Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Rapid Interplanetary Tether Transport SystemsIAF-99-A.5.10There is a trade in aerobraking capturebetween momentum gain by the capturing tetherand mission redundancy. To make up formomentum loss from outgoing payloads, thetether would like to capture incoming payloadsat similar velocities. That, however, involveshyperbolic trajectories in which, if the payloadis not captured, it is lost in space. Also, in theearly operations before extensive ballast mass isaccumulated, care must be taken that the tetheritself is not accelerated to hyperbolic velocitiesas a result of the momentum exchange.Payload ReleaseThe release orbit is tangential to the tethercircle in the tether frame of reference bydefinition, but it is not necessarily tangential tothe trajectory in the frame of reference of theorigin planet. The injection velocity vector issimply the vector sum of the motion of the tethertip and the tether center, displaced to thelocation of the tether tip. Note in the third partof Figure 18 that this does not generally lie alongthe radius to the tether center of mass. Formaximum velocity, if one picks up the payload a ttether periapsis, one must wait for the tether toswing the payload around to a point where its tipvelocity vector is near parallel to the tethercenter of mass orbital velocity vector. By thistime, the tether has moved significantly beyondperiapsis, and there will be a significant flightpath angle, which both orbits will share at theinstant of release. Large variations from thisscenario will result in significant velocity losses,but velocity management in this manner couldprove useful. If, on the other hand, maximumvelocity transfer and minimum tether orbitperiapsis rotation is desired, the payload can beretained and the tether arm length or periodadjusted to release the payload in a purelyazimuthal direction at the next periapsis.Rendezvous of Grapple with PayloadThe seemingly difficult problem of achievingrendezvous of the tether tip and payload isnearly identical to a similar problem solveddaily by human beings at circuses around theworld. The grapple mechanism on the end of arotating tether is typically subjected to acentrifugal acceleration of one gee by the rotationof the tether. Although the grapple velocityvector direction is changing rapidly, its speed isconstant and chosen to be the same speed as thepayload, which is moving at nearly constantvelocity in its separate free fall suborbitaltrajectory. The timing of the positions of thetether tip and the payload needs to be such thatthey are close to the same place (within a fewmeters) at close to the same time (within a fewseconds), so their relative spacing and velocitiesare such that the grapple can compensate for anydifferences. This situation is nearly identical tothe problem of two trapeze artists timing theswings of their separate trapeze bars so that thatthe "catcher," being supported in the 1 geegravity field of the Earth by his bar, meets upwith and grasps the "payload" after she has letgo of her bar and is in a "free fall" trajectoryaccelerating with respect to the "catcher" at onegee. They time their swings, of course, so thatthey meet near the instant when both are at nearzero relative velocity. The tether grapplesystem will have the advantages over the humangrapple system of GPS guidance, radar Dopplerand proximity sensors, onboard divert thrusters,electronic synapses and metallic grapples, whichshould insure that its catching performance iscomparable to or better than the demonstratedhuman performance.An essential first step in the development ofthe MERITT system would be the construction andflight test of a rotating tether-grapple system inLEO, having it demonstrate that it canaccurately toss a dummy payload into a carefullyselected orbit such that n orbits later the twomeet again under conditions that will allow thegrapple to catch the payload once again.The Automated Rendezvous and Capture(AR&C) Project Office at Marshal Space FlightCenter (MFSC) has been briefed on the AR&Crequirements for the capture of a payload by agrapple vehicle at the end of a tether with a onegeeacceleration tip environment. MSFC has beenworking AR&C for over six years and has a greatdeal of experience in this area. It is their opinion[14] that their present Shuttle-tested [STS-87 &STS-95] Video Guidance Sensor (VGS) hardware,and Guidance, Global Positioning System (GPS)Relative Navigation, and Guidance, Navigationand Control (GN&C) software, should, withsufficient funding, be able to be modified for thistether application.Tether ConsiderationsFor a tether transport system to beeconomically advantageous, it must be capable of18