The Open Sky is about twice the RK's diameter, and 60 RPM is perfect.
That RPM is good for a couple reasons. First, the more energetic the agitation, the better the heat transfer (in a perf drum) and the lower your environment temperature, if you wish. You can add more beans and/or ramp faster, if you have the heat for it. Second, there's a sweet spot where the tumbling beans no longer resist rotation, but are nearly neutral in their effect because
you're pitching the beans over to the downward-moving side of the drum, and even adding force to any vanes descending on that side.
Too fast, and the beans just hug the inner side of the drum. Too slow, and you're using mostly conduction (less efficient by far in a light perf drum).
The ideal, IMO, is to be sure the beans go airborne and land on the far side of the drum. If they only roll back down on the same side they're ascending, that's a terrible use of the fine airflows a perforated drum allows, and a worse reliance on predominantly conduction for heat transfer on a surface not ideal for that. It just doesn't make sense.
There's the story of the trucker who pulled over before he got to the weigh station and whacked the heck out of the side of his truck just before he pulled onto the scales. Turns out his load of pigeons put him over weight, so if he could just keep 'em in the air until they read his weight . . .
Ya wanna keep these pigeons in the air, not crowding each other in a slow-rolling pile.