# Tag Info

110

With a rocket you have to carry the fuel with you. You are not just propelling the mass of the payload, but also the mass of the fuel. Installing a space elevator is a one-time event that can then be used to propel payloads indefinitely. You no longer have to carry the fuel to get to orbit.

67

Because space isn't about going high; it's about going fast! For example, in a 400 km orbit (like ISS) you need a speed of about 27,500 km/h or 7.66 km per second. So if you would extend a pole, winch or anything else into the lower parts of the atmosphere, it would also move at about 27,500 km/h (if we ignore atmospheric drag and all other ...

65

In addition to not requiring fuel: A rocket has to accelerate to orbital speed. This takes a lot of energy. A space elevator can climb at a low, constant vertical speed (albeit for a very long climb), and gets its orbital speed almost for free, from Earth's rotation (see Tom Spilker's answer for far more detail on this). Because a rocket accelerates to ...

61

Here's a simple reason: Most of the rocket's fuel is used just to push the rest of fuel! It sounds strange for those unfamiliar with Rocket equation. The reality is, if we want to accelerate by exhausting something behind us - then we have a problem when the speed we need to reach (8 km/s orbital speed) is greater than exhaust speed (3-5 km/s). In this ...

53

A "space tower" could be built at the north pole, but only if materials capable of supporting its weight were available. The "space tower" should be supported by the Earth's crust below it, but the crust will be flexible under the enormous load and over a long time. A space elevator with cables makes use of the centrifugal forces caused by the rotation of ...

38

It boils down to efficiencies of energy conversion and the cost of the technologies doing the conversions. If you have a given mass at Earth's surface that you want in geostationary orbit, you have to raise it to the geostationary radius (or altitude, if you prefer to think in those terms), and you have to accelerate it to geostationary orbit velocity. Both ...

29

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 ...

27

Yes, GEO is a balance point for anchored to Earth space elevator. It has to be, to keep its rotation rate synchronized with Earth's own and keep tether stable. This necessarily means that at GEO altitude, the elevator rotates at GEO orbital speed. Stepping off it at that altitude then means you're in a stable equatorial geosynchronous orbit, or GEO (...

24

After being released from an elevator, the payload and earth are a 2-body system. The paths in 2 body systems are conic sections: ellipses, parabola, and hyperbolas. What sort of conic section depends on where it is released from the elevator. The elevator makes a revolution each sidereal day. The angular velocity (aka ω) is 2 π radians/sidereal day. ...

23

Supplemental to the other answers; you are correct that the net force on the tether would be minimal, since the rotation of the counterweight would counteract the force of gravity. But, the individual components of this net force aren't being distributed evenly. Consider, say, the first kilometer of tether from the ground. This is being pulled down by ...

22

Ultimately an elevator is going to be more efficient, because it doesn't have to deal with gravity losses. Let me pose a question to you. What does it take for a rocket to hover in place like Blue Origin's New Shepard? If you've watched any of their launches you know they don't shut off the engine completely, but keep them running the whole time while ...

20

At the geostationary orbit, the growth (gradient) of tension will be practically zero, but the value will be far, far from zero. Actually, it will be the highest in the whole tether. After all, the whole length of the tether, that goes all the way down to Earth surface, is moving progressively slower than what orbital speed at corresponding altitude is - ...

19

(with considerable help from "Why we'll probably never build a space elevator") You have laid out a good foundation, for the first, and largest challenge, namely the material for the cable itself. Carbon Nanbotubes are the best substance we know of to build a Space Elevator. In their purest form, they have a tensile strength of over 100 GPa. The exact ...

19

It's the exponential nature of the equation determining the thickness. Let's discretize the problem to make it simpler to picture: you have a thread of very strong material, that has a self-breaking length of two kilometers. That means, if you hang more than two kilometers of this rope on iself, it will break under own weight. Let's cut it in kilometer ...

18

In short, no. The reason is, the hardest part by far of getting to space is getting to Low Earth Orbit. As the saying goes, once you've done that, you're half way to anywhere. It would be difficult to make the elevator stay put to non-anchored locations. Furthermore, it would mess with the speed required to get to each of these locations, and in the end, ...

