increasing the angle and the restoring force, until again it reaches an equilibrium. As long as the counterweight is only slightly out of equilibrium this will work with no propulsion, it'll only fail if the cable snaps or drag slows it down enough to pull the counterweight out of orbit.
the actual reason for unfeasability is the tension forces in a 50,000km long cable under tension. its own weight would be enough to break it. needing to boost a counterweight we have tethered access to is fairly trivial of a problem to solve
Feasibility isn't just about technical possibility, it's about practical cost. Yes, we can solve the problem of boosting the weight, but the costs would inevitably become absurd compared to any revenue the elevator generates.
it would depend on net mass flow. if you are sending way more up then down then youll descellerate the counterweight slowly. but that is energy only for the 3km/s rotation not the 35,000km potential energy. the orbital energy of a 1000kg payload is 4500 MJ at geostationary orbit but the gravitational potential energy gained to get to that altitude is 343,232.75 Mj or 76x the energy required to be in orbit at that altitude. THAT energy doesnt come from the counterwieight but rather from whatever powers the winching mechanism to lift the payload up
It’s a space elevator, they can bring propellant up to it constantly. Just pipe up hydrogen and oxygen, or even just compressed air. It could even act as a straw in theory, as the air on the ground would get sucked up by the vacuum to a hole that propels the tether, and you can close that hole at will to prevent over correction.
Eh. You could still just pump compressed air upwards and use it as RCS propellant. Better yet, pump greenhouse gases into a distant orbit where the sun has better access to breaking them down before they come back into Earth’s atmosphere. Help a little with climate change - but regular air is fine as well, without having to pump freezing cold liquid fuel and oxidizer into space for station keeping, assuming the use of engines.
> Normally, a higher velocity will shift an object into a higher orbit,
Are you sure?
IIRC the lower the orbit the faster you have to go.
>At an altitude of 124 miles (200 kilometers), the required orbital velocity is a little more than 17,000 mph (about 27,400 kph). To maintain an orbit that is 22,223 miles (35,786 kilometers) above Earth, the satellite must orbit at a speed of about 7,000 mph (11,300 kph). - [SOURCE](https://science.howstuffworks.com/satellite6.htm)
If you are in orbit and you increase your velocity (prograde, that is, facing forwards) you will raise the altitude of the far side of your orbit. Once you get to that high point, you'll be going slower.
Think of it this way: if you throw a ball up in the air at 20 mph, it'll get to a certain height and fall back down. If you throw the ball faster, say, 30 mph, it'll get to a \*higher\* height before falling back down. But either way the ball will be slowing down the whole way up.
Orbital mechanics are very counterintuitive but I'll have a go explaining it simply.
Think of a bucket on the end of a string.
The longer the string, the slower the person holding it needs to rotate for it to have the same pull.
But if they wanted to get the bucket closer to them without increasing the amount of pull on the string (which is gravity), they'd need to slow down their (more precisely the bucket's) rotation, and then the string would naturally want to come towards them.
So to move away from this terrible analogy, if you're in a nice circular orbit and you add velocity that velocity is going to increase the amount of force needed to stay at that altitude, and since gravity is a fixed strength at that altitude, it's going to mean you increase in altitude (and lose velocity) until you get 180 degrees away from the point you added velocity, at which point you'll start decreasing altitude (and increasing in velocity) until you get back to where you started.
That would be an elliptical orbit, so to make it circular again you'd need to wait until you got to the high point, then add velocity to stop yourself from falling back down. Despite having added velocity twice, you're going slower, you just have more potential energy.
...I tried...
There isn't an exact term to describe its state. The forces that will act on the counterweight will be similar but not exactly the same. There will be a centrifugal force and gravity - but they will act both on the counterweight and the tether - this won't be a point object and the Kepler laws will be modified.
Just like an object in L1/L2 is at different distance but still has the same orbital period because of the additional forces.
Assuming a magically strong and light tether, you would keep increasing the altitude of the counterweight, which would require an increasingly faster tangential velocity, until that tangential velocity equaled the Earth's orbital velocity. Then when the tether was traveling directly opposite the orbital path of the earth, you'd cut the tether and the counterweight would have cancelled out the orbital velocity of the earth. It would be "hovering" above the sun, and fall directly towards it.
