Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Tethers Unlimited, Inc.Appendix K: Facility Reboost StudyAchieving the Desired Final Orbit:In order to maximize the payload throughput capacity of a tether boost facility, it is desirable toperform electrodynamic thrusting whenever the magnetic and plasma conditions allow it. The tetherboost facility will be located in or near the equatorial plane. At these low inclinations, the ionosphericplasma has a “bulge” which significantly enhances the plasma density compared to inclined orbits. As aresult, it will likely be possible to perform significant thrusting out to 2000 altitude. Figure 6 illustratesthe geometry of the tether facility’s orbit, and the portion of the orbit in which thrusting can beperformed.Figures 2-5 show results for reboosting operations in which the tether current is controlled to achievea net average thrust that is perpendicular to the vector between the facility’s center of mass and the centerof the Earth. This method maximizes the efficiency of adding both energy and momentum to the tetherfacility’s orbit. Because thrusting is performed during periods when the tether facility is well away fromperigee, however, this thrusting method results in significant boosting of the perigee altitude. Figure 3shows that the perigee altitude is raised above the desired final value of 385 km in less than 3 days.Consequently, it will be necessary to modify the reboost method in order to achieve the desired final orbitshape.The orbit eccentricity is plotted as a function of the semimajor axis in Figure 7. With no adjustment tothe reboost program, the eccentricity increases roughly as e ∝ 6e-8 a. If this relationship holds during theentire reboost maneuver, by the time the semimajor axis has been restored to 11,040 km, the eccentricitywill be approximately 0.3807, and the perigee altitude will be approximately 74 km too high.There are several methods that could be used to enhance the eccentricity of the orbit while stillachieving the same reboost time. First, the tether facility could perform thrusting at a higher level nearperigee and at a lower level when it is away from perigee. Second, the tether facility could perform tetherreeling operations to increase the orbit eccentricity. Third, the tether facility could vary the direction ofthe net thrust during its passage through the ionosphere. In the following paragraphs we will describeFigure 6. Geometry of the tether facility’s orbit after it has boosted a payload. The red linesindicate the limits of the portion of the orbit during which the facility altitude is below 2000 km andelectrodynamic thrusting is likely to be effective.K-7