Topic Three: Nuclear Propulsion
Old time fertliizer and diesel oil rockets have got to be replaced by more energy-efficient/weight efficient propulsion.
Recently, some start-up upstart company is claiming "blue gas" will doom Elon Musk and the Lithium battery.. "Blue gas" is hydrogen gas, one of the most dangerous chemicals we know for flammability. Remember the Hindenberg. So the idea here is the fuel cell, where hydrogen is burned to water in a electrical reaction. I mean, it gives off electric current/potential across the electrodes.
Not a good idea for space. Remember the Van Allen Belt and the various threats of space.
There is a very high energy technology being developed in the area of metal hydrides. I know a little about this. You don't want a nickel hydride fusion reactor in your car. They melt labs, windows, walls and all when something goes wrong..... Very unstable technology doesn't go well with Space.
So it's still nuclear fission, Uranium. Thorium maybe.
But by far, the greatest energy per mass fuel is the cold fusion/deuterium reaction. If we ever get this working, we can forever say goodbye to the combustion engine.