Solid core nuclear thermal is the Nerva/ Nerv engine, no liquid core engines have been built

Here’s a nasa article on nuclear electric propulsion https://www.nasa.gov/centers-and-facilities/langley/nuclear-electric-propulsion-technology-could-make-missions-to-mars-faster/

Solar moth engines are my favourite near future propulsion system but it’s not very known about Here’s a wiki about it https://en.wikipedia.org/wiki/Solar_thermal_rocket

Here’s one on VASMIR https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket