Robert Forward distinguishes two kinds of orbital slings:
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 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 is greater than the mass of the Earth! High-strength plastic cannot be used because it is vulnerable to space radiation and thermal fatigue.
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.