Caliber and stability

[Charles_McG]

The other rule of thumb of distance between CP and CG >= 10% of length is another good option that is closer to a ratio of restoring force to angular inertia. The challenge with calculating a true angular inertia is knowing where the weight is. Rockets with the same CG could have wildly different inertia values if one has a heavier motor and lots of nose weight to bring the CG to the same location.

Yep. And I list both in my preflight checklist.

It seems to me that software like Open rocket has a good idea of the axial positioning of both mass and aerodynamic force. At least it has the mass as long as you aren't using a master override.

Part of my checklist:

 

[tsmith1315]

The other rule of thumb of distance between CP and CG >= 10% of length is another good option that is closer to a ratio of restoring force to angular inertia

And with that rule of thumb, is there a rule of thumb for when to use the other rule of thumb?

i think i missed the swing test.

Stine, again.

 

[Off Grid Gecko]

I have a few estes rockets and a 3D printed rocket for a D motor. Ill re-search the threads of mine because i think i missed the swing test.
Ill also model the estes rockets i have in OR to see what i get. back in the day i used to tape a stick to just a rocket motor and launch it. they didnt go as high as my rockets and when i was younger, that puzzled me haha

I was the opposite, I would take bottle rockets apart and make a foil rocket airframe around them. Made those matchstick rockets too in college. Nice for my roomates to wake up with one coming at their face. Ah to be young and dumb again. I don't know how any of my friends survived, let alone myself.

Okay cool, I didn't know your inventory. Those little rockets can be treasure troves as teachers. The higher my cert level gets the more I'm drawn to little rockets. Maybe cause I can build a crate of them for the price and less time than a HPR.

 

[boatgeek]

ok, let me make sure i understood the way you wrote it. so CP should be 10% of the rockets length behind CG? ie, if rocket is 3 feet, cp should be (0.1)(36inches) behind the CG?

That's correct, see below for reasons why you'd use this instead of 1-2 calibers (plus some more if you're breaking mach).

Yep. And I list both in my preflight checklist.

It seems to me that software like Open rocket has a good idea of the axial positioning of both mass and aerodynamic force. At least it has the mass as long as you aren't using a master override.

Part of my checklist:
View attachment 538314

Let me tell you how to grow apples for cider, too. I bow before your checklist and info page.

And with that rule of thumb, is there a rule of thumb for when to use the other rule of thumb?

AIUI, the 1-2 caliber rule of thumb is great for a rocket where L-D ratio is approximately 10. If you have very long or very short rockets, it can underestimate or overestimate required stability, respectively. I'd start thinking hard about using the 10%L rule of thumb for anything over 15:1 L-D or under 5:1. YMMV and there are lots of people who know more than me about the topic. Lower L-D can also use the base drag hack in OR/Rocksim, but that's another discussion entirely.

[edit] If I type L : D without spaces, I get L

 

[rharshberger]

And with that rule of thumb, is there a rule of thumb for when to use the other rule of thumb?



Stine, again.

Reading that one little book would help you immensely, thats why its known as the Rocketry Bible nearly everything you need to know is in it...The Handbook of Model Rocketry by G. Harry Stine and Bill Stine.

 

[Walter Longburn]

If you go on open rocket and go to tools/component analysis, you can change Mach number and watch how speed changes the CP. It typically moves backwards until around M1.3 and then starts moving forward.

 

[Walter Longburn]

Another rule of thumb I use is at least 1.1 caliber of fin span.

 

[JackC]

Another rocket god along the lines of John Coker is Vern Knowles. He also has a website that shows his extensive CAD plans of many of his rockets along with helpful guides on things like the number and size of altimeter vent holes.

https://www.vernk.com/

 

[CoyoteNumber2]

If you go on open rocket and go to tools/component analysis, you can change Mach number and watch how speed changes the CP. It typically moves backwards until around M1.3 and then starts moving forward.

I'm glad someone brought this up. Stated another way: Center of Pressure is not fixed. It moves in flight, and the stability margin must be maintained for any/all CP locations. Fortunately, OpenRocket makes this easy to determine.

 

[user 30299]

Ok, reading some documentation in OR and a few books, i have seen the same term used over and over, and that is caliber. In units of measurement, that is inches, IE 50 caliber is 0.5 inches in diameter. So if my rocket has a 2.35 caliber, which according to OR basic rocket design documentation is over stable, does this mean 0.02 inches from CG to CP?

" Ensure at least 1.0 airframe caliber of separation between the (forward) center of mass and (aft) center of pressure. This is a rule of thumb, not a hard-and-fast stability solution."
what is deemed as "airframe caliber" ? is that meaning the diameter of the body tube in a measurement of caliber rather than inches? so a 3 inch diameter is a 300 caliber body tube?

I know QFactor gave me a tip of keeping the CP about 1-2 times the body diameter behind the CG.

thanks.

QFactor did not give you that tip - just to keep the record straight. But the 1 to 2 diameters is correct.