It's fun to estimate how hot the exhaust of a torch ship must be, and how much mass should it expend on constant 1G sustained for days.

Rockets exchange impulse with the exhaust at a linear rate (mv), but put the kinetic energy into it at a quadratic rate (mv²/2), so increasing mass efficiency by increasing exhaust speed / temperature costs more and more. Realistic with fusion reactors, to the extent to which compact fusion reactors themselves are realistic. Or even with fission reactors, in the form of bombs (see project Orion).

Nothing like the current simple, low-tech chemical rockets.