Momentum-exchange / electrodynamic reboost (MXER) tether systems show great promise for use in propellantless orbital transfer. In 1998, MSFC and Boeing conducted a simple, preliminary examination of the system requirements of a tether facility to boost payloads from LEO to GTO. Work conducted at MSFC and TUI over the last two years has updated and refined these results, and led to alternate configurations and concepts that show greater promise for successful utilization. Two appendices are included that detail analysis techniques and mathematical derivations that can be used in tether facility design.
IntroductionThe development of tether technology has opened up an exciting new possibility for spacecraft-propellantless propulsion. Rockets push against their own exhaust, but an electrodynamic tether pushes against the Earth's magnetic field, and in essence, the Earth itself, to enable payloads to acquire higher-energy orbits.A pure momentum-exchange (MX) tether does not create orbital energy; it only exchanges it. If it catches and throws a payload, its orbital energy will be reduced, and it will assume a lower orbit. Without reboost, it will soon lose too much orbital energy and enter the atmosphere and burn up.Any type of propulsion system, in theory, could be used to reboost an MX tether. Chemical, nuclear, and electric are all options, but if any rocket reboost technologies are chosen, the MX tether will have a payload fraction that is governed by the specific impulse of the propulsion system, according to the rocket equation.On the other hand, a pure electrodynamic (ED) tether is limited to the regions above the Earth where the ionosphere and magnetic field are relatively strong (<1000 km). It collects electrons from the ionosphere to flow current through its conductive tether 1 . That tether acts like a wire moving through the field lines of the Earth's magnetic field; consequently a J x B force is exerted on the system. The ED tether can passively generate power (at the expense of orbital energy) or use a power supply to drive current through the tether and generate motive force (increasing orbital energy).In theory, an ED tether could dock with a payload and slowly spiral up to a higher orbit, then release it and spiral back down. However, again the ED tether is limited to altitudes less than 1000 km, and achieves performance similar to other low-thrust, high-power propulsion systems that have very low thrust-to-weight ratios.The MX and ED tethers, by themselves, do not achieve exceptional improvements in performance over existing technologies, but a hybrid of the two, the momentumexchange/electrodynamic reboost (MXER) tether, may have capabilities far beyond either technology separately.In principle, a rotating MXER tether in an elliptical orbit could catch a payload in a low Earth orbit, carry it for a single orbit, and than throw it into a higher energy orbit, all in a short period of time. It can then employ electrodynamic reboost over a period of weeks to restore the orbital energy it gave to the p...
