If we still need rockets to reach the bottom of the cable 36,000 km above Earth's surface, doesn't that negate the point of a "space elevator"?
The line would of course need to be designed to handle that, as well as the increased load as the payload is hoisted up to the moon.
Or to think another way think of all the challenges with human powered helicopter where the record is reaching 3.3 meters in 64 second flight, whereas any moderately physically fit person can climb the same height on the rope much faster and hang out there indefinitely, because they did not have to lift the weight of the helicopter and send the big masses of swooshing air down from the blades just to combat the force of gravity.
Also, if we start with a rocket+fuel on earth, and end with a rocket on the moon, there's an inevitable input of energy to move the rocket to a location of higher gravitational potential, that would have to be provided with hoisting as well, but there's also be a lot of rocket exhaust (formerly fuel) that's now zipping around the solar system, carrying away energy.
The saturn 5 rocket weighed 3,000 tons and was 96% fuel.
Even if we only save 10% fuel, that's an extra 300 tons we can get to the moon.
The Saturn V was capable of delivering 50 tons to lunar insertion. Reducing the fuel requirement improves the payload amount by that percentage. It’s not a direct trade off between the total mass of the vehicle and fuel for more payload.
I don't think you have your maths correct, either.
Going from a rocket that is 2880T fuel and 120T for everything else to a rocket that uses 25% less fuel gives you 2160T fuel and 840T everything else. Decreasing the fuel by 25% increased the rest of your budget by 7x!
If the starting point is 2,880t fuel for 120t payload, so 24t of fuel per ton of payload, then 25% less fuel gives 18t of fuel per ton of payload and thus your 3,000t rocket can now carry 158t of payload, or 32% more.
Even "we can see that the spaceline cuts the fuel cost of such a mission by approximately two thirds" is not a game changer compared to the cost of building the cable.
Space elevators are touted as game changers because although building one would be extraordinarily expensive it would slash the cost of sending payload to space by orders of magnitude, making it worthwhile.
This article talks about a cable from geostationary orbit to the moon.
That still leave the ground-to-geostationary part for conventional lifters.
Dang, I totally misunderstood, I thought this thing was going to go much lower.
Of course, it still probably isn't worth doing, there's not huge dollar reasons for people to be on the moon, mostly just fun science reasons.
And this thing doesn't even help with the orbit part, it helps with getting to a different part of the solar system once you get to orbit.
It's not like the moon lacks resources, we won't be needing to push 300 tons of anything to the moon if we have the capability to build the cable from it.
Also checkout my project:
It's an active structure made of small parts that are held together with a stream of magnets.
If the fountain fails then each part can have its own parachute so that it can safely return to Earth. i.e. without destroying entire cities as would be the case for a traditional elevator design.
But on the Earth-side, it can support up to 2000kg! And launching from the Moon is cheaper, anyway, since it's a less massive object than the Earth, and there's no atmosphere to deal with.
Edit: On 2nd thought they are probably using kg in terms of mass, not weight.
To the height of geostationary orbit. The actual endpoint wouldn't be geostationary, as the moon orbits the Earth, it'd be pretty difficult to keep one end of a line in a stable spot above the earth's surface.
The math suggests that putting a rendezvous base 41% of the way from the Earth to the Moon would be better than extending more cable down into Earth's gravity well. Meeting the cable closer to Earth would always cost more fuel. You could save fuel to GEO by meeting the cable further out, and then climbing back towards Earth. But every bit of extra cable you add on the Earth side increases the tension at the max-tension point at L1.
So just stop at the lowest-fuel rendezvous height, and add enough mass there to keep the whole cable in tension, but not enough to break the cable at L1.
(The L1 point is at about 84% the distance from Earth to Moon.)
The same type of material would likely be usable to anchor a halo relay satellite slightly beyond Earth-Moon L2, just by bolting it down to the darkest antipodal point on the dark side of the Moon.
How would you use it to reach geostationary orbit?
Any circular orbit with height between 5% and 41% of the Earth-Moon distance could be placed more cheaply by flying out to rendezvous at 41% with fuel, then walking it back in along the cable with solar power.
