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Michael Wojcik Silver badge

the Em drive (which creates thrust without using fuel

Sigh. The EmDrive, if it works1, consumes energy, which has to come from something. Whatever that something is, it's going from a higher energy state to a lower one, which makes it "fuel" for all intents.

What the EmDrive doesn't need is reaction mass, since it gets its reaction from electromagnetic radiation, which has no (rest) mass. The advantage of such a drive is that you don't have to hoist reaction mass out of the planet's gravitational well, and don't have to lug around most of your reaction mass for the rest of the trip.

1Personally, I don't immediately see anything wrong with it. It's expending energy, so there's no obvious thermodynamics objection. Photons don't have rest mass but they do have momentum, so there's no obvious conservation-of-momentum objection. I don't see offhand how it's very different in principle from a standard photonic drive - you throw EM out the back and get a (small) impulse forward. But I haven't looked into it at all, just skimmed the Reg story.

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