Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Appendix F: Tether Boost Facility Design Final ReportThe flight control subsystems of various MMOSTT elements face qualitatively differentrequirements than previous flight control systems. Previous spacecraft have not had tocontrol their attitude or position while attached to a long, flexible tether that exertsconsiderable force and is as massive as the spacecraft. Orbital mechanics codes mustcontend with a spacecraft whose mass is so widely distributed that orbital speed variesappreciably as the system rotates. The magnitude and direction of electrodynamicthrust vary as the local magnetic field and plasma density change, so trajectory planningrequires more flexibility than systems that use well-defined rocket firings. All of thesechallenges can be solved, but they have not been solved and tested in flight, yet theTRL for flight control is 3.Another area with qualitatively new requirements is erosion protection for the tetheritself. The preferred tether structural material is Spectra 2000. Tests at MSFC suggestthat Spectra is susceptible to rapid erosion by atomic oxygen. (and degradation by UV?)Coatings protect other spacecraft surfaces from atomic oxygen and UV, but unlike thosesurfaces, the tether will stretch and bend. It is unknown whether any extant coatingswill adequately adhere to Spectra through many stretching cycles, and if so, how theymight affect the overall strength to mass ratio of the tether system. Erosion protectionfor the high-voltage insulation around an ED tether faces similar issues and is similarlyundefined. Erosion protection for both the tether structure and the current-carryingcomponent get a TRL of 2.Two MMOSTT subsystems are defined only at the functional level with no design yetdefined to implement the functions. These are the grapple mechanism and the collisionavoidance subsystem. Both currently rate a TRL of 2. The grapple concept will bedefined by the on-going HASTOL Phase Two contract. The HASTOL team is currentlydeveloping a rendezvous and capture simulation that will define requirements for agrapple system. Once grapple requirements are known, the HASTOL team will define agrapple concept, bringing the grapple TRL to 3. The collision avoidance subsystemcovers ground elements as well as flight elements, but the key challenge is a flightissue: maneuvering the tether system to avoid a predicted collision or close approach.Current spacecraft have only one reasonable approach to collision avoidance: changethe spacecraft's trajectory to provide a safe miss distance. In most cases, a tether canuse at least four approaches: change trajectory, change rotation rate or phase, changetether attitude (e.g. change libration angle or phase), or change tether shape (e.g.induce a bending motion different from the bend due to ED thrust.) Beyond definingthese options, no development has yet occurred in collision avoidance technology fortethers.F-13