Some approaches to mass reduction of the gasdynamic mirror fusion rocket

Previous studies have shown that the gasdynamic mirror (GDM) fusion propulsion system is capable of producing specific impulses in excess of 10 seconds and thrusts in the tens of kilonewtons. These propulsive capabilities arise from the ability of this magnetic fusion system to confine a hot plasma long enough to produce fusion energy while allowing a certain fraction of its charged particle population to escape through one end (a magnetic nozzle) to generate thrust. Earlier investigations have revealed that the optimum performance arise from the use of large mirror ratios which require large magnets and result in very massive vehicles. Major contributors to the large mass, in addition to the magnetic fields, are the large radiators required to dispose of waste heat. In this paper we address the question of mass reduction by investigating the role of magnetic field reversal near the mirror region, on the one hand, and the utilization of the liquid droplet radiator design on the other. We find that significant reduction in the mass of the vehicle can be achieved, and a reduction in cost and travel time are the direct beneficiaries of this reduction. Introduction The gasdynamic mirror fusion propulsion concept shown in Fig. 1 makes use of a magnetic mirror confinement system in which the plasma has a density and temperature that make the ion collision Copyright © 1998 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. mean free path much shorter than the length of the device. Under these conditions the plasma behaves like a continuous medium a fluid, and its confinement is governed by gasdynamic laws. The escape of the plasma from the system is analogous to the flow of a gas into a vacuum from a vessel with a hole. The governing equations are the particle and energy conservation equations which when solved self-consistently yield the length of the device as a function of the plasma parameters/' With the aid of a power flow diagram for a propulsion system that consists of the reactor, the injector, and a thermal and a direct converter, it has been shown that the energy multiplication factor of the fusion reactor (the Q-value) has to be slightly larger than unity to accommodate the efficiencies of the various components. A preliminary design of such a propulsion system, given in Table 1, reveals that for a deuterium-tritium (DT) fuel cycle, the rocket will be about 44 m long with a total vehicle mass of 422 mT. It will produce a specific impulse and thrust that allows a round trip mission to Mars to be accomplished in 169 days instead of several years. Of particular concern, however, is the mass of the magnets which can be very large especially at the mirror ends due to the large mirror ratio required for modest size devices. Also because of the relatively low efficiencies of the thermal converter (30% 45%) a large amount of waste heat has to be disposed of by a large radiator whose mass is calculated on the basis of an ability to radiate 5MW of thermal power per metric ton of mass. The magnet mass shown in Table 1 assumes MAGNETIC RELD LINES MAGNETS