Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Appendix N. MXER Tether for Deploying Microsatsprovide them with a total ∆V of approximately 2.3 km/s. Carroll’s design, however, assumed that the tether wouldbe placed in a circular LEO orbit. In order for this facility and tether to remain above the atmosphere after a payloadboost operation, the central facility had to mass 50-100 times the payload mass. This large mass would require avery large launch cost to set up the tether facility, which would likely hinder the economic viability of the concept.Hoyt (1997) investigated a concept proposed earlier by Forward (1991) for a tether system for transporting payloadsfrom LEO to the surface of the Moon. This design used two tethers in Earth orbits to minimize the total tether massrequired for the system. Hoyt proposed placing the tethers in elliptical orbits and performing all catch and tossoperations at or near perigee. Doing so minimized the drop in the tether’s perigee, enabling a tether facility to boosta payload and still stay above the atmosphere with facility masses as low as 5-10 times the payload mass.Bangham, Lorenzini, and Vestal (1998) developed a conceptual design for a two-tether system for boosting payloadsfrom LEO to GEO. The tether transport system was proposed to stage the ∆V operations using two tether facilitiesin elliptical orbits so as to minimize the required tether mass. Their design proposed the use of high specific impulseelectric thrusters to restore the orbit of the tether facilities after each payload boost operation. Even with thepropellant mass requirements for reboost, they found that this system could be highly economically advantageouscompared chemical rockets for GEO satellite deployment.In a Phase I NIAC effort in 1999, Hoyt and Uphoff studied the orbital mechanics of multi-tether systems fortransporting payloads between LEO and the surface of the Moon and found that orbital perturbations caused byEarth oblateness and other effects would make scheduling transfers in a staged system difficult or impossible (Hoyt1999). Consequently, they concluded that tether systems for transporting payloads from LEO to GTO or LTOshould use one tether facility in Earth orbit to provide all of the ∆V. Further study revealed that although a singletethersystem requires a much larger total tether mass than a staged two-tether system, the total system mass for aone-tether system, including the mass required for the control station and grapple assemblies, is the same or lessthan a multi-tether system because the total ballast mass required in a single-tether system is lower (Hoyt 1999).In a follow-on Phase II effort funded by NIAC, Hoyt (2000) developed a design for a tether facility optimized forboosting commercial communications satellites to geostationary transfer orbit. This paper extends that work byinvestigating the scaling of the system to handle small microsatellite-class payloads.µSAT TETHER BOOST FACILITY DESIGNThe ultimate goal of the NIAC-funded research effort is to develop an architecture for a fully reusable in-spacetransportation infrastructure capable of providing frequent rapid round-trip transport between Earth, the Moon, andMars. The technical development of such a transportation architecture must, however, follow a path that iscommensurate with a viable business plan, in which early components can serve useful functions to generaterevenue to fund the development of the rest of the system. The deployment of a tether boost facility requires thelaunching of a tether and control station which, together, mass roughly 10 times the mass of the payload. Fordeploying one or a few spacecraft, a tether facility thus would not be economically competitive with conventionalrocket systems. For applications where a large number of spacecraft must be deployed, however, a tether boostfacility can become highly advantageous because it eliminates the need to launch transfer propellant for eachspacecraft. One potential application of a tether boost facility is in the deployment of microsatellites. A small tetherfacility could provide a low-cost means for deploying swarms of microsatellites or delivering multiple smallsatellites to GEO to service and refuel communications and observation satellites. The same tether boost facilitycould also be capable of delivering numerous microsatellites to lunar orbit. In the following sections we describe aconcept for a tether boost facility designed to be deployed into LEO using an Athena-II class launch vehicle.System RequirementsPayload Mass: The baseline mission of a µSat Tether Boost Facility will be to pick 200 kg microsatellites up fromlow-LEO orbits and inject them into transfer orbits to GEO altitudes. To do so, the Tether Boost Facility willprovide each microsatellite with a total ∆V of 2.4 km/s. This same facility will also be capable of boostingapproximately 100 kg payloads to lunar transfer orbits.Safety Factor: To provide ample margin for error and degradation of the tether over time, the tether structure issized to provide a safety factor of 2 for the largest loads expected in the system. The largest loads will be due totransient oscillations immediately after the payload capture. These loads are predicted using numerical modelingwith TetherSim . Computed with respect to the nominal loads, the safety factor is chosen to be 3.5 over most of theN-2