If the counterweight were in a ballistic orbit at an altitude above geosynchronous, yes, its orbital period would be less than 24 hours. But it’s not in a ballistic orbit: it’s being flung around like a tennis ball at the end of a string, with the other end of the string being held by Earth. Its orbital period will be in lock-step with the rotation of the earth’s surface, regardless of its height (as long as it stays above geosynchronous altitude).
The usual design proposed is a ribbon. It needs to have a larger cross-section in the middle than near the ends, and the easiest way to accommodate that is to be a constant thickness one way, and variable width the other.
Not even that. Cargo going up is balanced out by cargo going up. Ultimately, all the energy comes from the rotation of the Earth.
Though if we treat that as a finite resource, and want to be renewable, we could balance cargo up and down anyway.
The main support tether won’t be moving. As you say.
But if the climber is gripping the cable while powering itself up or down at e.g. ~1,000 mph to give a ~24 hour ride to geosync altitude, then some aspect of the climber’s machinery is engaging with the tether at a rate of 1000 mph. Imagine a drive tape sandwiched between two powered wheels. Pretty common sort of linear motor at ordinary scales. Now scale it up and make those wheels turn for a radial velocity of 1000 mph. Pretty high speed high stress violent stuff.
IMO @Chronos was talking about a fixed tether for support and guidance. But with a pulley mounted up high and two (or more) cars dangling from opposite ends of a different cable strung over that top pulley. Some power source somewhere rotates that pulley to move that cable, causing the cars to rise and fall. Again if we’re positing a travel speed of 1000mph, those moving parts need to be large and robust and are still applying a lot of high-speed violence to wherever / however they engage with the moving structures.
There is a helpful (subterranean, rather than orbital) article showing this concept, the “man engine”.
No, a man engine doesn’t show the same concept, because if you have a net mass going up, you still have to pay the energy costs of raising that net mass. The closest terrestrial analogue you’ll find is a siphon, where there’s some small energy cost in getting started, but once started, it sustains itself indefinitely (so long as your starting reservoir holds out) with no further energy input.
Oh, and given that having a space elevator makes it much, much cheaper to build more space elevators, probably what would end up happening would be that you’d have one elevator that uses the most efficient, but very slow, processes, for freight where you don’t care if it takes weeks, and another one that’s less efficient, but faster, mostly for passengers. And possibly others in between, for things that are somewhat time-sensitive but not as sensitive as people.
Yes, a mass beyond the geosynch point would have an orbit greater than 24 hours. a mass going fast enough to do a circular orbit in 24 hours sitting at than point would have a tendency to fly away (hence, “centrifugal force”) and being tethered, that’s what keeps the tether cable taut. However, it (and the extra cable) also has to have enough of that “centrifugal force” to pull upward on the weight of the cable from below geosynch distance to earth, since the part of the cable below 22,300mi is going too slow to maintain its orbit level and will want to drop to a lower level.
(In quotes, because it’s not a real force, it’s just a manifestation of the dynamics of a rotating system)
Obviously, the ideal motive for a cable system would be magnetic linear induction motor, since there would be no physical contact (wear and tear) with the cable and you could presumably, once outside the earth’s atmosphere, hit theoretical speeds well in excess of 1000mph. But that makes for a more complex cable. The cable could maybe also supply the power to the car.
The trouble with any counterweight system is that unless you have precisely enough mass going down vs up, there will always be a need for energy. I suppose at some point, there may be resources from the rest of the solar system that can be fed to earth. But at least initially, most of the cargo will be going upward, except for returning travelers.
It also seems to me that for simplicity, there would be a counterweight station off beyond geosynch level, but the waystation for getting on and off would be geosynchronous, so that the ships to take people and goods to and from earth orbit would be able to rendezvous with minimum effort.
Can you explain this part a bit more? Is it because you’d have a quick way to move stuff up into space?
That, and as with any big project, finishing the first one means you’ve finished all of the R&D, which is the largest part of the costs. All that’s left is the material costs, which are probably fairly low.
That makes sense, thanks. Of course it’s easier to duplicate things than make something new.