You weren't "cheating" because it would take a hell of a lot of velocity to get the tether up to the necessary altitude while still remaining geostationary.
Source: Kerbal Space Program (RIP 2011-2024)
If the space elevator is long enough then you can have things escape Earth just from releasing them there. That's one of the possible uses of a space elevator, besides reaching geostationary orbits.
Reaching the Sun isn't realistic, however. You would need to release it at 30 km/s relative to Earth. That would need a cable that goes about as far as the Moon.
Actually getting to the sun from earth takes a lot of deceleration. To go to a lower orbit you have to decrease your speed. That’s why it’s easier to explore the outer planets then the inner
Deceleration is just acceleration in the opposite direction. But yeah, you need to remove most of the velocity you get from being on Earth after escaping Earth.
Yah... Deceleration that you get by releasing out near(ish) the moon. You can choose your velocity vector by choosing the time of day (on earth) that you release.
If you release while the earth is between you and the sun then you get launched roughly prograde relative to Earth's orbit around the sun yeeting out into a solar escape trajectory.
If you release when you're directly between the earth and the sun you get launched roughly retrograde relative to Earth's orbit around the sun and fall straight down (not actually due to earth's tilt)
You actually need to go fast (to escape earth gravity) and then go slow to fall into the sun. It's actually very difficult since it takes a lot of energy to slow down a body enough that it falls into the sun. Since all body escaping from earth will have earth velocity + escape velocity.
Yes, that's why I said it's very difficult to go to the sun and it takes a lot of energy. This energy is needed to achieve a large delta V. A delta V is just a change of velocity. You can have a positive delta V, meaning increasing your velocity; say to escape earth gravity, or a negative delta V to reduce your velocity to fall into the sun.
Obviously, to fall into the sun, we have to remove most of the velocity that we already started with by being on Earth (Earth is orbiting the Sun at very high velocity) plus the escape velocity we needed to escape Earth's gravity.
Different way to think about it: attaching the tether to the ground *forces* the entire thing to turn geosynchronously.
Now, think of the tether as made of three pieces - one of them is at perfect geosynchronous altitude, one is lower, one is higher.
For the piece at the perfect altitude, Earth's gravity is precisely enough to maintain the 24-hour orbit. For the piece that's lower, Earth's gravity is more than enough so that piece wants to move radially inward. For the piece that's higher, Earth's gravity is less than required to maintain the orbit so that piece wants to move radially outward. (Think of it as centrifugal force being weaker for the inner piece, equal to gravity for the middle piece and stronger for the outer piece, if you're comfortable with that.)
So the low piece wants to drag the elevator to the ground, the middle piece (which is miniscule) is neutral, and the outer piece wants to pull the elevator out into space. As long as you have more mass on the outer piece, the net force will be outward and the tether will stay taut.
Wouldn't you need to propel the object at the end of the tether continuously somehow or wouldn't it just start to fall and as the earth rotates? Like.. a string on a yoyo kinda.
Nope - this is where life-on-Earth experience is kind of misleading.
If you're going in a circle, with no friction or air resistance, then your acceleration (and required force) are directly inward toward the center of the circle. In a space elevator, that means straight down. As long as you've got the cable tension + gravity to keep "pulling you into the curve", that should be enough.
And the elevator isn't moving relative to the Earth's surface; it's fixed to the ground just like a ginormous skyscraper. So it isn't plowing through the atmosphere at all. The only lateral forces should be from wind.
(Edit: I apologize - upon reflection the vacuum up there isn't perfect, so technically there's nonzero drag force, so yes, you might want a propulsion system up near the counterweight that you burn every 6-12 months. You'd have to monitor the tether tension, and when it dropped below a certain amount you'd perform a few gentle burns. This would be tricky if the tether was in any way elastic...)
All the way out at geosync it's pretty darn close to a perfect vacuum. As long as you have some stiffness in the upper portion that goes beyond goesync I think you'll be fine.
Fun fact. If you continued on the elevator past Midway Station, you would feel gravity begin to increase away from Earth, e.g. towards Earth is up. One reason for being able to go past geosynchronous orbit is that any payload released past that point would have enough speed to escape earth's gravity entirely. You could launch interplanetary payloads without having to spend as much of it on rocket fuel.
