Talk-Polywell.org

created by tying a piece of fiber into a slip knot:

http://www.extremetech.com/extreme/1544 ... ial-thread

Would this significantly advance efforts to create tethers for a space elevator?

Probably not. The problem with space elevators is ultimate strength. Toughness will come into the mix when (if) the ultimate strength is high enough.

Space elevators may not be suitable for Earth. They may do much better on places without an atmosphere like the moon.

The problem isn't really the atmosphere. It's the size of the body (gravitational pull really) and the rotational speed (how much tension will be in the cable). The atmosphere is probably actually beneficial; it can help screen out micro-meteors which could cause problems on the moon.

They miss titled their study. What they are talking about is ductility, not toughness. Though often confused, ductility and toughness are not the same thing.

Toughness is the parameter describing the ability of a material to absorb energy.

energy per unit mass
or
energy per unit volume
or
energy per unit area

They are talking J/g. Dimensions of energy per unit mass.

In materials science, fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for many design applications. The linear-elastic fracture toughness of a material is determined from the stress intensity factor (K) at which a thin crack in the material begins to grow. It is denoted KIc and has the units of Pa(m^1/2) or ksi(in^1/2). Plastic-elastic fracture toughness is denoted by JIc, with the unit of J/cm2 or lbf-in/in2, and is a measurement of the energy required to grow a thin crack.

necoras wrote:The problem isn't really the atmosphere. It's the size of the body (gravitational pull really) and the rotational speed (how much tension will be in the cable). The atmosphere is probably actually beneficial; it can help screen out micro-meteors which could cause problems on the moon.

I was thinking oxygen.

MSimon wrote:I was thinking oxygen.

Think lightning.

KitemanSA wrote:

MSimon wrote:I was thinking oxygen.

Think lightning.

Works for me.

Space elevators are problematic for most moons, our own included, because they're typically tidally locked to their parent. They don't rotate fast enough to have a practical synchronous orbit station. It may be practical on these to run an elevator either straight toward the parent or straight away from it.

I looked into one of Saturn's moons, Phoebe, which is so far out it is not tidally locked, and I think has a period of rotation of about 9 hours. On that body, you could run a space elevator using rope from the hardware store.

I predict that our first space elevator will be rigged to an asteroid, likely by the first asteroid miners, and not in the far-distant future.

I predict that our first space elevator will be rigged to an asteroid, likely by the first asteroid miners, and not in the far-distant future.

Wait, are you saying you think we'll use an asteroid as a secondary mass on an Earth bound space elevator, or build a space elevator off of an asteroid? Because the second is a colossal waste of money. Even on Ceres, the largest rock you can still call an asteroid, a world class pitcher can almost throw a baseball out of orbit. On Vesta anyone in the MLB could do it. There's zero reason to build an elevator there from an economic standpoint. A linear accelerator is much more practical and useful. Even plain old rockets make more sense.

We seem to be wasting a lot of time talking about SEs in this toughest material topic when the main issue for SEs is strength.

I agree that whatever the current record-holder for "world's toughest material" is at any given time in the near future will be used for something other than a space elevator to Earth orbit. In order to pay for a space elevator you need a pre-existing space economy experiencing enough launch costs to make a dent in the cost, and promise of a strong enough space economy to make it turn a profit.

Thus, first somebody has to go out there and prove to the general public and financial communities what the futurists who hang around this forum believe in their bones: the main thing lacking for a space economy at the moment is more than a token human presence in space.

Asteroids generally rotate briskly enough to make landing on them dicey. De-spinning that much mass will not be a simple task. Rather than landing a large ship on one, I'd look into tethering to it in a scheme that essentially makes the tether a space elevator. The mass of the ship would require that the resulting two-body behavior would produce an "orbit" about a common center of mass, but the dynamics would be different than for just the mutual gravitational attraction. Tension in the tether would allow more force, and more net effect. The tether would provide a convenient way to move people and material between the two bodies, without requiring any landing legs, etc, on the ship. It may also offer possibilities for de-spinning the asteroid or deliberately producing tidal forces to mimic gravity, at least on the ship and on one side of the asteroid.

Space elevators extended well beyond synchronous orbit allow the possibility of a space catapult to launch cargo away from the main body. Cheap way to send stuff home. I expect we'll see them.

The space elevator is a variant of a skyhook which is a type of launch tether. Skyhooks are tidally locked tethers. There are also rotating tethers and tethers that don't pick up from a point but from a moving object. Each of these additions to the physics allow for more useful work with less strength required. Hypersonic skyhooks can be built with current materials. Hypersonic rotating tethers (HASTOL for one) can do even more with that strength.

My point is that you don't need to take one giant leap. Several smaller steps will do, even to Earth Orbit.

Asteroids are also generally light enough that despinning them wouldn't be too hard.