I've built several model kits with my kids for school and we have done well in the altitude sessions using conventional modeling technics but now we want to design our own rocket. We want to design with altitude in mind for A/B engines and C/D engines.

My question is why do we not see any tapered body tubes when most of the wind tunnel test seem to indicate that the optimum design to reduce Cd is a Tapered tube? Is the data faulty? Two reports I have seen show the Drag on a rocket with a 1.8X 20:1 tube as half of conventional tubes. I am going to build one and compare for myself, but I am curious why this obvious advantage of design is not being marketed.

Thanks Dave

From a commercial point-of-view, you're looking at mandrels with one specific use (or, one customer only), meaning it will be more expensive to produce. The more-common straight-sided tube can be sold to dozens (if not hundreds) of customers for more than one purpose (ie candy containers, model rockets, or electrical insulators using the same identical part), only requiring a single investment in tooling. Each tapered mandrel has to be sized not only for the start and stop diameters, but a specific length, and as such, only one unique item can be made from it.

Not to say it couldn't be done, nor that there's no company doing it. Just that it represents a more expensive component for the model rocket kit manufacturer to purchase compared to a standard tube, and that the final price to the customer (you and me) might be out of our wallet range or sensible justification.

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Building conical rockets or rocket parts using 110# cardstock isn't too hard. Look at the old Centuri Vulcan (not exactly a low drag design though) or the tailcone on the classic Estes Sprint.

The current Estes Bullpup has a plastic tailcone, though you really need a razor saw to build that one (so I might decide against it for classroom use).

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I just started building paper models and I do have access to some heavy coated paper, mainly 60 to 110 lb stock. I also have some mylar I might give a try to. I do want t,o give this body style a try. I tried to post a picture but the the format was wrong even thought it is only a 6K file but if you look at John S. DeMar's "Model Rocket Drag Analysis" report On the NAR R&D list # NAR 52094. It is body model # 12. http://web.syr.edu/~smdemar/rocketdrag.html



Thanks for your help Dave

Looking at the page you mention, I see what you mean. I'd modify the design somewhat to make it easier to build; probably around 2" of BT-60 and a Bertha nose cone, mated to around 10" or so of BT-20 for the lower sustainer, then wrap a long paper transition from the lower edge of the BT-60 to the tail of the rocket. Attaching fins will be a challenge, though... probably I'd slot the transition and attach the fins directly to the BT-20 tube. Elliptical fins like those on the Estes Sprint would be perfect for this model, I expect.

Or you could make the transition come up a bit short of the tail of the rocket, exposing some body tube for fin attachment. That might make the drag go up a bit though, since you'd lose the boattail effect.

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... now I'm going to have to build one. dang.

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NAR # 115523
Once upon a better day... SAM #0076
My site: http://rocketry.gonnerman.org

For a design where the frontal cross sectional area is mandated, like in egg-lofting, tapering back to the diameter of the minimum motor size makes overall sense. But in designs where the only size requirement is a cross sectional area large enough to accomodate a specific motor, increasing the nosecone cross sectional area to accomodate a taper behind it yields more of a drag penalty than a benefit.



RockSim is a good way to play around with various designs to see their effect on oveall drag.

Tapered rockets are used in specialized areas, such as egg-lofting, where some advantage can be gained by tapering from the diameter of the egg capsule (1.8-2.0" typical) to the diameter of the motor. The taper is usually custom made, precluding commercial parts. Many years ago, Apogee Components sold competition egglofters using pre-printed heavy cardstock shrouds. I still have three of those rockets. You can find the plans of similar egglofters around the net. Possibly over at George Gassaway's site you might find the Three-minute Egg or something similar.

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Roy
nar12605

Thanks Roy I'll give that a try.

Dave

There are several plans at www.nar.org

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