With an even number of engines, you could handle all your roll control, at least when you're under power, with brakes: apply the brakes on one or more engines, and it will apply a torque to the rocket.
With an even number of engines, you could even handle an engine out scenario by applying braking to the rest of the engines spinning in the same direction as the dead engine to keep their applied torque on a par with the other set, rather than having to shut down an engine. One might even be able to achieve a bit of throttling this way; slowing down the rate of spin would reduce chamber pressure and thus thrust. This would reduce efficiency slightly, but I don't think it would be much. I haven't really done the math yet, but I don't think an electromagnetic braking system would need to dump as much energy as some people think. The tangential force on the engines is nothing compared to the axial thrust they're making. -- Sean R. Lynch KG6CVV http://www.chaosring.org/~seanl/
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