STRUCTURAL MATERIALS

Slings, elevators, and many other orbital structures must be made of materials having high specific strength (strength-to-mass ratio). The specific strength of composites is only half of the specific strength of the reinforcing fibers. High-strength plastics, e.g., PBO, become brittle when exposed to thermal fatigue and space radiation. Atomic oxygen erosion, space junk, and meteoroids also damage orbital contraptions and towers. Contraptions protected by the atmosphere or a pile of rubble, can be made of plastic.

For details on atomic oxygen erosion see:
-John N. Stevens, "Method for Estimating Atomic Oxygen Surface Erosion in Space Environments," Journal of Spacecraft and Rockets, Vol. 27, No. 1, 1990, pp. 93-95.
-G. E. Caledonia and R. H. Krech, "Studies of the Interaction of 8 km/s Oxygen Atoms," in Materials Degradation in Low Earth Orbit (LEO), edited by V. Srinivasan and B. A. Banks, Minerals, Metals, and Materials Society, Warendale, PA, 1990, pp. 145-153.
-R. C. Tennyson, "Atomic Oxygen and Its Effects on Materials," in The Behavior of Systems in the Space Environment, edited by R. N. DeWitt, Kluwer Academic, Amsterdam, 1993, pp. 233-357.
For details on space junk and meteoroids see:
-William. A. Bacarat and C. L. Butner, Tethers in Space Handbook, NASA, 1986, page 4-32.
-Nicholas L. Johnson and D. S. McKnight, Artificial Space Debris, Orbit Books, 1991.
- Interagency Report on Orbital Debris, Office of Science and Technology Policy, National Science Technology Council, November 1995.
- http://elses1.msfc.nasa.gov/nee/meteo.html
- http://www.animatedsoftware.com/spacedeb/index.htm
- http://members.aol.com/earth2039/index.html
-Nicholas L. Johnson, "Monitoring and Controlling Debris in Space," Scientific American, Vol. 279, No. 2, August 1998, pp. 42-47.
Glass ribbons are cheap and resistant to oxygen erosion but fragile. For details see:
John V. Milewski (ed.), Handbook of Reinforcements for Plastics, Van Nostrand, New York, 1987, pp. 76-97.
Polycrystalline diamond has a tensile strength of about 700 megapascals. A small device using plasma chemical vapor deposition technique can deposit 25 micrometers of polycrystalline diamond per hour. The same device can deposit graphite at the rate of several millimeters per hour. A protective coating is needed to prevent erosion of carbon by atomic oxygen. Details:
J. J. Beulens, A. J. M. Buuron, and D. C. Schram, "Carbon Deposition Using an Expanding Cascaded Arc D.C. Plasma," Surface & Coatings Technology, Vol. 47, No. 1-3, 1991, pp. 401-417.
A carbon matrix produced by a chemical vapor deposition technique and reinforced with carbon fibers has a tensile strength of at least 1.5 GPa. Details:
John V. Milewski (ed.), Handbook of Reinforcements for Plastics," Van Nostrand, New York, 1987, p. 376.
A glass matrix reinforced with carbon fibers is fairly strong and resistant to oxygen erosion. Details:
-Brian C. Hoskins and Alan A. Baker, (eds.) Composite Materials for Aircraft Structures, AIAA, 1986, ISBN 0-930403-11-8.
-William K. Tredway and Karl M. Prewo, "Fiber Reinforced Glass Matrix Composites for Space Structures," in 23rd International SAMPE Technical Conference, Vol. 23, ed. Robert L. Carri, 1991, pp. 762-776.
Piano wire is cheap and has a tensile strength of about 3 GPa, while the strongest commercial steel wire attains 5 GPa. Details:
H. K. D. H. Bhadeshia and H. Harada, "High-strength (5 GPa) steel wire: an atom-probe study" Applied Surface Science, Vol. 67, 1993, pp. 328-333.
Buckytubes are microscopic carbon tubes. They are also known as carbon nanotubes. Their specific strength is 2 orders of magnitude greater than that of steel! Buckytubes are still too expensive to be used as a structural material, but fabrication techniques have been improving rapidly. Cheap buckytubes would make skyhook practicable. Professor Richard E. Smalley of Rice University is the leading buckytube expert. (Richard Smalley, Robert Curl and Harold Kroto received 1996 Nobel prize for chemistry for the discovery of a similar carbon structure called buckyball.) You can learn more about buckytubes from:
-David Tomanek's Nanotube Site.
-Web article: "From Fullerenes to Nanotubes".
-Associated Press article: "Carbon material could be used for space elevator".
-Smalley speech: "From Balls to Tubes to Ropes: New Materials from Carbon".


longitudinal speed of sound = (Y/R)0.5

where:
Y is the Young's modulus
R is the density of the solid
substance tensile
strength
[GPa]
Y
(Young's
modulus)
[GPa]
R
(density)
[kgm-3]
longitudinal
speed of sound
in thin rods
[m/s]
steel 1-5 200 7900 5000
berylium fiber 3.3 310 1870 12870
boron fiber 3.5 400 2450 12778
fused silica n. a. 73 2200 5760
pyrex glass n. a. 62 2320 5170
E-glass fiber 2.4 72.4 2540 5339
S-glass fiber 4.5 85.5 2490 5860
Kevlar 49 (aramid fiber) 3.6 130 1440 9502
Spectra fiber (gel-spun polyethylene) 3.0 170 970 13239
PBO (poly-paraphenylene benzobisthiazole, plastic fiber) 5.8 365 1580 15199
carbon fiber 2-5 250-830 1850 11600-21200
buckytube cable (theoretical data) 150 630 1300 22014
Table data compiled from:
-Dominic V. Rosato, Rosato's Plastics Encyclopedia and Dictionary, Hanser Publishers, Munich, 1993, p. 638.
-Alan S. Brown, "Spreading Spectrum of Reinforcing Fibers" Aerospace America, January 1989, pp. 14-18.
-CRC Handbook of Chemistry and Physics, 66th edition, page E-43.
-Theoretical buckytube cable data provided by Boris I. Yakobson (North Carolina State University, Department of Physics).