18

For a first order estimate, we can use Wikipedia's list of vehicle speed records. Let's look at ground vehicles, ignoring rocket-powered vehicles (which sort of defeats the point of using an elevator). For wheel driven vehicles, the speed records seems to be around 750 km/h (I'm rounding a bit). For maglev rail vehicles, the record is close to 600 km/h. To ...

18

Here's a pic of the tether proposed by Liftport. Go 160,000 km beyond EML1 and drop a payload from that point. It will follow an elliptical path whose perigee grazes low earth orbit. A 3 km/s burn at perigee would circularize the orbit at LEO. Doing a 3 km/s burn from LEO and you will get the same ellipse. At the apogee of this ellipse, the rocket is ...

18

As an assist to the current answers. Try imagining an analogy: First you hold a rope with your arms held out horizontally and you spin. The result would be the rope spinning with you, tending towards the horizontal the faster you go. Now imagine spinning around but with the rope above your head. It will only fall down onto your head. You would need a ...

18

As an alternative to DarkDust's answer, if you start higher, at the classic altitude for space elevators, the end of your cable is indeed stationary to the air. But your cable needs to reach from geostationary orbit to the upper atmosphere, something like 35,700 km. The clipping off the last 20-60 km does not make a big difference in the overall ...

17

Because a space elevator would connect to a geostationary satellite. Geostationary satellites can only exist above the equator.

15

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 ...

14

Earth Carbon nanotubes might endure the enormous stress of an earth elevator but only short lengths have been manufactured so far. It would be a mega engineering project that would dwarf earlier human endeavors. An earth elevator would need to extend at least to geosynchronous orbit at about 36,000 km altitude. And unless there were a truly enormous ...

13

The idea behind a Space Elevator is to build a structure, usually from the surface of the Earth (Though there are notions of ones that just touch the lower atmosphere) to Geosynchronous orbit (22,000Miles) and then to balance the weight out, a similar distance of structure continuing on a further 22,000 miles (or else a large counterweight)). It needs to be ...

13

This is a simple calculation in the conservation of angular momentum. The angular momentum of uniform sphere is ${2\over 5}MR^2\omega$. We will assume that Earth is a uniform sphere (close enough for this question), so $M=5.972\times 10^{24}\,\mathrm{kg}$, $R=6371\,\mathrm{km}$ (mean), and $\omega=7.292\times 10^{-5}\,\mathrm{s}$. So the angular momentum ...

13

Water has a well-defined surface. Everything below it is water, everything above it is air. The top of the atmosphere isn't like that; you just get a very gradual decrease of pressure until you're several hundred kilometers above Earth's surface. You could try to build a cylinder with a closed bottom and open top, and place this upright, so it'll float like ...

13

One scheme for building elevators is to start from an anchor mass in synchronous orbit and to extend tethers down and up. The tether above will pull the anchor mass up and the tether below would pull the mass down. Care is taken to balance the two and keep building until the tether reaches the planet's surface. Synchronous orbit for earth is about 36,000 ...

13

Your question in its present form is unanswerable. Tensile strength is not the limiting factor of how space elevators are designed. Space elevators are designed to have an arbitrary constant amount of tension. This is accomplished by varying the thickness of the cable as you move up to either L1 or a GEO orbit for Earth. Imagine it this way, when you ...

12

A Clarke style space elevator is a (very large) gravity gradient stabilized vertical tether. When in a rotating frame (as on a merry-go-round) you feel a tug. It's just inertia but feels like an acceleration. This so called acceleration is $\omega^2r$ where $\omega$ is angular velocity expressed in radians/time. Gravity's acceleration is $GM_{earth}/r^2$. ...

12

In theory, yes, one could build a bridge. There are several potential problems. The first problem is that no known natural orbit is circular; all of them are ellipses. If, for example, the semi-major axis of the co-orbital elipse is 11000km, and the semi-minor axis is 10000 km, you need 1000km of "flex" in your bridge. Further, the "stable" point in ...

12

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.

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