Sure, you could add another set of engines and more propellant and/or fuel for them, but given that (for example) a courier is already carrying 600 tons of Hydrogen, even adding a completely separate set of engines will be cheap and light as long as it can use the existing propellant. It's only if your secondary drive needs different fuel/propellant that you're adding a lot of mass. And I'm really interested in a secondary drive; I, too, would assume there is some small amount of maneuvering capability, probably chemical, but unless it can use the Hydrogen propellant it's not really what I'm looking for. I understand that antimatter is not Mr. Fusion or even diesel; you can't just dump any old crap in there and have it work fine, but it seems that if you're already generating precise and powerful electromagnetic fields, delivering Hydrogen, and have a chamber which can contain annihilation reactions, it's a relatively minor feat of engineering to get it to act as maybe a plasma rocket. So for example, if we assumed all that, and if we further assume a delta V of 1600km/s and mass ratio of 3.3 (from jsnead in the space naval combat thread), and allow the specific impulse to drop, say, a factor of 10 (again from jsnead in the DIY space travel thread relative specific impulse by drive type), then you're left with a delta V in the neighborhood of 80km/s. (rough, back-of-the nomogram calculation, going with specific impulse of about 160,000s, from the delta V and mass, not the 200,000s from the other thread, as I'm assuming that's just typical for the drive type and I'm using a specific craft for this example) That's using all 600 tons of H, of course. Maybe that is too generous, since modified-to-act-as-plasma should probably be worse, not as-good-as plasma, but the point is even if you just poke a hole in the H tanks and let it vent out the back you'll get some measureable thrust and delta V, and even with relatively modest performance you could still be looking at a significant delta V. Even if we cut my above estimate of specific impulse down another order of magnitude, 20km/s can nearly get you from orbiting Mercury to Mars, where you can easily pick up more H and without using any precious, precious AM. AM is simply too valuable and too dangerous for this not to have been a consideration in the design of AM rocket craft. And with 600 tons of Hydrogen you could THROW it out the back one handful at a time and get a respectable delta V, so in my mind it's less a question of whether you can do it and more a question of just how crappy is it. But of course, if there's one place to go to find out how wrong you are, it's an internet forum, so by all means if you think I'm wrong straighten me out. Thoughts? p.s. Are you the same nezumi from Dumpshock?