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.

Glass ribbons are cheap and resistant to oxygen erosion but fragile. For details see:

John V. Milewski (editor), Handbook of Reinforcements for Plastics, Van Nostrand, New York, 1987, pp. 76-97.

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 (editor), 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, (editors) 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.

Buckytubes are microscopic carbon tubes. They are also called single wall carbon nanotubes. Buckytubes have three kinds of crystallographic lattice: armchair, zigzag, and chiral. Armchair buckytubes are ballistic conductors, which means that their electric resistance at room temperature is relatively small (6500 Ohms) and independent of length. If the armchair buckytubes are free of defects, one millimeter long buckytube has the same electric resistance as a buckytube that is one thousand kilometers long. Superconductive contacts eliminate or greatly reduce the resistance. The maximum current density of the armchair buckytubes is about one billion amperes per square centimeter -- 3 orders of magnitude more than the maximum current density of copper! Professor Guo-Meng Zhao claims that armchair buckytubes are room temperature superconductors, rather than ballistic conductors. Zigzag and chiral buckytubes behave like semiconductors.

Buckytubes have greater tensile strength than any other material. Their specific strength is 2 orders of magnitude greater than that of steel! If they do not break under stress, their deformation is perfectly elastic and reversible. Buckytubes are extremely slippery, so they contribute little to the strength of the composite material unless they are least one centimeter long. Brad Edwards claims that cyanate ester is the best matrix (glue) for a buckytube composite used in outer space because it is resistant to atomic oxygen, gamma radiation, and charged particles. Cheap, long buckytubes would make skyhook and geomagnetic levitation practicable.

Crystallographic lattice of buckytubes (based on data published by Smalley Group and Groupe mésoscopie)

MatWeb - properties of metals, alloys, plastics, ceramics, and composites.

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longitudinal speed of sound = (Y/R)^0.5

where:

Y is the Young's modulus

R is the density of the solid

material	P (tensile strength) [GPa]	Y (Young's modulus) [GPa]	R (density) [kgm-3]	U (characteristic velocity) (2P/R)0.5 [m/s]	longitudinal speed of sound in thin rods [m/s]
steel	1-5	200	7900	503-1125	5032
aluminum alloys	0.1-0.7	72	2700	272-720	5270
titanium alloys	0.6-1.3	110	5000	490-721	4690
berylium fiber	3.3	310	1870	1879	12870
boron fiber	3.5	400	2450	1690	12778
fused silica		73	2200		5760
pyrex glass		62	2320		5170
E-glass fiber	2.4	72.4	2540	1375	5339
S-glass fiber	4.5	85.5	2490	1901	5860
Kevlar 49 (aramid fiber)	3.6	130	1440	2236	9502
Spectra 1000 fiber (gel-spun polyethylene)	3.0	170	970	2487	13239
Spectra 2000 fiber (gel-spun polyethylene)	3.51		970	2690
PBO (plastic fiber, Zylon is brand name of PBO)	5.8	280-365	1560-1580	2710-2727	13397-15199
carbon fiber	1-6.5	250-830	1850	1040-2651	11600-21200
buckytube cable (theoretical data)	150	630	1300	15191	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).

BIBLIOGRAPHY OF REFRACTORY MATERIALS

D. R. Tenney, W. B. Lisagor, and S. C. Dixon, "Materials and Structures for Hypersonic Vehicles," Journal of Aircraft, vol. 26, November 1989, pp. 953-970.

K. Upahya, J.-M Yang, and W. P. Hoffman, "Materials for ultrahigh Temperature Structural Applications," Ceramic Bulletin, December 1997, pp. 51-56.