Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Appendix N. MXER Tether for Deploying Microsatslength of the tether. To provide additional safety during rendezvous and capture dynamics, the safety factor isincreased to 4.0 for the 10 km portion closest to the grapple.Throughput: Because one of the primary advantages of momentum-exchange tethers is their reusability, tomaximize the cost-competitiveness of the system it will be designed to boost microsatellites as frequently as onceevery 30 days.Momentum-Exchange/Electrodynamic-Reboost Facility ConceptIn order for the tether facility to boost one payload per month, the tether must restore its orbital energy after eachpayload boost operation. If the tether facility operates at least partly within LEO, it can instead utilize electrodynamictether propulsion to perform reboost of its orbit. This concept, called the “High-strength ElectrodynamicForce Tether” (HEFT) Facility (also referred to as a “Momentum-Exchange/Electrodynamic-Reboost (MXER)Tether Facility), is illustrated in Figure 1 (Forward and Hoyt 1997). The Tether Boost Facility will include a controlstation housing a power supply, ballast mass, plasma contactor, and tether deployer, which would extend a long,tapered, high-strength tether. A small grapple vehicle will reside at the tip of the tether to facilitate rendezvous andcapture of the payloads. The tether will include a conducting core, and a second plasma contactor would be placednear the tether tip. By using the power supply to drivecurrent along the tether, the HEFT Facility will generateelectrodynamic forces on the tether. By properly varyingthe direction of the current as the tether rotates and orbitsthe Earth, the facility can use these electrodynamic forcesto generate either a net torque on the system to change itsrotation rate, or a net thrust on the system to boost itsorbit. The HEFT Facility thus can repeatedly boostpayloads from LEO to GTO, and in between each payloadboost operation it will use propellantless electrodynamicpropulsion to restore its orbital energy.Orbital DesignTo boost a microsatellite from LEO to GTO, the tetherfacility performs a catch and release maneuver to providethe microsatellite with two ∆V impulses of approximately1.2 km/s each. To enable the tether to perform two“separate” ∆V operations on the payload, the facility isplaced into a highly elliptical orbit with its perigee inOrbitalVelocityTorqueEarth's MagneticFieldPlasma ContactorJxB ForcePlasma ContactorCenter of MassCurrentHigh StrengthConducting TetherPayload CaptureVehiclePayloadLEO. The tether facility’s initial orbit is chosen so that when the tether is near perigee, its center of mass is movingapproximately 1.2 km/s faster than the payload in circular LEO. It can then catch the payload, hold it for half arotation, and then release it at the top of the tether’s rotation. This injects the payload into the high-energy transfertrajectory.Table 1 shows the orbital design for the µSat Tether Boost Facility. The orbital parameters and system massesshown in Table 1 are chosen so that the payload’s orbit and the facility’s initial orbit are harmonic. For this designthe resonance is 41:20. This enables the tether facility to have multiple opportunities to capture the payload. If thepayload and tether do not succeed in achieving docking during the first rendezvous attempt, they will wait for 2.6days, adjusting the tether spin and correcting any trajectory errors, and then a second rendezvous will be possiblewithout any significant maneuvering. The resonance design shown in Table 1 accounts for regressions of bothorbits due to the Earth’s non-ideal gravitational potential, up to the J4 term.ThrustGrapple AssemblyControlStationHigh StrengthNonconducting TetherFigure 1. Schematic of the HEFT Facility concept.N-3