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When I talk about this and people ask why, and when people comment "why", this is my response.
The space elevator is the Panama Canal of our time.
People dreamed of building it for close to a hundred years before it was finally achieved. Multiple companies and countries tried and failed. Once it was achieved, it changed the world.
This is so much more important. Instead of just linking two hemispheres, the space elevator opens up all of space to us. It's like the Bering land bridge 20k years ago or first cheap ships that could reliably cross the Atlantic. It's all of those things a thousand fold.
An alternative to a full space elevator is the rotovator or rotating sky hook. Instead of building a single tethered cable that reaches up to geostationary orbit, you instead build shorter, untethered cables that rotate end over end in their orbit.
These structures can then be used to assist conventional rockets. Place a rotovator in low orbit, then send up a suborbital rocket to hook onto the end of the cable when at its lowest point. Detach half a rotation later, and the rocket is flung into a higher orbit.
There's a more detailed discussion here:
https://www.reddit.com/r/technology/comments/2h07ku/japanese...
People got enthused about this after Brad Edwards, mentioned in the article, led a NASA study on space elevators about a decade ago, which found it to be surprisingly feasible. Here's their final report (pdf): http://www.niac.usra.edu/files/studies/final_report/521Edwar...
The material would be carbon nanotubes a couple centimeters long, bound together by a reasonably strong epoxy. They estimate a cost in the low tens of billions of dollars, by using seven shuttle flights to deploy a minimal elevator, and using that to pull up additional elevator material. They examine a lot of practical issues and find solutions for them.
We live in a world where an individual has the resources to make a credible stab at a space elevator....that is amazing.
i wonder why before various unobtanium requiring structures we wouldn't try some rail/coil gun in a [close to] vacuum tube going several kilometers up a mountain. After all, 2000 years ago it was possible to build that http://en.wikipedia.org/wiki/Roman_aqueduct , so building something like it up the Kilimanjaro (or any other mountain near equator like this http://en.wikipedia.org/wiki/Nevado_Sajama) should be pretty doable today.
Or how about just this http://en.wikipedia.org/wiki/Project_Babylon for starters - was worked on by a guy who knew what he was doing:
"The barrel was to be 156 metres (512 feet) long, with a bore of 1 metre (3.3 feet).[3] Originally intended to be suspended by cables from a steel framework, it would have been over 100 metres (300 feet) high at the tip. The complete device weighed about 2,100 tonnes (the barrel alone weighed 1,655 tons). It was a space gun intended to shoot projectiles into orbit, a theme of Bull's work since Project HARP."
One cool proposal is StarTram [1], which has a long (100km+) maglev run in an evacuated tunnel to a linear accelerator up the side of a mountain, ending in a magnetohydrodynamic "window" to keep the atmosphere out, launching out the end fast enough to reach orbit. The acceleration of the first plan (30g) is too much for humans, so it'd initially be for cargo only.
The second stage is a crazy-awesome magnetically levitated tube that exits at an altitude of 22km, and gives low enough accelerations that a human could survive.
Nice, somewhat like Jules Verne's From the Earth to the Moon's:
Wait... most of the energy of going into orbit is in accelerating to orbital speed (lateral), not getting away from earth (vertical).
An elevator's top, in a geosynchronous orbit) rotates much faster than the bottom (describing a larger circle, covered in the same time).
Therefore, a payload moving up the elevator would either need to be accelerated laterally somehow, or it would bend the elevator over. There's no getting away from needing the energy for lateral acceleration - it must be supplied somehow.
Piping fuel up (instead of blasting it up in a rocket) may be more efficient, but consider that its mass too will need lateral acceleration.
\edit
Another alternative, for the reaction mass, is earth-encompassing rings at different altitudes. You can accelerate it without throwing it away.
Of course, at lower altitudes, you can just use air... maybe there's even enough (for this purpose) at quite high altitudes...
Finally, I think the basic solution has been a counter-weight: as one goes up (needs to speed up to orbital velocity), one comes down (needs to slow down from orbital velocity). If it was just one elevator each, the accelerations would be imparted at different points, twisting the evelator. But I guess a series of elevators - or even a continous elevator (like a conveyer-belt... or escalator...) would smoothen out this effect as much as you like, and only needs electricity to power it... and, any acceleration of the electron mass would also balance out, because they also go up and down in a circuit (or down and up for space solar panels). Sorry, nothing to see here. EDIT I see now a reply mentions this solution, upvoted.
Wait... most of the energy of going into orbit is in accelerating to orbital speed (lateral), not getting away from earth (vertical).
That's true for Low Earth Orbit, but as the radius of a circular orbit increases the kinetic energy decreases and the potential energy increases.