Edit: it gets even weirder, if you consider the Moon's distance from Earth goes from 363,300Km at perigee to 405,500Km at apogee... just how long should the line be? If it's 327,300Km, we might only be able to operate it near perigee, sure if it's a bit longer it'd be more usable, but it cannot extend past 363,300Km - or it would touch down here :) - that means there's no way we might use it near apogee, since it would always be too far away.
No, it would not have to reach orbit, it would have to reach the altitude but not the horizontal speed. Huge difference.
A 747 cruises at 893 km/h.
Concorde cruised at 2140 km/h.
The ISS orbital speed is 27580 km/h — or 36 megameters in 1 hour 18 minutes.
Space elevators are particularly strange to think about because they combine the weirdness of orbital mechanics with some, but not all, of the intuitions we have about terrestrial behavior -- and then add in the complications of being a megastructure on top of it, which humans have basically zero experience building. They defy easy reasoning.
The cars are limited by human reaction time, the aircraft are limited by air resistance, the ISS is a space station and all the constituent parts had to be brought up to that speed in the first place from the ground by rockets.
The main advantage of the category that space elevators are a part of is that energy and reaction mass doesn’t need to be carried with you as when you use a rocket, which makes them much easier.
Several of these winners achieved less than 5mph and this is near earth so without the issues of supplying power over thousands of miles.
But relative to the feasibility of this, how far are we from building automated systems to mine raw materials, refine and process them, and build stuff? Initially, of course, to build more such automated systems.
It usually takes millions of years of tidal forces from a planetary body acting upon its satellite for the latter to be tidally locked to the former.
What is the force on a rotating structure in space. Is it possible to build a space station that rotates with 1G force on people at the rim, and if so what scale of structure could be built with current, or plausible technology? Where does a structure go from "imagined future" to "magic"?
But merely getting up to 1G is trivial. I think the reason we don't have it today is that while getting "something" up to 1G is trivial, all the other engineering things that it interacts with get to be non-trivial, and right now, we're pretty tapped out with what we've got in space. If we continue to get more comfortable with space engineering and we continue to get better at putting stuff up there, I wouldn't be surprised we start seeing some spin-for-gravity relatively soon now.
This is important because even things like convection in the air stop working well at 0g, so for example, you can suffocate on your own CO2 if you fall asleep and there isn’t a fan running.
Funny enough the hardest part about this doesn’t even seem to be the tethering or rotation, but two things; 1) you have to make slight course corrections along the way because the initial burn is never precisely accurate enough to target something as far away as a planet, and 2) occasionally you have to put as much mass as possible between you and the sun due to solar storms spewing radiation, so you have to stop the rotation occasionally to point the engines sunward to use your fuel as a radiation shield, or to course correct.
The challenge in realizing it almost entirely an issue of getting materials into orbit, which is the problem space tethers are targeting. The cost of getting to geosynchronous orbit is around $30k per kilo, so at the moment economics is limiting us rather than tech when it comes to building crazy space stations. If we could bring the cost per kilo down 100x or so then so many more possibilities open up.
There is a size limit, though--the required material strength scales linearly with the radius. Note that if you'll accept some fancy engineering this can be avoided: Instead of making it self-supporting you surround your station with a non-rotating ring. You can make it as thick as you need to provide the required strength. Wheels can't take the speeds involved, the connection will have to be magnetic levitation. Yes, this means maintenance but so long as you have an adequate safety margin you can power down and take apart any given piece for upkeep or even replacement. Just don't take apart too many at once.
The math pencils out, but the station has to have a fairly large radius away from the axis of rotation. For example a station rotating once a minute with a radius of 500m produces 0.6g. This means that you are looking at a 1km diameter if the station is symmetrical, or possibly less if some sort of counterweight is setup.
Not at all implausible with current technology. Two habitats connected with a tether would do it.
My sense is that the most valuable information to come from ISS is of astronaut adaptation and long-term space effects on health.
Specifically this video: https://youtu.be/86JAU3w9mB8
tl;dr: it's trivial from a materials point of view, but hard from a "but you have to build it in space" point of view. We're still not great at building large structures in space.
I believe this idea is just incompatible with reality.
Huge charges across long distances mean very large currents. That tends to create difficulties, such as massive heating.
Whether or not this will be the case I don't know, though experiments with simple long tethers (several km to 10s or 100s of km) should be relatively easy to test: try them in some empty space and instrument for charges and magnetic fields.
