Nuclear Thermal Rocket Engines are an alternative to chemical propulsion, and can offer far better specific impulses. Real world example: NERVA.

The basic gist of an NTR is that, though they have about the same Thrust Power as a chemical engine, they can use different propellants and get specific impulses which are far better than chemical propulsion, without having the abysmal TWR of ion engines.

A handy property of NTRs is that depending upon the propellant you use, you can trade off TWR for Isp. So if you use the stock standard LH2-Only propellant, you can get Isp of around 900 seconds, although the TWR will be less than one. If you use Water, however, you can get the same specific impulse as a Hydrogen/Oxygen rocket. That's because the maximum temperatures for an NTR's nuclear reactor core and that of a chemical rocket combustion chamber are about the same, and water, when split up at those temperatures, has the same molar mass as burnt hydrolox. The NTR will also have about the same thrust as the Hydrolox chemical rocket for the same reason. However, water is far, far denser than Hydrolox propellants, so you can get away with less dry mass.

Solid Core NTRs are not the only kind. There are also Gas Core NTRs, both in open cycle and closed cycle modes, which can get much larger thrust powers than chemical rockets, meaning better Isp AND better thrust. Open Cycle Gas Core NTRs are practically overpowered torchdrive monstrosities, but they also spew radioactive waste out of their bottoms. Closed Cycles, meanwhile, are less powerful but are far safer.

NTRs will need shapeable radiator parts and/or deployable radiators to allow them to cool down, else they'd overheat and stop working/explode.

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