[Humabout]

Again, an interesting analysis, but a few things to consider when discussing torches. While ISP is a measure of efficiency, it should be noted that rockets generally cannot be throttled down terribly far (iirc, the Merlin has the widest range at present and can be reduced to 30% max thrust). This means that a torch will not only provide constant acceleration (meaning high-dv transfers), but they also have higher thrusts (meaning even more dv consumption). The end result is that torches eat far more fuel than non-torches, but they do get you there faster.

Why do we care? Economics. It actualy turns out that, at TL 11 (because that's the TL I did the analysis at a while back), the most economical engine for anything outside of cislunar space is a fusion torch, with advanced nuclear pulse drive a close runner up (im doing this from memory; those might be reversed. I'll see if i still have the spreadsheet for this). Inside cislunar space, an ion drive is cheapest. And beyond Neptune, you have to use a pion drive and it's going to wreck your budget.

This analysis assumed single stage craft. Beyond Neptune might be achieved more cheaply with staged fusion engines/anpu.

If this doesn't square with the FTL drive requirements, ships may need a set of auxiliary high-G engines, or the FTL drive might need tweaking to "just work".

As for combat, again, a set of high-g engines might be useful for maneuvering. This does address two issues at once.

Also, recall that all of the above engines, excepting the ion drive, are probably too hazardous for populated space. This would indicate the presence of tugs that tow inyerplanetary ships into and out of port, so they can fire their engines safely. These tugs would likely use either high-g HEdM rockets or ion engines (again, cost).

The alternative to tugs are ion-engine-shuttles that transfer passengers and cargo from inhabited space to parking orbits further out where its safe to fire up a the kind of radioactive nightmare that is a ANPU drive or fusion drive.