Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Tethers Unlimited, Inc.MMOSTT Final ReportD. Development and Simulation of Tether Rendezvous MethodsOf the technology needs identified in the TRL evaluation, the most significant challenge is toenable payloads to successfully and reliably rendezvous with the grapple on a rotating tether.To begin addressing this challenge, we used a numerical simulation that includes models fororbital mechanics and tether dynamics to study the rendezvous between a payload in orbit anda rotating tether facility. In a tether-payload rendezvous, the relative motion between the tethertip and payload is primarily along the local vertical direction. The relative acceleration isconstant, so, from the perspective of the payload, the tether tip descends to the payload, haltsinstantaneously, then accelerates away. We have developed a method for using tetherdeployment to increase the length of time that the payload and grapple are near each other.This method is illustrated in Figure 8. As shown in Figure 9, numerical simulations indicatedthat this tether deployment maneuver can extend the “instantaneous” rendezvous to a windowof tens of seconds, without need for propellant usage. We also studied the effects of the payloadcapture on the tether tension. The simulations indicated that for an ideal rendezvous, tensionwave behavior will cause tension excursions roughly double that of the steady-state loads. Ifthe rendezvous is not ideal, that is, if the tether must be deployed for several seconds while thepayload and tether tip vehicle maneuver to achieve a docking, the resultant tension spikes canfurther increase the peak tether loads. Additional tether deployment maneuvers can help toameliorate the peak tension excursions and damp the longitudinal oscillations.A more detailed discussion of the study of Tether Rendezvous Methods is presented inAppendix G.Payload Capture Vehicledescends towards PayloadPCV releasestethered grapplePCV pays out tetherand Payload maneuversto dock with grapplePCV engagestether brake andbegins to lift payloadFigure 8. Schematic of rendezvous method where the Payload Capture Vehicle drops a tethered grappleinto free fall.13