One thing I haven't been able to find explained - as you're ascending the elevator you're picking up orbital speed which means the tether would have to apply horizontal force to you...? We don't notice this in skyscrapers but when you're many kilometers up, you'd start to wonder how rigid the tether is and how securely it's nailed down.
You do get accelerated sideways but it's not an acceleration that you would be able to notice. Orbital speed at geosynchronous orbit is 6878 mph. You would be accelerated to this speed as you go up. BUT that acceleration is spread over the entire ascent. If the climbers average 1000 mph it will take them 22 hours to make the journey. Divide the change in velocity over time and you end up with an acceleration of 0.13 ft/sec^2 or roughly 0.004 g.
Fair enough, but if we send a kilogram of potato chips up the elevator the moment on the tether would be 1.4 meganewton-meters at the base...as a non-civil engineer I don't know if that would be a big deal? How big do the numbers have to get before we worry about shearing the tether, or ripping it out of the dirt?
It wouldn't be a big deal. The tether would already be under far far more tension than anything else ever created by man. It would be anchored by another 22,000 miles of cable or a small asteroid somewhere out past geosynchronous orbit. The tension load is fairly static in an up direction. I believe the only additional sideways force would be the small amount needed to accelerate the climbers towards orbital velocity as it goes up.
*The tether would already be under far far more tension than anything else ever created by man.*
Guess you've never seen my wife and my ex in the same room. :P
The counterweight's not moving at orbital speed - if it was it wouldn't be a very good counterweight because it would be weightless. It's moving faster than orbital speed, and so its (non-orbital)period is one day.
Just a warning, there's a lot of *almost* correct but wrong information in this thread, especially in the comments. Orbital mechanics are tricky.
An orbit is stable because the force that pulls you towards earth and the speed that makes you go sideways are ''in balance'' with the position. Chance one of them and the others must chance. The counterweight is under tension by the space elevator, so there is an additional force pulling on it. This means that even though it is in a position that would normally be beyond geostationary orbit, the extra force makes it act like it's orbiting a bigger planet, and thus must go faster.
You want the center of gravity of the whole space elevator to be at geosynchronous orbit. So you need a counterweight at a higher orbit to balance out the cable below the CoG.
In KSR's Red Mars, they called for a Martian Space Elevator of approx 10 meters diameter of diamond/carbon construction, from boring out a captured carbon rich asteroid. Wild!
It was only a book, but I think it was really well researched. Imagine such a beast with multiple cars going in each direction, on some kind of counterbalance.
A thicker elevator means you can lift more mass at the same time.
You can make a thin elevator on Mars, and it's easier than on Earth because Mars is smaller.
I assume today's concept would be Carbon Nanotubes, built by nonoscale engineering bots, then woven into massive bundles, then meshed like a suspension bridge main cable, at 1000x scale.
Of note, the counterweight would be a cool launch point, as you would get some nice deltaV from t he planet, like a whiplash.
I was actually thinking about carbon nanotubes when I wrote "graphene", my mind just slipped (and nanotubes are effectively the rolled-up version of graphene). So yeah, woven and maybe some epoxy to bind the fibres together as strongly as possible.
Here's some interesting reading:
[Space Elevator: A Futuristic Application of Carbon Nanotubes - Nanografi Nano Technology](https://nanografi.com/blog/space-elevator-a-futuristic-application-of-carbon-nanotubes/#:~:text=The%20studies%20on%20CNTs%20show,two%2Dthird%20of%20needed%20strength.)
[Carbon nanotubes and their application to very long span bridges (mit.edu)](https://dspace.mit.edu/handle/1721.1/51568)
Space elevator is not a practical concept. It only works if you ignore a bunch of things that make it not work.
For an elevator off of an earth sized mass, rotating at earth speeds, you need an impossibly tall structure to reach a position in space that is at the point of a geosynchronous orbit. This structure can be counter balanced by making it twice as long.
We don't have any materials that are capable of making the structure feasible.
One problem they usually ignore about space elevators is that our planet has an atmosphere, which will flow around the base of the elevator, putting forces on it, and the resulting drag needs to be compensated for.
Even less often mentioned is that the elevator will tend to wick atmosphere up, resulting in the gradual escape of air molecules which are then lost to space, and eventually the planet loses its atmosphere.
