"base drag" and tailcones?

In poking around the forums, I've seen references to "base drag", and seen an article talking about the center-of-pressure of a flat plate, and treating the base of a short/fat rocket as a flat plate and adding an imaginary cone to rocksim to properly model the stability of such a rocket.

My question, then, is does adding a tailcone to a short/fat rocket make it less stable? It will clearly move the CP forward some, since there is less tube at the back of the rocket now, but will it also negate the "plate" or "base drag" effect?

I'm wondering because I plan to build just such a rocket... imagine, if you will, a fat-boy nosecone butted up against a phoenix nosecone, with just enough body tube to connect the two... cut off the tip of the phoenix nosecone and stick a 24mm motor mount up it...

If the whole contraption is similar in length to a fatboy, uses fatboy shaped fins (extended such that the entire length of the root is attached to the motor mount tube), will it be unstable?


thanks,
Rick (prodigal rocketeer, rocksimless)

It sounds kind of like a teardrop with fins. Am I right? If that's the case, then my intuition says it should be stable. I'm not at home (where my rocksim is), but my mental image says it's good to go. Take away the fins, and you've got a problem, but the fins (of Fat Boy size/shape) will keep the CP far enough aft that you'll be okay.

WW

I ran this and used a 29mm mount. It is marginally stable with 4oz of nose weight. Sorta cool design.
Picture attached.

Here it is as a 24mm.

Originally posted by Sailorbill
I ran this and used a 29mm mount. It is marginally stable with 4oz of nose weight. Sorta cool design.
Picture attached.

Click to expand...

Hey Chief, could you run that through for him again, but with a 24mm, since that's what his intended build was? I think he might be able to get away with not even having any noseweight. I agree that it looks pretty cool, and would absolutely ROCK on a G.

WW

OOPS that was the 3D version.
Here is the 2D.

It looks almost like the old WWII bombs, but with 3 fins instead of the 4-in-a-box fin arrangement. Pretty interesting.

WW

I could not get rid of the nose weight. It just went crazy.
This is with 2 oz nose weight and an F39-6. About 900 ft.
As you said it does look like and old bomb but it is cool.

I'm kinda surprised, considering that Estes designed the Phoenix to fly on just a single 24mm BP motor, and it's a heavier design with 2 full pats of noseweight clay.

I created a simulation file for this design with and aft transition to take into account the base drag on this short wide rocket. It balances out with 1.5 ounces of nose weight if you want to fly this rocket on a G55 motor (4544' AGL).

Bruce S. Levison, NAR #69055

Thanks for all the help, guys! I am graphically challenged, so have been unable to get a good electronic image of my plan . I was actually planning to fly it on E-6 RC reloads (if it had a 24mm mount), with an altimeter somehow built into the nosecone. I'm a big fan of short/fat rockets, but I'm also a sucker for the teardrop shape.

It needed noseweight with an F, would it still need noseweight with an Estes E-9, or an aerotech E6-RCT? If I build it for the E6 it then the altimeter may function as noseweight anyways, but if it gets heavier than the Estes phoenix then it's not likely to lift well on the E6.

If I were going to fly it on G's, though, I'd be so very tempted to use a BT-55 for the engine mount so it can take the 32mm G12-RCT...

thanks,
Rick

It would definitely need noseweight - the E6 is as heavy as an F39.

Neat design though, and if you build it, definitely post pics