With the expected advances in reusable rocket technology within the next decade, will constructing a space elevator become meaningless? Or will it still be way cheaper, safer and convenient to use them?
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8$\begingroup$ Space elevators are science fiction at this point @harsh99. We don't have the technology or know-how to build one, so how would we know if it's going to be cheaper to use one or not? $\endgroup$– GdDNov 2, 2017 at 13:17
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1$\begingroup$ @GdD according to projected costs? $\endgroup$– harsh99Nov 2, 2017 at 14:00
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2$\begingroup$ @harsh99 it's certainly possible that we'll find out something new after a few decades, but until we have some idea of the physical processes that might make so strong materials, we can't make even remote estimates on what activities, resources and thus what costs would be required to make one. E.g. if carbon nanotubes might be sufficient, then estimating the future cost of mass-producing carbon nanotubes would help; but since they can't ever be sufficient (the C-C molecular bond is not strong enough) then that estimate doesn't help us say how much effort that future material might need. $\endgroup$– PeterisNov 3, 2017 at 10:27
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1$\begingroup$ And it is worth noting that there are just as many things that we thought that might be possible and with more knowledge in physics turned out to be permanently impossible. Thermodynamics conservation laws put a big stop to all kinds of ideas on how to achieve perpetual motion or free energy; relativity put a stop to imagined high-speed travel by finding out the light speed barrier. $\endgroup$– PeterisNov 3, 2017 at 10:33
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4$\begingroup$ There's a limit to how strong the strongest possible material can be - science doesn't improve that limit, science finds out where the limit lies by approaching it as closely as possible; and the true limit (which we don't know yet) might be sufficient for an earth-sized space elevator, or it might well be below that, as far as we know. $\endgroup$– PeterisNov 3, 2017 at 10:33
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7 Answers
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A Space Elevator would still be amazingly useful
The two factors that come to mind are forms of power and scale:
Power
With a space elevator connected to the ground, you could use the energy in your power grid to lift everything up. By doing this, we can use green and renewable energies.
With rockets, baring any massive advancements, we are restricted to using chemical energy. So not only to we have to specifically manufacture the fuel, but the entire process maybe less efficient. Not to mention the environmental effects of burning millions of tons of methane if you want something big in orbit.
Scale
Most likely, we'd be able to bring much larger masses into orbit at a time with a space elevator. Even with SpaceX's BFR system, a large ship in orbit has to refuel multiple time to get anywhere. With a space elevator, we could potentially bring a fully fueled ship (without any need to have in-atmosphere propulsion on-board) into orbit in one go.
All this being said, we still need HUGE advances in material science for any space elevator to be viable.
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28$\begingroup$ Another advantage worth considering, but too minor for its own answer--a space elevator's slower ascent profile also has a much kinder acceleration/vibration environment for its payloads, reducing engineering and construction costs for several payload types. $\endgroup$ Nov 2, 2017 at 18:29
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3$\begingroup$ Many launch systems don't burn methane but liquid oxygen + liquid hydrogen which creates nothing but water vapor. But the energy use argument applies here too, of course. $\endgroup$– PhilippNov 2, 2017 at 22:09
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1$\begingroup$ Not only can you use energy from the electric grid to lift stuff into orbit, you can recover most of that energy (minus friction losses &c) when you lower it back to the surface. Indeed you could turn it into a power generator by docking a small asteroid to it, and lowering pieces to the ground :-) $\endgroup$– jamesqfNov 3, 2017 at 3:32
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6$\begingroup$ Space elevators can actually get things in orbits, but only higher orbits. If you drop something off at GEO it will actually stay there, and you will have some kind of an orbit as you get closer. Also, no current launch system uses methane, most common is kerosene or hydrogen. $\endgroup$– PearsonArtPhoto ♦Nov 3, 2017 at 16:28
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3$\begingroup$ @Jens: Elevators would save a LOT of energy. If you launch something with a rocket, most of the energy is spent lifting the fuel. See e.g. en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation or reflect that the Saturn V used for lunar missions delivered about 4% of its liftoff mass to Earth orbit. OTOH, if your space elevator uses electric power, the "fuel" is essentially massless. $\endgroup$– jamesqfNov 4, 2017 at 5:18
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Rockets suffer from the tyrrany of the rocket equation. While a reusable rocket is great, you need to burn a lot of propellant for each kg of mass to reach orbital velocity.
A space elevator can steal momentum from the planet itself to provide the speed required to reach orbital velocity (or beyond).
In general, fast energy expenditure is less efficient than slow. Elevators allow efficient energy (solar power or whatever) to be used to lift cargo (electric climbing cars). Rockets require high energy density fuel (rocket fuel) to be burned and lifted to the spot where it is used by earlier rocket fuel.
The 'hard' part of a space evevator is the materials science required; we lack matter that has the tensile strength required on human scales in a controlled lab, let alone industrial scales in the open air. Once that is solved, we have the problem of building it in place, building a factory in orbit, dodging debris and weather, designing lifters, etc, each of which are problems larger in scale than the Apollo Program.
