Nuclear Electric Rockets

If the nuclear reactor powers an electric thruster, the propulsion system becomes a ''pure'' nuclear electric propulsion (NEP) system, or nuclear electric rocket (NER), in which acceleration is not based on expanding a fluid, but on the presence and strength of electric or magnetic fields. In juxtaposition, thermodynamic expansion has an efficiency, r, that depends on the ratio between the maximum and the minimum propellant temperature. r can be enhanced only up to a point, because of materials temperature limitations already discussed.

Both magneto-hydro-dynamic (MHD) acceleration based on the Lorentz force, and electrostatic acceleration based on the Coulomb force, as in ion thrusters for commercial TLC satellites, look very convenient thrust-producing mechanisms, because per se they do not imply thermodynamic efficiencies. In both strategies reactor and propulsion system are separate objects, lending themselves to separate optimization of each, see Figure 7.7.

However, the electricity must be generated somehow: a nuclear reactor produces (so far) only heat. If electricity is from conventional generators, the r issue reappears: this time r is not that of the electric thruster, but that associated to the thermal to electric energy conversion process. Alternatives to conventional (thermodynamic) electricity generation have been proposed, but the step from proven physics to engineering is still a long one (e.g., see [Bidault et al., 2004; Backhaus et al., 2004]). In this area the group of Professor S. Anghaie at the University of Florida has proposed MHD power generation, by utilizing the ionized plasma from a gas-core reactor, see for instance [Smith and Anghaie, 2004]. A more promising concept is solid-state alkali metal thermal to electric conversion (AMTEC) [Schock et al., 2002], with r ~ 25%. The conversion process comes also with a high price in terms of mass: for instance, stated goals at NASA-Glenn for the JIMO mission are a mass/ electric power ratio less than 40 kg/kWe (the subscript indicates electric power, not the reactor-generated power). Payload and trip duration depend critically on this ratio, see [Oleson and Katz, 2003]. This ratio should be compared with NASA's same goal for NTR, that is 0.08 kg/kW! The stunning difference is the result of the naturally low efficiency of energy conversion, and of the mandatory space radiator. NTR do not need either.

Ion and MHD-based thrusters have been studied for many years; their main features can be found, for instance, in [Sutton, 1992] and will not be reported here. Almost invariably, all electric thrusters have been powered by solar cell arrays, that is, at low power. What is new in the context of NER is the scale of the power available when switching from solar arrays to nuclear reactors. Scaling thruster power from kilowatts to megawatts involves opportunities as well as engineering and technology challenges. These are still far from having been satisfactorily analyzed. A recent workshop has begun to focus on some [Alta, 2003].

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