No, I don't mean jet engines. I mean rockets: Bussard ramjets! They're less infeasible than previously thought.
So the two big problems with the design (other than having to get up to ridiculously high speeds to use 'em) are that the massive magnetic field will kill you, and that the field itself creates drag.
Only, when Bussard—and Larry Niven, who may well have been the main popularizer of the ramjet idea, and definitely is the reason we call 'em "ramscoops"—did their work, the Halbach array hadn't been invented yet. It strengthens magnetic force on one side, while virtually nullifying it on the other: that, I predict, will be important for not just ramjets, but also magnetic nozzles for all sorts of atomic rockets. Or magnetic pusher plates in the case of IC fusion rockets (which are basically Orion on steroids).
As for drag, apparently using electrostatic fields instead of electromagnetic ones means there is no drag. I'm not sure if such strong static will kill you (if it hits you all at once, e.g. lightning, it will, but I doubt even an interstellar scoop would be that strong), but there's probably a way to do with static what Halbach does with magnets. Magnetic and electric fields are neat that way.
Personally, I wouldn't use the ramjet for fusion—the original design called for, essentially, achieving fusion by shoving hydrogen into a funnel at relativistic speed. That's not only inelegant, it's unlikely to ever hit a break-even point. But you might be able to use the ramjet to achieve partial, or conceivably even full, in-flight refueling (okay, "propellant replenishment"), on ships that use more conventional methods to attain their fusion. What I mean is, with a ramjet refilling your tanks as you go, you could have, say, an effective mass-ratio of 16 or 20 despite only carrying enough tankage for a mass ratio of 5 to 9.
If you're not trying to get to a speed where gas fuses on its own, you also might not need to achieve quite as high of velocities before the ramjet works. The minimum is usually given as 1% c, but if you're not brute-force funnel-compressing it for fusion, like that, you might be able to do it at the top speed of the HOPE magnetized-target fusion rocket, which the delta-v calculator puts at .188% c, with a mass ratio of 5 (assuming you also want enough fuel to, y' know, stop).
PS. One thing that's interesting is, if you fiddle with the delta-v calculator, you quickly notice that changing the mass ratio results in tiny changes in delta-v—but changing the exhaust velocity results in huge ones. It's because (in the rocket equation), the number varies with the natural logarithm of the mass ratio, but directly with the exhaust velocity.
I.e.:
| (2) | Δv = | Ve× | ln | m0 m1 |
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