Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

--Commercial Tether Transport AIAA 2000-3842components that will provide the payload with guidanceand maneuvering capabilities in excess of whatwould be required of it in a conventional system.These components will be an additional expense, anduntil full round-trip traffic is established, will likelyrepresent a significant recurring cost in the system.Prior Work on Tether Transport ArchitecturesSeveral prior research efforts have investigated conceptualdesigns for momentum-exchange tether systems.In 1991, Carroll proposed a tether transport facilitythat could pick payloads up from suborbital trajectoriesand provide them with a total ÆV of approximately2.3 km/s. 4Soon thereafter, Forward 5proposed combining thissystem with a second tether in elliptical Earth orbit anda third tether in orbit around the Moon to create a systemfor round-trip travel between suborbital Earth trajectoriesand the lunar surface. In 1997, Hoyt 6developeda preliminary design for this ÒLEO to Lunar SurfaceTether Transport System.ÓIn 1998, Bangham, Lorenzini, and Vestal developeda conceptual design for a two-tether system for boostingpayloads from LEO to GEO. 7Their design proposedthe use of high specific impulse electric thrusters torestore the orbit of the tether facilities after each payloadboost operation. Even with the propellant mass requirementsfor reboost, they found that this systemcould be highly economically advantageous comparedchemical rockets for GEO satellite deployment.Under a Phase I NIAC effort, Hoyt and Uphoff 1 refinedthe LEO⇒Lunar system design to account for thefull three-dimensional orbital mechanics of the Earth-Moon system, proposing a ÒCislunar Tether TransportationSystemÓ illustrated in Figure 10. This architecturewould use one tether in elliptical, equatorial Earth orbitto toss payloads to minimum-energy lunar transfer orbits,where a second tether, called a ÒLunavator ª Ówould catch them and deliver them to the lunar surface.The total mass of the tether system, could be as smallas 27 times the mass of the payloads it could transport.The same NIAC effort also resulted in a preliminarydesign by Forward and Nordley 3for a ÒMars-EarthRapid Interplanetary Tether Transport (MERITT)Ó systemcapable of transporting payloads on rapid trajectoriesbetween Earth and Mars.Momentum-exchange tethers may also provide ameans for reducing the cost of Earth-to-Orbit (ETO)launches. This architecture would use a hypersonic airplaneor other reusable launch vehicle to carry a payloadup to 100 km altitude at Mach 10-12, and handing itoff to a large tether facility in LEO which would thenpull it into orbit or toss it to either GTO or escape. 8,9Building a Tether Transport SystemIf a tether-based transportation architecture is to bedeveloped in part or in whole by a commercial venture,the deployment of the system must follow a path thatis commensurate with a viable business plan. AnEarth-Moon-Mars Tether Transportation System willrequire at least three tether facilities, one in Earth orbit,a second in lunar orbit, and a third in Martian orbit.Each of these will require a significant investment intechnology development, system fabrication, and facilitylaunch. To keep the capital investments smallenough for a business plan to close, the system architecturemust be designed in a manner in which the firstcomponents can immediately serve useful functions togenerate revenue to fund the development of the rest ofthe system. This would be quite analogous to the developmentof the cross-continental railroads, where eachextension of the rail line was used to generate revenueto help build the rest of the line.In this document we will attempt to lay out a roadmapfor developing a full Tether Transportation System,beginning by discussing the technology developmentneeded to prepare for the deployment of tetherboost facilities, and then describing a possible sequencefor building a tether transport system to service commercialtransport markets.First Steps: Technology Developmentand DemonstrationWe have conducted an evaluation of the TechnologyReadiness Levels (TRLÕs) of the components and technologiesrequired for the tether facilities and other subsystemsof a tether transportation system. Many of therequired technologies, such as communications & control,solar power systems, thermal control, power storage,and plasma contactors are already at relatively advancedreadiness levels, or are expected to be brought tohigh levels within the next few years by ongoingNASA and commercial programs. Several key technologies,however, are unique to momentum-exchangeand electrodynamic tether systems, and will requireinvestment in technology development and risk reductiondemonstrations in order to enable the commercialdevelopment of tether transportation systems. Thesetechnologies are:• Tether Rendezvous & GrapplingAs mentioned previously, the rendezvous and grappling maneuver is currently the Òtall technology tentpoleÓ for momentum-exchange tether systems. For apayload to successfully grapple with a rotating tether,the system must first obtain a very accurate predictionfor the position and velocity of the tether tip grappleassembly at the appropriate pick-up time. The payloadmust then maneuver into an orbit properly phased sothat it will be at that position at the pick-up time.When the tether grapple and the payload do come intoproximity, the payload must then maneuver to meet up3