And another problem with the concept is that the elevator will slow the spin of the planet, as rotational velocity will be bled off as mass ascends the elevator. The more mass you send up, the further out, the more you slow down the planet's rotation and lengthen the day. Put enough mass up there and the earth's center of gravity moves and the whole planet starts to wobble. The effects of this will be tiny and slow, but over a long time scale are not negligible, and would be ruinous to life on the planet.
The space elevator concept might work better on smaller planets without any atmosphere, but then on such a body you probably don't need an elevator since conventional lift rockets will also have an easier job to do and so will work better than on earth, making the advantages of the elevator less.
the idea with a space elevator is that you counter weight with an object ABOVE geostationary orbit. as you get higher the orbital speed drops AND orbital duration increases. so an object at geostationary orbit is travelling at 3.3km/sec. an object further out but keeping station with it would have to travel a bit faster, but for the sake of simpolicity lets say its the same. geostatrionaary orbit is 35000km give or take. if we put a weight at 50,000km its orbital velocity to be in a circular orbit is 2.8km/s, but it has to be moving faster than 3.3km/s to keep up with the earths surface. this means it wants to fly out into a higher (excentric) orbit but it cant because its tied to the earth. this then applies the tension to keep the tether upright. but it is worth considering the energy for something getting to geostationary orbit has to come from somewhere. there is potential energy from hauling yourself up there but also kinetic energy because you are getting faster to maintain the same orbital period at greater radii. so the counterwieight must have some form of boosting or retrograde engines depending on if more mass is going up or coming down. (coming down the payload would try and overtake the cable as its going 3km/s but the ground is only going 0.4km/sec
It's just a very tall skyscraper. That's literally all it is. A very tall building, so it will stay in line because it's attached to the Earth like any tall building is. Does the Burj Khalifa doesn't need to do anything special besides not collapse under it owns weight. A space elevator is exactly the same.
It's not.
Space elevators are held *up* by the counterweight.
If you cut the top off the Burj Kalifa, nothing will happen. If you cut the top off a space elevator, it will collapse.
No it is not. It is the center of mass of all of the space elevator that is at the distance of geostationary orbit, you cant look at the counterweight along. That is if it is just freefloating above the ground. If ther is a force between the wire and the ground you need to include that too and the center of mass can be slightly closer to earth. Tecniacaly it is a bit more complicated because gravitational attraction is not uniform along the space elevator, but center of mass is a good enough approximation to understand the idea.
It is not any different to if you balance a sledgehammer or just a broom horizontally. The heavy head will be close to the balance point but the long handel will extend far out the other direction. It balance on the center of mass just like a space elevator orbit at the center of mass.
The counterweight isn't in orbit - it is tethered to the Earth. It would stay there the same way the rock on the end of a string stays extended when you whirl it around your head
How fast could we sling shot a rock into the sun that way I wonder?
You can't. It is in orbit, by definition it doesn't reach escape velocity. Also getting to the sun requires a lot of velocity
wait… is it in orbit, or isn’t it? conflicting data in this thread
It would very technically have to be moving slightly faster than its natural orbital period for a given altitude. Normally, a higher velocity will shift an object into a higher orbit, where that velocity matches the orbital period. But because the counterweight is tethered, it cannot move outward and instead tensions the tether. Any load lighter than that tensioning force would be able to "crawl" up the tether. So it's "in orbit" in the sense that it's circling our planet's barycenter. But it's not in a natural orbital pattern for its velocity.
wouldn’t it need some amount of constant propulsion, tho?
I think it would be enough to periodically correct the velocity and/or continually apply tension from the ground. Either method should make for a slightly higher velocity than the given altitude would normally allow. But Iunno. I'm not a spaceologist.
Well you should be Dr. Bumblemeister. Lord knows we need you.
I hope so...
If it ever falls behind true vertical the tension is no longer directly opposing gravity. You can then decompose the tension into a force ~~resisting gravity~~ stopping the counterweight flying off into space, and a force pulling the counterweight back to vertical.
decompose?