Reusable rockets would reduce the cost of lifting stuff to orbit, but the fuel costs would remain high. Such cheap launch technology would open up the possibility of building (expensive) factories in space, and/or move asteroids into position to provide the other end of the space elevator. So quite possibly they are a prerequisite for the elevator.
Once you have the elevator, lifting to orbit energy cost plummets and it becomes basically electricity (at current prices of 1$ per kg to orbital velocity). Currently it costs on the order of 10000\$ per kg. Reliable reusable rockets might cut that down, but probably to 1000\$ or 100\$s of dollars per kg, not single-dollars per kg.
Space elevators have limited capacity, so while the energy costs might be 1\$ per kg, prices may be similar to competing technology until demand saturates the pipe. However, the first space elevator can then be used to build the second, and you'd have something similar to an industrial revolution exponential economic expansion as the cost of reaching space converges on the energy cost.
At 100\$-1000\$/kg to orbit, space travel of humans requires a small fortune per person. Space may open up to industry and robots, but not to migration.
At 1\$/kg to orbit, and 2\$/kg to Lithobraking on Mars (not including travel time), the solar system becomes our back yard.
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1$\begingroup$ LOL @ lithobraking! Hopefully that's not the deceleration plan when we are sending humans to Mars :) $\endgroup$– BrianNov 2, 2017 at 18:10
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2$\begingroup$ @Brian RyanMars offers premium transport options at a higher price point. $\endgroup$– YakkNov 2, 2017 at 18:41
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$\begingroup$ @Yakk "each of which are problems larger in scale than the Apollo program" Really? $\endgroup$– harsh99Nov 3, 2017 at 10:12
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2$\begingroup$ @harsh99 Yes, by far. The Apollo program was to move a single small craft to the moon, land, take off, and get back to Earth. We are talking about building a megastructure, something in many ways bigger than humanity has ever built, mostly in outer space (unless you count systems like interstate highways or rail systems as single structures). Building a beanstalk on Earth (lying down, say) would be a ridiculously huge undertaking; building it in orbit has "minor details" that are themselves huge. $\endgroup$– YakkNov 3, 2017 at 11:47
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1$\begingroup$ @harsh99 The practical way to build a beanstalk is to start with mass in geosynchronous orbit and extrude structure both down and possibly up. At the ground, you build a mounting facility (which by itself is a megaengineering project, but minor as part of this project). Hand-waving difficulty, you dangle the constantly tapering cable down through the atmosphere until it it connected to the mounting facility. There are precursors to thi kind of technology, like skyhooks and hyde fountains, that may be easier than a full beanstalk yet cheaper than reusable rockets. $\endgroup$– YakkNov 3, 2017 at 13:46
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Whatever concept of space elevator you want to build, it requires you to transport massive amounts of material into space, so no. Having cheap launch vehicles is actually a requirement to building a space elevator. Whether the elevator could ever break even given its high mass is another matter altogether.
answered Nov 2, 2017 at 12:24
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$\begingroup$ If we use resources available in space or atmospheric carbon to do it, then? $\endgroup$– harsh99Nov 2, 2017 at 12:57
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1$\begingroup$ @harsh99 lot of equipment would be needed anyway no matter where you get the material from. $\endgroup$– jkavalikNov 2, 2017 at 14:19
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16$\begingroup$ The comments are approaching Worldbuilding.SE ;) $\endgroup$– EzenhisNov 2, 2017 at 15:10
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1$\begingroup$ @harsh99 impossible to know - at that point we might have fusion powered rockets, or not. Too much fi for the sci. $\endgroup$– jkavalikNov 2, 2017 at 15:25
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1$\begingroup$ I specifically prefaced my answer with "Whatever concept of space elevator you want to build" because there are lots of concepts, but they all require enormous amounts of material to be transported to space, generally as a trade-off between technological complexity of the approach to mass required. It's a hard question to answer if you allow for technology that isn't there yet, so I didn't want to go into it. $\endgroup$ Nov 2, 2017 at 18:17
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A space elevator, no matter what sort of unobtanium it's built from or how it works, has one unbeatable advantage over a rocket: the elevator cars don't need to carry their fuel with them. The vast majority of the energy involved in a rocket launch goes into carrying rocket fuel to ever-higher speeds and altitudes; being able to leave your energy source on the ground is a huge efficiency gain.
The phrase tyranny of the rocket equation isn't just poetry.
