A fusion propulsion system for rapid interplanetary travel

Abstract

A propulsion system with capabilities that far exceed conventional and nuclear thermal systems is proposed for human exploration of the solar system and beyond. It makes use of fusion nuclear reactions, and is based on the magnetic mirror configuration which was studied extensively as a potential terrestrial power reactor. Unlike the conventional mirror machine, however, the proposed device will confine a plasma of such density and temperature as to make the ion-ion collision mean free path much shorter than the plasma length. Under these conditions the plasma behaves like a fluid and its escape from the machine is analogous to the flow of a gas into vacuum from a vessel with a hole, hence the name gasdynamic mirror (GDM) (See Fig. 1). Recent studies of this system reveal that the conditions for its performance as a propulsion system are much less stringent than those imposed by an economically viable power reactor. This is reflected by the small energy magnification factor Q (ratio of fusion power to injected power) required by the propulsion system when compared to that of the power reactor. The plasma confinement in this device is provided by a magnetic field that is stronger at the ends than it is at the center thereby providing the necessary reflecting force for containment while allowing a certain fraction of the energetic particles to escape to generate the thrust. The plasma dynamics and the corresponding propulsion characteristics of GDM can be obtained from a set of coupled conservation equations which yield, among other things, the specific impulse and thrust of such an engine. Typical values of the specific impulse is 1.26 × 10 5 seconds and a thrust of tens of kilonewtons for a system with a dry mass of about 230mT. Such a propulsion system is shown to be about 44 m long and to make a round trip to Mars in less than five months. In this paper we examine the various physics and engineering considerations that lead to the modest vehicle mass noted above.