Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Tethers Unlimited, Inc.Cislunar Tether Transportpayloads both to and from the Moon, and thus canuse the orbital energy of inbound payloads toboost outbound payloads, it is possible for thefirst component of the system, the Earth-orbitTether Boost Facility, to repeatedly boostpayloads into lunar transfer trajectories withoutpropellant expenditure or return traffic needed.The key to achieving this is the combination ofmomentum-exchange tether techniques withelectrodynamic tether propulsion.HEFT Tether Boost FacilityThis concept, the ÒHigh-strength ElectrodynamicForce TetherÓ (HEFT) Facility, 6 isillustrated in Figure 3. The HEFT Facility wouldinclude a central facility housing a power supply,ballast mass, plasma contactor, and tetherdeployer, which would extend a long, tapered,high-strength tether. A small grapple vehiclewould reside at the tip of the tether to facilitaterendezvous and capture of the payloads. Thetether would include a conducting core, and asecond plasma contactor would be placed near thetether tip. By using the power supply to drivecurrent along the tether, the HEFT Facility couldgenerate electrodynamic forces on the tether. Byproperly varying the direction of the current asthe tether rotates and orbits the Earth, thefacility can use these electrodynamic forces togenerate either a net torque on the system tochange its rotation rate, or a net thrust on thesystem to boost its orbit. The HEFT Facility thuscould repeatedly boost payloads from LEO to theMoon, and in between each payload boostoperation it would use propellantless electrodynamicpropulsion to restore its orbital energy.Design of a Tether Boost Facility forLunar Transfer InjectionThe first stage of the Cislunar Tether TransportSystem will be a Tether Boost Facility inelliptical, equatorial Earth orbit. The mission ofthis facility is to pick up a payload from low-Earth orbit and inject it into a near-minimumenergy lunar transfer orbit. The desired lunartransfer trajectories have a C 3 of approximatelyÐ1.9 (km/s) 2 . A payload originating in a circularorbit at 350 km altitude has an initial velocity of7.7 km/s and a C 3 of Ð60 (km/s) 2 . To impulsivelyinject the payload into the lunar transfer orbitwould require a ∆V of approximately 3.1 km/s.OrbitalVelocityThrustTorqueEarth's MagneticFieldGrapple VehicleJxB ForcePlasma ContactorCenter of MassCurrentFacilityHigh StrengthConducting TetherPlasma ContactorPayloadFigure 3. Schematic of the HEFT Facility design.Orbital DesignIn the Cislunar Tether Transport System, thetransfer of payloads between a low-LEO andlunar transfer orbits is performed by a singlerotating tether facility. This facility performs acatch and release maneuver to provide thepayload with two boosts of approximately1.5Êkm/s each. To enable the tether to performtwo ÒseparateÓ ∆V operations on the payload,the facility is placed into a highly ellipticalorbit with its perigee in LEO. When the tether isnear perigee, its center of mass is movingapproximately 1.5 km/s faster than the payloadin circular LEO. The tether rotation is arrangedsuch that when the facility is at perigee, thetether is swinging vertically below the facilityso that it can catch a payload moving moreslowly than the facility. After it catches thepayload, it holds the payload for half a rotationand then releases it at the top of the tetherÕsrotation, injecting the payload into the highenergytransfer trajectory.Table 1 shows the orbital design for theLEO⇒LTO Tether Boost Facility. To minimizethe mass of the tether, it is tapered along itslength to maintain a constant load level; Figure 4illustrates this tapering.3