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1$\begingroup$ And if "22500 mile long carbon nanotubes that can hold dozens of tons of weight while not being ripped to shreds by the friction of the crawler" does actually turn out to be unobtanium, youtube.com/watch?v=iAXGUQ_ewcg a laser which gets at most 2.5% efficiency isn't too much better -- if at all -- than the rocket equation. $\endgroup$– RonJohnNov 3, 2017 at 2:05
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$\begingroup$ @RonJohn: Not sure exactly how the video derived that, but it was probably considering using PV panels driven by lasers. Very recent research has shown it's possible to create direct rectenna power generation at optical wavelengths, with much greater efficiency at a particular wavelength. Here, that should give a solid fourfold increase in efficiency, or at least 10% assuming the other (unknown) calculations were correct. $\endgroup$ Nov 3, 2017 at 2:59
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$\begingroup$ @harsh99 you'll have to ask the engineers and scientists who study what would be needed to develop space elevators. The reason that lasers are given as the energy source is that they are ground-based and thus the crawler won't have to carry the huge extra mass of it's energy source with it (which is the whole purpose of getting rid of rockets). $\endgroup$– RonJohnNov 3, 2017 at 12:24
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Another interesting point - the hard part isn't getting up, it's going fast.
Presumably, since the elevator is tethered to the ground it's already going fast - fast enough to stay in geostationary uh... orbit?
Since by the time whatever you were moving into space got to the top of the elevator, it's going to be going the same speed, it should be a lot easier since you just need to keep moving.
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$\begingroup$ For small payloads, slowly, one at a time, from one and the same point on the surface at the equator. Logistically very sensitive. Strings are by design highly exposed and unprotected. It is too big and inflexible an investment to be worthwhile. Power can be beamed. Fuel is very cheap and plentiful. Low gravity asteroids one can jump off of in a space suit, so the concept is useless in low gravity and undoable in Earth gravity. The Moon would be the only potential candidate (unless its eccentricity messes that up). If it can be built there without using any carbon (not available on the Moon). $\endgroup$ Nov 5, 2017 at 15:22
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In the book "The Fountains of Paradise", by Arthur C. Clarke, it is mentioned that a space elevator can regenerate some of the electricity it uses to move material up by electronically braking as it brings material down, like regenerative braking on electric cars.
There is also the possibility of actual rescue, should something go wrong. There is a very intense section of the book that involves a rescue maneuver, which also involves "thinking outside the box". I'd explain more, but I don't want to drop any spoilers.
https://en.wikipedia.org/wiki/The_Fountains_of_Paradise
I believe the book also mentions that with the space elevator, you don't need to reach escape velocity until you reach the end of the elevator. Since that's in space, escape velocity from that point should be considerably less than it is from the surface. Less air resistance should also make it easier to reach.
Rockets work on a exponential thrust restriction. The more you want to put into space, the way more fuel you need to lift off. It's not just that you are lifting 10 lbs into orbit, but you are also lifting the all fuel to get you there. The rocket crews have to make decisions of how important items are to a mission, because that 10 lbs of cargo may mean something like 10,000 lbs more fuel. Maybe this is explained by the tyranny of the rocket equation, but I don't have the chops to understand that.
Because there's so much fuel going into a rocket launch, it's expensive. Electricity, on the other hand, is relatively cheap. Renewable and nuclear generators have pretty much guaranteed that. Plus, with the extra height of the space elevator to get past the atmosphere, solar panels might be able to be added to the tower itself, allowing it to generate some of it's own power, beyond the regenerative braking mentioned earlier.
Beyond launch things into space, a space elevator could be a starting point for a ring around the earth, at orbital height. This could in turn be a platform for a full planet enclosure. Both ideas are referenced in several books I've read but can't recall right now. Both have their advantages in economy and beyond.
Going back to "The Fountains of Paradise", it tells about families going for sight seeing trips, including elderly people. Sightseeing is very big business, and if even the baby and the older generation of a family can join in, then you've really got something that'll draw visitors. Well, you know, besides being able to see a significant portion of the earth all at once, being "in space", and lots of other "cool factors."
The book also mentions being able to launch capsules in very short order, like multiples a day. Right now, that's not possible with rockets, even with airplane launched rockets.
The book is basically an essay in how space elevators are pretty much necessary for us to continue to be in space, couched in a science fiction novel. It's a great book, winning both the Hugo and Nebula awards. I've enjoyed reading it multiple times.
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Escape velocity is greater than geosynchronous velocity. The asteroid or whatever at the end of a tether would have to travel a bit faster than geosynchronous velocity, because of all the mass below the orbit of the asteroid moving at slower than (for the altitude of each bit) orbital velocity. Perhaps this would be close, or at least close enough, to escape velocity to allow easy exit from Earth's gravity well.
But even so, the tether itself would have to be very strong to resist the tension it would have to bear. A lot of people have suggested that carbon nanotubes would do the trick; I suspect even that would not be enough.
And even if we could invent the materials, how would we construct such a thing? If we capture an asteroid, put it into low-Earth orbit, and move the materials to it, we would still have to make a connection somehow. We can't just lower the cable into the atmosphere -- it would be whipping around until we (somehow) grab onto the end and attach it onto the Earth station.
Perhaps we will solve all these problems someday. But it's a little more difficult than I believe most people think.
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$\begingroup$ sources would be better than hunches $\endgroup$– user20636Nov 5, 2017 at 21:31