Copyright © 2006 by Frank Crossman. Published by the Mars Society with permission
THE WEAKEST LINK IN THE SPACE ELEVATOR VISION -
Frank W. Crossman
The space elevator, hypersonic tether, and related concepts offer the promise of reducing the cost of reaching earth orbit by a factor of 10. There has been growing interest in the development of concepts for a space elevator ribbon made from carbon nanotubes (CNTs) that supports robotic climbers capable of traveling a few hundred km/hr on a several days journey to earth geosynchronous orbit (GEO) and beyond. Although the proponents readily agree that the only material from which an elevator can be made is CNT material with a theoretical strength of ~1/10 its measured stiffness of approximately 1 TPa, nearly everyone has ignored the critical fact that theoretical strength is based on the assumption of perfect crystalline structure not only at the nanometer scale, but also extending to the macroscopic (meter scale) and even to the "GEOscopic" (36,000 km scale).
The observed facts are that all materials are found to contain defects at several scales that reduce the effective macroscopic strength of materials to ~ 1/100 of the crystalline stiffness. The proponents of space elevator would like to believe that CNTs are an exception to this rule. But since the year 2000, experiments on CNTs have shown that this material is not an exception to the rule - carbon nanotubes do contain defects. Furthermore, the shear strength and shear stiffness of parallel arrays of CNTs are two orders of magnitude less than tensile strength and stiffness along the tube axis. This paper will review the physics of CNT crystalline anisotropy and the fracture mechanics of several types of defects found in CNTs and in macroscopic aggregates (ropes, ribbons) made from CNTs. The implications of these defects on the projected strength of macroscopic CNT structures and their use in space elevators and tethers for Earth and Mars will be discussed.