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The basic idea is a development of existing [[radioisotope thermoelectric generator]], or RTG, systems, in which the heat generated by decaying nuclear fuel is used to generate power. In the rocket application the generator is removed, and the working fluid is instead used to produce thrust directly. Temperatures of about 1500 to 2000 °C are possible in this system, allowing for [[specific impulse]]s of about 700 to 800 seconds (7 to 8 kN·s/kg), about double that of the best chemical engines such as the [[LH2]]-[[LOX]] [[Space Shuttle Main Engine]].
However the amount of power generated by such systems is typically fairly low. Whereas the full "active" reactor system in a [[nuclear thermal rocket]] can be expected to generate over a gigawatt, a radioisotope generator might get 5 kW. This means that the design, while highly efficient, can produce thrust levels of perhaps 1.3 to 1.5 N, making them useful only for thrusters. In order to increase the power for medium-duration missions, engines would typically use fuels with a short [[half-life]] such as [[polonium|Po-210]], as opposed to the typical RTG which would use a long half-life fuel such as [[plutonium]] in order to produce more constant power over longer periods of time. The even shorter half-life element [[fermium]] has also been suggested<ref>[http://pdf.aiaa.org/preview/CDReadyMSPACE06_1393/PV2006_7272.pdf AIAA meeting paper comparing fermium, polonium and plutonium as power sources]{{dead link|date=November 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
Another drawback to the use of radioisotopes in rockets is an inability to change the operating power. The radioisotope constantly generates heat that must be safely dissipated when it is not heating a propellant. Reactors, on the other hand, can be throttled or shut down as desired.
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