Anti-matter rockets without anti-matter

6 posts / 0 new
Last post
Moon-Hawk Moon-Hawk's picture
Anti-matter rockets without anti-matter
So as I understand them, anti-matter (AM) rockets use lots and lots of Hydrogen as propellant, and they burn small amounts of AM/Hydrogen as fuel to accelerate the bulk of the Hydrogen as reaction mass. But I can imagine a number of situations where a fast courier or other AM ship might fight itself with plenty of Hydrogen but no AM. Maybe the AM containment had to be jettisoned (I have to assume this is possible, considering what it is) or maybe due to the extreme expense they were not able to refill their AM completely, or maybe they don't want any more weapons of mass destruction than absolutely necessary, or whatever. The point is, even with the AM tanks dry, you may still find yourself with lots and lots of Hydrogen which could, presumably, be flung out the back of the ship with some kind of efficiency. Even in non-emergency situations, running an AM rocket in a non-AM mode (if possible) would be desirable, for example if you want to move a craft slowly but cheaply. So it seems that if there is any way to engineer such a drive to be capable of running without AM, it would be done. You've already got the Hydrogen and powerful electrical fields, so you've got the basic workings of a plasma rocket or even a fusion rocket already, even without the extra oomph of annihilation, so it seems plausible. Alternately, but for all the same reasons, AM craft might have a smaller non-AM drive which utilizes the same Hydrogen propellant. It seems logical, but is not addressed one way or the other that I can find. Any thoughts on this, and if plausible what kind of acceleration, deltaV, etc it might be capable of? tl;dr What kind of performance can you get from an anti-matter drive using only the Hydrogen?
nezumi.hebereke nezumi.hebereke's picture
Re: Anti-matter rockets without anti-matter
All sorts of AM vehicles will likely find themselves wanting to decelerate, accelerate or maneuver, without dosing the local area with deadly levels of gamma radiation. Because of that, I assume that all vehicles will come also with chemical rockets. I don't think that it would be cost effective to have a dual-fuel fusion/AM engine. I doubt it's easy to switch from one nuclear reaction to another, and if we need a low-radiation source, using nuclear power may not be the best alternative. But chemical rockets are dirt cheap, require no special equipment to operate, and create no gamma radiation. Chemical rockets use the information listed in the book. But since they're just for maneuvering, it's unlikely to have the fuel necessary for a high speed (unless you were expecting it and added more fuel).
Moon-Hawk Moon-Hawk's picture
Re: Anti-matter rockets without anti-matter
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?
icekatze icekatze's picture
Re: Anti-matter rockets without anti-matter
hi hi Venting hydrogen out the back will give you measurable ∂V, but it is going to be even weaker than chemical rockets by far. If you're just opening the nozzle, the exhaust velocity of the tank will depend on how much pressure it is under, and that will likely drop as the tank empties. I would estimate that you would be lucky to get 30 meters of ∂V out of that. If you want to actively propel the hydrogen out the back, it is going to depend on what kind of energy source you have. If anti-matter is out, is there a fusion reactor onboard?
jsnead jsnead's picture
Re: Anti-matter rockets without anti-matter
nezumi.hebereke wrote:
All sorts of AM vehicles will likely find themselves wanting to decelerate, accelerate or maneuver, without dosing the local area with deadly levels of gamma radiation. Because of that, I assume that all vehicles will come also with chemical rockets. I don't think that it would be cost effective to have a dual-fuel fusion/AM engine. I doubt it's easy to switch from one nuclear reaction to another, and if we need a low-radiation source, using nuclear power may not be the best alternative. But chemical rockets are dirt cheap, require no special equipment to operate, and create no gamma radiation. Chemical rockets use the information listed in the book. But since they're just for maneuvering, it's unlikely to have the fuel necessary for a high speed (unless you were expecting it and added more fuel).
The additional mass of liquid oxygen would seriously reduce the possible payload of the AM rocket. Instead, I'd think that the ship might carry equipment to rework the AM reaction chamber into the reaction chamber for a fusion rocket - smart materials would make this far easier.
nezumi.hebereke nezumi.hebereke's picture
Re: Anti-matter rockets without anti-matter
Moon-Hawk wrote:
p.s. Are you the same nezumi from Dumpshock?
Only when there are no contracts out on me. There are a few of us DSers bouncing around (and can you blame us? We're just following Adam and Rob :P )