Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Tether Boost Facility Design AIAA 2000-3866boosted by the tether facility, a payload accommodationadapter (PAA) will be fitted to the payloadÕs standardmounting fixtures. The PAA will provide the rendezvousmaneuvering and docking capabilities to the payload, and may also provide the apogee kick ÆV.Safety Factor:To provide ample margin for error and degradation ofthe tether over time, the tether structure is sized to provide a safety factor of 2 for the largest loads expected inthe system. The largest loads will be due to transientoscillations immediately after the payload capture.These loads are predicted using numerical modelingwith TetherSim ª . Computed with respect to thenominal loads, the safety factor is roughly 3.5.Throughput:Because one of the primary advantages of momentum-exchange tethers is their reusability, to maximizethe cost-competitiveness of the system it will be designed to boost payloads as frequently as once every 30days.Momentum-Exchange/Electrodynamic-Reboost FacilityConceptIn order for the tether facility to boost one payloadper month, the tether must restore its orbital energyafter each payload boost operation. Previous effortshave proposed using ion thrusters or other electric propulsion to accomplish this reboost; 4,7electric thrusters,however, require propellant expenditure and thus wouldincur launch mass costs and resupply operations costswhich would limit the competitiveness of the tethersystem.If the tether facility operates at least partly withinLEO, it can instead utilize electrodynamic tether propulsionto perform reboost of its orbit. This concept,called the ÒHigh-strength Electrodynamic Force TetherÓ(HEFT) Facility (also referred to as a ÒMomentum-Exchange/Electrodynamic-Reboost (MXER) TetherFacility), 8 is illustrated in Figure 2. The Tether BoostFacility will include a control station housing a powersupply, ballast mass, plasma contactor, and tether deployer,which would extend a long, tapered, highstrengthtether. A small grapple vehicle would reside atthe tip of the tether to facilitate rendezvous and captureof the payloads. The tether would include a conductingcore, and a second plasma contactor would be placednear the tether tip. By using the power supply to drivecurrent along the tether, the HEFT Facility could generateelectrodynamic forces on the tether. By properlyvarying the direction of the current as the tether rotatesand orbits the Earth, the facility can use these electrodynamicforces to generate either a net torque on thesystem to change its rotation rate, or a net thrust on thesystem to boost its orbit. The HEFT Facility thuscould repeatedly boost payloads from LEO to GTO,and in between each payload boost operation it wouldOrbitalVelocityThrustTorqueEarth's MagneticFieldPlasma ContactorGrapple AssemblyJxB ForcePlasma ContactorCenter of MassCurrentHigh StrengthConducting TetherPayload CaptureVehiclePayloadControlStationHigh StrengthNonconducting TetherFigure 2. Schematic of the HEFT Facility concept.use propellantless electrodynamic propulsion to restoreits orbital energy.Orbital DesignTo boost a payload from LEO to GTO, the tetherfacility performs a catch and release maneuver to providethe payload with two ÆV impulses of approximately1.2Êkm/s each. To enable the tether to performtwo ÒseparateÓ ÆV operations on the payload, the facilityis placed into a highly elliptical orbit with its perigeein LEO. The tether facilityÕs initial orbit is chosenso that when the tether is near perigee, its center ofmass is moving approximately 1.2 km/s faster than thepayload in circular LEO. It can then catch the payload,hold it for half a rotation, and then release it at the topof the tetherÕs rotation. This injects the payload intothe high-energy transfer trajectory.Table 1 shows the orbital design for the LEO⇒GTOTether Boost Facility. To minimize the mass of thetether, it is tapered along its length to maintain a constantload level; Figure 3 illustrates this tapering.The orbital parameters and system masses shown inTable 1 are chosen so that the payloadÕs orbit and thefacilityÕs initial orbit are harmonic. For this design theresonance is 41:20. This enables the tether facility tohave multiple opportunities to capture the payload. Ifthe payload and tether do not succeed in achievingdocking during the first rendezvous attempt, they willwait for 2.6 days, adjusting the tether spin and correctingany trajectory errors, and then a second rendezvouswill be possible without any significant maneuvering.The resonance design shown in Table 1 accounts forregressions of both orbits due to the EarthÕs non-idealgravitational potential, up to the J4 term.3