non-round rockets

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Gunstar

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I'm scratch building a rocket that will look like a pencil (minus bite marks, at least to start with)

The main body tube is a bt55, but i'm going to put a hexagonal wrap around it using yellow bristol board (might glue two layers of it together for strength. How does it affect CP when the body cross section is not round? Do you base things on the corner to corner width, side to side wdith, or an average of those or what?
 
My gut says calc the perimeter of the hex, then back out the diameter.

We're mostly worried about skin drag here, which is mostly surface area. Probably a little less in the middle of your faces, a little more at the corners.

If you attach the find on the flats you'd get an increase in interference drag, a decrease by mounting at the corners.

All in all it's not going to make a dramatic subsonic difference. Stay away from the extremes your first flight or three and use those results to tune your model.
 
My guess is the same as dhbarr; you won't see much difference. Given that motors can vary by + or - 10% thrust (if I recall correctly (and don't bet anything on my recall)), you will see a greater difference based on any given motor than on the shape you are proposing. I have square rockets made by New Way and they fly fine; not as high as a round rocket, but still fine.

And if you are doing a pencil I assume it will be long and skinny, so that works in your favor too. Long and skinny is more stable than short and wide. You aren't doing one of those stupid golf pencils which are really short, are you?
 
When I did my square rocket, I simulated with a diameter that gave the same cross-section area as the square. For a hex, equal perimeter is going to be pretty close to equal area.
 
When I did my square rocket, I simulated with a diameter that gave the same cross-section area as the square. For a hex, equal perimeter is going to be pretty close to equal area.
+1, area is less incorrect / a better choice.
 
Because cross sectional area is a influence for drag force calculations by fluid mechanics. Set your non round area equal to the cross sectional area of a circle then solve for diameter to simulate at. Velocity, area, density, and drag coefficient are all the variables in a drag force calculation. Plate drag is same. You may have some minor skin drag differences but other than that, it'll be good enough. Velocity term is squared. So if your pencil goes Mach 10 like Sprint ABM expect a larger program error by a skin drag error.
 
If you wanted to get really OCD nit picky about it you'd realize the program only does some plate drag by cross sectional area. You could 3D model it and sim it at whatever Mach you want then pull a proper drag coefficient off of it, but for a hobby rocket outside of a university or other competition there's no practical point. That's the difference between a real rocket and a hobby toy at some point people stop caring about data and say good enough. There's going to be a slight discrepancy in the drag coefficients between a hex and circle. But most people don't have the knowledge, time, or tools to find out. You'd need a wind tunnel capable of the Mach number the rocket goes or a CFD and CAD model with fluid mech knowledge and more to get a real drag coefficient. Wish there was a simple one size fits all chart but there's this reality bit that pisses engineers off and requires tests. And for practical purposes below say Mach 1 the error is really small a few decimals places.
 
It will be 3 feet tall. I'm hoping I can put enough weight in the nose that it will be stable without fins, or at least really small fins.. I don't care if that means it doesn't set altitude records. I care about my rockets looking good and and flying stable. If they don't go super high then the clouds are less likely to eat them.
 
Worry less about how the rocket looks and more about the stability margin cal is. You want a range between 1.5-2.0 for some general rockets. Look up apogee articles on sizing parachute and weather cocking and they will explain more info. You always want the center of gravity ahead of center of pressure. Stability cal is a ratio of tube diameter in length distance between the two points. CP will be area resisted to surface body time area and fin area. Trapezoidal and triangle fins are easy to make.
 
It will be 3 feet tall. I'm hoping I can put enough weight in the nose that it will be stable without fins, or at least really small fins.. I don't care if that means it doesn't set altitude records. I care about my rockets looking good and and flying stable. If they don't go super high then the clouds are less likely to eat them.
There is no amount of nose weight you can add that will make a finless pencil-shaped rocket stable. Somebody mentioned polycarbonate, which is the typical choice.
 
As others have said, a finless rocket will not be stable. Nose weight added to a finless rocket just makes a heavier unstable rocket.
 
As others have said, a finless rocket will not be stable. Nose weight added to a finless rocket just makes a heavier unstable rocket.

Qualifier" a finless rocket without base drag, spin stabilization, or other modification beyond a simple body tube will not be stable.
 
I'm scratch building a rocket that will look like a pencil (minus bite marks, at least to start with)

The main body tube is a bt55, but i'm going to put a hexagonal wrap around it using yellow bristol board (might glue two layers of it together for strength. How does it affect CP when the body cross section is not round? Do you base things on the corner to corner width, side to side wdith, or an average of those or what?

Here's an easy way to find out how the CP changes (easy for me, you're going to do the work!).

Calculate the location of the CP using a round body tube the diameter of the hexagon (pencil) across the flats. Calculate the CP using a body tube the diameter of the hexagon across the points. What's the difference in absolute value (inches, cm, etc.)? What's the difference in percent? Does the CP location even change?
 

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