The Quest to Build an Elevator into Space
gizmodo.com
gizmodo.com
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.
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...
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.
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."
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.
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.
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%.
[0] http://www.dnaindia.com/analysis/comment-space-debris-consta...
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.
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.
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.
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
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.
The best "ground powered" structure would be the space fountain: http://en.wikipedia.org/wiki/Space_fountain
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.
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.
use it to make cheaper/lighter rockets.
Making the rocket from lighter materials won't matter, since the rocket is almost entirely fuel anyway.
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.
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).
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?