ORBITAL SLINGS

Robert Forward distinguished two kinds of orbital slings:

• The rotovator picks cargo from the Earth surface and accelerates it to orbital velocity.
• The bolo cannot accelerate cargo from standstill to orbital velocity, but it can change cargo velocity and direction. For example, it can circularize the orbit of a gun projectile as described by Pearson.

The following article describes two bolos boosting cargo from low Earth orbit to geostationary orbit and back:
Enrico C. Lorenzini, M. L. Cosmo, M. Kaiser, M. E. Bangham, D. J. Vonderwell, and L. Johnson, "Mission Analysis of Spinning Systems for Transfers from Low Orbits to Geostationary," Journal of Spaceraft and Rockets, Vol. 37, No. 2, March-April 2000, pp. 165-172.

Bibliography of space tethers.

TETHER BOOTSTRAP PROPULSION

Tether bootstrap propulsion is a bolo which can change its eccentricity without propellant expenditure. The feat is accomplished by changing length of the bolo at perigee and apogee.

BIBLIOGRAPHY OF TETHER BOOTSTRAP PROPULSION

Geoffrey A. Landis and Frank J. Hrach, "Satellite Relocation by Tether Deployment," NASA Technical Memorandum 101992, April 1989.

Geoffrey A. Landis, "Tether Methods for Reactionless Orbital Propulsion," Space Manufacturing, Vol. 8, AIAA, 1991, pp. 387-391.

Geoffrey A. Landis, "Reactionless Orbital Propulsion Using Tether Deployment," Acta Astronautica, Vol. 26, No. 5, 1992, pp. 307-312.

ROTOVATOR

The sling orbits the Earth and spins about its center of mass. Movement of the sling resembles the movement of a wheel spoke riding on the Earth. A hook on the end of the sling picks up cargo from the surface of the Earth. The end of the sling heats up when it plunges into the atmosphere. A low-thrust propulsion replenishes orbital energy. Gravity severely perturbs the sling. The minimum mass of a steel sling (for 1-ton cargo) is greater than the mass of the Earth! Tethers made of high-strength plastics are vulnerable to space radiation and thermal fatigue. The intensity of space radiation below the Van Allen belts (below the altitude of 2000 km) is much smaller than its intensity inside the belts and above the belts. It seems that a shaded plastic tether would survive long enough in low Earth orbit to do useful work. Coating one side of an arcuous tether with a reflective aluminum film can also reduce its temperature and is easier to fabricate than the shade. The reflective coating always faces the Sun. Perhaps the most practical material for an orbital sling is a rope made of S-glass fibers fused together under high tension and high temperature.

ROTOVATOR BIBLIOGRAPHY

Yuri Artsutanov, "V Kosmos bez Raket (in Russian, Into Space without Rockets)," Znanije-Sila Vol. 7, 1969, p. 25 (English translation: Report No. ADA084507, Air Force Systems Command, Wright Patterson AFB, Ohio, 1969).

Hans Moravec, "A Non-Synchronous Orbital Skyhook," Journal of the Astronautical Sciences, Vol. 25, No. 4, October-December 1977, pp. 307-322.

Rotovator