Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Tethers Unlimited, Inc.Tether RendezvousTether Deployment to Extend Rendezvous WindowAlthough the rendezvous shown in Figures 1 & 2 was an ÒidealÓ case where the payload and grapplemet perfectly, in real life, the payload and grapple will likely meet with errors in their position and orvelocities. Consequently, it will be necessary to enable the system to deal with reasonable errors and stillachieve rendezvous. Although automated systems may be capable of matching the position and velocitywithin the several-second window in the baseline rendezvous scenario, a tether transportation systemmay be made more technically feasible and salable to potential customers if the rendezvous window canbe extended to a longer period.While constant thrusting by either the payload vehicle or the tether tip vehicle could be used toextend the rendezvous window, the high acceleration levels between the payload in free-fall and therotating tether tip would require high thrust levels from the rockets used and would present large total∆V requirements on the system. Since the primary objective of the tether transport system is to minimizethe need for propellant usage for in-space propulsion it is highly desirable to minimize the propellantrequirements of the rendezvous method.Tethered Grapple DeploymentOne method for extending the rendezvous window that does not require the use of propellant is atether deployment maneuver illustrated in Figure 3. In this method, the payload would be guided torendezvous with a Payload Capture Vehicle (PCV) at the tether tip. When the payload and PCV reachtheir closest point of proximity, the PCV releases a tethered grapple fixture. This grapple tether will bedeployed at the minimum deployment tension possible. This will place the grapple fixture in a nearlyfree-fall trajectory which will closely match the payloadÕs free-fall trajectory until the grapple tether isfully deployed. The grapple fixture might contain some maneuvering capability; because it would berelatively light, a small thruster system could provide it with the ability to maneuver quickly to interceptthe payload. With this deployment maneuver, it will be possible to extend the deployment window toseveral tens of seconds, depending upon the length of the tether and the tether tip acceleration level. Anapproximate rule-of-thumb for the resulting rendezvous window duration is∆t≈2 l , (1)aPayload Capture Vehicledescends towards PayloadPCV releasestethered grapple3PCV pays out tetherand Payload maneuversto dock with grapplePCV engagestether brake andbegins to lift payloadFigure 3. Schematic of rendezvous method where the Payload Capture Vehicle drops a tethered grappleinto free fall.