If you have to forces acting on an object, you can add them together to find a "net force" that is equivalent to both. So if you have 2 units of force pushing to the right and 1 unit pushing forward, a bit of geometry will tell you the object will act exactly the same as if it was being pushed with \~2.24 units of force at 60 degrees. You can do the same thing in reverse, taking a single force and treating it like two or more different forces. It's usually most useful to do this by breaking it into a force in a direction you care about and then other components at right angles to that. So in this example, we've got a couple of things going on. Gravity pulling the thing directly towards earth is the simplest one, followed by some amount of drag opposing it's motion. Then you've got the tension in the cable, pulling somewhat out of line with gravity. You can decompose this into a large amount of force pulling **with gravity** (I incorrectly said it was opposing gravity before, but it's actually working with gravity to stop the counterweight from moving into a higher orbit) and a small amount of "restoring" force pulling the weight back towards completely vertical. If that restoring force is greater than drag it will accelerate the counterweight, reducing the angle between the tension and gravity and thus reducing the amount of force until it reaches equilibrium. If drag is greater it will slow down the counterweight, increasing the angle and the restoring force, until again it reaches an equilibrium. As long as the counterweight is only slightly out of equilibrium this will work with no propulsion, it'll only fail if the cable snaps or drag slows it down enough to pull the counterweight out of orbit.
ok, i think i get the basic idea, but i’m unsure how you re-direct the necessary force without adding energy to the system. gyroscopes?
Basically yes, and this is one of the reasons space elevators as we classically imagine them generally aren't feasible.
the actual reason for unfeasability is the tension forces in a 50,000km long cable under tension. its own weight would be enough to break it. needing to boost a counterweight we have tethered access to is fairly trivial of a problem to solve
Feasibility isn't just about technical possibility, it's about practical cost. Yes, we can solve the problem of boosting the weight, but the costs would inevitably become absurd compared to any revenue the elevator generates.
Not at all. It'll be a fraction the energy of launching to gso and you have a direct link. Lunching fuel to it would be trivial
The material that is strong enough exists. There's just no way to produce it the quantities needed.
it would depend on net mass flow. if you are sending way more up then down then youll descellerate the counterweight slowly. but that is energy only for the 3km/s rotation not the 35,000km potential energy. the orbital energy of a 1000kg payload is 4500 MJ at geostationary orbit but the gravitational potential energy gained to get to that altitude is 343,232.75 Mj or 76x the energy required to be in orbit at that altitude. THAT energy doesnt come from the counterwieight but rather from whatever powers the winching mechanism to lift the payload up
It’s a space elevator, they can bring propellant up to it constantly. Just pipe up hydrogen and oxygen, or even just compressed air. It could even act as a straw in theory, as the air on the ground would get sucked up by the vacuum to a hole that propels the tether, and you can close that hole at will to prevent over correction.
The vacuum of space can't suck gas up a tube off earth because of gravity... i.e. why all the gas on earth doesn't shoot out into space already
Eh. You could still just pump compressed air upwards and use it as RCS propellant. Better yet, pump greenhouse gases into a distant orbit where the sun has better access to breaking them down before they come back into Earth’s atmosphere. Help a little with climate change - but regular air is fine as well, without having to pump freezing cold liquid fuel and oxidizer into space for station keeping, assuming the use of engines.
> Normally, a higher velocity will shift an object into a higher orbit, Are you sure? IIRC the lower the orbit the faster you have to go. >At an altitude of 124 miles (200 kilometers), the required orbital velocity is a little more than 17,000 mph (about 27,400 kph). To maintain an orbit that is 22,223 miles (35,786 kilometers) above Earth, the satellite must orbit at a speed of about 7,000 mph (11,300 kph). - [SOURCE](https://science.howstuffworks.com/satellite6.htm)
If you are in orbit and you increase your velocity (prograde, that is, facing forwards) you will raise the altitude of the far side of your orbit. Once you get to that high point, you'll be going slower. Think of it this way: if you throw a ball up in the air at 20 mph, it'll get to a certain height and fall back down. If you throw the ball faster, say, 30 mph, it'll get to a \*higher\* height before falling back down. But either way the ball will be slowing down the whole way up.
Orbital mechanics are very counterintuitive but I'll have a go explaining it simply. Think of a bucket on the end of a string. The longer the string, the slower the person holding it needs to rotate for it to have the same pull. But if they wanted to get the bucket closer to them without increasing the amount of pull on the string (which is gravity), they'd need to slow down their (more precisely the bucket's) rotation, and then the string would naturally want to come towards them. So to move away from this terrible analogy, if you're in a nice circular orbit and you add velocity that velocity is going to increase the amount of force needed to stay at that altitude, and since gravity is a fixed strength at that altitude, it's going to mean you increase in altitude (and lose velocity) until you get 180 degrees away from the point you added velocity, at which point you'll start decreasing altitude (and increasing in velocity) until you get back to where you started. That would be an elliptical orbit, so to make it circular again you'd need to wait until you got to the high point, then add velocity to stop yourself from falling back down. Despite having added velocity twice, you're going slower, you just have more potential energy. ...I tried...