Might produce an interesting de-orbiting spectacle as well.
"The Space Tether Experiment"
> The space tether experiment, a joint venture of the US and Italy, called for a scientific payload--a large, spherical satellite--to be deployed from the US space shuttle at the end of a conducting cable (tether) 20 km (12.5 miles) long. The idea was to let the shuttle drag the tether across the Earth's magnetic field, producing one part of a dynamo circuit.
The experiment failed, for a fascinating reason...
I was vaguely aware that the experiment had been tried. I'd either forgotten or never known the specific reasons for the failure. Though it rather closely matches the scenario I'd suggested above.
Let's ignore that.
What if the cable attached to the moon and then locking the other end to earth orbit alters the month's orbit? There was some stuff I read that the moon doesn't neccesarily orbit the earth. It's the earth and moon orbit each other. Essentially our orbit is significantly affected by our moon compared to other planets and their moons. We have a ton of wildlife that depends on the current orbit pattern. Wouldn't this cable greatly alter said orbit?
Plus, 380,000km worth of material? Along with they just hand wave that all failure will happen on the moon side and never affect earth... um okay? Because of testing?
This seems more like a joke paper in all honesty. Kind of like the paper based on Silicon valley where they dev an algorithm to find out how long it would take to give an audience handjobs. d2f ratio
They don't handwave it, they calculate it. It's significantly less handwavy than your back-of-napkin objections.
Now let's do some basic arithmetic. Moon is about 10^22 kg. Saying it will move as a result is like worrying that a semi will stop because of a fly.
The cable in not in geosynchronous orbit, it's in geosynchronous altitude. If it is cut, it will fall into Earth.
I imagine it would be nailed down at the Moon, even though it 's going "up" on that end too, the Earth end will very likely weight much more than the Moon and pull the entire cable. But the fact that it must be much fatter near the Moon does indeed complicate things.
Only if it's extremely massive.
I think scrapping together enough material on earth to build this will be the first problem.
2.36 x10^8m for rough cylinder volume.
Steel weight would be 1,864,400,000,000 kg. Yes itd be a hybrid material. So, half of that? Maybe a quarter? That's not an insignificant amount of material at a 1m diameter cable. We haven't even gotten to labor costs and energy required.
I get astronomically, its insignificant. But I doubt it's going to have no effect.
That's 37,545,591,074 times your projected weight of the cable.
I would have assumed such measurements would deal exclusively with the moon's mass.
Are the two terms used interchangeably in this context?
(honest question, I really have no idea)
Massive to a man, not a moon...
Sorry that's functionally just not possible. More so the moon is already moving away from the Earth at a couple centimeters per year. You're not even going to slow that down.
But everyone knows the moon orbits the earth with a cycle sort of similar to what we call a month.
How is an earth-moon cable feasible again?
That's the concept. They could have fit that within in the title of the post.
If you're scared about plutonium from reprocessing there are two things:
1) Reactor fuel has been in the reactor for too long. It's going to have far too much Pu-240. Reactors don't care, bombs do. You can't make a reliable bomb if you have too much Pu-240.
2) Deliberately leave hot stuff in the reprocessed fuel. So long as it's not a neutron absorber the reactor won't care. However, it will leave the fuel deadly hot. The industry has the equipment to handle it (after all, it's no hotter than the spent fuel they are removing), the terrorist does not. Nobody's smuggling it out because they won't live to accomplish it, not to mention that it will set off alarms from far away.
Of course the cheapest part of getting to the moon is the bit between the edge of space and moon orbit. Approximate current cost: $0. Approximate cost after placing 40,000kg fishing line in the way....
I'm also pretty sure crawling along a fishing line will be prohibitively slow compared to letting a rocket coast with its engines off.
No, it costs 4.1km/s of delta-v. https://en.wikipedia.org/wiki/Delta-v
Of course the way I worded this is a bit lose, and it's not completely free, but according to the figure in that Wikipedia article, the difference between getting into geosynchronous orbit and getting to lunar orbit from earth seems to be only about 0.1km/s, which is negligible compared to the total cost. So my basic point still stands.
It's actually really expensive to get to GEO.
you should play some Kerbal Space Program to get an intuition for delta-Vs and the cost of getting to the Mun.