Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Tethers Unlimited, Inc.Tether Rendezvouswhere l is the length of the grapple tether and a is the tether tip acceleration.Deployment of Additional Tether by PCVAlternatively, instead of the PCV releasing the grapple on an additional tether, the PCV could releaseitself into free-fall by deploying a length of the main tether, as illustrated in Figure 4. The disadvantageof this second method would be that the heavier PCV would be less maneuverable than a smaller grapplefixture.However, this second method could have an advantage in that it might be able to achieve a trajectorycloser to Òfree-fallÓ than the previous method. When a tether is pulled off a SEDS deployer withoutbraking, the deployment tension varies with the deployment speed roughly as: ii[ 0 ]2 − ET = T + IρL ˙ A rel(2)where the first term in the brackets is a static (or minimum) tension and the second term is a tensiondependent upon the square of the deployment velocity that is due to the inertia of the tether being pulledoff of the deployer spool. Here, A rel = 1-AL/L end , L end = total tether length, A = tether wind solidity (=0.942for the SEDS winding), E is an area exponent = 0.8, I is an inertia multiplier = 4.1, and ρ is the lineardensity of the tether.Equation (2) describes the tension on the tether. The force experienced by the deployer, however, canbe significantly lower, because the deployer will not experience the force due to the inertia of the tether.As a result, a tethered grapple deployed from a PCV, where the deployer remains on the PCV, willexperience a larger tether tension, and thus a larger acceleration relative to the payload, than a PCV thatdeploys more of the main tether.Simulation ResultsUsing TetherSim, we modeled a rendezvous maneuver where the PCV begins with 1 km of tetherstored on a spool. When it reaches closest proximity to the payload, it releases a brake on the deployerand deploys tether at low tension, dropping the PCV into free fall with the payload. Figure 5 shows thePayload Capture Vehicledescends towards PayloadPCV DeploysMore TetherPCV pays out tetherand Payload maneuversto dock with grapplePCV engagestether brake andbegins to lift payloadFigure 4. Schematic of rendezvous method where the Payload Capture Vehicle deploys tether to followa free fall trajectory.4