The Fusion pulse motor deuterium or helium3. Central motor with dark iceball periphery laser pulse for directional control.
My preferred motor for interplanetary travel orbit to orbit is the laser pulse motor that is a simple bell or parabolic that pulses small ice balls well below nuclear fusion temperatures.
Power coming from a nuclear fission reactor. Actually four blast bells. The ice balls being small made by an ice machine in the ship. Why ice? Very abundant in the solar system at ones destination. Why small ice balls? Because tests show that blast pulse is problematic if the blast frequency is low. Whereas fast pulse like the hum of a fifty blast per second gives good handle on maneuverability. For that small ice balls. Moreover this is an excellent way to dispense of the plutonium excess very dangerous to stockpile as bombs on earth and in hundreds of fission reactors around the world, as cleaner nuclear fusion is around the corner.
The orbit to orbit spaceship and the to and from planetary shuttle were separate vehicles whose motors were different. The orbit to orbit interplanetary vehicle used pulse lasers to fire on dark ice balls as I described because of redundancy. Eventually nuclear fusion substituted for nuclear fission power source as power is needed for internal heating, instrumentation. Later on nuclear fusion pulse substituted ice ball laser pulse.
Fission then Fusion:
Past the orbit of Mars solar panels become worthless the interior of spaceships must be room temperature and that alone requires nuclear energy for missions to the asteroid belt and beyond. The asteroid Ceres alone contains more water and ice than in all of the earth’s oceans combined. From a self supporting base on Mars easy access for missions further out. Hence the laser explosions of little darkened ice balls at the reflector focal point an ideal propulsion motor if the pulse frequency is high. The problem with fusion pulse is I’m big mother. Colossal. Not for amatures in space travel technology. Hence fission power source a buffer necessity.