You could define something with a plausible exhaust velocity and mass flow rate without defining exactly how that exhaust velocity is obtained, or vaguely defined ("antimatter primed pulse fusion"). It feels easier to swallow because spaceships feel more like spaceships - they are actually using propellant and mass is changing over time, and you can keep the exhaust velocity to something within physical possibility but enough that things like lifting to orbit or making 100 diameters is not a major project needed to be conducted by a government. You could arbitrarily say that the engines can tune exhaust velocity from 10 km/sec to 300 km/sec at a maximum flow rate of 1 kg/sec per engine, so they could be used close to the surface of a planet without any particular risk but would become "highly efficient" (compared to TL8) as soon as it was safe to be that way, yet each engine would provide 300 kN thrust at most efficient. 10,000 kg of propellant would give 2h46m of thrust. You'd probably have to tune that a bit to fit what you're looking for.
The traveller rules don't really want that kind of thing though.
‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
... as posed in my previous question to the list, what do you use to get a
ship from dirtside to space, without having to completely snap the
suspenders of disbelief?
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