On the surface of the Earth, we have:
-(398600 km^3/s^2 [1]) / (6378 km [2])) = -62.5 km^2/s^2 of potential energy
(0.465 km/s [3])^2/2 = 0.1 km^2/s^2 of kinetic energy
At LEO, we have:
-(398600 km^3/s^2 [1]) / (6678 km [4])) = -59.7 km^2/s^2 of potential energy
(7.8 km/s [5])^2/2 = 30.4 km^2/s^2 of kinetic energy
At GEO, we have:
-(398600 km^3/s^2 [1]) / (42164 km [6])) = -9.5 km^2/s^2 of potential energy
(3.07 km/s [7])^2/2 = 4.7 km^2/s^2 of kinetic energy
Notice how LEO has only 2.8 km^2/s^2 of additional potential energy compared to the surface, but 30.3 km^2/s^2 of additional kinetic energy. However, at GEO there is 53 km^2/s^2 additional potential energy compared to the surface and only 4.6 km^2/s^2 additional kinetic energy.
[1] Earth's gravitational parameter [2] Earth's equatorial radius [3] Earth's equatorial rotation speed [4] A 300 km altitude orbit [5] Velocity at LEO [6] GEO radius [7] Velocity at GEO
I thought centrifugal force held it straight up and you're taking a tiny amount of energy from the earths rotation when sending up payloads. Please correct me if I'm wrong.
This is the correct answer. Angular momentum in the system is conserved; as you move mass up the space elevator, the earth spins (imperceptibly) slower.
Note that the tether only moves at orbital velocity at one point. Below that point, if you want to get into orbit, you must accelerate laterally after releasing the tether.
So the cable pulls the payload across, to (lateral) orbital speed: this was my point about the payload bending the cable over.
1. The cable has to be strong enough to do this (which may be reasonable, given how strong it needs to be anyway - plus it's in tension)
2. The cable will still be bent over, by a force imparted at the payload. If the bottom is attached and the top is "fixed" in geostationary orbit, the force will move the payload laterally (lagging the orbit - west), looking like a kind of arrow, or "V" on its side. The force on the cable itself might be OK, as the force is in tension; and the curve of the bend might be gradual enough. (Or maybe it would end up straight, but tilted westward?)
It will also pull on both earth and satellite, slowing both.
Therefore, a payload moving up the elevator would either need to be accelerated laterally somehow, or it would bend the elevator over.
If the cable is not anchored to the Earth, this causes the cable to slow down, and wobble a bit since the force is applied sequentially along the cable's length, rather than always on the center of mass, though tidal effects will tend to damp the wobble and keep it oriented practically straight up and down. For any one payload, the effect is negligible, as a space elevator cable would be freakin' massive, but without some sort of active stationkeeping it would eventually come down.
If the cable is anchored to the Earth, however, as soon as it begins to be pulled over even a little bit by the rising payload, the lateral force is transmitted to the ground. The cable pulls on the Earth and the Earth pulls back, re-accelerating the cable with no need for active stationkeeping and ever so slightly slowing the rotation of the Earth. Since the Earth is Gigantic and has ridiculously large quantities of angular momentum and rotational kinetic energy, no one will ever notice this loss in practice.
I like the extension to this which is to make a cable that stretches much further out than geostationary, then you clip payloads to the cable above geostationary and let the earth catapult them up the cable and slingshot them into the outer solar system.
the math actually works, given a strong enough cable. You would lower a tether from geosynchronous orbit, and simultaneously extend a tether into space. Thus, the total mass stays centered in geosynchronous orbit, and the lower tether gets pulled towards earth because of gravity, whole the upper tether gets pulled away because of centrifugal forces.
Thus, in the end, you have a tether that lightly touches the ground without force and stays in the same place, and an outer tether that extends about a third of the way out to the moon. The tensile forces would be strongest in geosynchronous orbit and weakest at the ends of the tether.
Another benefit of this construction is that you can use the outer tether to launch spaceships into space at escape velocity without any need for propulsion. You can even use tethers on other planets to catch such payloads again.
This construction does not eliminate the need for propulsion to reach orbit. But, you now can push against the tether instead of slippery and ever-thinning air, and you don't need to worry about lateral acceleration.
What if there's constant up/down traffic on two opposite sides of the ribbon? Wouldn't ascending cars try to pull the ribbon "back" (from orbital direction), whereas descending cars would try to push the ribbon "forward"?
I'm pretty tired now and I can't think very well, but my gut feeling tells me the two effects would compensate each other to a pretty large fraction of 100%.
But where will that extra downward traffic come from? The total weight of all space debris is only around 6000 tons[0] so that'll get exhausted within a matter of years (assuming we're even capable of collecting said debris). The only other alternative seems to be pulling near-Earth asteroids into geosynchronous orbit and using that.
[0] http://www.dnaindia.com/analysis/comment-space-debris-consta...
There could be bounty hunters, who wrangle asteriods and other debris to serve the ever-hungry fleet of elevators. Also, there's legit asteriod/moon mining possibilities to at least partially counter the up-traffic.
I been trying to think about this from a different angle and have been trying to work out how long you could make a self-supporting fuel and oxygen line by putting a lightweight triangular gantry with three small adjustable rocket engines and a fuel pump every hundred meters or so. This wouldn't need stupid materials as it would not be under the same tensile loads, but it will have a maximum length that depends on overall fuel consumption of the structure vs the flow rate of pipe you can haul.
