Space Elevator Terminology
A diagram showing the types of carbon nanotubes.
Created by Michael Ströck (mstroeck) on February 1, 2006.
Released under the GFDL.
The (n,m) nanotube naming scheme can be thought of as a vector (Ch) in an infinite graphene sheet that describes how to 'roll up' the graphene sheet to make the nanotube. T denotes the tube axis, and a1 and a2 are the unit vectors of graphene in real space.
If m=0, the nanotubes are called zigzag. If n=m, the nanotubes are called armchair. Otherwise, they are called chiral.
So what's all this about "MegaYuris"?
The short version is that a Mega-Yuri (or MYuri) is a measure of how strong (in tension) a material is relative to how dense (mass per volume) the material is.
The technical version is that 1 Mega-Yuri = 1 GPa-cc/g.
Where GPa is Giga Pascals, cc is cubic centimeters, and g is grams.
The "Yuri" is named after Yuri Artsutanov, co-inventor of the SE. It has not yet been adopted as an officially named unit.
The reason the Space Elevator community talks about Mega-Yuris is because what it measures is THE critical attribute for a material capable of constructing a feasible Space Elevator. This is because most of the tension of an SE is caused by its own mass. Mass which has weight below geosynchronous orbit (geosync), increasingly so as you get closer to the Earth’s surface, and mass which has centrifugal force into space above geosync, increasingly so as you get further away from geosync. In both cases, those forces are creating tension on the ribbon at the “center” (geosync). More mass means more tension, so the material has to be strong enough to withstand all that tension – but the less dense it is, the less mass involved, and thus the lower those forces that tension will be.
This required attribute - being very strong yet very light - is why Carbon Nanotubes are currently thought to be the only material with which to build a feasible Space Elevator. The carbon-carbon bonds of the tubes makes for very high strength, while the open structure of the tubes gives them very low density.
Here's an excellent explanation of what MYuri is all about on the space elevator wiki: http://keithcu.com/wiki/index.php/Specific_Strength_in_Yuris.
The table below gives some insight into why the potential for CNTs to have very high strength with low density is so critical to the feasibility of a Space Elevator. As noted above, the center of the ribbon has to be strong enough to hold the two Earth-ward and space-ward halves together, which can be thought of as how wide that point has to be compared to the bottom end at the Earth's surface, which only has to be wide enough to hold your payload. Thus the ribbon tapers from the middle out. The lower the MYuri of your favorite material, the higher the taper - dramatically so. The higher the taper, the larger the mass of the Space Elevator. Around 30 MYuri, we begin to enter feasibility, as a starter ribbon would weigh about 955 tons - for comparison, the Space Station masses 417 tons. At even higher MYuri materials, things get much better, such that a starter ribbon only weighs 44 tons - for comparison, the Space Shuttle could carry a 24 ton payload to LEO.
Tons of ribbon
needed to lift 1 ton
Note that 1054 is 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000.
*The density and breaking tension of CNTs is not well defined since the cross section of them is not well defined, as they walls are so then and the empty space within the tubes is so large by comparison. Until large scale ribbon material composed of CNTs (and probably other materials) is actually developed, these numbers are only suggestive of the potential strength/density values.