Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

-Commercial Tether Transport AIAA 2000-3842with the grapple and a secure, high-strength connectionmust be made between the payload and grapple within arelatively short period of time Ð typically 5-15 seconds.While this is a much shorter time period than has beendemonstrated in space to date, other systems have demonstrated rendezvous and capture on equivalent or evenshorter timescales. One example would be the landingof jets on an aircraft carrier; this maneuver occurs withhigh relative velocities, unpredictable relative accelerations, and small physical windows for successful capture of the aircraftÕs hook by the arresting rope, yet it isperformed successfully many times every day. A second example would be the mid-air capture of film cannisters dropped by surveilance satellites. This systemagain had short (~2 seconds) rendezvous windows andhigh relative velocities, yet this maneuver was performed many times with a 100% success rate.• Tether Dynamics Control and StabilizationThe dynamics of flexible tethers in orbit are complex,and system that utilize electrodynamic propulsion mustbe controlled to avoid problems with dynamical instabilities. TUI has already developed a simple methodfor stabilizing the dynamics of the Terminator Tether ª ,an electrodynamic tether drag system. 10A momentumexchange/electrodynamic-reboosttether facility, however, will require a more complex dynamics controlsystem to maintain optimum performance of the tetherthrusting and ensure that tether dynamics do not adversely impact the rendezvous and capture maneuvers.• High-Strength Survivable Tether with IntegratedElectrodynamic TetherTUI has already demonstrated fabrication of multikilometerlengths of conducting multiline tethers andnonconducting tethers made of high-strength fibers suchas Spectra 2000. However, a tether boost facility willrequire a very high strength-to-weight micrometeoroidsurvivable tether structure that has both high-strengthfibers and conducting elements for electrodynamicthrusting. Furthermore, the electrodynamic componentof this tether must be designed to reliably operate atmany kilovolts of potential relative to the tether facilityand ambient plasma.• High-Power, High-Voltage SystemsIn order to perform electrodynamic thrusting on aTether Boost Facility that has a tether length of many10Õs of kilometers, the power system on the facilityÕscontrol system must be capable of processing manykilowatts of powers and converting them to voltages onthe order of 20 kV, while ensuring that no electricalarcing can occur to threaten the integrity of the tether orother systems.• Tether Orbit Propagation & Collision AvoidanceA Tether Boost Facility will be a very large objectmoving through altitudes where there are many existingsatellites and space debris objects. Although a survivable space tether structure such as the Hoytether ª canenable the tether to withstand degradation by impactswith small pieces of space debris, the tether system willstill have to deal with large objects that may get in itsway. One of the significant issues for this is developing accurate and fast methods for propagating the orbitand dynamical behavior of a tethered system so that thetether system controllers can reliably predict closeencounterevents and command avoidance maneuvers.Working to our advantage, however, is the fact that amomentum-exchange/electrodynamic-reboost tetherfacility will have significant ÆV capabilities using itselectrodynamic thrusting. Thus if close encounters canbe predicted with sufficient advanced notice, the tetherfacility can avoid these encounters.Suggested Technology Development Efforts:In order to address the technology needs listed above,there are several development efforts that could significantly advance the technology readiness levels of appropriate solutions with relatively low investment requirements.Grapple Mechanism Develo p mentThe payload-tether rendezvous is the most significantchallenge for a momentum-exchange tether system.There are, however, several grappling concepts thatcould make this problem more tractable. One concept,originally suggested by Tillotson and recently improvedby Sorenson, 11 is illustrated in Figure 2. In thisconcept, the tether grapple assembly at the end of thetether would open a net structure, providing a very largetarget area for the payload. The task for the payloadwould then be to intersect this net and secure itself tothe net. To minimize chances of the net damaging thepayload, rather than intersecting the net, the payloadmight instead maneuver to come within a short distanceof the net and shoot a tethered ÒharpoonÓ into the net.The payload would then ride the net for half a revolutionof the tether. To release itself from the net, thepayload would retract the barbs on its harpoon, therebyinjecting itself into its transfer orbit.GRASP DemoSome of the methods for achieving the rendezvousbetween the payload and the rotating Tether Boost Facilitycould be demonstrated in a low-cost ground experimentthat would utilize existing Automated Ren-Figure 2. SorensenÕs ÒNet and GrappleÓ concept forfacilitating payload-tether rendezvous.4