So we'll have three reaction engine types in Hard Space:
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 (or ion? or plasma?) 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.
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. Realistic.
3) Chemical rockets - used almost exclusively for 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 if they have a lifting body. Realistic.
By the way, do fusion rockets necessarily mean shipboard fusion reactors? Or are they "simpler" to make (as this is not very fine-controlled fusion)? I have the potential vision of ships with fission reactors and fusion-torch rockets...
I am also going to research Paul Elliott's Orbital: 2100 rules (which are OGL) regarding reaction drives...
Your Fusion drive wouldn't be a power plant as such, it would use magnetic fields etc, to compress a stream of hydrogen (tritium?) to make it fuse and create the exhaust velocity you need for acceleration.
HOWEVER, once the plasma is flowing, it will create a magnetic field of its own. That magnetic field can be tapped to generate electricity (just wrap the exhaust tube with conductive wire like copper). It slightly reduces the velocity of your exhaust, but probably not significantly. This type of power plant, which can only work when the plasma drive is operating, is called a Magneto-Hydrodynamic Drive (MHD) and is explained in 2300.
So, I would expect that ships with plasma drives, have an MHD built into them, so that when the torch drive is operating, the fission reactor can be placed in standby - thus extending its life quite a bit. The fission plant is then powered up when the torch drive is shut down. So your Maneuver Drive provides power when it is being used, and your Power Plant produces energy when the M-Drive is idle.
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