In writing space opera there’s two ways you can go.
The most difficult is hard science, or rather diamond hard science, where you try to get all the rocket science exactly right, according to the current laws of physics. An excellent site in this regard is Winchell Chung’s Atomic Rockets, which I cannot recommend too highly. Especially with ships there are nuclear powered torchships, which output literally terawatts of power.
I mean, you are basically sitting on a series of mini nuclear bombs, or riding a continuous nuclear torch. Forget the pissy ships of tv and movie sci-fi, they barely glow.
The problem, I found, is if I wanted a freighter of, say, 30,000 tonnes and even moderate 0.03 g acceleration (as fast as a freight train accelerating, almost nothing) it would require about a third of a Hiroshima bomb’s worth of energy a second. Every second for as long as the ship gets to speed, say 2 weeks. No engine could handle that without vaporising. In fact even if it didn’t, the whole outside of the ship would glow and be almost incandescent.
So goodbye hard science…
The alternative is handwavium, which is a fancy nerd word for “making things up”. This means you assume some amazing magical breakthrough in science that’ll let you get around all these sticky problems of real universe energy requirements.
So you can have the pissy little puny rocket exhaust of pop sci-fi after all! Because at some point in the future some Plott-DeVice Drive is invented which lets you have something for nothing. And then you can whizz around the universe and still call it science
By the way, I was later pointed at a space opera setting that does use torchships, John Lumpkin’s Human Reach, though they are smaller than my freighter and accelerate more slowly at cruising speed, they can also put on bursts of high G acceleration (though that’d wreck their delta v (range) something chronic)