On the performance of a motorized tether using a ballistic launch method

In order to launch mankind into deep space a staging base must be built on the moon, or in space, to gain experience of the extra-terrestrial environment and to test the critical technologies needed. In view of this, this paper outlines a low-cost, reusable method for payload transfer to the moon using a Motorized Momentum Exchange Tether. The performance of the Motorized Momentum Exchange Tether (MMET) is predicted by simulation of the equations of motion of a pair of identical propulsion tethers emanating from the rotor of a central motorized facility with two identical payloads, one at the end of each tether. Suitable reaction is provided by a pair of similarly structured counter-rotating stabilizing stators comprising tethers attached to the central facility, conventionally known as the outrigger system. The outer payload gains the required ΔV by means of the motor torque which angularly accelerates both the payloads with respect to a reference frame fixed at the centre of rotation, and then converts the resultant angular momentum to linear momentum by releasing both payloads from the propulsion tethers. Thus, the absolute velocity of the outer payload is the orbital velocity plus this increment due to motorized spinup. The inner payload's absolute velocity is the orbital velocity minus the increment due to motorized spin-up. Momentum is, of course, conserved across the system. A demonstration mission to transfer a payload from LEO to Lunar Capture is proposed in this paper. It uses the Weak Stability Boundary (WSB) method, successfully demonstrated by the Hiten mission, to launch ballistically to Lunar Capture Orbit with a relatively minor 50ms^-1 capture burn. This method is particularly suited to launching with tethers, because the small capture burn replaces the large circularization burn traditionally used with Hohmann transfers. The propulsion time will be acceptable for most payloads; 100 days using ballistic transfer compared to 4 days with chemical propulsion and 500 days for the ESA SMART1 mission using ion propulsion. It is important to note that transfers based on motorized tethers would be unsuitable for live organic payloads given the relatively high centripetal accelerations involved in the spin-up.