Perhaps the biggest of all was Project Orion, a 1960s concept for nuclear-lofted space launch that could put up to 8 Million tons into space and at up to 10% lightspeed.
It was deemed practical, feasible and more importantly doable at the time and nothing has changed that view. It was allegedly not followed up as no one could figure out any application for multi-million ton spacecraft.
Just like the rubber deck it turned out not to have a practical use/need. At the time.
Maybe its time will come?