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The nuclear rocket is potentially capable of much higher impulse than any chemically fueled rocket, because of the high energy content per unit mass of the fissionable material. While a part of this potential can be realized by use of a low molecular weight propellant heated by solid fuel plates, it seems clear that the full potential can be realized only if the fissionable material can be retained in gaseous form, and its fission energy transferred directly to the propellant.
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Kerrebrock et al. (1961) studied this question.
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