|Artwork by Philippe Bouchet AKA "Manchu"|
for Robert Heinlein's Time For the Stars
Torchship "Lewis & Clark"
I have decided to make interplanetary travel in Hard Space more nuanced than I have originally thought.
So we now have three types of rocket engines used in this setting:
1) Fusion torches. Used by starships and fast interplanetary ships. Can maintain constant acceleration/deceleration at high G (typically 1-G). Highly destructive exhaust. Ships with fusion torches use chemical thrusters for fine maneuvering (such as docking) where a fusion torch would be too dangerous. Such ships do not land, at least not in most cases but can "dock" with smaller asteroids. Unobtanium (i.e. physically possible but we don't know how to build them yet) but not handwavium (unlike J-Drives).
Note that the fusion torch is not a fusion power plant; in fact, torchships ships carry fission reactors for their energy needs (especially when the rocket is turned off). Controlled, contained fusion reactors are massive planetside affairs, to large and heavy to include in a starship.
2) Closed-cycle gas-core fission rockets ("Nuclear Lightbulbs"). Used by slower interplanetary craft and interface craft not intended for atmospheric use. Much safer than fusion torches while providing significantly better performance and endurance than chemical rockets. Such ships can land on airless worlds if they have a standard - rather than distributed - hull. However, still unsafe to use in an atmosphere due to the risk of radioactive gas leakage in case of accident or combat hits; thus, used for airless worlds where everything is sealed and radiation-shielded anyway. Realistic.
3) Chemical rockets. Used almost exclusively by atmospheric craft, as well as for fine maneuvering on ships with fusion (or even fission?) rockets. Inefficient but safe. Can land anywhere if they have a streamlined hull and can fly like an airplane in an atmosphere if they have a lifting body. Realistic.