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Why do we care? Economics. It actually 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.
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I've got my spreadsheet handy: The best ISP is the advanced Fusion pulse drive. Its got better ISP than the torch at all TL's. The non-advanced fusion pulse drive is slightly under the torch. Of course, the torch has 10 times the power of the non-advanced and 100 times the power of the advanced drives. Freighters probably use the advanced Fusion Pulse drives.
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Originally Posted by 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.
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I don't think the math there follows. You can't burn a rocket at 10% of total thrust, but you can burn a rocket for 10% as long, assuming its not a solid booster and you have reasonable tolerances.