Ideal Tail Cone Shape

I am working on a tail cone assembly for a project at school, and I was wondering if any of you had information on finding the ideal cone angle. I remember this being a topic on Rocketry Planet a few years ago. There were some really good documents posted that I can't find now. If I recall correctly, it was a variable angle based off of the speed of the rocket.

Ogive a la Silver Comet?

dave carver said:

Ogive a la Silver Comet?

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I'm cool with that... however I'd be more inclined towards the Velociraptor.

Use a transition shape that smoothly curves from the straight side of the body tube inward to the taper of the aft tail cone. Tail cone angle should be in-swept only to a maximum of 10 to 15 degrees.....less angle is better.....you could do your own smoke-tunnel tests to find the angle where you start to get flow separation and turbulence.

For supersonic flight, airflow can follow a sharp corner between the BT and the tailcone. There will be an expansion wave that forms from that corner. The greater the angle of the in-swept tail cone, the more intense that expansion wave will be (and the more drag).

All of this is pretty much trumped by weight, looks (a.k.a. styling) and by ease of construction. If you can build it lighter using a paper tailcone, your rocket could well perform better than a heavier rocket with an aerodynamically "pretty" tailcone (like a balsa or basswood tail shape turned on a lathe).

Estes Technical Report (TR11) discusses boat-tails and their effect on drag reduction (sub sonic)

I would consider a tail cone that tapers to the minimum diameter it can. The smoother the taper the smoother and less turbulent the airflow will be. Smoother airflow will lower your drag.

Consider an ogive or VK nose cone with the tip cutoff at the appropriate diameter.

FWIW, with a quick search I found this paper which investigates the effects of boattail parameters on a 155mm shell.

https://www.mtc.edu.eg/ASAT13/pdf/FM05.pdf

So, we have tracking smoke. I have wondered if it's enough for a base bleed effect. If so then a 14* boattail with a base bleed tracking smoke should substantially lower the base drag which according to the paper is approx. 35% of the total drag. A 50-60% lowering is significant.

This will be an aluminum cone, similar to an Aeropack tail cone. Unfortunately, the machinist would not like it if I sent him an order for an ogive piece. The artillery shell paper is promising, though. The top speed for the rocket I'm working on is about Mach 1.3, and nitrogen will be venting through the nozzle for a good portion of the coast. My current cone angle is 12.6 degrees, which seems to be a good angle based on the data.