Robert Forward distinguished two kinds of orbital slings:
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 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.
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.
The sling orbits the Earth and spins about its center of mass. Movement of the sling resembles the movement of a spoke of a wheel 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 would be vulnerable to space radiation and thermal fatigue. Perhaps the best material for any orbital sling is a rope made of strong (6.5GPa) carbon fibers coated with a thin layer of aluminum and fused together in a hot press.
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.