Tethers have been the subject of much analysis and speculative advanced missions; spaceelevators have captured the imagination of science fiction writers and space scientists for manyyears. There have been many successful and partially successful tether missions; most have beenbeset by minimal funding, consequently, they have operated without redundancy, operated assecondary payloads and the failures which have occurred have been due to, at times, relativelyinsignificant items in significant areas failing; items which could have enjoyed better quality orredundancy with adequate funding. There are several classes of momentum exchange tethers,the Extensible Tethered Upper Stage is one of a class of such devices. The proposed innovativetechnology reduces the insertion velocity necessary to achieve low earth orbit (or low Mars orlow Lunar orbit) by ～two km/sec. This results in huge reductions in the rocket mass ratiosnecessary, which is reflected as doubling of the payload for existing rockets, or as reduced riskfor Fully Reusable Earth to Orbit Systems (FRETOS) capable of enabling cheap access to space(＜ $200/kg). A hanging, momentum-exchange tether is proposed that rapidly contracts when itcaptures a payload to lift it above 90% of the Earth's debris field and re-circularizes above 99%of the debris field using advanced electric thrusters (Electrodeless Lorentz Force or ELFThrusters), for example.Recently, a team analyzed the impact of a Mid-Earth Orbiting tether employing near-term andavailable technologies on a fully reusable single stage to orbit (SSTO) vehicle. The E-TUSallows a realistic reduction of the orbital DV by 1.5-2.0 km/s. This reduction virtually enablesSSTO of LOX-LH_2 and even LOX-Kerosene SSTO vehicles are within the range of feasibility.Analyses on tether stability were conducted, power requirements were analyzed, methods tomeliorate certain longitudinal and transverse vibrational modes were proposed and analyzed.Methods to dramatically reduce the power required to winch up the tether were proposed andevaluated. We conclude that a MEO tether can be built with existing technologies and will resultin major improvements in reliable access to space and at dramatically reduced cost. This paperreviews the results of our analysis and propose some follow-on work.
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