Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Tethers Unlimited, Inc.Tether RendezvousTETHER RENDEZVOUS METHODSRob HoytTethers Unlimited, Inc.AbstractUsing a numerical simulation that includes models for orbital mechanics and tetherdynamics, we have studied the dynamics of rendezvous between a payload in orbit and arotating 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. The simulations indicated that tether deploymentmaneuvers can extend this ÒinstantaneousÓ rendezvous to a window of tens of seconds, withoutneed for propellant usage. We also studied the effects of the payload capture on the tethertension. The simulations indicated that for an ideal rendezvous, tension wave behavior willcause tension excursions roughly double that of the steady-state loads. If the rendezvous is notideal, that is, if the tether must be deployed for several seconds while the payload and tether tipvehicle maneuver to achieve a docking, the resultant tension spikes can further increase the peaktether loads. Additional tether deployment maneuvers can help to ameliorate the peak tensionexcursions and damp the longitudinal oscillations.IntroductionRotating momentum-exchange tethers hold great potential for reducing the costs of in-spacetransportation by eliminating the need for transfer propellant for many missions. One of the primarytechnical challenges that must be accomplished if momentum-exchange tethers are to achieve theirpotential is the need to enable a payload to rendezvous with a grapple mechanism at the tip of therotating tether. This rendezvous and capture maneuver is significantly more challenging than a standardorbital rendezvous, such as that between the Space Shuttle and the International Space Station, becausewhereas the Shuttle can take many orbits to gradually match its position and velocity with the ISS, atether and payload must achieve rendezvous at a specific location, velocity, and time. The rendezvouswindows available in a rotating tether system will be very short, requiring that the system must be able topredict and control the tether location to a very high degree of accuracy, and must be able to guide thepayload to the desired location with the right terminal velocity. Once the payload and tethered grapplehave come into proximity, they must then be able to maneuver and complete a secure docking within avery short window of time. In order to make this rendezvous and capture maneuver more feasible, inthis document we investigate the possibility of using tether deployment maneuvers to extend therendezvous window.Baseline Tether-Payload RendezvousTo illustrate the challenge of the rendezvous maneuver between the payload and tether, we havecalculated the relative positions and velocities of a payload and tether tip grapple during a rendezvoususing the TetherSim simulation. i In these simulations, we modeled a 100 km long tether, rotating with atip velocity of 1 km/s, picking up a payload from a LEO orbit. Figure 1 shows the relative vertical andhorizontal separations of the payload and grapple during an ideal rendezvous, where the trajectory of thepayload has been specified so that it meets with the grapple at just the right position and velocity. Thefigure shows that the relative motion is predominantly along the local vertical direction; this verticalmotion is illustrated more clearly in Figure 2, which plots the relative separations at 0.1 second intervals.In this rotating tether system, the tether tip experiences an acceleration level of approximately 1 gee, soone way to visualize this relative motion is to picture oneself reaching out from a fire escape and having afriend toss a ball up vertically so that it just reaches oneÕs hand. There is just a brief second or so when theball is close enough and moving slowly enough that you can catch it. Nonetheless, this Ò1-geerendezvous and captureÓ is quite feasible for a low-tech human, and thus it should be feasible for anadvanced autonomous rendezvous and capture technology.1