FOC, arrow vanes and rocket stability.

[Senior Space Cadet]

I could have put this under a previous post, but decided it might need a separate discussion.
In my opinion, model rockets and arrows have a lot in common.
People have been making arrows for thousands of years. Rockets, not that long.
An important term in putting arrows together is front of center (FOC). Basically, this is referring to the balance point being ahead of the mid point of the shaft. If it isn't, the arrow won't be stable.
I've been using this in my rocket design which has lead some to think that I'm not using any method at all, for making sure my rocket is stable.
I've posted a couple rocket designs, not complete rockets, just designs, that had arrow vanes for fins. I'll attach a photo of the most recent one. The body is 12" long and I'm using 24mm tubes.
I was told that the fins were too small and didn't stick out far enough. Well, OK, maybe. I know arrows but not rockets.
But using my FOC method, I first did a spin test of just the body with weighted nose cone. It passed with flying colors. Flew beautifully.
Believing that a more complete rocket might perform differently, I installed a C6-5 motor and again balanced it so the balance point was ahead of center. At least half an inch.
I did the spin test and again it performed perfectly.
It seems pretty obvious to me that the arrow vanes work OK. Maybe I'm insane, but that's what I think. I want to play it safe, so a working model might have an 18" body, or I might space the vanes out from the body with some balsa. Kind of a hybrid fin design. I'd, of course, start with a much smaller motor and work my way up, watching for any signs of instability. And obviously, since I'm 66 and not some kid, I'll try to be responsible.
Hybrid fin designs are coming, waiting for a fletching clamp and some other stuff.

 

[ToneDeafJunior]

That's a good looking rocket SSC. Very sharp looking. With your attention to detail, I'd suggest downloading the program OpenRocket. You should love playing with all the parameters... OR is very good for single tube rockets. Boosters, not so much. I'm sure there are better programs out there you pay for, but OR is free and has worked well for me most of the time. One of the things I was told when I started with rocketry was to make the fins between 2 and 2.5 times the diameter of the tube. Maybe this isn’t the case, but this is what I was told. When I downloaded OR and started playing with it, I found that I needed to reduce the size of my fins. Larger fins pushed the center of pressure further back in reference to the center of mass exceeding 1 to 2 times the diameter of rocket which allowed for more 'weather cocking' in flight. I have experienced this firsthand in many builds. OR teaches me to pay attention to the fin scale to obtain a more optimum flight. I sure someone else in the forum with more experience can let us know where I might be wrong, but I find smaller fins work better for me in many applications.

 

[dhbarr]

Qty. 4x fins about 1.1 times as "tall" as the body tube is "wide" are recommended for sounding rockets that are 10-12 "widths" "long".

The reason for this is so that when the rocket is not going perfectly straight, the end of the fin sticks out into the clean air and can still do it's job.

That being said, more shorter fins will work to a certain extent if you have enough of them, you should just expect a squirrellier flight if you have any crossbreeze, rodwhip, etc.

EDIT: instead of FOC, we say CP and CG; the distance between them is typically given in "calibers" ( of stability ) , and they both move around during flight.

 

[dr wogz]

Senior, one thing I've seen in all your posts, is that you tend to lump "flying things" together. While they are similar, they are not the same, not interchangeable. (Well, for the most part, not interchangeable)

While we are disproving some of your ideas & thoughts, we are also suggesting corrections and trying to help you out.

 

[Steve Shannon]

I could have put this under a previous post, but decided it might need a separate discussion.
In my opinion, model rockets and arrows have a lot in common.
People have been making arrows for thousands of years. Rockets, not that long.
An important term in putting arrows together is front of center (FOC). Basically, this is referring to the balance point being ahead of the mid point of the shaft. If it isn't, the arrow won't be stable.
I've been using this in my rocket design which has lead some to think that I'm not using any method at all, for making sure my rocket is stable.
I've posted a couple rocket designs, not complete rockets, just designs, that had arrow vanes for fins. I'll attach a photo of the most recent one. The body is 12" long and I'm using 24mm tubes.
I was told that the fins were too small and didn't stick out far enough. Well, OK, maybe. I know arrows but not rockets.
But using my FOC method, I first did a spin test of just the body with weighted nose cone. It passed with flying colors. Flew beautifully.
Believing that a more complete rocket might perform differently, I installed a C6-5 motor and again balanced it so the balance point was ahead of center. At least half an inch.
I did the spin test and again it performed perfectly.
It seems pretty obvious to me that the arrow vanes work OK. Maybe I'm insane, but that's what I think. I want to play it safe, so a working model might have an 18" body, or I might space the vanes out from the body with some balsa. Kind of a hybrid fin design. I'd, of course, start with a much smaller motor and work my way up, watching for any signs of instability. And obviously, since I'm 66 and not some kid, I'll try to be responsible.
Hybrid fin designs are coming, waiting for a fletching clamp and some other stuff.

You’re clever for applying what you know, but this is an excellent opportunity to extend what you know into what you might not know.
An arrow consists of an arrowhead, a homogeneous shaft, and the fletching. Because the fletching are just slightly larger than broadhead arrowheads, the center of pressure will frequently end up near the center of the arrow. Arrowheads (especially flint arrowheads) are sometimes heavy though which places the center of gravity ahead of the center of the arrow. Smaller types of arrowheads, such as field tips, result in a shift rearward of the center of pressure. If you look at the native arrows used by some indigenous tribes which simply sharpen wooden shafts, you’ll see much larger fletching used on the arrow in order to make up for the lack of forward weight.
The relationship of center of gravity and center of pressure is crucial for aerodynamic stabilization. Simply making sure that the balance point (center of gravity or center of mass) is ahead of center of length is a very simplistic way of maintaining that relationship, as long as a rocket doesn’t do much different than an arrow, but limits your design choices. The balance point or CG really doesn’t have to be that far forward. It just has to be forward of the center of pressure CP, typically by the diameter of the rocket, which we call a caliber. Having it too far forward can result in a rocket that is very twitchy and overreacts to wind shear.

 

[ToneDeafJunior]

I can't wait to use that first line in a business meeting. I'm saying it to myself over and over. Thank you.