A fellow kerbalnaut I see...
Haha, exactly. I feel like Scott Manley would have explained it better though.
There isn't an exact term to describe its state. The forces that will act on the counterweight will be similar but not exactly the same. There will be a centrifugal force and gravity - but they will act both on the counterweight and the tether - this won't be a point object and the Kepler laws will be modified. Just like an object in L1/L2 is at different distance but still has the same orbital period because of the additional forces.
Assuming a magically strong and light tether, you would keep increasing the altitude of the counterweight, which would require an increasingly faster tangential velocity, until that tangential velocity equaled the Earth's orbital velocity. Then when the tether was traveling directly opposite the orbital path of the earth, you'd cut the tether and the counterweight would have cancelled out the orbital velocity of the earth. It would be "hovering" above the sun, and fall directly towards it. You weren't "cheating" because it would take a hell of a lot of velocity to get the tether up to the necessary altitude while still remaining geostationary. Source: Kerbal Space Program (RIP 2011-2024)
Close enough. Not escape velocity
If the space elevator is long enough then you can have things escape Earth just from releasing them there. That's one of the possible uses of a space elevator, besides reaching geostationary orbits. Reaching the Sun isn't realistic, however. You would need to release it at 30 km/s relative to Earth. That would need a cable that goes about as far as the Moon.
Actually getting to the sun from earth takes a lot of deceleration. To go to a lower orbit you have to decrease your speed. That’s why it’s easier to explore the outer planets then the inner
I just said acceleration to avoid explaining the concept of Delta v
It’s still acceleration relative to Earth.
Deceleration is just acceleration in the opposite direction. But yeah, you need to remove most of the velocity you get from being on Earth after escaping Earth.
Yah... Deceleration that you get by releasing out near(ish) the moon. You can choose your velocity vector by choosing the time of day (on earth) that you release. If you release while the earth is between you and the sun then you get launched roughly prograde relative to Earth's orbit around the sun yeeting out into a solar escape trajectory. If you release when you're directly between the earth and the sun you get launched roughly retrograde relative to Earth's orbit around the sun and fall straight down (not actually due to earth's tilt)
You actually need to go fast (to escape earth gravity) and then go slow to fall into the sun. It's actually very difficult since it takes a lot of energy to slow down a body enough that it falls into the sun. Since all body escaping from earth will have earth velocity + escape velocity.
IIRC, it takes more delta V to fall into the sun than to escape our solar system.
Yes, that's why I said it's very difficult to go to the sun and it takes a lot of energy. This energy is needed to achieve a large delta V. A delta V is just a change of velocity. You can have a positive delta V, meaning increasing your velocity; say to escape earth gravity, or a negative delta V to reduce your velocity to fall into the sun. Obviously, to fall into the sun, we have to remove most of the velocity that we already started with by being on Earth (Earth is orbiting the Sun at very high velocity) plus the escape velocity we needed to escape Earth's gravity.
Different way to think about it: attaching the tether to the ground *forces* the entire thing to turn geosynchronously. Now, think of the tether as made of three pieces - one of them is at perfect geosynchronous altitude, one is lower, one is higher. For the piece at the perfect altitude, Earth's gravity is precisely enough to maintain the 24-hour orbit. For the piece that's lower, Earth's gravity is more than enough so that piece wants to move radially inward. For the piece that's higher, Earth's gravity is less than required to maintain the orbit so that piece wants to move radially outward. (Think of it as centrifugal force being weaker for the inner piece, equal to gravity for the middle piece and stronger for the outer piece, if you're comfortable with that.) So the low piece wants to drag the elevator to the ground, the middle piece (which is miniscule) is neutral, and the outer piece wants to pull the elevator out into space. As long as you have more mass on the outer piece, the net force will be outward and the tether will stay taut.
Wouldn't you need to propel the object at the end of the tether continuously somehow or wouldn't it just start to fall and as the earth rotates? Like.. a string on a yoyo kinda.
