Thrust to weight of a nuclear engine is fairly poor, so they are best suited for upper stage or in-space work. A heavy-planet rocket might use chemical propulsi

Thrust to weight of a nuclear engine is fairly poor, so they are best suited for upper stage or in-space work. A heavy-planet rocket might use chemical propulsion in a lower stage just like we do and a high-energy nuclear upper stage (or two) where the really high Isp would be quite useful.