Motorised momentum exchange space tethers: the dynamics of asymmetrical tethers, and some recent new applications

Abstract

This paper reports on a first attempt to model the dynamics of an asymmetrical motorised momentum exchange tether for spacecraft payload propulsion, and it also provides some interesting summary results for two novel applications for motorised momentum exchange tethers. The asymmetrical tether analysis is very important because it represents the problematic scenario when payload mass unbalance intrudes, due to unexpected payload loss or failure to retrieve. Mass symmetry is highly desirable both dynamically and logistically, but it is shown in this paper that there is still realistic potential for mission rescue should an asymmetry condition arise. Conceptual designs for tethered payload release from LEO and lunar tether delivery and retrieval are also presented as options for future development.

A numerical study of orbit correction for an asymmetrical motorised momentum exchange tether

“…The planar tether system on orbit about Earth is shown in orbital plan view in Fig. 2(a) and it can be seen there that the natural symmetry of the tether enables a simple dumb-bell system on orbit, as considered in previous publications [1][2][3][4][5][6][7][8][9], and more widely still. The Earth-fixed frame, EXY, provides the overall reference for the problem and the unlabeled position vector from the centre of the Earth, E, to the mass centre of the tether, which is geometrically co-located with the central facility at C, points to the origin of a local rotating frame of reference, ox0y0, and the tether is seen to rotate about the origin of this frame through angle 𝜓.…”

“…We also note the position vector to the remaining payload mass 𝑀 "& and this is denoted by 𝑟 & . It is also important to see that the displacement axially along the (instantaneously) lower sub-span 𝑙 # is defined by 𝑧 and, as first shown in [4], this can be a substantial distance of many km. The aim of any correction that we now apply is to reduce this distance back to zero so that the tether is effectively reset on orbit, even though it is still asymmetrically laden, so that it can then be re-balanced and continue with the mission, notwithstanding the likely need for further reset correction after the re-balancing payload has been attached.…”

“…Furthermore, analysis of the geometry of the immediate post-release configuration in Fig. 2(b) results in a cubic equation in 𝑟 $ , [4],…”

“…It has already been shown [4] that geometrical, and hence mass asymmetry causes an immediately large displacement of the centre of mass of the tether system from the central axis of rotation of the motor-drive within the central facility, to some point far outside the periphery of the central facility. Although this situation is generally retrievable, as shown in [4], the specific de-orbit circumstances for the tether have to be taken closely into account, and that is the principal subject of this paper. It is shown here that different orbits and release asymmetries have different repercussions for the remaining tether components and subsequent recovery scenarios.…”