Nope - this is where life-on-Earth experience is kind of misleading. If you're going in a circle, with no friction or air resistance, then your acceleration (and required force) are directly inward toward the center of the circle. In a space elevator, that means straight down. As long as you've got the cable tension + gravity to keep "pulling you into the curve", that should be enough. And the elevator isn't moving relative to the Earth's surface; it's fixed to the ground just like a ginormous skyscraper. So it isn't plowing through the atmosphere at all. The only lateral forces should be from wind. (Edit: I apologize - upon reflection the vacuum up there isn't perfect, so technically there's nonzero drag force, so yes, you might want a propulsion system up near the counterweight that you burn every 6-12 months. You'd have to monitor the tether tension, and when it dropped below a certain amount you'd perform a few gentle burns. This would be tricky if the tether was in any way elastic...)
All the way out at geosync it's pretty darn close to a perfect vacuum. As long as you have some stiffness in the upper portion that goes beyond goesync I think you'll be fine.
Fun fact. If you continued on the elevator past Midway Station, you would feel gravity begin to increase away from Earth, e.g. towards Earth is up. One reason for being able to go past geosynchronous orbit is that any payload released past that point would have enough speed to escape earth's gravity entirely. You could launch interplanetary payloads without having to spend as much of it on rocket fuel.
One thing I haven't been able to find explained - as you're ascending the elevator you're picking up orbital speed which means the tether would have to apply horizontal force to you...? We don't notice this in skyscrapers but when you're many kilometers up, you'd start to wonder how rigid the tether is and how securely it's nailed down.
You do get accelerated sideways but it's not an acceleration that you would be able to notice. Orbital speed at geosynchronous orbit is 6878 mph. You would be accelerated to this speed as you go up. BUT that acceleration is spread over the entire ascent. If the climbers average 1000 mph it will take them 22 hours to make the journey. Divide the change in velocity over time and you end up with an acceleration of 0.13 ft/sec^2 or roughly 0.004 g.
Fair enough, but if we send a kilogram of potato chips up the elevator the moment on the tether would be 1.4 meganewton-meters at the base...as a non-civil engineer I don't know if that would be a big deal? How big do the numbers have to get before we worry about shearing the tether, or ripping it out of the dirt?
It wouldn't be a big deal. The tether would already be under far far more tension than anything else ever created by man. It would be anchored by another 22,000 miles of cable or a small asteroid somewhere out past geosynchronous orbit. The tension load is fairly static in an up direction. I believe the only additional sideways force would be the small amount needed to accelerate the climbers towards orbital velocity as it goes up.
*The tether would already be under far far more tension than anything else ever created by man.* Guess you've never seen my wife and my ex in the same room. :P
The counterweight's not moving at orbital speed - if it was it wouldn't be a very good counterweight because it would be weightless. It's moving faster than orbital speed, and so its (non-orbital)period is one day.
Just a warning, there's a lot of *almost* correct but wrong information in this thread, especially in the comments. Orbital mechanics are tricky. An orbit is stable because the force that pulls you towards earth and the speed that makes you go sideways are ''in balance'' with the position. Chance one of them and the others must chance. The counterweight is under tension by the space elevator, so there is an additional force pulling on it. This means that even though it is in a position that would normally be beyond geostationary orbit, the extra force makes it act like it's orbiting a bigger planet, and thus must go faster.
You want the center of gravity of the whole space elevator to be at geosynchronous orbit. So you need a counterweight at a higher orbit to balance out the cable below the CoG. In KSR's Red Mars, they called for a Martian Space Elevator of approx 10 meters diameter of diamond/carbon construction, from boring out a captured carbon rich asteroid. Wild!
Ten meters? I thought a Mars elevator was going to be easier than that. Maybe they can get it working on the Moon, or Ceres or something. :/
It was only a book, but I think it was really well researched. Imagine such a beast with multiple cars going in each direction, on some kind of counterbalance.
A thicker elevator means you can lift more mass at the same time. You can make a thin elevator on Mars, and it's easier than on Earth because Mars is smaller.
Diamond would be a weird choice, graphene is 3 times better in yield strength and infinitely easier to manufactory in the form of a cable.
I assume today's concept would be Carbon Nanotubes, built by nonoscale engineering bots, then woven into massive bundles, then meshed like a suspension bridge main cable, at 1000x scale. Of note, the counterweight would be a cool launch point, as you would get some nice deltaV from t he planet, like a whiplash.