If I’m understanding your points correctly, you’re more closely describing a ‘sky hook’, rather than an ‘elevator’. The folk at Kurzgesagt have done rather a nice little video about it:
Not so much quick, but cheap. Rockets that can take a payload into orbit are insanely expensive. SpaceX has made it cheaper by making the rocket reusable, but “reusable” in this context doesn’t mean tens of thousands of cycles like a commercial airliner, and it’s still expensive because that vehicle needs to be built to very tight specifications. Right now SpaceX is charging $97M for a Falcon Heavy launch, and that price only gets your 27,300 kg payload to a geosynchronous transfer orbit, after which your payload will need to thrust itself into a proper circular geostationary orbit using its own propulsion system and fuel (meaning the actual useful thing you want up there in GSO needs to weigh considerably less than 27,300 kg). If you have a space tether and a tether-crawling elevator or conveyor belt that’s actually reusable for thousands of cycles, and it runs on conventional electric motors instead of rockets, the cost to get a kilo of payload up to GSO goes way, way down. So you use super expensive rockets to construct your first space tether (because you have no other options), and then you use that tether and its elevator to cheaply hoist additional materials up there to build the next tether.
A rocket ride to GSO happens pretty quick, but you probably wouldn’t want those kinds of speeds for a tether elevator. High-speed trains here on Earth are currently hitting around 320 km/h. GSO is at an altitude of 35,800 km, but ground level is already at 6400 km; at 320 km/h, it would take about 90 hours to get to that altitude, less than four days. If you assume your space tether’s counterweight is twice as high as GSO, then it would take about eight days to reach that altitude. For reference, the Queen Mary 2 takes about 7 days to sail across the Atlantic, so this wouldn’t be a ridiculously long/arduous journey, especially if passengers are similarly accorded the freedom to move around, sleep in a real bed, have regular meals, and so on.
There sort of are other options. You don’t start with a full-scale elevator cable that can handle hundred-ton cars. You start with a tiny, tiny cable, that can only support an extremely small car, and then you send up that extremely small car carrying a spool of more cable material as payload, which it grafts onto the existing cable as it climbs, to increase the cross-section size and hence strength of the cable by, say, 5%. And then repeat with another small car, 5% larger, and repeat. So by the time that you get to the first full-sized cable, most of the mass of the cable has been lifted by space elevator, not by rocket.
That said, even that tiny “starter cable” is still one hell of a big rocket launch.
If I remember Larry Niven’s The Integral Trees a long object in orbit will orient itself radially due to tidal forces. Pieces closed than the center of gravity are going too slow and will be pulled downward, further than the center of gravity will try to fly away, hence stretch outward.
To create a space elevator, you send up strands of cable to geosynch orbit. you then unspool them, keeping the center of gravity at geosynchronous distance. (You can cheat by adding simple weight to the outside end so that it does not need an equal length of cable to match the down side.) This can be done in stages, unspooling the cable (and running successive counterweights outward to balance). The fun part is the last one or two hundred miles where the atmosphere comes into play. The cable should be synchronous with the earth’s rotation, but any significant winds (or jet streams) will need to be dealt with. Also, no dawdling during this phase as winds can subtract from orbital momentul… So really fancy hovering rocket to capture the last bit, or be dangling an aircraft from orbit, that can do basic maneuvers once in the atmosphere.
The trick is of course manufacturing the cable made of pure unobtanium - over 22,300 miles worth. Launching that to orbit, plus enough equivalent dead weight to act as a counterweight. And make sure the cable doesn’t get hit by a Starlink satellite once there are 30,000 of them in a near-Earth halo.
(Fun calculation - how many kg per km? Therefore, what’s the tensile strength required of a cable that can dangle that far, when gravity and the cancelling orbital velocity are taken into account? People with better physics backgrounds have done the math, I’m sure… But the first few hundred miles it’s basically pure gravity)
Again, your elevator does not go up to the counterweight well above geosynch but to the point where it matches geosynchronous orbit, so cargo offloading or waiting to onload simply floats free adjacent to this waystation, instead of flying off due to centrifugal tendencies.
That depends on where your cargo is going. If all you need is just “Space, somewhere” (and there are a lot of applications for which that’s all you need), then sure, you can unload it at geosynch height. But if you’re trying to send something to the Moon, say, or to another planet, then it might be very useful indeed to send it higher, so it flies off centrifugally. Of course, you’d need to release it at the right time, so it goes in the direction you want, but that’s not too tough to calculate. And even if you can’t get it in precisely the direction you want, due to the inclination of the Earth’s rotation, you could still save a lot of fuel costs.
This is more or less how they spin suspension-bridge cables… but even the tiny starter cable needs the bridge towers and anchors in place to attach to.
I’ve heard of suspension bridges where the construction started with someone shooting an arrow, or flying a kite, to get a thin string across the gap.
I just happened across this video (done by a physicist…I think). I queued the video to where he talks about the tether being broken:
