System analysis of the expected electrodynamic tether performance for the ProSEDS mission

This paper reviews some expected systems performance aspects of NASA’s ProSEDS (Propulsive Small Expendable Deployer System) electrodynamic (ED) tether mission after recently being required to lower its initial orbit from 360 km to 285 km. In addition, the ProSEDS tether, which has conductive and non-conductive sections, shortened its nonconductive section thereby reducing overall tether length from 15-km to 12-km long. The International Reference Ionosphere (IRI) model is not as accurate as previously predicted when the altitude is less than 300 km and it was found that a factor of 0.65 should be multiplied to the electron plasma density on the IRI 1990 model to compensate for this effect. The ED characteristics of ProSEDS are being theoretically predicted using software called TEMPEST developed at the University of Michigan. Using OML (Orbital Motion Limited) theory for tether collection TEMPEST demonstrated that ProSEDS will de-orbit in 90 hours instead of the originally predicted 160 hours. The induced EMF (electromotive force) range will remain approximately the same from 400 V to 1000 V as will the collected current range, which varies according to altitude. Also, it takes 50 hours for the atmospheric drag to become stronger than the electromagnetic drag from the tether at 285 km as opposed to the 130 hours at 360 km. Various errors in the potential measurement of ProSEDS stem from: (1) the electron sheath at the upper end of the tether, (2) a “phantom current” occurring throughout the tether, and (3) the measurement of the potential used for current collection on the nozzle of the Delta II module. The “phantom current” also causes an extra 2 km per day de-orbit. In addition, there is a 0.733-mA theoretical current that travels into the Delta II from the tether that must be emitted. This potential is plotted versus the atmospheric density to show how much current is collected. The total potential error in the ProSEDS system ranges from 4–71% without hollow cathode operating, depending on the density of the electron plasma.