How would you make a Bussard ram's scoop?

Forget for a moment that a Bussard ramship design would generate more drag than the thrust it could produce. Pretend we’ve come up with a way to make the collected hydrogen undergo nuclear fusion without slowing it down, a la a scramjet, so that the drag is negligible.

Now, given that caveat … how would you go about making a scoop with an effective cross-section that was hunreds, if not thousands, of kilometers across? How would you project a magnetic field (or an electric field, or some other kind of field) that far, without requiring a ginormous amount of electrical power?

Magnetic Monopoles, of course…


Okay, maybe I should ask … what kind of real-world technology did Bussard, or one of his intellectual heirs, consider using to make the collection field?

I thought Bussard’s whole point was nuclear fusion to solve every problem you mention. You won’t get any real acceleration without fusion. And you can take some of that fusion energy to power the electromagnet.

I have no doubt that it does require ginormous amounts of power to keep the scoop going, but as long as you produce ginormous+x amounts of power from fusion, you’ve got energy left over for acceleration. It’s very likely that only a tiny fraction of the total fusion energy goes into propulsion.

But even a small +x is enough. As long as you’re producing more energy that it takes to collect material (including the effect of drag that the material causes), then you’re still accelerating.

You’re missing the point. The act of collecting hydrogen from the interstellar medium and funnelling it into a jet tight enough to cause sustained fusion results in “astrodynamic drag” from the momentum transfer of the hydrogen to the ship via the scoop. There is therefore an upper bound to the velocity a ramscoop-propelled ship can attain, which is dependent upon the density of the local interstellar medium. In the local neighborhood of Earth, with a hydrogen density of ~10[sup]5[/sup]H/m[sup]3[/sup], the theoretical maximum limit is around 15% c. In the average interstellar medium which is an order of magnitude greater in density, your limit is about 8-9% c, and all of this assumes complete fusion and propulsive efficiency approaching 100%. As with an air-sucking chemical ramjet/scramjet, the faster you go the longer your reaction chamber needs to be in order to extract most of the release energy, and at some point it becomes prohibitively long.

You also have the problem of needing to get up to a minimum speed in order to achieve and sustain fusion via ramjet pressure. This again depends on the density of the medium, efficiency of the ramjet, and the size/drag ratio of the scoop, but it is generally estimated as being a minimum of 2-3% of c. This may not sound like much until you realize that it is about five orders of magnitude faster than we can propel any vessel with conventional propulsion technology. And while the numbers you get out of fusion reactions seem vast compared to existing sources of fossil and fission energy, they are dwarfed by the kinetic energy necessary to move even a tiny mass from one star to another in anything like a human lifespan. The amount of energy required is literally astronomical. [post=11726059]Here[/post] is a thread about the practical difficulties of interstellar transit using any extant or foreseen technologies.

As for the question posed by the o.p.: we don’t really know. Such a scoop would have to be made from a web of very lightweight conductor, capable of continuously varying its topology to maintain a parabolic flow shape given variances in gas density and the reaction from drag, plus it would have to cope with the occasional cloud of OH[sup]+[/sup] gas or Li ions. Magnetic monopoles are likely the only way you could display such fine control of magnetic flux lines over such a large area, and we don’t even know how (or indeed, if) we can manufacture those.


Was this just a joke answer? I’m not seeing how magnetic monopoles are helping here. If you want to compress Hydrogen nuclei you’re approaching, you’d want a magnetic field in front of you that circled your flight path. If you had a magnetic monopole on your ship, the magnetic field would just make the Hydrogen corkscrew around your flight path.

Yes, of course it was a joke.
Haven’t you read any Niven?

If you really wanted to do this, you probably wouldn’t want to use magnetic fields at all, instead using electric fields, which could be contrived to deflect the protons into your fusion reactor.

Actually, in the ideal case, it’s independent of density, since both drag and thrust are directly proportional to the density. Last time I ran the numbers, for idealized H to He fusion (no energy wasted in neutrinos, etc.), the speed limit is just about exactly 0.12 c.

Even if we didn’t need such fine control over such a large area, we’d need some means of making the field wide enough to have an effective collection radius in the hundreds (if not thousands) of kilometers.

In theory, how big could such a magnetic/electric/field-y scoop be? Could you really build a field-scoop the diameter of Mars?

What are you using as the function for drag? At speeds of more than a few percent of c in a region of higher HII density the interstellar medium no longer behaves like thin (ideal) gas, and you actually have to treat it as a slightly viscous fluid due to magnetohydrodynamical effects; in other words, the mass of ionized hydrogen that you end up pushing “ahead” (relative to the static medium) creates a Lorentz force reaction which is analogous to the viscosity due to intermolecular electrostatic repulsion in fluid continua. This means that your drag function is no longer straight kinematic viscosity, and is instead nonlinear.

Of course, as you point out, H-H fusion (which, given that you are using the predominately neutral hydrogen interstellar medium for fuel) isn’t very inefficient (by the Lawon or triple point criterion), and it is unlikely that you would be able to build a chamber long enough to permit the entire p-p fusion chain to complete. If you can achieve D-H fusion you can get about 7 MeV per reaction, substantially less than you could theoretically obtain from D-T fusion, although it is essentially aneutronic and therefore all immediately available for use as a charged propellant.

But basically, the Bussard-type ramscoop has a number of theoretical difficulties on top of the limitations of extant and foreseeable technology which make it unlikely to ever be suitable for interstellar propulsion. It thus remains in the same bag of science fiction conventions as “warp drive”, “interstellar wormholes”, and “hyperspace”; something that is conceptually possible but we have no idea how or even if it can be actualized. My money is still on the Infinite Improbability Drive, despite some of the disturbing side effects.


No matter what kind of magnetic generator you have on the ship, with distance the field will just be a dipole. I don’t know enough about EM to tell you the effect of a dipole zooming through a cloud of charged particles but I’m pretty sure it will not suck them into the center. To get a real “scoop” you would need some kind of framework of conductors extending out from the ship. Again, I couldn’t begin to tell you how it would be designed to generate a scoop field, but I’m pretty sure it would have to extend out as far as the cross section you want to scoop (if not beyond). If you have the materials to build an antenna with the diameter of Mars - that’s extremely lightweight and won’t collapse in operation - then, yes, your field could be that big.

I did not know that. I was, in fact, assuming an ideal noninteracting gas.

On this point, I would disagree. A ramscoop is an engineering problem, albeit a rather large one: We do know it would be possible to build a ramscoop, just very difficult. A wormhole or warp drive, however, is a problem of fundamental physics: We have no reason to suspect that they’re possible at all, and in fact reason to suspect that they’re not.