Also, it would be constantly burning fuel, so it has pretty high running costs.
edit - you could also make an electric version of the same thing that pumps a reaction mass like water, but then you need to be able to fire it very fast out of the nozzles otherwise the water line becomes unwieldy very quickly, so I figured using chemical fuel might make things simpler.
There was an informal JPL study that was posted to rec.arts.sf.science a very long time ago, that found that a 100km high exponentially tapering tower could be practically built by using highly pressurized balloon tanks of boron. (Practical meaning, conceivably affordable by a superpower, with a chance in hell of actually happening.)
People have also studied using dynamically supported structures. We could probably build the Lofstrom Loop and it would bring the cost of going into LEO to space elevator levels.
http://en.wikipedia.org/wiki/Launch_loop
Still, not a chance in hell of it happening in the US. China might be crazy enough to build it.
Yeahhhhhh that is not an effective strategy at all. Rockets upon rockets. It sounds like an idea hatched in Kerbal Space Program.
The best "ground powered" structure would be the space fountain: http://en.wikipedia.org/wiki/Space_fountain
Back of an envelope calculations seem to indicate that a 200km one is possibly feasible using normal petrol, tho I wouldn't like to bet on geostationary. The space fountain has it's own issues, such as engineering the deflector and making the giant electromagnetic tube. What I am suggesting is far simpler.
edit - and the I had the idea before Kerbal came out, so if it is total madness, I promise you they are not to blame.
So what happens if the cable snaps? I assume this would happen at the height of a geostationary orbit where tension is the highest. And this is an ungodly tension so I assume it would be pretty violent. On top of that the cable will not fall straight down because the top of the cable is traveling much faster than the bottom (you could view this as the coriolis effect). It seems you would have 22,236 miles of cable wrapping itself around the earth.
It's more of a ribbon than a cable - it would float gently down.
The tip would be going over 5000 mph relative to earth, not including the speed because of the snap. I don't know if it would float gently, but it probably would burn up once it hit the atmosphere.
I would like to know if electromagnetic flux pinning is strong enough to use instead of a cable. I'm imagining a scenario where pairs of rotating superconductor panels in orbit lift a giant solenoid into space by flux pinning its magnetic field as they rotate upwards.
You're probably going to have to draw a diagram to get your basic concept across a wee bit better.
I've asked about the concept here as well:
http://forums.xkcd.com/viewtopic.php?f=18&t=109979
I suggested a two designs and the one I mentioned on hn has an ascii drawing about halfway down the page.
I believe lifting cargo outside orbit is not the greatest strength of an orbital elevator, instead it most useful feature would be on gathering sun rays which have not been diminished by the atmosphere. But there are many challenges to achieve that: http://io9.com/5984371/why-well-probably-never-build-a-space...
The atmosphere only attenuates sunlight by a factor of order unity. Won't it always be much, much cheaper to build slightly larger solar panels?
I think the counter argument to the space elevator magic material was always that if you could make something that light and that strong...
use it to make cheaper/lighter rockets.
The Space Shuttle is already 96% propellant to 4% structure (http://www.nasa.gov/mission_pages/station/expeditions/expedi...). You're going to get much appreciable gains from lighter rockets.
The 96% number refers to the orange fuel tank. Once you factor in the shuttle, the payload, and rocket engines, it's more like... 94% fuel. Some rockets are 88% fuel, but still only ~2-3% payload. A lighter rocket could mean 10% payload.
I was being kind of sparse in details, sorry.
Having a material that can self support itself into orbit can potentially unlock practical variations of:
http://en.wikipedia.org/wiki/Steam_rocket
whose main problem is: The simplest design has a pressurised water tank where the water is heated before launch, however, this gives a very low exhaust velocity since the high latent heat of vapourisation means that very little actual steam is produced and the exhaust consists mostly of water, or if high temperatures and pressures are used, then the tank is very heavy.
Although, that would also require the material to be very heat resistant as well. Which might be problematic. Compressed air might also work at that sort of level of material as well. At least for initial launch stages until you need to switch over to another type of propellant (or a more controllable engine).
I thought the Space Shuttle was already primarily a steam rocket. It held liquid oxygen and hydrogen, which was combined, ignited, and the result is high temperature water (steam).
Another way I wondered about, is to have a cold water tank with a nuclear reactor in it. As long as you ignore the safety part of the equation, do the physics work out? That is, would you get more thrust than from recombining hydrogen/oxygen?
The energy per kilogram lifted to orbit is vastly different. It's order(s) of magnitude cheaper (in terms of energy) to lift stuff into orbit.
Making the rocket from lighter materials won't matter, since the rocket is almost entirely fuel anyway.
The issue with rockets is that it takes x amount of fuel to get to your destination once in orbit and x amount of fuel to get the craft and the destination fuel to orbit.
It's why rockets generally work best in stages (you just drop the extra support structure needed for the fuel). It's really an exponential problem. Every lb of infrastructure you remove from the rocket makes it _much_ more practical.
See my comment above about having magic space elevator materials available anyway. Steam rockets and the like start to seem very practical at that point. And probably less insane than a cable all the way to space.