Spaceship Propulsion Units
The main propulsion for planetary system vessels is reactionless (inertial) drive, which gives thrust without ejecting mass (fuel), presumably in contradiction with the law of conservation of momentum. However, this only applies to a closed system and it can be argued (and it has), that as inertia is the result of interaction with distant mass in the universe these drives are not a closed system and that’s what I’m sticking with. Acceleration is constant and anywhere from 0.1 to 0.4 m/s2 depending on craft (freighter, passenger, private, military, etc.). Currently the two best contenders are Mach Effect Gravity Assist (MEGA) and Rotating mass on Eccentric Trajectories. It’s not stipulated in the stories which system is used but if pushed I’d opt for Rotating mass on Eccentric Trajectories
The propulsion for interstellar vessels is a bit more complicated. It consists of an inertial drive in conjunction with electric/magnetic and or electromagnetic pulsed, rotating and modulated disks that spin interstellar medium (gas, dust and various others) backwards to give thrust (a bit like a squid or aircraft propeller). Electromagnetic disks also deviate the destructive radiation that would impinge on any vessel traveling close to c (speed of light).
Space Vessel Power
Power for all space vessels is fusion of deuterium with helium-3 and both can be mined from regolith. This is a highly efficient form of direct electricity production (without water exchangers to drive turbines), with virtually no waste and no radiation. Both deuterium and helium-3 are contained in the interstellar medium but in very low concentrations, so it may not be feasible for electric/magnetic or electromagnetic disks or propellers to harvest it while traveling. However deuterium can be used to produce helium-3 (and a beta partial that can be used to produce electricity). Hydrogen-1 (a proton) and boron can also be used for the same result, so maybe this is what the starships use.