[Spaceships] balance with a) interstellar wars, b) super-science and c) size mods

[muduri]

Still haven't managed to break the Spaceships addiction - particularly propulsion systems. So I'm curious about the balancing act between some different publications and systems.

a) I've tried building some sample vehicles from Spaceships but have only skimmed the Interstellar Wars rules. So I'd like to ask - let's say you started with a similar concept (and presumably setting, budget, TL and SM) - would a spacecraft built with the ISW rules come out similar to Spaceships? Or has anyone found significant break points worth keeping in mind?

b) This one's a bit trickier. Given the addiction to propulsion, I'd like to keep both the normal and super-science drives both available and locally optimal. But right now the super-science engines seem like game-changers - there's not much reason to use a nuclear thermal rocket if a fusion torch or a reactionless drive are available. I was thinking of some kind of GM fiat - maybe "super-science is available, but only TL10^ in a TL11 world, and at 10x cost." It seems like then the torches only go to the richest, and the PCs might have to start with NTR or fusion pulse drives as they work their way up. Am I understating the cost differences that already exist, or risking unbalancing the game for other reasons, or is this something that people have already thought about tweaking?

c) Obviously the atmospheric scaling principles that cause a massive cargo aircraft < a bomber < fighter for speed don't apply for the same reasons in space. But could something be justified? I don't know, limited surface area means that only so much reaction mass can be tossed out at a time per engine? Really it's just the instinct to stop carriers and battleships from buying double-hot-reactionless drives and keeping up with the fighters. But, assuming it could be tweaked out, is there any way to justify this or would it just be fiat?

Many thanks for your thoughts on any or all three of these!