Originally posted by powderburner
While I don't have any hard data at my disposal (here is yet another great spot for someone to do a research project), I can tell you that the benefits would be substantially reduced for sharp corners or steeper aft fairing angles. Maybe not completely useless, probably somewhere in between like half useful?
Subsonic airflow does not like sharp corners and will readily transition from smooth to turbulent flow. Even if you must limit your transition length to something relatively abrupt like 1/4 inch or less, if you can round off that corner and keep it smooth you will get a much greater benefit in smooth flow and reduced drag. You don't have to use a 50-foot-long transition but every bit that you can work in will help.
Zackly. The alternative to any amount of boat tail is big ole flat butt "base drag", worse yet on those birds that have a hollow base with the bottom centering ring set in a way from the aft enbd of the tube (yuck). This is what the rear transition is supposed to reduce.
The factors that go into a good nose design for a given rocket design should apply. For subsonic, a 4:1 to 5:1 parabolic/ogive (minus the cut-off tip) would probably prove to be best. For supersonic, where number of/amount of transition is more important, a long (10:1) conical would probably be best. Of course for supersonic, you'd have to have a real great need to have a body larger than minimum diameter.
I've used PML tail cones where a long boat tail wasn't feasible (see my Mirage mod on EMRR), based on the intuitive "it's GOT to be better than not". The sharp transition is less than optimal certainly, but it's less than a perpendicular plate would be.
For most rockets I've simply used plastic nose cones. I applied the slotted-tube idea to the cone, making the fins go through a slot in the cone roughly equal to half the fin chord, and slotted the body to fit the other half. Here's one constructed along those lines.
(Note the slot for the Estes style motor hook. The cone was a plastic Bertha style. The tip was blunter than an optimal subsonic shape, but of course that was cut off. What remained is nicely parabolic with very little curvature at first. There's a smaller centering ring (BT55 size; motor tube is BT50, main is BT60) on the end of the motor tube for gluing into the end of the tail cone. It's far enough up the tube that the engine hook is able to flex sufficiently.)
I worked with a second assumption here, and I'd be glad to hear if others think it's valid: fins set forward from the base reduce drag. Even though here they're along the length of the boat tail, it just seemed to me that having some "flat" surface for relaminarizing the flow would be beneficial. It might be less relaminarizing, but still there. Again, intuitively, it just seemd the that low pressure being pulled by the decreasing diameter would pull the airflow into laminar more so along the boat tail than it would on a straight tube. Such a phenomenon would be part of the "equal area" body/fin design, would it not?