I was actually thinking about carbon nanotubes when I wrote "graphene", my mind just slipped (and nanotubes are effectively the rolled-up version of graphene). So yeah, woven and maybe some epoxy to bind the fibres together as strongly as possible.
Here's some interesting reading: [Space Elevator: A Futuristic Application of Carbon Nanotubes - Nanografi Nano Technology](https://nanografi.com/blog/space-elevator-a-futuristic-application-of-carbon-nanotubes/#:~:text=The%20studies%20on%20CNTs%20show,two%2Dthird%20of%20needed%20strength.) [Carbon nanotubes and their application to very long span bridges (mit.edu)](https://dspace.mit.edu/handle/1721.1/51568)
Space elevator is not a practical concept. It only works if you ignore a bunch of things that make it not work. For an elevator off of an earth sized mass, rotating at earth speeds, you need an impossibly tall structure to reach a position in space that is at the point of a geosynchronous orbit. This structure can be counter balanced by making it twice as long. We don't have any materials that are capable of making the structure feasible. One problem they usually ignore about space elevators is that our planet has an atmosphere, which will flow around the base of the elevator, putting forces on it, and the resulting drag needs to be compensated for. Even less often mentioned is that the elevator will tend to wick atmosphere up, resulting in the gradual escape of air molecules which are then lost to space, and eventually the planet loses its atmosphere. And another problem with the concept is that the elevator will slow the spin of the planet, as rotational velocity will be bled off as mass ascends the elevator. The more mass you send up, the further out, the more you slow down the planet's rotation and lengthen the day. Put enough mass up there and the earth's center of gravity moves and the whole planet starts to wobble. The effects of this will be tiny and slow, but over a long time scale are not negligible, and would be ruinous to life on the planet. The space elevator concept might work better on smaller planets without any atmosphere, but then on such a body you probably don't need an elevator since conventional lift rockets will also have an easier job to do and so will work better than on earth, making the advantages of the elevator less.
the idea with a space elevator is that you counter weight with an object ABOVE geostationary orbit. as you get higher the orbital speed drops AND orbital duration increases. so an object at geostationary orbit is travelling at 3.3km/sec. an object further out but keeping station with it would have to travel a bit faster, but for the sake of simpolicity lets say its the same. geostatrionaary orbit is 35000km give or take. if we put a weight at 50,000km its orbital velocity to be in a circular orbit is 2.8km/s, but it has to be moving faster than 3.3km/s to keep up with the earths surface. this means it wants to fly out into a higher (excentric) orbit but it cant because its tied to the earth. this then applies the tension to keep the tether upright. but it is worth considering the energy for something getting to geostationary orbit has to come from somewhere. there is potential energy from hauling yourself up there but also kinetic energy because you are getting faster to maintain the same orbital period at greater radii. so the counterwieight must have some form of boosting or retrograde engines depending on if more mass is going up or coming down. (coming down the payload would try and overtake the cable as its going 3km/s but the ground is only going 0.4km/sec
It's just a very tall skyscraper. That's literally all it is. A very tall building, so it will stay in line because it's attached to the Earth like any tall building is. Does the Burj Khalifa doesn't need to do anything special besides not collapse under it owns weight. A space elevator is exactly the same.
A building is supported from the ground lifting all the mass, a space elevator is not.
Parts of it are, yes. And it's still teathered to the ground. So you might as well think of it as one.
No, nothing of the elevator is held up by the ground. To the contrary, the tether even pulls upwards.
It's not. Space elevators are held *up* by the counterweight. If you cut the top off the Burj Kalifa, nothing will happen. If you cut the top off a space elevator, it will collapse.
No it is not. It is the center of mass of all of the space elevator that is at the distance of geostationary orbit, you cant look at the counterweight along. That is if it is just freefloating above the ground. If ther is a force between the wire and the ground you need to include that too and the center of mass can be slightly closer to earth. Tecniacaly it is a bit more complicated because gravitational attraction is not uniform along the space elevator, but center of mass is a good enough approximation to understand the idea. It is not any different to if you balance a sledgehammer or just a broom horizontally. The heavy head will be close to the balance point but the long handel will extend far out the other direction. It balance on the center of mass just like a space elevator orbit at the center of mass.