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Nuclear reactors in rockets, specifically nuclear-thermal rockets, provide the energy needed to generate thrust. Most rockets generate thrust through combustion, where fuel is burned to release energy. Nuclear-thermal rockets generate thrust through nuclear fission, where atoms are split to release energy. While fuel is still needed for nuclear-thermal rockets, nuclear fuel releases 107 times more energy than combustion fuel. This means that with the same amount of fuel, a nuclear-thermal rocket can carry more payload than a combustion rocket.