This concept has been called ''hybrid'' by its proponents [Dujarric, 1999; Project 242 WG, 1999], in the sense that is neither a pure NTR, nor an electric thruster concept. Its thermodynamics is in fact closer to that of an arcjet as suggested by the work of Auweter-Kurtz and Kurtz, 2005] in Section 7.16. In the first version of this concept, part of the nuclear power heats the propellant as in any conventional NTR; the rest heats it by means of induction coils located along the conical portion of the expansion nozzle. The induction power is generated by the waste heat rejected by the nuclear reactor. This arrangement was proposed mainly to reduce space radiator size and mass, and raised Isp by an (estimated) 132 s, to a total Isp = 1,041s [Dujarric, 1999].
Alternatively, the nuclear reactor could simply generate electricity feeding the induction loops that heat the propellant. The reactor would generate all the electric power needed by SC induction coils. This second concept is more radical, and performance will depend much on the specifics of the design. In both original and alternative concepts, success holds on the balance between energy inductively deposited in the propellant, and that lost by plasma through radiative heat transfer.
All these propulsion systems producing thrust power via conventional machinery suffer a substantial q penalty: it is inefficient to generate thermal nuclear power, convert it into electricity (with q no higher than perhaps 50%) and then convert the electricity back into heat. The only advantages conceivable at this early stage are probably the ability to manage power, and especially to control the power distribution/injection along the engine system: it is much easier to handle electric rather than heat power.
No estimates are available for the total mass of such systems. However, their general philosophy and layout resemble modern so-called ''clean'' high enthalpy wind tunnels (for instance, the Plasmatron wind tunnel at the Von Karman Institute in Belgium [Bottin et al., 1998a, 1998b]). A mature Russian technology, Plasmatrons have shown to have good performance and little or no problem in inductively heating air to form air plasma at 7,000 to 9,000 K. By replacing air with hydrogen, for the same temperatures the Isp attainable should be in the 2,000-2,500 s range, including radiation losses. One of the problems in designing inductive heaters is predicting the effect of scaling from relatively small power and sizes up to the power required for a large engine, e.g., for a Mars mission. However, clustering individual thrusters of 1-2 MW power each appears feasible with an adequate cooling strategy, and 1-MW Plasmatrons are an established technology.
In conclusion, inductive NTR heating of propellant, either alone or in combination with conventional nozzle expansion is a concept worth investigating further for interplanetary missions. That is probably one of the reasons why ESA has acquired the patent rights to this technology.
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