A fictional starship that I am designing functions via a form of Bussard ramjet that uses hydrogen from the interstellar medium as reaction mass, not as fuel.

In order to accelerate the propellant, the starship utilises a fictional mechanism by which it is able to convert a fraction of the baryons entering the drive into antimatter. The resulting annihilation provides thrust exceeding 1G, and the drive is able to sustain this thrust long enough to attain velocities of a large portion of c.

What I need to know, is whether the physics checks out. I know that the original concept of a Bussard Ramjet was not feasible since the drag produced by the scoop was greater than the thrust, but I am curious: If you had a ram augmented interstellar rocket that could convert around ~1% of incoming baryons into antimatter, would this drive be an efficient method of interstellar travel?

NOTE: The mechanism by which baryons are transformed into antimatter can be assumed to use no more energy than a reasonable sized nuclear reactor.

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    $\begingroup$ You do realize there are likely to be zero research papers on the RAIR concept where 1% of incoming baryons are converted to antimatter by a 'magic' mechanism. I suggest it is unwarranted to ask for [hard-science] answers. The level of analysis, solving the appropriate equations, might approach that of a research paper. Somewhat more than answers usually are here. Perhaps, [science-based] would be the sensible tag choice. It will improve the probability of its being answered. $\endgroup$
    – a4android
    Commented Feb 21, 2019 at 4:46
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    $\begingroup$ In case you wondered, this is a concept that interests me as a possible interstellar propulsion system. I would be very interested in the answers. I haven't pursued the idea to the analytic depth of your question. It sounds feasible in principle, but practical problems can derail the most beautiful concept. $\endgroup$
    – a4android
    Commented Feb 21, 2019 at 4:51
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    $\begingroup$ Actually, this is the first hard-science question I've seen in a while where I actually thought someone might be able to foot the bill. I'm looking forward to the answers on this one because the fundamental concept of a ramjet is sound: bring in 1kg of hydrogen, use 1g of it to create a whomping big energy explosion that heats up the rest of it, forcing it to squirt out the back end at a high velocity. Even if no one can demonstrate feasibility to the hard-science mandate, I love the idea! $\endgroup$
    – JBH
    Commented Feb 21, 2019 at 5:04
  • $\begingroup$ Even without the magic conversion this is a quite nice concept. It reduces your fuel to just the antimatter, but you should be able to achieve stupidly high exhaust velocities. I pity the poor solar system you’re headed away from though... $\endgroup$
    – Joe Bloggs
    Commented Feb 21, 2019 at 8:17
  • $\begingroup$ @JoeBlogs I don't think a few grams of matter-antimatter annihilation per second are going to produce much gamma radiation to be a problem for anyone farther than a few kilometers away. The main problem is shielding the spaceship itself. :) $\endgroup$
    – Rekesoft
    Commented Feb 21, 2019 at 9:19

1 Answer 1


DISCLAIMER: My sources are not all scholarly, but I'm assuming the information provided is accurate for the sake of my answer. I'm also not a physicist. Take these answers with a rather large grain of salt.

With that out of the way, let me try my best to answer this...

First, let's take a look at interstellar medium. If there's (MAX) .1 particles per cubic centimeter, and 75% of the medium is hydrogen, that gives us .075 particles per cubic centimeter. And that's probably a a high estimate. So you then convert 1% to antimatter. That gives us... a whopping .00075 particles per cubic centimeter. That doesn't sound like much, but it adds up fast. That's 750 useful particles per cubic meter, which is quite a lot! I'm totally ignoring all the problems with just holding on to antimatter for long enough to use it (check out the Atomic Rockets page linked below if you're curious)! Also, I don't really know what to do with this information, but it may be useful if combined with more technical information about your rocket engine.

OK so there's a lot of different ways to harness this antimatter, but none of them are a ramjet, apparently because "antimatter ramjet" is not something that really exists, since ramjets and antimatter don't mix well. Oh well. Apparently Michio Kaku covers this in his book Physics of the Future (found here on amazon) if you're curious and really want to dive deep into this. That's kind of the end of my formal answer to your question, but read on if you want some theorycrafting and more guesswork.

Relatedly, you're going to need some pretty darn thick shielding, since hydrogen's antimatter pions are remarkably annoying as far as trying to murder you goes. Additionally, you might be better off trying some other material, since Nitrogen, for example, has a charged pion to propellant efficiency of 95%

The Atomic Rockets page has a "Antimatter Rocket Equation", and so I'll just use that for fun to see if you really could accelerate at 1 g. Well, here's the thing- the equation doesn't use the antimatter as reaction mass. Oh well. Back to the internet drawing board. (There is a group of wonderful charts for antimatter reactant for different propulsion speeds but quite honestly I'm not sure that helps you, since you're looking for acceleration, not delta-v. Onwards and upwards...)

My quest ends here. I'd highly recommend you check out Atomic Rockets, it's a wonderful source for all things sci-fi. I was unfortunately unable to find anything calculated for antimatter ramjets, so I'd definitely look more into that. I did, however, find an article claiming 1 g of acceleration using RAIR (that's the "nextbigfuture" link if you're curious), and the paper cited in the article may provide some answers. I didn't read the whole thing, but from my skim I didn't notice any glaringly useful information. It also details a sort of mixing antimatter in with other reactant, so that's a little bit different from your engine.

This turned into an absolute mess of an answer, but I hope that provides at least a little bit more information for you, and I wish you the best of luck in your spaceship design process!


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    $\begingroup$ Thanks for that paper! It's right on the money. :) $\endgroup$ Commented Feb 23, 2019 at 5:14

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